Ligand-stabilized dilithium (C6F5)2Li2 featuring two planar tetracoordinate lithium and carbon centers
Guo Y., Li Y., Qiao Y., Shan Y., Ding C., Pino-Rios R., Pan S.
Article, Journal of Chemical Physics, 2026, DOI Link
View abstract ⏷
The design of planar hypercoordinate Li represents a significant challenge because the stabilization in such molecules arises exclusively from electrostatic interactions, while covalent glue, particularly delocalized π/σ bond, is needed to stabilize a planar conformer. Here, we report a computational study of a novel system, two pentafluorophenyl ligands stabilized dilithium, (C6F5)2Li2, featuring two planar tetracoordinate lithium (ptLi) atoms and two planar tetracoordinate carbon (ptC). The design strategy was inspired by the recent synthesis of tolyl–lithium complexes and refined through systematic structural modifications to achieve a fully planar geometry corresponding to a true minimum on the potential energy surface. Both thermodynamic and kinetic analyses demonstrate that the structure is stable under static and dynamic conditions. A thorough bonding analysis using different methods reveals that the stabilization of the ptLi atoms arises primarily from the electrostatic interactions, while the orbital contributions are comparatively weak. On the other hand, the stabilization of the ptCs is the interplay of both electrostatic and covalent interactions. Aromaticity analysis based on magnetically induced current densities indicates that aromatic character is confined to the benzenoid rings, whereas the Li-containing core is non-aromatic. These findings expand the conceptual framework for hypercoordinate species in s-block elements and highlight the role of selecting proper ligands that can lead to the realization of such planar hypercoordinate Li, not only in the cluster form but also in molecular materials.
Molecular Boron-Phosphides: From Stable Monomers to Aromaticity-Tunable Smallest Neutral Metallacycles
Purushothaman A., Liang H., Salam F.A., Parashar A., Francis M., Pan S., Sun D., Roy S.
Article, Inorganic Chemistry, 2026, DOI Link
View abstract ⏷
Unlike the conventional polymeric boron-phosphide-based (BP) semiconductors, which exhibit limited reactivity, the neutral monomeric BP motif (1) is extremely reactive and nonexistent under ambient conditions. Herein, we depict the ligand-engineering strategies for stabilizing the elusive species 1, initially by employing stereoelectronically tuned donor-based ligands, followed by their successive incorporation into the smallest metallacycles with induced aromaticity, and hence excellent stability. The electron density distribution and chemical bonding of homo- and heterobileptic ligand-stabilized monomers [(L′)BP(L)] (2–7) [L′, L = singlet carbenes], and the corresponding neutral 3-membered metal(II)dihalide complexes [BP(MX2)] (8–9′) and [((L′)BP(L))(MX2)] (10–17) [M = Pd/Pt, X = Cl, Br], are investigated by various quantum chemical methods. The remarkable ligand-switched σ and π aromaticity in the unprecedented mixed d- and p-block planar metallacycles is unambiguously confirmed by NICSzz calculations, ELF, AdNDP, GIMIC, and EDDB analyses.
Quadruple Bonding of Alkaline Earth Atoms in AeCLi4 (Ae = Be − Ba) Complexes
Li Y., Ding C., Pan S., Frenking G.
Article, Journal of Computational Chemistry, 2026, DOI Link
View abstract ⏷
The results of quantum chemical calculations of the complexes AeCLi4 (Ae = Be − Ba) are reported at the BP86-D3(BJ)/def2-QZVPP and CCSD(T)/def2-QZVPP level. The calculated equilibrium geometries with Ae = Be, Mg have a trigonal bipyramidal geometry (C3v symmetry) as the energetically lowest-lying form. A slightly higher-lying isomer has a square pyramidal geometry (C4v symmetry), which is only < 1 kcal/mol less stable than the C3v form. In contrast, only the square pyramidal structure is an energy minimum of the heavier homologues with Ae = Ca, Sr, Ba. The calculated bond dissociation energies of the Ae-CLi4 bond are very high. The strongest bond is computed for the Be-CLi4 bond (De = 82.9 kcal/mol at CCSD(T)/def2-QZVPP). The weakest bond is calculated for the Mg-CLi4 bond (De = 40.6 kcal/mol). The heavier homologues have values between De = 63.1 kcal/mol (Sr-CLi4) and De = 72.0 kcal/mol (Ba-CLi4). Inspection of the occupied valence orbitals and the AdNDP results suggests that there are four Ae-CLi4 bonds in the complexes. This is supported by the EDA-NOCV analysis, which reveals that there is a dominant Ae → CLi4 σ-donation, which is enhanced by weaker Ae ← CLi4 σ-backdonation and degenerate Ae ← CLi4 π-backdonation. The best signature of the chemical bonds is Ae (Figure presented.) CLi4. The lighter atoms, Be, Mg, use their (n)s and (n)p AOs for the covalent bonds, whereas the heavier atoms, Ca, Sr. Ba, employ their (n)s and (n-1)d AOs for the covalent interactions. The NBO method does not provide a reasonable account of the covalent bonds, because it does not consider the (n)p and (n-1)d AOs of Ae atoms as genuine valence orbitals.
A Cerium Yldiide Complex with a Ce←←[jls-end-space/]C Double Dative Bond
Su W., Li Y., Sun N., Ding C., Pan S.
Article, Inorganic Chemistry, 2026, DOI Link
View abstract ⏷
Methandiides, bisylides, and yldiides are geminal dianions having two lone pairs of electrons at the central carbon atom and are applicable to construct f-block carbon multiple-bond complexes. However, cerium yldiide complexes possessing a cerium–carbon double bond are not known to date. Herein we report cerium yldiide complex 4 which bears a short Ce–C bond of 2.461(5) Å, exhibiting significant cerium–carbon multiple-bond character. The nature of the Ce–Cmethine bond in 4 was probed by DFT, unveiling a rare Ce←←[jls-end-space/]C double dative bond. Therefore, 4 is the first cerium yldiide double-bond complex. Complex 4 underwent nucleophilic addition toward ClBPh2 to give 5 incorporating a borate-functionalized ylide. These findings may provide straightforward access to lanthanide yldiide multiple-bond complexes from lanthanide ortho-metalated ylides.
Dinitrogen complexes N2L2 (L = N2, CO, CS, NO+, CN−)
Li Y., Ding C., Xie L., Pan S., Frenking G.
Article, Chemical Science, 2026, DOI Link
View abstract ⏷
Quantum chemical calculations using ab initio methods and density functional theory have been carried out on the equilibrium structures and the vibrational spectra of the (valence) isoelectronic compounds N2L2 (L = N2, CO, CS, NO+, CN−). The molecules have a trans-periplanar arrangement of the L2 ligands at the N2 unit. The complexes with L = N2, CO, NO+, CN− are predicted as thermodynamically unstable for dissociation into N2 + 2L with ΔG298 value lying in between −257 kcal mol−1 (L = NO+) and −73 kcal mol−1 (L = CO), but the adduct N2(CS)2 is calculated as slightly stable with ΔG298 = 4 kcal mol−1. The homolytic dissociation reaction into two fragments N2L2 → 2 NL is energetically less favorable than the heterolytic fragmentation N2L2 → N2 + 2 L, which proceeds synchronously but asymmetrically. The activation barriers for the fragmentation reaction N2L2 → N2 + 2L have values between ΔG≠(298 K) = 17 kcal mol−1 for L = N2 and ΔG≠(298 K) = 84 kcal mol−1 for L = CS. The calculated vibrational frequencies suggest that the molecules N2L2 can be identified by the IR active antisymmetric stretching mode νas of the ligands L, which is blue shifted for L = CO (Δ = 55 cm−1) and L = NO+ (Δ = 118 cm−1) but it is red shifted for L = CS (Δ = −242 cm−1) and L = CN− (Δ = −133 cm−1) relative to the νas mode of L = N2. The analysis of the bonding situation reveals that there is a total charge donation L→(1Γ-N2)←L in all complexes, ranging between 1.38 e (L = CN−) and 0.56 e (L = N2), except in the dication with L = NO+, where a small backdonation in reverse direction L←(1Γ-N2)→L with 0.10 e is calculated. EDA-NOCV calculations of N6 show that the best description of the bonding situation is given in terms of dative interactions N2→(1Γ-N2)←N2 between central N2 in the excited (1)1Γg singlet state and the terminal N2 fragments in the 1Σg+ electronic ground state. In contrast, the best description of the complexes with L = CO, CS, NO+ is calculated for the interactions between the central N2 in the 5Σu+ quintet state and the terminal ligands in the symmetry-adapted (L)2 quintet state. For N2L2 with L = CN−, it is found that the bonding is best described for the interaction between N2− in the electronic quartet (4Σu+) state and the terminal (L)2− ligand as symmetry-adapted quartet. In contrast to the common bonding model for N6 using Lewis structures N−N+N–NN+=N−, the donor–acceptor model N2→(N2)←N2 explains that the lowest activation barrier is found for the concerted cleavage of the two formal double bonds, leading to the experimentally observed dissociation into 3 N2.
Manifestations of Boron-Alkali Metal and Boron-Alkaline-Earth Metal Romances
Cui Z.-H., Cui L.-J., Barroso J., Guo J.-C., Zhai H.-J., Pan S., Merino G.
Article, Accounts of Chemical Research, 2026, DOI Link
View abstract ⏷
Conspectus: The electron deficiency of boron promotes the formation of multicenter σ and π bonds that endow its clusters and solids with exceptional structural diversity. While bulk boron favors cage-like frameworks, clusters often adopt planar or quasi-planar motifs composed of triangles that evolve into tubular and cage-like architectures as their size increases. Many of these clusters are stabilized by delocalized σ and π bonds that are associated with fluxional behavior and multiple aromaticity.Metal doping enriches this chemistry. Transition metals use their d or f orbitals to couple with the boron framework, generating metal-centered rings, metallo-boron nanotubes, and metalloborophenes. In contrast, alkali and alkaline-earth metals have long been viewed as simple counterions, yet recent findings reveal that they can orchestrate deep structural reorganizations by combining charge transfer with efficient orbital overlap. Lithium, for example, leads to a quasi-planar → tubular → cage evolution in B12 clusters via strong electrostatic attraction to the boron framework, whereas beryllium engages in pronounced covalent Be–B interactions that yield rare architectures such as the Archimedean Be4B12+ cage, the B–Be sandwich B7Be6B7, and four-ring tubular forms like Be2B24+.In heavier alkaline-earth systems, the participation of (n–1)d orbitals (Ca, Sr, Ba) introduces transition-metal-like covalent interactions, producing highly symmetric rings and tubular clusters. This Account summarizes how electrostatic and covalent interactions jointly control geometry and bonding in boron–metal systems, defining the rich landscape of boron chemistry.
An isolable germa-isonitrile featuring a terminal nitrogen–germanium triple bond
Wang Z., Ding C., Chen Y., Huang M., Wang D., Xu L., Pan S., Ye S., Tan G.
Article, Nature Chemistry, 2026, DOI Link
View abstract ⏷
Isonitriles (R–N≡C), first discovered by Lieke in 1859, are well-established functional molecules in organic and organometallic chemistry. By contrast, the synthesis and investigation of tetrela-isonitriles (R–N≡E, E = Si, Ge, Sn or Pb), their heavier group 14 analogues, remain challenging due to their high reactivity. The characterization of such species has largely relied on spectroscopic data collected at cryogenic temperatures or under gas-phase conditions. Here we report the synthesis and characterization of a germa-isonitrile (Ar–N≡Ge) stabilized by a bulky aryl ligand. This compound, which features a terminal N≡Ge triple bond with a Ge‒N bond length of 1.6395(19) Å, has been characterized through X-ray crystallographic, solid-state ¹⁵N nuclear magnetic resonance spectroscopic and computational studies. The highly polarized N≡Ge moiety exhibits versatile reactivity towards organic substrates and transition metal precursors, underscoring its potential use in synthetic chemistry. (Figure presented.)
Quadruple bonding between carbon and transition metal in the global minimum geometry of CM(BO)(CO)2− (M = Ru, Os)
Liu Y.-Q., Hou X.-Y., Yan B., Pan S., Cui Z.-H.
Article, Journal of Chemical Physics, 2025, DOI Link
View abstract ⏷
Prompted by the previous report of BFe(CO)3− possessing a B≣Fe quadruple bond, the detailed potential energy surface exploration for the BMC3O3− (M = Fe, Ru, Os) formulation reveals that the most stable isomer for M = Ru, Os has a C s-symmetric CM(CO)2(BO)− (M = Ru, Os) structure in a singlet electronic state with an ultra-short C–M bond along the center axis, whereas for M = Fe, the global minimum is a C s-symmetric isomer in the triplet electronic state where C of (OC)C(BO) binds with Fe of the FeCO unit. BM(CO)3− is a kinetically stable high-lying isomer for all cases. Detailed bonding analyses on CM(CO)2(BO)− (M = Ru, Os) reveal that the C–M bond can be described as a quadruple bond consisting of a strong electron-sharing C–M(CO)3− σ and π bonds, accompanied by a strong C←M(CO)3− π bond and a weak C→M(CO)3− σ bond. These bonding motifs expand the landscape of high-order multiple bonding between main-group elements and transition metals, particularly in the context of heavier transition-metal carbonyl complexes.
Theoretical Prediction of a Stable Xenon Bis(diazaborolyl) Complex: A Donor–Acceptor Complex
Xie L., Li Y., Leyva-Parra L., Ding C., Tiznado W., Pan S.
Article, Inorganic Chemistry, 2025, DOI Link
View abstract ⏷
Complexes with bulky ligand-supported low-valent elements are very well-known in chemistry. However, because of their little reactivity, such complexes are unknown so far for noble gas (Ng) atoms. Here, the viability of a xenon complex with the ligand diazaborolyl ((L = HCN(dipp))2B) in the form of HCN(dipp)2B–Xe–B(HCN(dipp))2(1) is assessed through quantum chemical calculations. Complex 1 is thermochemically stable at room temperature against dissociation, 1 → Xe + 2L. Although the dissociation process that leads to the formation of ligand dimer [(HCN(dipp))2B]2and Xe, 1 → Xe + L2, is exergonic in nature, the scrutiny of the corresponding mechanism through B–Xe–B bending reveals that this process eventually leads to the formation of free Xe and two (HCN(dipp))2BH units with one isopropyl group in dipp being converted into an isopropenyl group. This process involves a significant potential energy barrier to occur. 1 can be described as a donor–acceptor complex between ligand and Xe, L⇄Xe⇆L, where Xe is in zero oxidation state. Despite being a donor–acceptor complex, the electrostatic interaction in the B–Xe–B bond plays a crucial role in the stabilization of the complex.
Revisiting aromaticity and stability in the diboron actinide compound Pa2B2
Ding C., Ruiz L., Vasquez-Espinal A., Pino-Rios R., Paez-Hernandez D., Pan S., Leyva-Parra L., Alvarez-Thon L., Tiznado W.
Article, Chemical Science, 2025, DOI Link
View abstract ⏷
Clusters composed of heavy elements, particularly actinides, provide a compelling platform for exploring unconventional bonding and the role of relativistic effects in electronic structure and stability. In this study, we critically reassess the D2h-symmetric Pa2B2 cluster, previously claimed to exhibit double Möbius-Craig aromaticity through delocalization of 4σ and 4π electrons. Our potential energy surface (PES) analysis disproves this assignment by showing that the D2h structure is a higher-energy isomer; the most stable form adopts a distorted tetrahedral structure. Magnetically induced current density (MICD) analysis—based on fully relativistic four-component Dirac-Coulomb calculations—further reveals the absence of a net diatropic ring current. Instead, a weak net paratropic response and a localized vortex are observed, associated with a σ Pa-Pa bond via dz2 orbitals. Multiconfigurational analysis using CASSCF(16,16) confirms that the D2h structure is dominated by a single-reference configuration (88%), supporting the reliability of our DFT computations. As a point of contrast, we evaluated the ReB4− cluster—experimentally observed and computationally confirmed as the global minimum—which exhibits a strong diatropic ring current (16.3 nA T−1), demonstrating that MICD reliably captures aromaticity when transition-metal d-orbitals are genuinely involved in cyclic delocalization. These findings underscore the importance of rigorous PES validation, multiconfigurational treatment, and fully relativistic analysis, including spin-orbit coupling, when assessing aromaticity in clusters of heavy elements. More broadly, this work reinforces the need to critically reassess the growing number of ‘unconventional’ aromatic motifs, many of which arise from incomplete analysis or mischaracterization of electronic structure rather than genuine bonding novelty.
Ng7Be2B5+: Binding of Noble Gas Through Both Cationic Beryllium and Anionic Boron Centers
Li Y., Liu Y.-Q., Ding C., Saha R., Cui Z., Pan S.
Article, Journal of Computational Chemistry, 2025, DOI Link
View abstract ⏷
Quantum chemical calculations have been performed to investigate the structure, stability, and bonding in noble gas (Ng) bound Be2B5+ complexes. The present results show that Be2B5+, a charge-separated [Be]2+[B5]3−[Be]2+ cluster, can employ both its cationic Be center and anionic B center to bind Ng atoms. It can bind a total of seven Ng atoms, resulting in the formation of a highly symmetric (NgBe)2Be2(NgB)5B5+ complex, having D5h point group. The thermochemical analyses reveal that the Ng-Be bonds are stronger than the Ng-B bonds. (NgBe)2Be2B5+ (Ng = Ar-Rn) complexes are stable against the dissociation of Ng atoms even at room temperature. But, (NgBe)2Be2B5+ (Ng = He and Ne) and (NgBe)2Be2(NgB)5B5+ (Ng = Ar-Rn) complexes are stable only at very low temperatures. Therefore, they can be suitable candidates for low-temperature matrix isolation. A thorough bonding analysis, through charge and energy decomposition methods, discloses that despite the Ng-B interaction being weaker than the Ng-Be interaction, the former bond is more covalent than the latter one. In fact, in the Ng-B bonds, both the orbital and electrostatic interactions are larger in magnitude than the Ng-Be bonds; however, significantly larger Pauli repulsion in the former bonds makes them weaker than the latter bonds. In both Ng-Be and Ng-B bonds, the covalent interaction originates from a strong Ng(pσ) → Be2B5+ σ donation, complemented by two weak Ng(pπ) → Be2B5+ π donations.
Clarification of Some Bonding Concepts: Virial Theorem, Electron Pair Repulsion, and Rotational Barriers
Schwarz W.H.E., Frenking G., Pan S.
Article, Journal of Computational Chemistry, 2025, DOI Link
View abstract ⏷
The molecular virial theorem relates kinetic and potential energies (T & V) to total energy and forces (E & R·∂E/∂R); it is a useful tool for analyzing the data, but does not provide clues on the origin of the stability of the “bonded” state. A strict conceptual distinction between cause and effect is recommended. Depending on the physical relationships, the induced change of one variable of the system leads to a resulting change of another variable; relaxation or response of the system can either moderate this change (in the sense of Le Chatelier's principle), enhance it, or even reverse it. Such unexpected, paradoxical behavior is common in reality and in daily life. As two examples of conceptual mix-up in molecular chemistry, we discuss details of the origin of the steric pair-pair repulsion and of the internal rotation barrier in ethane.
Synthesis and Structure of Uranium Disilyl-Substituted Alkylidene Complexes
Li Y., Ding C., Zhao Q., Wang S., Xie J., Pan S., Zhu C.
Article, Journal of the American Chemical Society, 2025, DOI Link
View abstract ⏷
Understanding the participation of f-orbitals of actinide elements in covalent bond formations is less explored, compared to the well-studied d-orbitals of transition metals, leading to the significant interest in actinide-carbon multiple bonds. Uranium alkylidene complex, containing an alkylidene linkage of the form U═CR2 (R = H, alkyl, silyl), represents a key milestone in actinide-ligand multiple bonding, but their isolation and characterization have remained elusive. Herein, we present the synthesis of an unprecedented uranium disilyl-substituted alkylidene complex, achieved through sequential dehydrogenation reactions of a methyl group under mild conditions. Single-crystal X-ray diffraction reveals the U═C double bond length of 2.332(4) Å. Quantum chemical calculations suggest that both 5f and 6d orbitals of uranium play a key role in the U═C double bond formation.
From Bis(borylene)-Substituted Xanthenes as Reactive Intermediates to Diboraoxirane Complexes
Fan J., Pan S., Yao S., Ding C., Frenking G., Driess M.
Article, Journal of the American Chemical Society, 2025, DOI Link
View abstract ⏷
The first N-heterocyclic carbene (NHC)-stabilized diboraoxirane complex 4 [NHC = IPr = C{N(iPr)CMe}2] was synthesized through the reduction of the corresponding bis(dichloroboryl-IPr)xanthene 3 with potassium graphite. Intriguingly, its formation stems from a diboron(I)-mediated C-O-C deoxygenation of the xanthene spacer via a bis(borylene)xanthene as a reactive intermediate. Consistent with the proposed pathway, bis(borylene)xanthene 6 with three-coordinate B(I) atoms could be isolated when the sterically less demanding NHC ligand IMe [IMe = C{N(Me)CMe}2] was employed. Due to its ring strain, the B-B bond of the B2O ring in 4 undergoes versatile ring-expansion reactions with small molecules to engender new boron-containing heterocycles. In fact, oxidation of 4 with trimethylamine N-oxide, O2, and elemental sulfur afforded the unprecedented 1,3-dioxa-2,4-diboretane 7, 1,3,4-trioxa-2,5-diborolane 8, and 1-oxa-3,4-dithio-2,5-diborolane 9, respectively. Moreover, 4 activates isocyanide to produce 1-oxa-2,4-diborete 10 and readily reacts with the C═O groups of benzophenone and CO2 to generate the ring-expansion products 11 and 12, respectively.
Synthesis and characterization of neutral and cationic 1-tris(pyrazolyl)borate organo-beryllium complexes
Berthold C., Stebens G., Butschke B., Bischoff I.-A., Schafer A., Ding C., Pan S., Buchner M.R.
Article, Inorganic Chemistry Frontiers, 2025, DOI Link
View abstract ⏷
The neutral and cationic 1-tris(pyrazolyl)borate (Tp) organo-beryllium complexes TpBe(R) (R = Ph, nBu, Me, Cp, Cp*) and [TpBe(carbene)]+ (carbene = IMe, IiPr, IDipp, CAAC(Dipp)) have been synthesized. These compounds were analyzed via NMR and IR spectroscopy, mass spectrometry as well as X-ray diffraction. A comparison of the Be-C bonds in solution and the solid state revealed no significant differences in the nature of this bond. Extensive quantum chemical evaluation of the bonding within the DFT framework showed that the Be-C bonds in all cases are dative covalent.
Planar Pentacoordinate Halogens
Cui L.-J., Miao L.-H., Orozco-Ic M., Li L., Pan S., Merino G., Cui Z.-H.
Article, Angewandte Chemie - International Edition, 2025, DOI Link
View abstract ⏷
Planar hypercoordinate motifs represent an intriguing frontier in chemistry, challenging traditional bonding norms. As electronegativity of the central atom increases, achieving planar hypercoordination becomes more difficult due to restricted delocalization, making the design of planar hypercoordinate halogens particularly puzzling. Here, we conduct an extensive computational survey of LinXn+1− (n=4, 5, 6; X=F, Cl, Br, I) clusters, revealing a starlike D5h-symmetry global minimum in Li5X6− (X=F, Cl, Br) with a planar pentacoordinate halogen (ppX), where X− is located at the center of Li5X5 crown. The clusters are stabilized predominantly through electrostatic interactions between X− and Li5X5, complemented by weak covalent bonding from dative interaction. Due to the weak orbital overlap, ppX clusters exhibit localized diatropic ring currents around X and Li.
Planar tetracoordinate beryllium in σ-aromatic Li4Be and Na4Be clusters: A missing member in first-octal row planar tetracoordinate family
Miao L.-H., Cui L.-J., Zhang H., Orozco-Ic M., Yang Y.-F., Pan S., Cui Z.-H.
Article, Journal of Chemical Physics, 2024, DOI Link
View abstract ⏷
While planar tetracoordinate (pt) centers have been extensively explored from carbon to other octal-row elements or their heavier analogs, their counterparts involving alkali (A) and alkaline-earth metals (Ae) remain elusive due to the large atomic radius and absence of p orbitals. In this work, we found six hitherto unknown anionic ptA (A4A−) and neutral ptAe (A4Ae) centers through an extensive exploration of potential energy surfaces. The D4h-symmetry ptBe structures in Li4Be and Na4Be emerge as the lowest-energy configurations, and all the other ptA/ptAe structures are higher in energy or saddle points. The global-minimum ptBe structure can be described as Be− with a 2s12px12py1 electronic configuration, forming three σ electron sharing interactions with quartet Li4+/Na4+ motifs. The delocalized σ orbitals contribute to σ aromaticity, thereby enhancing the overall stability of these intriguing title ptBe species. Furthermore, these ptBe systems can be encapsulated within the [n]cycloparaphenylene nanoloop (n = 7, 8) thermochemically spontaneously, without any disturbance in planarity in the ptBe moiety, where the systems get stabilized by a predominant electrostatic interaction between Li4/Na4 and the nanoloop.
Chemical Bonding in [Fe(η4-P4)2]2- and Related Complexes
Ding C., Pan S., Frenking G.
Article, Inorganic Chemistry, 2024, DOI Link
View abstract ⏷
Quantum chemical calculations of the six valence isoelectronic complexes [FeL2]2-, [CoL2]−, and NiL2 with L = η4-P4, η4-C4H4 using density functional theory have been carried out. The molecular structures were investigated with a variety of methods. The analysis of the electronic structure in [Fe(η4-P4)2]2- shows that the bonding situation is very similar to valence isoelectronic Ni(η4-C4H4)2. The orbital interactions in the 18 electron complexes [TML2]q (TMq = Fe2-, Co-, Ni) come mainly from TM(dπ)→L2 backdonation, enhanced by smaller contributions from TM(dδ)→L2 backdonation and TM(s)←L2 donation. Calculations of the six TML2 species indicate that all of them are viable candidates for synthetic work. The bonding situation is very similar and can straightforwardly be explained with the Dewar-Chatt-Duncanson bonding model in terms of dative bonding between d10 metal atoms and the ligands in the electronic singlet state. EDA-NOCV calculations using the ligands and the metal atoms with different charges and electronic states indicate that the metal-ligand bonds in the charged complexes [FeL2]2- and [CoL2]− are best described with fragments in the electronic triplet state between the metal atoms with d8 configuration and triplet ligands. The singlet fragments give the degenerate TM(dπ)→L2 π backdonation as the strongest component, whereas the triplet fragments have the related electron-sharing TMq (dπ)-(L2)2- π bonding as the major component, differing only by the assignment of the bonded two electrons to one or both fragments. The calculations of the charge distribution using the Hirshfeld and Voronoi partitioning methods suggest that the metal atoms are nearly neutral or carry small negative charges in all complexes. The NBO method gives erratic charges, because of the neglect of the 4p AOs of the transition metals as genuine valence orbitals.
Unusual quadruple bonds featuring collective interaction-type σ bonds between first octal-row atoms in the alkaline-earth compounds AeOLi2 (Ae = Be-Ba)
Cui L.-J., Liu Y.-Q., Pan S., Cui Z.-H., Frenking G.
Article, Chemical Science, 2024, DOI Link
View abstract ⏷
Quantum chemical calculations are reported for the complexes of alkaline earth metals AeOLi2 (Ae = Be-Ba) at the BP86-D3(BJ)/def2-QZVPP and CCSD(T)/def2-QZVPPQZVPP levels. The nature of the Ae-OLi2 bond has been analyzed with a variety of methods. The AeOLi2 molecules exhibit an unprecedented σ donor bond Ae→OLi2 where the (n)s2 lone-pair electrons of the Ae atom are donated to vacant O-Li2 antibonding orbitals having the largest coefficient at lithium. This is a covalent bond where the accumulation of the associated electronic charge is located at two positions above and below the Ae-OLi2 axis. The bifurcated component of orbital interactions is structurally related to the recently proposed collective bonding model, but exhibits a completely different type of bonding. The most stable isomer of AeOLi2 has a C2v geometry and a singlet (1A1) electronic ground state. The bond dissociation energy (BDE) of the Ae-OLi2 bonds exhibits a zig-zag trend from BeOLi2 to BaOLi2, with BeOLi2 having the largest BDE (De = 73.0 kcal mol−1) and MgOLi2 possessing the lowest BDE (De = 42.3 kcal mol−1) at the CCSD(T) level. The calculation of the atomic partial charges by the Hirshfeld and Voronoi methods suggests that Be and Mg carry small negative charges in the lighter molecules whereas the heavier atoms Ca-Ba have small positive charges. In contrast, the NBO and QTAIM methods give positive charges for all Ae atoms that are larger for Ca-Ba than that calculated by the Hirshfeld and Voronoi approaches. The molecules AeOLi2 have large dipole moments where the negative end is at the Ae atom with the polarity Ae→OLi2. The largest dipole moments are predicted for the lighter species BeOLi2 and MgOLi2 and the smallest value is calculated for BaOLi2. The calculation of the vibrational spectra shows a significant red-shift toward lower wave numbers for the Ae-OLi2 stretching mode in comparison to diatomic AeO. Besides the Ae→OLi2 σ-donor bonds there are also three dative bonds due to Ae←OLi2 backdonation which consist of one σ bond and two π bonds. The appearance of strong Ae→OLi2 σ donation leads to quadruple bonds AeOLi2 in all systems AeOLi2, even for the lightest species with Ae = Be, Mg. The valence orbitals of Ca, Sr, and Ba, which are involved in the dative interactions, are the (n)s and (n−1)d AOs whereas Be and Mg use their (n)s and (n)p AOs. The EDA-NOCV results are supported by the AdNDP calculations which give four 2c-2e bonding orbitals. Three bonding orbitals have occupation numbers ∼2. One σ orbital has smaller occupation numbers between 1.32 and 1.73 due to the delocalization to the lithium atoms. The analysis of the electronic structure with the ELF method suggests multicenter bonds with mainly trisynaptic and tetrasynaptic basins, which also support the results of the EDA-NOCV calculations.
InnTl4-nH+ (n = 0∼4): Tetracoordinate Hydrogen in a Planar Fashion?
Cui L.-J., Liu X.-B., Zhang H.-Y., Yan B., Orozco-Ic M., Pan S., Cui Z.-H.
Article, Inorganic Chemistry, 2024, DOI Link
View abstract ⏷
The recent report of planar tetracoordinate hydrogen (ptH) in In4H+ is very intriguing in planar hypercoordinate chemistry. Our high-level CCSD(T) calculations revealed that the proposed D4h-symmetric ptH In4H+ is a first-order saddle point with an imaginary frequency in the out-of-plane mode of the hydrogen atom. In fact, at the CCSD(T)/aug-cc-pV5Z/aug-cc-pV5Z-PP level, the C4v isomer, with the H atom located 0.70 Å above the In4 plane, is 0.5 kcal/mol more stable than the D4h isomer. However, given the small perturbation from planarity and essentially barrierless C4v ↔ D4h ↔ C4v transition, the vibrationally averaged structure can still be considered as a planar. Extending our exploration to the InnTl4-nH+ (n = 0-3) systems, we found all these ptH structures, except for In2Tl2H+, to be the putative global minimum. The single σ-delocalized interaction between the central hydrogen atom and InnTl4-n ligand rings proves pivotal in establishing planarity and aromaticity and conferring substantial stability upon these rule-breaking ptH species.
In Silico Design and Characterization of a New Molecular Electride: Li@Calix[3]Pyrrole
Saha R., Skjelstad B.B., Pan S.
Article, Chemistry - A European Journal, 2024, DOI Link
View abstract ⏷
Electrides, in which anionic electrons are localized independently of the atoms in the compound, have shown promise, especially as catalysts and optoelectronic materials. Here, we present a new computationally designed molecular electride, Li@calix[3]pyrrole (Li@C3P). Electron density and electron localization function analyses unequivocally confirm the existence of localized electride electron density, outside the system, independent of any specific atoms. Non-covalent interaction plots further validate the character of the isolated localized electron, suggesting that the system can be accurately represented by Li+@calix[3]pyrrole ⋅ e−, denoting its distinct charge separation. The remarkable non-linear optical properties of Li@C3P, including average polarizability, (Formula presented.) =412.4 au, first hyperpolarizability, β=4.46×104 au, and second hyperpolarizability, (Formula presented.) =18.40×106 au, are unparalleled in the previously reported and similar Li@C4P molecular electride. Furthermore, energy decomposition analysis in combination with natural orbital for chemical valence theory sheds light on the mechanism of electron density transfer from Li to the C3P cage, yielding the charge-separated Li@C3P complex. In addition to the electron transfer, a key factor to its electride nature is the electronic structure of the CnP cage, which has its lowest unoccupied molecular orbital located in the void adjacent to the N−H groups at the back of the bowl-shaped CnP cage.
Revisiting the Structure and Bonding in Li5H6- and the Exploration of Reactivity: Planar Pentacoordinate Hydrogen
Cui L.-J., Li Y., Leyva-Parra L., Tiznado W., Pan S., Cui Z.-H.
Article, Journal of Physical Chemistry A, 2024, DOI Link
View abstract ⏷
Recently, Guha and co-workers (Sarmah, K.; Kalita, A.; Purkayastha, S.; Guha, A. K. Pushing The Extreme of Multicentre Bonding: Planar Pentacoordinate Hydride. Angew. Chem. Int. Ed. 2024, e202318741) reported a highly intriguing bonding motif: planar pentacoordinate hydrogen (ppH) in Li5H6-, featuring C2v symmetry in the singlet state with two distinct H-Li (center-ring) bond distances. We herein revisited the potential energy surface of Li5H6- by using a target-oriented genetic algorithm. Our investigation revealed that the lowest-energy structure of Li5H6- exhibits a ppH configuration with very high D5h symmetry and a 1A1′ electronic state. We did not find any electronic effect like Jahn-Teller distortion that could be responsible for lowering its symmetry. Moreover, our calculations demonstrated significant differences in the relative energies of other low-lying isomers. An energetically very competitive planar tetracoordinate hydrogen (ptH) isomer is also located, but it corresponds to a very shallow minimum on the potential energy surface depending on the used level of theory. Chemical bonding analyses, including AdNDP and EDA-NOCV, uncover that the optimal Lewis structure for Li5H6- involves H- ions stabilized by the Li5H5 crown. Surprisingly, despite the dominance of electrostatic interactions, the contribution from covalent bonding is also significant between ppH and the Li5H5 moiety, derived from H-(1s) → Li5H5 σ donation. Magnetically induced current density analysis revealed that due to minimal orbital overlap and the highly polar nature of the H-Li covalent interaction, the ppH exhibits local diatropic ring currents around the H centers, which fails to result in a global aromatic ring current. The coordination of Li5H6- with Lewis acids, BH3 and BMe3, instantly converts the ppH configuration to (quasi) ptH. These Lewis acid-bound ptH complexes show high electronic stability and high thermochemical stability against dissociation and, therefore, will be ideal candidates for the experimental realization.
Exploring the Use of “Honorary Transition Metals” To Push the Boundaries of Planar Hypercoordinate Alkaline-Earth Metals
Liu X.-B., Tiznado W., Cui L.-J., Barroso J., Leyva-Parra L., Miao L.-H., Zhang H.-Y., Pan S., Merino G., Cui Z.-H.
Article, Journal of the American Chemical Society, 2024, DOI Link
View abstract ⏷
The quest for planar hypercoordinate atoms (phA) beyond six has predominantly focused on transition metals, with dodecacoordination being the highest reported thus far. Extending this bonding scenario to main-group elements, which typically lack d orbitals despite their larger atomic radius, has posed significant challenges. Intrigued by the potentiality of covalent bonding formation using the d orbitals of the heavier alkaline-earth metals (Ae = Ca, Sr, Ba), the so-called “honorary transition metals”, we aim to push the boundaries of planar hypercoordination. By including rings formed by 12-15 atoms of boron-carbon and Ae centers, we propose a design scheme of 180 candidates with a phA. Further systematic screening, structural examination, and stability assessments identified 10 potential clusters with a planar hypercoordinate alkaline-earth metal (phAe) as the lowest-energy form. These unconventional structures embody planar dodeca-, trideca-, tetradeca-, and pentadecacoordinate atoms. Chemical bonding analyses reveal the important role of Ae d orbitals in facilitating covalent interactions between the central Ae atom and the surrounding boron-carbon rings, thereby establishing a new record for coordination numbers in the two-dimensional realm.
Multiple Bonding in AeN− (Ae=Ca, Sr, Ba)
Cui L.-J., Liu Y.-Q., Wang M.-H., Yan B., Pan S., Cui Z.-H., Frenking G.
Article, Chemistry - A European Journal, 2024, DOI Link
View abstract ⏷
Quantum chemical calculations using ab initio methods at the MRCI+Q(8,9)/def2-QZVPPD and CCSD(T)/def2-QZVPPD levels as well as using density functional theory are reported for the diatomic molecules AeN− (Ae=Ca, Sr, Ba). The anions CaN− and SrN− have electronic triplet (3Π) ground states with nearly identical bond dissociation energies De ~57 kcal/mol calculated at the MRCI+Q(8,9)/def2-QZVPPD level. In contrast, the heavier homologue BaN− has a singlet (1Σ+) ground state, which is only 1.1 kcal/mol below the triplet (3Σ−) state. The computed bond dissociation energy of (1Σ+) BaN− is 68.4 kcal/mol. The calculations at the CCSD(T)-full/def2-QZVPPD and BP86-D3(BJ)/def2-QZVPPD levels are in reasonable agreement with the MRCI+Q(8,9)/def2-QZVPPD data, except for the singlet (1Σ+) state, which has a large multireference character. The calculated atomic partial charges given by the CM5, Voronoi and Hirshfeld methods suggest small to medium-sized Ae←N− charge donation for most electronic states. In contrast, the NBO method predicts for all species medium to large Ae→N− electronic charge donation, which is due to the neglect of the (n)p AOs of Ae atoms as genuine valence orbitals. Neither the bond orders nor the bond lengths correlate with the bond dissociation energies. The EDA−NOCV calculations show that the heavier alkaline earth atoms Ca, Sr, Ba use their (n)s and (n-1)d orbitals for covalent bonding.
Mono-Ortho-Beryllated Carbodiphosphoranes: Synthesis, Structure, Bonding and Reactivity
Buchner M.R., Kreuzer L.K., Thomas-Hargreaves L.R., Muller M., Ivlev S.I., Frenking G., Pan S.
Article, Chemistry - A European Journal, 2024, DOI Link
View abstract ⏷
The reaction of organoberyllium compounds with hexaphenylcarbodiphosphorane yields mono-ortho-beryllated complexes, which feature a double dative Be=C bond. The bonding situation in these compounds together with a simple carbodiphosphorane and an N-heterocyclic carbene adduct was analysed with energy decomposition analysis in combination with natural orbital for chemical valence as well as with quantum theory of atoms-in-molecules. Furthermore, the driving forces accountable for mono-ortho-beryllation were elucidated along with the reactivity of the Be=C bond.
BeM(CO)3− (M = Co, Rh, Ir) and BeM(CO)3 (M = Ni, Pd, Pt): Triply bonded terminal beryllium in zero oxidation state
Liu Y.-Q., Kalita A.J., Zhang H.-Y., Cui L.-J., Yan B., Guha A.K., Cui Z.-H., Pan S.
Article, Journal of Chemical Physics, 2024, DOI Link
View abstract ⏷
We perform detailed potential energy surface explorations of BeM(CO)3− (M = Co, Rh, Ir) and BeM(CO)3 (M = Ni, Pd, Pt) using both single-reference and multireference-based methods. The present results at the CASPT2(12,12)/def2-QZVPD//M06-D3/def2-TZVPPD level reveal that the global minimum of BeM(CO)3− (M = Co, Rh, Ir) and BePt(CO)3 is a C3v symmetric structure with an 1A1 electronic state, where Be is located in a terminal position bonded to M along the center axis. For other cases, the C3v symmetric structure is a low-lying local minimum. Although the present complexes are isoelectronic with the recently reported BFe(CO)3− complex having a B-Fe quadruple bond, radial orbital-energy slope (ROS) analysis reveals that the highest occupied molecular orbital (HOMO) in the title complexes is slightly antibonding in nature, which bars a quadruple bonding assignment. Similar weak antibonding nature of HOMO in the previously reported BeM(CO)4 (M = Ru, Os) complexes is also noted in ROS analysis. The bonding analysis through energy decomposition analysis in combination with the natural orbital for chemical valence shows that the bonding between Be and M(CO)3q (q = −1 for M = Co, Rh, Ir and q = 0 for M = Ni, Pd, Pt) can be best described as Be in the ground state (1S) interacting with M(CO)30/− via dative bonds. The Be(spσ) → M(CO)3q σ-donation and the complementary Be(spσ) ← M(CO)3q σ-back donation make the overall σ bond, which is accompanied by two weak Be(pπ) ← M(CO)3q π-bonds. These complexes represent triply bonded terminal beryllium in an unusual zero oxidation state.
Analysis of the Unusual Chemical Bonds and Dipole Moments of AeF− (Ae=Be−Ba): A Lesson in Covalent Bonding
Qin L., Liu Y.-Q., Liu R., Yang X., Cui Z.-H., Zhao L., Pan S., Fau S., Frenking G.
Article, Chemistry - A European Journal, 2024, DOI Link
View abstract ⏷
Quantum chemical calculations of the anions AeF− (Ae=Be−Ba) have been carried out using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory employing BP86 with various basis sets. The detailed bonding analyses using different charge- and energy partitioning methods show that the molecules possess three distinctively different dative bonds in the lighter species with Ae=Be, Mg and four dative bonds when Ae=Ca, Sr, Ba. The occupied 2p atomic orbitals (AOs) and to a lesser degree the occupied 2s AO of F− donate electronic charge into the vacant spx(σ) and p(π) orbitals of Be and Mg which leads to a triple bond Ae F−. The heavier Ae atoms Ca, Sr, Ba use their vacant (n-1)d AOs as acceptor orbitals which enables them to form a second σ donor bond with F− that leads to quadruply bonded Ae F− (Ae=Ca−Ba). The presentation of molecular orbitals or charge distribution using only one isodensity value may give misleading information about the overall nature of the orbital or charge distribution. Better insights are given by contour line diagrams. The ELF calculations provide monosynaptic and disynaptic basins of AeF− which nicely agree with the analysis of the occupied molecular orbitals and with the charge density difference maps. A particular feature of the covalent bonds in AeF− concerns the inductive interaction of F− with the soft valence electrons in the (n)s valence orbitals of Ae. The polarization of the (n)s2 electrons induces a (n)spx hybridized lone-pair orbital at atom Ae, which yields a large dipole moment with the negative end at Ae. The concomitant formation of a vacant (n)spx AO of atom Ae, which overlaps with the occupied 2p(σ) AO of F−, leads to a strong covalent σ bond.
Stabilizing Monoatomic Two-Coordinate Bismuth(I) and Bismuth(II) Using a Redox Noninnocent Bis(germylene) Ligand
Xu J., Pan S., Yao S., Lorent C., Teutloff C., Zhang Z., Fan J., Molino A., Krause K.B., Schmidt J., Bittl R., Limberg C., Zhao L., Frenking G., Driess M.
Article, Journal of the American Chemical Society, 2024, DOI Link
View abstract ⏷
The formation of isolable monatomic BiI complexes and BiII radical species is challenging due to the pronounced reducing nature of metallic bismuth. Here, we report a convenient strategy to tame BiI and BiII atoms by taking advantage of the redox noninnocent character of a new chelating bis(germylene) ligand. The remarkably stable novel BiI cation complex 4, supported by the new bis(iminophosphonamido-germylene)xanthene ligand [(P)GeII(Xant)GeII(P)] 1, [(P)GeII(Xant)GeII(P) = Ph2P(NtBu)2GeII(Xant)GeII(NtBu)2PPh2, Xant = 9,9-dimethyl-xanthene-4,5-diyl], was synthesized by a two-electron reduction of the cationic BiIIII2 precursor complex 3 with cobaltocene (Cp2Co) in a molar ratio of 1:2. Notably, owing to the redox noninnocent character of the germylene moieties, the positive charge of BiI cation 4 migrates to one of the Ge atoms in the bis(germylene) ligand, giving rise to a germylium(germylene) BiI complex as suggested by DFT calculations and X-ray photoelectron spectroscopy (XPS). Likewise, migration of the positive charge of the BiIIII2 cation of 3 results in a bis(germylium)BiIIII2 complex. The delocalization of the positive charge in the ligand engenders a much higher stability of the BiI cation 4 in comparison to an isoelectronic two-coordinate Pb0 analogue (plumbylone; decomposition below −30 °C). Interestingly, 4[BArF] undergoes a reversible single-electron transfer (SET) reaction (oxidation) to afford the isolable BiII radical complex 5 in 5[BArF]2. According to electron paramagnetic resonance (EPR) spectroscopy, the unpaired electron predominantly resides at the BiII atom. Extending the redox reactivity of 4[OTf] employing AgOTf and MeOTf affords BiIII(OTf)2 complex 7 and BiIIIMe complex 8, respectively, demonstrating the high nucleophilic character of BiI cation 4.
Li6E5Li6: Tetrel Sandwich Complexes with 10-π-Electrons
Inostroza D., Leyva-Parra L., Pino-Rios R., Solar-Encinas J., Vasquez-Espinal A., Pan S., Merino G., Yanez O., Tiznado W.
Article, Angewandte Chemie - International Edition, 2024, DOI Link
View abstract ⏷
When (4n +2) π-electrons are located in single planar ring, it conventionally qualifies as aromatic. According Hückel's rule, systems possessing ten π-electrons should be aromatic. Herein we report a series of D5h Li6E5Li6 sandwich structures, representing the first global minima featuring ten π-electrons E510− ring (E=Si−Pb). However, these π-electrons localize as five π-lone-pairs rather than delocalized orbitals. The high symmetry structure achieved is a direct consequence of σ-aromaticity, particularly favored in elements from Si to Pb, resulting in a pronounced diatropic ring current flow that contributes to the enhanced stability of these systems.
Transition Metal Behavior of Heavier Alkaline Earth Elements in Doped Monocyclic and Tubular Boron Clusters
Cui L.-J., Dong X., Liu Y.-Q., Pan S., Cui Z.-H.
Article, Inorganic Chemistry, 2024, DOI Link
View abstract ⏷
Quantum chemical calculations are carried out to design highly symmetric-doped boron clusters by employing the transition metal behavior of heavier alkaline earth (Ae = Ca, Sr, and Ba) metals. Following an electron counting rule, a set of monocyclic and tubular boron clusters capped by two heavier Ae metals were tested, which leads to the highly symmetric Ae2B8, Ae2B18, and Ae2B30 clusters as true minima on the potential energy surface having a monocyclic ring, two-ring tubular, and three-ring tubular boron motifs, respectively. Then, a thorough global minimum (GM) structural search reveals that a monocyclic B8 ring capped with two Ae atoms is indeed a GM for Ca2B8 and Ba2B8, while for Sr2B8 it is a low-lying isomer. Similarly, the present search also unambiguously shows the most stable isomers of Ae2B18 and Ae2B30 to be highly symmetric two- and three-ring tubular boron motifs, respectively, capped with two Ae atoms on each side of the tube. In these Ae-doped boron clusters, in addition to the electrostatic interactions, a substantial covalent interaction, specifically the bonding occurring between (n - 1)d orbitals of Ae and delocalized orbitals of boron motifs, provides the essential driving force behind their highly symmetrical structures and overall stability.
Structure, Stability and Bonding in Ligand Stabilized C3 Species
Book chapter, Electron Density: Concepts, Computation and DFT Applications, 2024, DOI Link
View abstract ⏷
The persistent carbenes stabilized C n species for n = 1 and 2 are quite well-explored. However, the corresponding C 3 homologs have only been little explored so far. Herein, we presented our recent report about the thorough scrutiny of structure, stability and bonding in the complexes L–C 3 –L with L = PPh 3 (1), NHC Me (2) and cAAC Me (3) through Quantum chemical studies using density functional theory and ab initio methods. The results show that in the minimum energy geometries of 1 and 2 , the ligands are bonded with rather acute bonding angles at the linear C 3 moiety. While 1 prefers to have a synclinal (gauche) conformation, 2 has a trans conformation of the ligands. However, in 3 , two cAAC Me ligands bind with C 3 fragment, making a nearly linear arrangement at the central C 5 core. The bond dissociation energies with respect to the dissociation of the ligands have the order 1 < 2 < 3 . The bonding analysis using natural bond orbital and energy decomposition analyses in combination with natural orbital for chemical valence theory implies that 3 can be best represented as a cumulene with electron-sharing double bonds between neutral fragments (cAAC Me)=C 3 =(cAAC Me), whereas 1 and 2 have a mixing of electron-sharing and dative bonds between positively charged ligands [(PPh 3) 2 ] + and [(NHC Me) 2 ] + and negatively charged [C 3 ] − .
Chemical Bonding
Pan S., Frenking G.
Book chapter, Exploring Chemical Concepts Through Theory and Computation, 2024, DOI Link
View abstract ⏷
This chapter discusses fundamental aspects of chemical bonding in molecules, highlighting the difference between the physical mechanism of bond formation and bonding models. The historical development of the most important bonding models is critically discussed, and the current understanding of the nature of chemical bonding is presented. The crucial importance of orbital symmetry for the structure and reactivity of molecules is emphasized. Further topics concern the length and strength of a chemical bond, the difference between the electron-sharing bond A-B and the dative bond A?B as well as the nature of polar bonds. The difference between the bond formation process between the original fragments A and B, which takes into account the deformation of the electronic structures, and the description of the bond finally formed, which is often confused and leads to controversy, is emphasized. A few selected model compounds are analyzed using modern methods of bond analysis to demonstrate the advances in sophisticated bond analysis that have been made. They illustrate the differences in chemical bonds between the main group atoms of the first octal row of the periodic table and the heavier homologs, as well as the transition metals (TMs).
Stabilization of Cyclic C4 by Four Donor Ligands: A Theoretical Study of (L)4C4 (L = Carbene)
Ding C., Pan S., Yan G.-R., N V T Gorantla S.M., Cui Z.-H., Frenking G.
Article, Journal of Physical Chemistry A, 2023, DOI Link
View abstract ⏷
Quantum chemical studies using density functional theory were carried out for the (L)4C4 complexes with L = cAAC, DAC, NHC, SNHC, MIC1, and MIC2. The results show that the title complexes are highly stable with respect to dissociation, (L)4C4 → C4 + 4L. However, their stability with respect to (L)4C4 → 2(L)2C2 is crucial for the assessment of their experimental viability. The (L)4C4 complexes with L = cAAC and DAC dissociate exergonically at room temperature into two (L)2C2 units. In contrast, the other (L)4C4 complexes with L = NHC, SNHC, MIC1, and MIC2 are thermochemically stable with respect to dissociation, (L)4C4 → 2(L)2C2. The computed adiabatic ionization potentials of (L)4C4 complexes with L = NHC, MIC1, and MIC2 are lower than those for the cesium atom. Particularly, (MIC1)4C4 and (MIC2)4C4 will very easily lose electrons to form cationic complexes. The SNHC ligand is the best for the experimental realization of (L)4C4 complexes, followed by NHC. The bonding analysis using charge and energy decomposition methods suggests that the (L)3C4-CL bond can be best described as a typical electron-sharing double bond with a strong σ-bond and a weaker π-bond. Therefore, the core bonding pictures in the title complexes resemble a [4]radialene. Larger substituents at the carbene ligands enhance the stability of the complexes (L)4C4 against dissociation.
Mimicking the C2 molecule: M2B2 and M3B2+ clusters (M = Li, Na) and the reactivity of the N-heterocyclic carbene bound Li2B2 complex
Liu Y.-Q., Yan G.-R., Cui L.-J., Yan B., Pan S., Cui Z.-H.
Article, Physical Chemistry Chemical Physics, 2023, DOI Link
View abstract ⏷
C2 has attracted considerable attention from the scientific community for its debatable bonding situation. Herein, we show that the global minima of M2B2 and M3B2+ (M = Li, Na) possess similar covalent bonding patterns to C2. Because of strong charge transfer from M2/M3 to B2 dimer, they can be better described as [M2]2+[B2]2− and [M3]3+[B2]2− salt complexes with the B22− core surrounded perpendicularly by two and three M+ atoms, respectively. The energy decomposition analyses in combination with the natural orbital for chemical valence theory give four bonding components in C2, M2B2, and M3B2+ clusters. However, the fourth component does not arise from a bonding interaction but from polarization/hybridization. Considering the effect of Pauli repulsion in σ-space, the attractive covalent interaction in these molecules mainly comes from the two π-bonds. We further presented stable N-heterocyclic carbene (NHC) and triphenylphosphine (PPh3) ligands bound Li2B2(NHC)2 and Li2B2(PPh3)2 complexes. A comparative study of reactivity towards L = CO2, CO, and N2 between Li2B2(NHC)2 and B2(NHC)2 is also performed. L-Li2B2(NHC)2 is highly stable against L dissociation at room temperature for L = CO2 and CO, and the stability is markedly higher than that in L-B2(NHC)2. The larger B2→L π-backdonation in L-Li2B2(NHC)2 also makes L more activated than in L-B2(NHC)2
Clusters and bulky Lewis acid protected complexes with planar hexacoordinate beryllium and magnesium
Yan G.-R., Liu Y.-Q., Liu X.-B., Wang M.-H., Cui Z.-H., Pan S.
Article, Journal of Chemical Physics, 2023, DOI Link
View abstract ⏷
Planar hexacoordination (ph) is only rarely reported in the literature. So far, only a few neutral and cationic molecules possessing phE (E = C, Si, B, Al, Ga) in the most stable isomer are predicted theoretically. Present electronic structure calculations report hitherto unknown anionic planar hexcoordinate beryllium and magnesium, phBe/Mg, as the most stable isomer. Global minimum searches show that the lowest energy structure of BeC6M3− (M = Al, Ga) and MgC6M3− (M = Ga, In, Tl) is the D3h symmetric phBe/Mg clusters, where beryllium/magnesium is covalently bonded with six carbon centers and M is located in a bridging position between two carbon centers. These global minimum phBe/Mg clusters are highly kinetically stable against isomerization, facilitating the experimental confirmation by photoelectron spectroscopy. Noteworthy is the fact that the phBe/Mg center is linked with carbon centers through three 7c-2e delocalized σ bonds and three 7c-2e π bonds, making the cluster double aromatic (σ + π) in nature. The bonding between the Be/Mg and outer ring moiety can be best expressed as an electron-sharing σ-bond between the s orbital of Be+/Mg+ and C6M32− followed by three dative interactions involving empty pπ and two in-plane p orbitals of Be/Mg. Furthermore, Lewis basic M centers of the title clusters can be passivated through the complexation with bulky Lewis acid, 9-boratriptycene, lowering the overall reactivity of the cluster, which can eventually open up the possibility of their large-scale syntheses.
Bonding Analysis of the Ge-Ge Bonds in the Octagermacubane Ge8(Sit-butyl2methyl)6
Pan S., Frenking G.
Article, Israel Journal of Chemistry, 2023, DOI Link
View abstract ⏷
Quantum chemical calculations have been carried out at the BP86/def2-SVP level on Ge8(Sit-butyl2methyl)6 (1) and the bonding situation has been analyzed with a variety of methods. The calculated equilibrium geometry of 1 is in good agreement with the reported x-ray structure analysis. The D3 correction for dispersion interactions as a sum of pairwise attractions leads to an overestimate of the effect of dispersion forces. Calculations at BP86-D3(BJ)/def2-SVP give shorter bonds for Ge(I)−Ge(I) than for Ge(0)−Ge(I), which is in contrast to the experimental values and the BP86/def2-SVP results. The NBO analysis suggests that the best Lewis structure of 1 has lone-pair orbitals at the Ge(0) atoms with occupation numbers of 1.70 e. A lone-pair character at Ge(0) albeit with less weight is also suggested by the shape of the HOMO, which is an antibonding orbital between the Ge(0) atoms with small contributions from the Ge(I) atoms. The LUMO of 1 is the corresponding bonding combination of the Ge(0) AOs, which can be explained with the reluctance of the heavier main-group atoms to s/p hybridization of the valence orbitals. The calculated bond order values suggest significant direct Ge(0)−Ge(0) interactions. This is supported by the shape of the HOMO and by the results of EDA-NOCV calculations. The deformation densities and the orbitals associated with the pairwise orbital interaction show that there is a direct charge flow between the Ge(0) atoms of the two fragments, but it is not completely separated from the Ge(0)−Ge(I) and Ge(I)−Ge(I) bond formation. The QTAIM calculations suggest that 1 has a cubic structure with a cage critical point but not a bond critical point for the Ge(0)−Ge(0) interactions. The dispersion interactions of the large substituents in 1 have a significant influence on the stability of the compound.
Global Planar Tetra-, Penta- and Hexa-coordinate Silicon Clusters Constructed by Decorating SiO3 with Alkali Metals
Wang M.-H., Fei D.-H., Chen C., Liu Y.-Q., Pan S., Cui Z.-H.
Article, ChemPhysChem, 2023, DOI Link
View abstract ⏷
The achievement of the rule-breaking planar hypercoordinate motifs (carbon and other elements) is mainly attributed to a practical electronic stabilization mechanism, where the bonding of the central atom pz π electrons is a crucial issue. We have demonstrated that strong multiple bonds between the central atom and partial ligands can be an effective approach to explore stable planar hypercoordinate species. A set of planar tetra-, penta- and hexa-coordinate silicon clusters were herein found to be the lowest-energy structure, which can be viewed as decorating SiO3 by alkali metals in the MSiO3−, M2SiO3 and M3SiO3+ (M=Li, Na) clusters. The strong charge transfer from M atoms to SiO3 effectively results in [M]+SiO32−, [M2]2+SiO32− and [M3]3+SiO32− salt complexes, where the Si−O multiple bonding and structural integrity of the Benz-like SiO3 framework is maintained better than the corresponding SiO32− motifs. The bonding between M atoms and SiO3 motif is best described as M+ forming a few dative interactions by employing its vacant s, p, and high-lying d orbitals. These considerable M←SiO3 interactions and Si−O multiple bonding give rise to the highly stable planar hypercoordinate silicon clusters.
B7Be6B7: A Boron-Beryllium Sandwich Complex
Dong X., Tiznado W., Liu Y.-Q., Leyva-Parra L., Liu X.-B., Pan S., Merino G., Cui Z.-H.
Article, Angewandte Chemie - International Edition, 2023, DOI Link
View abstract ⏷
Planar boron clusters have often been regarded as “π-analogous” to aromatic arenes because of their similar delocalized π-bonding. However, unlike arenes such as C5H5− and C6H6, boron clusters have not previously shown the ability to form sandwich complexes. In this study, we present the first sandwich complex involving beryllium and boron, B7Be6B7. The global minimum of this combination adopts a unique architecture having a D6h geometry, featuring an unprecedented monocyclic Be6 ring sandwiched between two quasi-planar B7 motifs. The thermochemical and kinetic stability of B7Be6B7 can be attributed to strong electrostatic and covalent interactions between the fragments. Chemical bonding analysis shows that B7Be6B7 can be considered as a [B7]3−[Be6]6+[B7]3− complex. Moreover, there is a significant electron delocalization within this cluster, supported by the local diatropic contributions of the B7 and Be6 fragments.
Structural Characterization and Bonding Analysis of [Hg{Fe(CO)5}2]2+ [SbF6]−2
Rupf S.M., Pan S., Moshtaha A.L., Frenking G., Malischewski M.
Article, Journal of the American Chemical Society, 2023, DOI Link
View abstract ⏷
The non-classical carbonyl complex [Hg{Fe(CO)5}2]2+ [SbF6]−2 is prepared by reaction of Hg(SbF6)2 and excess Fe(CO)5 in anhydrous HF. The single-crystal X-ray structure reveals a linear Fe-Hg-Fe moiety as well as an eclipsed conformation of the eight basal CO ligands. Interestingly, the Hg-Fe bond length of 2.5745(7) Å is relatively similar to the corresponding Hg-Fe bonds in literature-known [Hg{Fe(CO)4}2]2- dianions (2.52-2.55 Å), which intrigued us to analyze the bonding situation in both the dications and dianions with the energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV) method. Both species are best described as Hg(0) compounds, which are also confirmed by the shape of the HOMO-4 and HOMO-5 of the dication and dianion, respectively, in which the electron pair is located mainly at the Hg. Furthermore, for the dication and the dianion, the σ back-donation from Hg into the [Fe(CO)5]22+ or the [Fe(CO)4]22- fragment is the most dominant orbital interaction and surprisingly these interaction energies are also very similar even in absolute values. The fact that both iron-based fragments are missing two electrons explains their prominent σ-acceptor properties.
Planar pentacoordinate s-block metals
Wang M.-H., Kalita A.J., Orozco-Ic M., Yan G.-R., Chen C., Yan B., Castillo-Toraya G., Tiznado W., Guha A.K., Pan S., Merino G., Cui Z.-H.
Article, Chemical Science, 2023, DOI Link
View abstract ⏷
The presence of a delocalized π-bond is often considered an essential criterion for achieving planar hypercoordination. Herein, we show that σ-delocalization could be sufficient to make the planar configuration the most stable isomer in a series of planar pentacoordinate s-block metals. High-level ab initio computations reveal that the global minimum of a series of interalkali and interalkali-alkaline earth clusters (LiNa5, Li5Mg+, Na5Mg+, K5Ca+, CaRb5+, Rb5Sr+, and SrCs5+) adopts a singlet D5h structure with a planar pentacoordinate lithium or alkaline earth metal (AE = Mg, Ca, Sr). These clusters are unusual combinations to stabilize a planar pentacoordinate atom, as all their constituents are electropositive. Despite the absence of π-electrons, Hückel's rule is fulfilled by the six σ-electrons. Furthermore, the systems exhibit a diatropic ring current in response to an external magnetic field and a strong magnetic shielding, so they might be classified as σ-aromatic. Therefore, multicenter σ-bonds and the resulting σ-delocalization stabilize these clusters, even though they lack π-aromaticity.
Quest of Quadruple Bonding Between Two Main-Group Atoms in AeB− and AeC (Ae=Ca, Sr, Ba) and the Role of d Orbitals of Heavier Alkaline-Earth Atoms in Covalent Interactions
Liu Y.-Q., Wang M.-H., Yan B., Li L., Pan S., Cui Z.-H., Frenking G.
Article, Chemistry - A European Journal, 2023, DOI Link
View abstract ⏷
Quantum chemical calculations using ab initio methods at the MRCI+Q(6,8)/def2-QZVPP and CCSD(T)/def2-QZVPP levels as well as density functional theory are reported for the diatomic molecules AeB− and isoelectronic AeC (Ae=Ca, Sr, Ba). The boride anions AeB− have an electronic triplet (3Σ−) ground state. The quintet (5Σ−) state is 5.8–12.3 kcal/mol higher in energy and the singlet (1Δ) state is 13.1–15.3 kcal/mol above the triplet. The isoelectronic AeC molecules are also predicted to have a low-lying triplet (3Σ−) state but the quintet (5Σ−) state is only 2.2 kcal/mol (SrC) and 2.9 kcal/mol (CaC) above the triplet state. The triplet (3Σ−) and quintet (5Σ−) states of BaC are nearly isoenergetic. All systems have rather strong bonds. The calculated bond dissociation energies of the triplet (3Σ−) state are between De=38.3–41.7 kcal/mol for AeB− and De=49.4–57.5 kcal/mol for AeC. The barium species have always the strongest bonds whereas the calcium and strontium compounds have similar BDEs. The bonding analysis indicates that there is little charge migration in AeB− in the direction Ae→B− where the alkaline earth atoms carry positive charges between 0.09 e–0.22 e. The positive charges at the Ae atoms are much larger in AeC where the charge migration Ae→C is between 0.90 e–0.91 e. A detailed analysis of the interatomic interactions with the EDA-NOCV method shows that all diatomic species AeB− and AeC are built from dative interactions between Ae (1S, ns2) and B− or C (3P, 2 s22pπ12pπ′1). The eventually formed bonds in AeC are better described in terms of interactions between the ions Ae+ (2S, ns1)+C− (4S, 2 s22pπ12pπ′12pσ1). Inspection of the orbital interactions suggests that the alkaline earth atoms Ca, Sr, Ba use mainly their (n-1)d AOs besides the (n)s AOs for the covalent bonds. This creates a second energetically low-lying σ-bonding MO in the molecules, which feature valence orbitals with the order ϕ1 (σ-bonding)<ϕ2 (σ-bonding)<ϕ3 (degenerate π-bonding). All four occupied valence MOs of AeB− and AeC are bonding orbitals. Since the degenerate π orbitals ϕ3 are only singly occupied, the formal bond order is three.
BH4Ng+ (Ar−Rn): Viable Compounds with a B−Ng Covalent Bond
Pino-Rios R., Vasquez-Espinal A., Pan S., Cerpa E., Tiznado W., Merino G.
Article, ChemPhysChem, 2023, DOI Link
View abstract ⏷
In this work, we explore, using high-level calculations, the ability of BH4+ to interact with noble gases. The He system is energetically unstable, while the Ne system could only be observed at cryogenic temperatures. In the case of the Ar, Kr and Xe systems, all are energetically stable, even at room temperature. The different chemical bond descriptors reveal a covalent character between B and the noble gas from Ar to Rn. However, this interaction gradually weakens the multicentric bond between the boron atom and the H2 fragment. Thus, although BH4Rn+ exhibits a strong covalent bond, it tends to dissociate at room temperature into BH2Rn++H2.
Bonding situations in tricoordinated beryllium phenyl complexes
Thomas-Hargreaves L.R., Liu Y.-Q., Cui Z.-H., Pan S., Buchner M.R.
Article, Journal of Computational Chemistry, 2023, DOI Link
View abstract ⏷
The bonding situation in the tricoordinated beryllium phenyl complexes [BePh3]−, [(pyridine)BePh2] and [(trimethylsilyl-N-heterocyclic imine)BePh2] is investigated experimentally and computationally. Comparison of the NMR spectroscopic properties of these complexes and of their structural parameters, which were determined by single crystal X-ray diffraction experiments, indicates the presence of π-interactions. Topology analysis of the electron density reveals elliptical electron density distributions at the bond critical points and the double bond character of the beryllium-element bonds is verified by energy decomposition analysis with the combination of natural orbital for chemical valence. The present beryllium-element bonds are highly polarized and the ligands around the central atom have a strong influence on the degree of π-delocalization. These results are compared to related triarylboranes.
Synthesis of a rhodium(iii) dinitrogen complex using a calix[4]arene-based diphosphine ligand
Emerson-King J., Pan S., Gyton M.R., Tonner-Zech R., Chaplin A.B.
Article, Chemical Communications, 2023, DOI Link
View abstract ⏷
The synthesis and characterisation of the rhodium(iii) dinitrogen complex [Rh(2,2′-biphenyl)(CxP2)(N2)]+ are described, where CxP2 is a trans-spanning calix[4]arene-based diphosphine and the dinitrogen ligand is projected into the cavity of the macrocycle.
Comment on “The oxidation state in low-valent beryllium and magnesium compounds” by M. Gimferrer, S. Danés, E. Vos, C. B. Yildiz, I. Corral, A. Jana, P. Salvador and D. M. Andrada, Chem. Sci. 2022,13, 6583
Pan S., Frenking G.
Note, Chemical Science, 2023, DOI Link
View abstract ⏷
We challenge the assignment of the oxidation state +2 for beryllium and magnesium in the complexes Be(cAACDip)2 and Mg(cAACDip)2 as suggested by Gimferrer et al., Chem. Sci. 2022, 13, 6583 in a recent study. A careful review of the data in the ESI contradicts their own statement and shows that the results support the earlier suggestion that the metals are in the zero oxidation state. The authors reported wrong data for the excitation energies of Be and Mg to the 1D (np2) state. We also correct some misleading statements about the EDA method.
Energy Decomposition Analysis of the Chemical Bond: Scope and Limitation
Zhao L., Pan S., Frenking G.
Book chapter, Comprehensive Computational Chemistry, First Edition: Volume 1-4, 2023, DOI Link
View abstract ⏷
We introduce and discuss the basics of the energy decomposition analysis (EDA), which is a powerful method that connects the results of accurate quantum chemical calculations with the Lewis electron-pair bonding model. The breakdown of the calculated interaction energy between two or more fragments into well-defined terms makes it possible to model the nature of the chemical bond in a physically meaningful way. The EDA focuses on the formation of the chemical bond rather than on the mere description of the finally formed electronic structure of a molecule. This distinguishes the EDA from the most of the other approaches of analysing a chemical bond. The consideration of various electronic states, charges and electron configurations of the fragments in EDA makes it possible to identify the best-suited fragments for the description of the bond and it provides deep insight into the interatomic interactions during bond formation. The combination of the EDA with natural orbitals for chemical valence (NOCV) connects the heuristic Lewis picture with quantitative MO theory complemented by Pauli repulsion and Coulombic interactions. The results of the EDA-NOCV method provide a physically sound picture of the chemical bonds of atoms across the periodic table. This review discusses the scope but also the limitation of the EDA-NOCV method. Results are presented for first-row diatomic molecules and for compounds of main-group atoms, transition metals, lanthanides and actinides.
A Multidimensional Approach to Carbodiphosphorane-Bismuth Coordination Chemistry: Cationization, Redox-Flexibility, and Stabilization of a Crystalline Bismuth Hydridoborate
Obi A.D., Dickie D.A., Tiznado W., Frenking G., Pan S., Gilliard R.J.
Article, Inorganic Chemistry, 2022, DOI Link
View abstract ⏷
Bismuth complexes stabilized by carbon-based donor ligands are underserved by their instability, often due to facile ligand dissociation and deleterious protonolysis. Herein, we show that the ortho-bismuthination of hexaphenylcarbodiphosphorane enables a robust framework with geometrically constrained carbone-bismuth bonding interactions, which are highly tunable by cationization. The carbodiphosphorane bismuth halides (1 and 2) are remarkably air-stable and feature unprecedented trans carboneC-Bi-X ligation, resulting in highly elongated Bi-X bonds. In contrast to known carbone-bismuth complexes, hydrolytic activation of the carbone yields well-defined organobismuth complexes, and subsequent dehydrohalogenation is feasible using potassium bis(trimethylsilyl)amide or N-heterocyclic carbenes. The redox-flexibility of this framework was evaluated in the high catalytic activity of 1 and 2 for silylation of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) under mild conditions (50 °C, 24-96 h) and low catalyst loadings (5-10 mol %), which suggests the accessibility of short-lived hydridic and radical bismuth species. The reaction of 1, PhSiH3, and tris(pentafluorophenyl)borane (BCF) yields the first crystallographically characterized bismuth hydridoborate complex as an ionic species (9), presumably by BCF-mediated hydride abstraction from an unobserved [Bi]-H intermediate. All isolated compounds have been characterized by heteronuclear NMR spectroscopy and X-ray crystallography, and the bonding situation in representative complexes (1, 2, 5, and 9) were further evaluated using density functional theory.
Hitting the Bull’s Eye: Stable HeBeOH+ Complex
Yun G.-R., Li H.-X., Cabellos J.L., Tiznado W., Cui Z.-H., Pan S.
Article, ChemPhysChem, 2022, DOI Link
View abstract ⏷
It is now known that the heavier noble gases (Ng=Ar-Rn) show some varying degrees of reactivity with a gradual increase in reactivity along Ar−Rn. However, because of their very small size and very high ionization potential, helium and neon are the hardest targets to crack. Although few neon complexes are isolated at very low temperatures, helium needs very extreme situations like very high pressure. Here, we find that protonated BeO, BeOH+ can bind helium and neon spontaneously at room temperature. Therefore, extreme conditions like very low temperature and/or high pressure will not be required for their experimental isolation. The Ng−Be bond strength is very high for their heavier homologs and the bond strength shows a gradual increase from He to Rn. Moreover, the Ng−Be attractive energy is almost exclusively originated from the orbital interaction which is composed of one Ng(s/pσ)→BeOH+ σ-donation and two weaker Ng(pπ)→BeOH+ π-donations, except for helium. Helium uses its low-lying vacant 2p orbitals to accept π-electron density from BeOH+. Previously, such electron-accepting ability of helium was used to explain a somewhat stronger helium bond than neon for neutral complexes. However, the present results indicate that such π-back donations are too weak in nature to decide any energetic trend between helium and neon.
Lewis Superacidic Heavy Pnictaalkene Cations: Comparative Assessment of Carbodicarbene-Stibenium and Carbodicarbene-Bismuthenium Ions
Warring L.S., Walley J.E., Dickie D.A., Tiznado W., Pan S., Gilliard R.J.
Article, Inorganic Chemistry, 2022, DOI Link
View abstract ⏷
We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and-bismuthenium ions using the Gutmann-Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr3and [(pyCDC)PnBr2][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr3(1) with one or two equivalents of AgNTf2(NTf2= bis(trifluoromethanesulfonyl)imide) or AgSbF6gives stibaalkene mono- A nd dications of the form [(CDC)SbBr3-n][A]n(2-4; n = 1,2; A = NTf2or SbF6). The stibaalkene trication [(CDC)2Sb][NTf2]3(5) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6-9. With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene (pyCDC) was used as a ligand to access [(pyCDC)PnBr2][Br] (10, 12) and trications [(pyCDC)Pn][NTf2]3(Pn = Sb (11), Bi (13)), forgoing the need for a second CDC as used in the synthesis of 5. The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC-Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf2]2(4) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF3, and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF5(85.1 kcal/mol), which suggest that these cations are Lewis superacids.
B3Al4+: A Three-Dimensional Molecular Reuleaux Triangle
Bai L.-X., Orozco-Ic M., Zarate X., Sundholm D., Pan S., Guo J.-C., Merino G.
Article, Molecules, 2022, DOI Link
View abstract ⏷
We systematically explore the potential energy surface of the B3Al4+ combination of atoms. The putative global minimum corresponds to a structure formed by an Al4 square facing a B3 triangle. Interestingly, the dynamical behavior can be described as a Reuleaux molecular triangle since it involves the rotation of the B3 triangle at the top of the Al4 square. The molecular dynamics simulations, corroborating with the very small rotational barriers of the B3 triangle, show its nearly free rotation on the Al4 ring, confirming the fluxional character of the cluster. Moreover, while the chemical bonding analysis suggests that the multicenter interaction between the two fragments determines its fluxionality, the magnetic response analysis reveals this cluster as a true and fully three-dimensional aromatic system.
E6C15 (E = Si-Pb): polycyclic aromatic compounds with three planar tetracoordinate carbons
Inostroza D., Leyva-Parra L., Vasquez-Espinal A., Contreras-Garcia J., Cui Z.-H., Pan S., Thimmakondu V.S., Tiznado W.
Article, Chemical Communications, 2022, DOI Link
View abstract ⏷
A systematic exploration of the potential energy surface reveals two global minima with three planar tetra coordinate carbons (ptCs) and two global minima with three quasi-ptCs for E6C15 (E = Si-Pb) combinations. These consist of aromatic polycyclic templates suitable for further design of different materials without hindering the ptC texture.
Be4B12+: A Covalently Bonded Archimedean Beryllo-Borospherene
Dong X., Liu Y.-Q., Liu X.-B., Pan S., Cui Z.-H., Merino G.
Article, Angewandte Chemie - International Edition, 2022, DOI Link
View abstract ⏷
A new class of beryllium-boron clusters, beryllo-borospherene, is described herein theoretically. When beryllium is gradually added to the B12 motif, it undergoes drastic structural modifications. The global minimum of the Be4B12+ cluster is an Archimedean beryllo-borospherene in a 2A1 electronic ground state, composed of four boron triangles linked at each corner, resulting in a truncated tetrahedron with four B6 rings capped with four beryllium atoms. Beryllium forms strong bonding with the boron clusters through strong electrostatic and covalent interactions. For instance, the bonding between a beryllium atom and Be3B12 unit is best described as a Be+ fragment in a 2P excited state forming a strong and polarized electron-sharing bond with Be3B12, followed by several dative interactions by employing its vacant s, p, and very high-lying d orbitals. Counterintuitively, for an s-block element, the p orbitals of beryllium are the most crucial atomic orbitals for bonding rather than s orbitals.
The Heaviest Bottleable Metallylone: Synthesis of a Monatomic, Zero-Valent Lead Complex (“Plumbylone”)
Xu J., Pan S., Yao S., Frenking G., Driess M.
Article, Angewandte Chemie - International Edition, 2022, DOI Link
View abstract ⏷
The elusive plumbylone {[SiII(Xant)SiII]Pb0} 3 stabilized by the bis(silylene)xanthene chelating ligand 1, [SiII(Xant)SiII=PhC(NtBu)2Si(Xant)Si(NtBu)2CPh], and its isolable carbonyl iron complex {[SiII(Xant)SiII]Pb0Fe(CO)4} 4 are reported. The compounds 3 and 4 were obtained stepwise via reduction of the lead(II) dibromide complex {[SiII(Xant)SiII]PbBr2} 2, prepared from the bis(silylene)xanthene 1 and PbBr2, employing potassium naphthalenide and K2Fe(CO)4, respectively. While the genuine plumbylone 3 is rather labile even at −60 °C, its Pb0→Fe(CO)4 complex 4 turned out to be relatively stable and bottleable. However, solutions of 4 decompose readily to elemental Pb and {[SiII(Xant)SiII]Fe(CO)3} 5 at 80 °C. Reaction of 4 with [Rh(CO)2Cl]2 leads to the formation of the unusual dimeric [(OC)2RhPb(Cl)Fe(CO)4] complex 6 with trimetallic Rh−Pb−Fe bonds. The molecular and electronic structures of 3 and 4 were established by Density Functional Theory (DFT) calculations.
Designing a Four-Ring Tubular Boron Motif through Metal Doping
Dong X., Liu Y.-Q., Tiznado W., Cabellos-Quiroz J.L., Zhao J., Pan S., Cui Z.-H.
Article, Inorganic Chemistry, 2022, DOI Link
View abstract ⏷
Tubular boron clusters represent a class of extremely unusual geometries that can be regarded as a key indicator for the 2D-to-3D boron structural evolution as well as the embryos for boron nanotubes. While a good number of pure boron or metal-doped boron tubular clusters have been reported so far, most of them are two-ring tubular structures, and their higher-ring analogues are very scarce. We report herein the first example of a four-ring tubular boron motif in the cagelike global minimum of Be2B24+. Global-minimum searches of MB24qand M2B24q(M = alkali/alkaline-earth metals; q = 1+, 0, 1-) reveal that the most stable structure of Be2B24+is a C2v-symmetric cage having a four-ring tubular boron moiety, whereas it is a high-lying isomer for those having a two/three-ring tubular boron motif for all other systems. The B24framework in Be2B24+can be viewed as consisting of two two-ring B12tubular structures linked together at one side of the B6rings along the high-symmetry axis and two offside B6rings capped by two Be atoms. The Be2-B24bonding is best described as Be22+in an excited triplet state, forming two highly polarized covalent bonds with B24-in a quartet spin state. The unique ability of beryllium to make strong covalent and electrostatic interactions makes the Be2B24+cluster stable in such an unusual geometry.
[SMe3]2[Bi2Ag2I10], a silver iodido bismuthate with an unusually small band gap
Mobs J., Pan S., Tonner-Zech R., Heine J.
Article, Dalton Transactions, 2022, DOI Link
View abstract ⏷
Iodido metalates of heavy main group elements have seen much research interest in the last years due to their possible application as absorbers in photovoltaics. However, for materials based on the non-toxic element bismuth one challenge lies in narrowing the optical band gap for sufficient solar absorption. Here, we present a new iodido silver bismuthate, [SMe3]2[Bi2Ag2I10] (1), which is prepared from solution and characterized regarding its structure, thermal stability and optical absorption. While compounds with similar anion compositions are known, the band gap of 1.82 eV is the smallest in chain-like Bi/Ag/I-compounds that has been reported to date. To support our experimental findings we carried out computational investigations and were able to reproduce the surprisingly narrow band gap, highlighting the subtle influence of the connectivity of different building units in multinary bismuthates. We also prepared and characterized the simple iodido pentelates [SMe]3[E2I9] (E = Bi, Sb; 2, 3) to provide a point of comparison.
The nature of the polar covalent bond
Zhao L., Pan S., Frenking G.
Article, Journal of Chemical Physics, 2022, DOI Link
View abstract ⏷
Quantum chemical calculations using density functional theory are reported for the diatomic molecules LiF, BeO, and BN. The nature of the interatomic interactions is analyzed with the Energy Decomposition Analysis-Natural Orbitals of Chemical Valence (EDA-NOCV) method, and the results are critically discussed and compared with data from Quantum Theory of Atoms in Molecules, Natural Bond Orbital, and Mayer approaches. Polar bonds, like nonpolar bonds, are caused by the interference of wave functions, which lead to an accumulation of electronic charge in the bonding region. Polar bonds generally have a larger percentage of electrostatic bonding to the total attraction, but nonpolar bonds may also possess large contributions from Coulombic interaction. The term "ionic contribution"refers to valence bond structures and is misleading because it refers to separate fragments with negligible overlap that occur only in the solid state and in solution, not in a molecule. The EDA-NOCV method gives detailed information about the individual orbital contributions, which can be identified by visual inspection of the associated deformation densities. It is very important, particularly for polar bonds to distinguish between the interatomic interactions of the final dissociation products after bond rupture and the interactions between the fragments in the eventually formed bond. The bond formation in LiF is dominated by orbital interactions (90%) between Li and F yielding a single bond, but the eventually formed bond comes mainly from the electrostatic attraction between Li+ and F-, where the minor orbital interactions (10%) have equally strong σ and πcomponents. The symmetry allowed bond formation of BeO between Be in the 1S ground state and O in the excited 1D state is dominated (90%) by a strong dative Be → O σ bond with negligible πinteractions. The final bond situation in BeO is best described by the interaction between Be+ and O-, where the Coulombic forces provide 60% of the attraction and the orbital interactions give equally strong σ and πbonds. The chemical bond in BN is analyzed in the X3Π ground state and the a1ς+ excited state. Both states have triple bonds with strong πbonds, which are in the a1ς+ state even stronger than the σ bond.
Bonding analysis of the C2precursor Me3E-C2-I(Ph)FBF3(E = C, Si, Ge)
Gorantla S.M.N.V.T., Pan S., Chandra Mondal K., Frenking G.
Article, Pure and Applied Chemistry, 2022, DOI Link
View abstract ⏷
A series of possible precursors for generating C2 with the general formula Me3E-C2-I(Ph)FBF3 [E = C (1), Si (2), and Ge (3)] has been theoretically investigated using quantum chemical calculations. The equilibrium geometries of all species show a linear E-C2-I+ backbone. The inspection of the electronic structure of the Me3E-C2 bond by energy decomposition analysis coupled with the natural orbital for chemical valence (EDA-NOCV) method suggests a combination of electron sharing C-C σ-bond and v weak π-dative bond between Me3C and C2 fragments in the doublet state for species 1 (E = C). For species 2 (Si) and 3 (Ge), the analysis reveals σ-dative Me3E-C2 bonds (E = Si, Ge; Me3EC2) resulting from the interaction of singly charged (Me3E)+ and (C2-IPh(BF4))- fragments in their singlet states. The C2-I bond is diagnosed as an electron sharing σ-bond in all three species, 1, 2 and 3.
Triple bonding between beryllium and nitrogen in HNBeCO
Wang L., Pan S., Wang G., Zeng X., Zhou M., Frenking G.
Article, Chemical Communications, 2022, DOI Link
View abstract ⏷
The HNBeCO complex is generated via the reaction of a beryllium atom with a HNCO molecule in a solid neon matrix, which is identified via infrared absorption spectroscopy with isotopic substitutions. The complex is characterized to have a linear structure with a very short Be-N bond distance. Bonding analyses indicate that the complex involves an unprecedented HNBeCO triple bond consisting of two degenerate electron-sharing π bonds and a dative σ bond with the π bonds being much stronger than the σ bond.
Bare and ligand protected planar hexacoordinate silicon in SiSb3M3+ (M = Ca, Sr, Ba) clusters
Chen C., Wang M.-H., Feng L.-Y., Zhao L.-Q., Guo J.-C., Zhai H.-J., Cui Z.-H., Pan S., Merino G.
Article, Chemical Science, 2022, DOI Link
View abstract ⏷
The occurrence of planar hexacoordination is very rare in main group elements. We report here a class of clusters containing a planar hexacoordinate silicon (phSi) atom with the formula SiSb3M3+ (M = Ca, Sr, Ba), which have D3h (1A1′) symmetry in their global minimum structure. The unique ability of heavier alkaline-earth atoms to use their vacant d atomic orbitals in bonding effectively stabilizes the peripheral ring and is responsible for covalent interaction with the Si center. Although the interaction between Si and Sb is significantly stronger than the Si-M one, sizable stabilization energies (−27.4 to −35.4 kcal mol−1) also originated from the combined electrostatic and covalent attraction between Si and M centers. The lighter homologues, SiE3M3+ (E = N, P, As; M = Ca, Sr, Ba) clusters, also possess similar D3h symmetric structures as the global minima. However, the repulsive electrostatic interaction between Si and M dominates over covalent attraction making the Si-M contacts repulsive in nature. Most interestingly, the planarity of the phSi core and the attractive nature of all the six contacts of phSi are maintained in N-heterocyclic carbene (NHC) and benzene (Bz) bound SiSb3M3(NHC)6+ and SiSb3M3(Bz)6+ (M = Ca, Sr, Ba) complexes. Therefore, bare and ligand-protected SiSb3M3+ clusters are suitable candidates for gas-phase detection and large-scale synthesis, respectively.
Isolation of Stable Borepin Radicals and Anions
Hollister K.K., Yang W., Mondol R., Wentz K.E., Molino A., Kaur A., Dickie D.A., Frenking G., Pan S., Wilson D.J.D., Gilliard R.J.
Article, Angewandte Chemie - International Edition, 2022, DOI Link
View abstract ⏷
Borepin, a 7-membered boron-containing heterocycle, has become an emerging molecular platform for the development of new materials and optoelectronics. While electron-deficient borepins are well-established, reduced electron-rich species have remained elusive. Herein we report the first isolable, crystalline borepin radical (2 a, 2 b) and anion (3 a, 3 b) complexes, which have been synthesized by potassium graphite (KC8) reduction of cyclic(alkyl)(amino) carbene-dibenzo[b,d]borepin precursors. Borepin radicals and anions have been characterized by EPR or NMR, elemental analysis, X-ray crystallography, and cyclic voltammetry. In addition, the bonding features have been investigated computationally using density functional theory.
Clarifying notes on the bonding analysis adopted by the energy decomposition analysis
Bickelhaupt F.M., Fonseca Guerra C., Mitoraj M., Sagan F., Michalak A., Pan S., Frenking G.
Article, Physical Chemistry Chemical Physics, 2022, DOI Link
View abstract ⏷
We discuss the fundamental aspects of the EDA-NOCV method and address some critical comments that have been made recently. The EDA-NOCV method unlike most other methods focuses on the process of bond formation between the interacting species and not just only on the analysis of the finally formed bond. This is demonstrated using LiF as an example. There is a difference between the interactions between the initial species which form the bond and are also the final product of bond cleavage, and the interactions between the fragments in the eventually formed molecule. The flexibility of the method allows the choice of the interacting fragments which helps to identify the charge and electron configuration of the fragments which describe the bond. This is very helpful in cases where the bond may be described with several Lewis structures. We reject the idea that it would be a disadvantage to have “bond path functions” as the energy components in the EDA, which actually indicate the variability of the method. The bonding analysis in a different sequence of the bond formation gives important results for the various questions that can be asked. This is demonstrated by using CH2, CO2 and the formation of a guanine quartet as examples. The fact that a bond is always defined by the bound molecule, the fragments, and their states is universal and deeply physical, as we show here again for various examples. The results of the EDA-NOCV method are in full accordance with the physical mechanism of the chemical bond as revealed by Ruedenberg.
Complex Featuring Two Double Dative Bonds Between Carbon(0) and Uranium
Fang W., Pan S., Su W., Wang S., Zhao L., Frenking G., Zhu C.
Article, CCS Chemistry, 2022, DOI Link
View abstract ⏷
The uranyl with two U=O double bonds is a well-known and predominant form of uranium in the environment, but the carbon-based analog with two U=C double bonds has rarely been synthesized. Here, we describe the formation of an unprecedented uranium complex [(PyPh2P)2C]2UCl2]2+·2(BPh4−) from the reaction of UCl4 with carbodiphosphorane in the presence of NaBPh4. The nature of the U–C bonds was revealed by density functional theory calculations, which show that the 5f and 6d orbital electrons of uranium are remarkably involved in the U=C double bonds. The inspection of the bonding characteristics with an energy decomposition analysis suggests that the uranium-ligand bond may be alternatively described with double dative bonds [CUC] or strong electron-sharing π bonds and weak σ bonds.
Generation and Characterization of the Charge-Transferred Diradical Complex CaCO2 with an Open-Shell Singlet Ground State
Zhou Y., Pan S., Dong X., Wang L., Zhou M., Frenking G.
Article, Journal of the American Chemical Society, 2022, DOI Link
View abstract ⏷
The CaCO2 complex is generated via the reaction of excited-state calcium atom with carbon dioxide in a solid neon matrix. Infrared absorption spectroscopy and quantum chemical calculations reveal that the complex has a planar four-membered ring structure with a strongly bent CO2 ligand side-on coordinated to the calcium center in an η2-O, O manner. The complex has an open-shell singlet ground state, which can be described as the bonding interactions between a Ca+ (4s1) cation in the doublet ground state and a doublet ground state CO2- anion. The analysis of the bonding situation suggests that the Ca-O2C bonds have a large (75%) electrostatic character. The covalent (orbital) interactions come from the coupling of the unpaired electrons of Ca+ and CO2- giving rise to electron-sharing bonding and a stronger contribution from dative bonding (Ca+)←(CO2-). The atomic orbitals (AOs) of Ca+ that are engaged in the covalent bonds are the 4s AO for the electron-sharing bonds and the 3d AOs for the dative bonds. This is further evidence for the assignment of the heavier alkaline-earth atoms as transition metals rather than main-group elements.
Relative Populations and IR Spectra of Cu38 Cluster at Finite Temperature Based on DFT and Statistical Thermodynamics Calculations
Buelna-Garcia C.E., Castillo-Quevedo C., Quiroz-Castillo J.M., Paredes-Sotelo E., Cortez-Valadez M., Martin-del-Campo-Solis M.F., Lopez-Luke T., Utrilla-Vazquez M., Mendoza-Wilson A.M., Rodriguez-Kessler P.L., Vazquez-Espinal A., Pan S., de Leon-Flores A., Mis-May J.R., Rodriguez-Dominguez A.R., Martinez-Guajardo G., Cabellos J.L.
Article, Frontiers in Chemistry, 2022, DOI Link
View abstract ⏷
The relative populations of Cu38 isomers depend to a great extent on the temperature. Density functional theory and nanothermodynamics can be combined to compute the geometrical optimization of isomers and their spectroscopic properties in an approximate manner. In this article, we investigate entropy-driven isomer distributions of Cu38 clusters and the effect of temperature on their IR spectra. An extensive, systematic global search is performed on the potential and free energy surfaces of Cu38 using a two-stage strategy to identify the lowest-energy structure and its low-energy neighbors. The effects of temperature on the populations and IR spectra are considered via Boltzmann factors. The computed IR spectrum of each isomer is multiplied by its corresponding Boltzmann weight at finite temperature. Then, they are summed together to produce a final temperature-dependent, Boltzmann-weighted spectrum. Our results show that the disordered structure dominates at high temperatures and the overall Boltzmann-weighted spectrum is composed of a mixture of spectra from several individual isomers.
Structural transformations in boron clusters induced by metal doping
Barroso J., Pan S., Merino G.
Review, Chemical Society Reviews, 2022, DOI Link
View abstract ⏷
In the last decades, experimental techniques in conjunction with theoretical analyses have revealed the surprising structural diversity of boron clusters. Although the 2D to 3D transition thresholds are well-established, there is no certainty about the factors that determine the geometry adopted by these systems. The structural transformation induced by doping usually yields a minimum energy structure with a boron skeleton entirely different from that of the bare cluster. This review summarizes those clusters no larger than 40 boron atoms where one or two dopants show a radical transformation of the structure. Although the structures of these systems are not easy to predict, they often adopt familiar shapes such as umbrella-like, wheel, tubular, and cages in various cases. This journal is
πback-Donation from a Beryllium Dibromide Fragment at the Expense of Its σ Strength
Thomas-Hargreaves L.R., Pan S., Ivlev S.I., Frenking G., Buchner M.R.
Article, Inorganic Chemistry, 2022, DOI Link
View abstract ⏷
It is common knowledge that metal-to-ligand πback-donation requires filled atomic orbitals at the metal center. However, we show through a combined experimental and theoretical approach that Be(II)→N-heterocyclic carbene (NHC) πback-donation is present in the two carbene adducts [(iPr)BeBr2] (1) and [(iPr)2BeBr2] (2) (iPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). These complexes were characterized with NMR, IR, and Raman spectroscopy as well as with single-crystal X-ray diffractometry. The unusual bonding situation is understood from the results of energy decomposition analysis in combination with natural orbital for chemical valence and quantum theory of atoms-in-molecules analysis. The obtained findings shed light on the unusually high Be-C bond strength in carbene adducts to beryllium compounds and rationalize their geometry and reactivity.
Planar hypercoordinate carbon
Das P., Pan S., Chattaraj P.K.
Book chapter, Atomic Clusters with Unusual Structure, Bonding and Reactivity: Theoretical Approaches, Computational Assessment and Applications, 2022, DOI Link
View abstract ⏷
In this chapter, we aim to deliver a brief review of the planar hypercoordinate carbon compounds. The conventional idea of carbon centers in organic molecules is that they have a maximum of four coordination numbers with tetrahedral geometries, i.e., the attached atoms or groups occupy four vertices of a tetrahedron. But the planar tetracoordinate carbon (ptC) molecules violate this conventional tetrahedral concept of tetracoordinate carbons. In the case of planar geometry with carbons, the maximum coordination is usually three. So, when four or more atoms or groups are attached to a planar carbon in the same plane, the system is considered to be a planar hyper coordination.
Atomic Clusters with Unusual Structure, Bonding and Reactivity: Theoretical Approaches, Computational Assessment and Applications
Chattaraj P.K., Pan S., Merino G.
Book, Atomic Clusters with Unusual Structure, Bonding and Reactivity: Theoretical Approaches, Computational Assessment and Applications, 2022, DOI Link
View abstract ⏷
Atomic Clusters with Unusual Structure, Bonding and Reactivity: Theoretical Approaches, Computational Assessment and Applications reviews the latest computational tools and approaches available for accurately assessing the properties of a cluster, while also highlighting how such clusters can be adapted and utilized for the development of novel materials and applications. Sections provide an introduction to the computational methods used to obtain global minima for clusters and effectively analyze bonds, outline experimental approaches to produce clusters, discuss specific applications, and explore cluster reactivity and usage across a number of fields. Drawing on the knowledge of its expert editors and contributors, this book provides a detailed guide to ascertaining the stability, bonding and properties of atomic clusters. Atomic clusters, which exhibit unusual properties, offer huge potential as building blocks for new materials and novel applications, but understanding their properties, stability and bonding is essential in order to accurately understand, characterize and manipulate them for further use. Searching for the most stable geometry of a given cluster is difficult and becomes even more so for clusters of medium and large sizes, where the number of possible isomers sharply increase, hence this book provides a unique and comprehensive approach to the topic and available techniques and applications.
Application of frustrated Lewis pairs in small molecule activation and associated transformations
Jiang D., Ghara M., Pan S., Zhao L., Chattaraj P.K.
Book chapter, Atomic Clusters with Unusual Structure, Bonding and Reactivity: Theoretical Approaches, Computational Assessment and Applications, 2022, DOI Link
View abstract ⏷
The chemistry of frustrated Lewis pair (FLP) is enriching rapidly. The present chapter provides a survey of several experimental work on FLPs and mechanistic insights into their reactivity from electronic structure theory calculation. The results of quantum chemical calculations in understanding the mechanism of H2 activation is clearly demonstrated in this chapter, which would help in designing more effective catalysts of H2 activation. NO, CO, CO2, SO2, N2O, alkenes, alkynes, etc., small molecules become activated by cooperative action of both the Lewis centers of FLP as demonstrated by different computational study. Nucleus-independent chemical shift (NICS) analysis illustrates the role of aromaticity in decreasing the activation barrier for the activation of H2 and other small molecules by FLP. Hydrogenation of imine, nitrile, enamine, aziridine, aldehyde, ketone, alkene, alkyne catalyzed by FLP and the mechanisms of hydrogenation process are discussed here. The term boron-ligand cooperation (BLC) in analogy to the metal ligand cooperation (MLC) has been suggested in order to demonstrate a specific reactivity of some FLPs in the activation of chemical bonds. FLPs containing Al(C6F5)3 as Lewis acid (LA) can polymerize a monomer molecule, which is described in the last section of this chapter.
M(L)8 complexes (M = Ca, Sr, Ba; L = PH3, PF3, N2, CO): Act of an alkaline-earth metal as a conventional transition metal
Li H.-X., Cui Z.-H., Jiang D., Zhao L., Pan S.
Book chapter, Atomic Clusters with Unusual Structure, Bonding and Reactivity: Theoretical Approaches, Computational Assessment and Applications, 2022, DOI Link
View abstract ⏷
Alkaline-earth elements have usually been treated as classical main group elements, with the occasional exception in the case of the heaviest element, Ba, which brings the suggestion with renaming it as “honorary transition metal.” However, the conventional transition metal-like behavior of Ca and Sr and the relevance of 18-electron rule to decide the overall structure and stability are counter-intuitive. We, through a series of studies, showed that alkaline-earth metals act as conventional transition metal where the metal-ligand bonding involves dominant interplay of d orbitals of M. Our previous studies on M(CO)8 and M(N2)8 in triplet electronic ground state and M(Bz)3 (M=Ca, Sr, Ba; Bz=benzene) in singlet electronic ground state showed that the complexes are stable with respect to single ligand dissociation and they satisfy the 18-electrons rule like transition metal complex. For L=CO and N2, M in an excited triplet state with ns0(n−1)d2 valence electronic configuration, and for L=Bz, M in an excited singlet state with ns0(n−1)d2 valence electronic configuration interacts with L predominantly via M(d)→(L)8 π-backdonation. Moreover, herein we also show that this behavior is not only exclusive to these ligands but also can be extended to PH3 and PF3 ligands. These ligands (i.e., L=PH3, PF3, and N2), which have somewhat lower π-accepting ability than CO, can also compensate the high excitation energy needed for the transition, ns2→ns0(n−1)d2, inducing enough stability in the title complexes to be viable. Therefore, the present results imply that the transition metal-like behavior of alkaline-earth, Ca-Ba, is more common than previously thought, provided only proper ligands are needed!
Planar hexacoordinate gallium
Wang M.-H., Chen C., Pan S., Cui Z.-H.
Article, Chemical Science, 2021, DOI Link
View abstract ⏷
We report the first planar hexacoordinate gallium (phGa) center in the global minimum of the GaBe6Au6+ cluster which has a star-like D6h geometry with 1A1g electronic state, possessing a central gallium atom encompassed by a Be6 hexagon and each Be-Be edge is further capped by an Au atom. The electronic delocalization resulting in double aromaticity (both σ and π) provides electronic stability in the planar form of the GaBe6Au6+ cluster. The high kinetic stability of the title cluster is also understood by Born-Oppenheimer molecular dynamics simulations. The energy decomposition analysis in combination with the 'natural orbitals for chemical valence' theory reveals that the bonding in the GaBe6Au6+ cluster is best expressed as the doublet Ga atom with 4s24p⊥1 electronic configuration forming an electron-sharing π bond with the doublet Be6Au6+ moiety followed by Ga(s)→[Be6Au6+] σ-backdonation and two sets of Ga(p‖)←[Be6Au6+] σ-donations. This journal is
OsB9−: An Aromatic Osmium-Centered Monocyclic Boron Ring
Yu R., Pan S., Cui Z.-H.
Article, Frontiers in Chemistry, 2021, DOI Link
View abstract ⏷
Transition-metal-centered monocyclic boron wheels are important candidates in the family of planar hypercoordinate species that show intriguing structure, stability and bonding situation. Through the detailed potential energy surface explorations of MB9− (M = Fe, Ru, Os) clusters, we introduce herein OsB9− to be a new member in the transition-metal-centered borometallic molecular wheel gallery. Previously, FeB9− and RuB9− clusters were detected by photoelectron spectroscopy and the structures were reported to have singlet D9h symmetry. Our present results show that the global minimum for FeB9− has a molecular wheel-like structure in triplet spin state with Cs symmetry, whereas its heavier homologues are singlet molecular wheels with D9h symmetry. Chemical bonding analyses show that RuB9− and OsB9− display a similar type of electronic structure, where the dual σ + π aromaticity, originated from three delocalized σ bonds and three delocalized π bonds, accounts for highly stable borometallic molecular wheels.
Effects of temperature on enantiomerization energy and distribution of isomers in the chiral cu13 cluster
Castillo-Quevedo C., Buelna-Garcia C.E., Paredes-Sotelo E., Robles-Chaparro E., Zamora-Gonzalez E., Martin-Del-campo-solis M.F., Quiroz-Castillo J.M., Del-Castillo-Castro T., Martinez-Guajardo G., De-Leon-flores A., Cortez-Valadez M., Ortiz-Chi F., Gaxiola T., Castillo S.J., Vasquez-Espinal A., Pan S., Cabellos J.L.
Article, Molecules, 2021, DOI Link
View abstract ⏷
In this study, we report the lowest energy structure of bare Cu13 nanoclusters as a pair of enantiomers at room temperature. Moreover, we compute the enantiomerization energy for the interconversion from minus to plus structures in the chiral putative global minimum for temperatures ranging from 20 to 1300 K. Additionally, employing nanothermodynamics, we compute the probabilities of occurrence for each particular isomer as a function of temperature. To achieve that, we explore the free energy surface of the Cu13 cluster, employing a genetic algorithm coupled with density functional theory. Moreover, we discuss the energetic ordering of isomers computed with various density functionals. Based on the computed thermal population, our results show that the chiral putative global minimum strongly dominates at room temperature.
Confinement induced chemical bonding: Case of noble gases
Pan S., Merino G., Zhao L.
Book chapter, Chemical Reactivity in Confined Systems: Theory, Modelling and Applications, 2021, DOI Link
View abstract ⏷
Chemical bonding is a fuzzy concept in chemistry defined based on different models since it is neither an experimentally observable quantity nor there is any Hermitian quantum mechanical operator corresponding to this. This chapter presents some examples to show how confinement can even induce chemical bonding in between two noble gas (Ng) atoms in true sense. The Xe-Xe bond can undoubtedly be assigned as a genuine chemical covalent bond and the Ar-Ar and Kr-Kr bonds have at least some partial covalent character. Depending on the size of cavitand and size of the Ng, the degree of covalent bond formation either between Ng and cage centers or between two Ng atoms gets formed. Confinement of Ng2 becomes a playground for the application of different bonding models and each model has their own advantages and limitations which further create debate.
A critical look at linus pauling’s influence on the understanding of chemical bonding
Pan S., Frenking G.
Article, Molecules, 2021, DOI Link
View abstract ⏷
The influence of Linus Pauling on the understanding of chemical bonding is critically examined. Pauling deserves credit for presenting a connection between the quantum theoretical description of chemical bonding and Gilbert Lewis’s classical bonding model of localized electron pair bonds for a wide range of chemistry. Using the concept of resonance that he introduced, he was able to present a consistent description of chemical bonding for molecules, metals, and ionic crystals which was used by many chemists and subsequently found its way into chemistry textbooks. However, his one-sided restriction to the valence bond method and his rejection of the molecular orbital approach hindered further development of chemical bonding theory for a while and his close association of the heuristic Lewis binding model with the quantum chemical VB approach led to misleading ideas until today.
Bonding in M(NHBMe)2 and M[Mn(CO)5]2 complexes (M=Zn, Cd, Hg; NHBMe=(HCNMe)2B): divalent group 12 metals with zero oxidation state
Pan S., Zhao L., Frenking G.
Article, Theoretical Chemistry Accounts, 2021, DOI Link
View abstract ⏷
Quantum chemical studies using density functional theory were carried out on M(NHBMe)2 and M[Mn(CO)5]2 (M=Zn, Cd, Hg) complexes. The calculations suggest that M(NHBMe)2 and M[Mn(CO)5]2 have D2d and D4d symmetry, respectively, with a 1A1 electronic ground state. The bond dissociation energies of the ligands have the order of Zn > Cd > Hg. A thorough bonding analysis using charge and energy decomposition methods suggests that the title complexes are best represented as NHBMe⇆M0⇄NHBMe and Mn(CO)5⇆M0⇄Mn(CO)5 where the metal atom M in the electronic ground state with an ns2 electron configuration is bonded to the (NHBMe)2 and [Mn(CO)5]2 ligands through donor–acceptor interaction. These experimentally known complexes are the first examples of mononuclear complexes with divalent group 12 metals with zero oxidation state that are stable at ambient condition. These complexes represent the rare situation where the ligands act as a strong acceptor and the metal center acts as strong donor. The relativistic effect of Hg leads to a weaker electron donating strength of the 6s orbital, which explains the trend of the bond dissociation energy.
Metal-CO Bonding in Mononuclear Transition Metal Carbonyl Complexes
Frenking G., Fernandez I., Holzmann N., Pan S., Krossing I., Zhou M.
Article, JACS Au, 2021, DOI Link
View abstract ⏷
DFT calculations have been carried out for coordinatively saturated neutral and charged carbonyl complexes [M(CO)n]qwhere M is a metal atom of groups 2-10. The model compounds M(CO)2(M = Ca, Sr, Ba) and the experimentally observed [Ba(CO)]+were also studied. The bonding situation has been analyzed with a variety of charge and energy partitioning approaches. It is shown that the Dewar-Chatt-Duncanson model in terms of M ← CO σ-donation and M → CO π-backdonation is a valid approach to explain the M-CO bonds and the trend of the CO stretching frequencies. The carbonyl ligands of the neutral complexes carry a negative charge, and the polarity of the M-CO bonds increases for the less electronegative metals, which is particularly strong for the group 4 and group 2 atoms. The NBO method delivers an unrealistic charge distribution in the carbonyl complexes, while the AIM approach gives physically reasonable partial charges that are consistent with the EDA-NOCV calculations and with the trend of the C-O stretching frequencies. The AdNDP method provides delocalized MOs which are very useful models for the carbonyl complexes. Deep insight into the nature of the metal-CO bonds and quantitative information about the strength of the [M] ← (CO)8σ-donation and [M(d)] → (CO)8π-backdonation visualized by the deformation densities are provided by the EDA-NOCV method. The large polarity of the M-CO πorbitals toward the CO end in the alkaline earth octacarbonyls M(CO)8(M = Ca, Sr, Ba) leads to small values for the delocalization indices δ(M-C) and δ(M···O) and significant overlap between adjacent CO groups, but the origin of the charge migration and the associated red-shift of the C-O stretching frequencies is the [M(d)] → (CO)8π-backdonation. The heavier alkaline earth metals calcium, strontium and barium use their s/d valence orbitals for covalent bonding. They are therefore to be assigned to the transition metals.
Chemical Bonding in Homoleptic Carbonyl Cations [M{Fe(CO)5}2]+ (M=Cu, Ag, Au)
Pan S., Gorantla S.M.N.V.T., Parasar D., Dias H.V.R., Frenking G.
Article, Chemistry - A European Journal, 2021, DOI Link
View abstract ⏷
Syntheses of the copper and gold complexes [Cu{Fe(CO)5}2][SbF6] and [Au{Fe(CO)5}2][HOB{3,5-(CF3)2C6H3}3] containing the homoleptic carbonyl cations [M{Fe(CO)5}2]+ (M=Cu, Au) are reported. Structural data of the rare, trimetallic Cu2Fe, Ag2Fe and Au2Fe complexes [Cu{Fe(CO)5}2][SbF6], [Ag{Fe(CO)5}2][SbF6] and [Au{Fe(CO)5}2][HOB{3,5-(CF3)2C6H3}3] are also given. The silver and gold cations [M{Fe(CO)5}2]+ (M=Ag, Au) possess a nearly linear Fe-M-Fe’ moiety but the Fe-Cu-Fe’ in [Cu{Fe(CO)5}2][SbF6] exhibits a significant bending angle of 147° due to the strong interaction with the [SbF6]− anion. The Fe(CO)5 ligands adopt a distorted square-pyramidal geometry in the cations [M{Fe(CO)5}2]+, with the basal CO groups inclined towards M. The geometry optimization with DFT methods of the cations [M{Fe(CO)5}2]+ (M=Cu, Ag, Au) gives equilibrium structures with linear Fe-M-Fe’ fragments and D2 symmetry for the copper and silver cations and D4d symmetry for the gold cation. There is nearly free rotation of the Fe(CO)5 ligands around the Fe-M-Fe’ axis. The calculated bond dissociation energies for the loss of both Fe(CO)5 ligands from the cations [M{Fe(CO)5}2]+ show the order M=Au (De=137.2 kcal mol−1)>Cu (De=109.0 kcal mol−1)>Ag (De=92.4 kcal mol−1). The QTAIM analysis shows bond paths and bond critical points for the M−Fe linkage but not between M and the CO ligands. The EDA-NOCV calculations suggest that the [Fe(CO)5]→M+←[Fe(CO)5] donation is significantly stronger than the [Fe(CO)5]←M+→[Fe(CO)5] backdonation. Inspection of the pairwise orbital interactions identifies four contributions for the charge donation of the Fe(CO)5 ligands into the vacant (n)s and (n)p AOs of M+ and five components for the backdonation from the occupied (n-1)d AOs of M+ into vacant ligand orbitals.
Editorial: “Changing the Perspective of the Noble Gas Reactivity”
Pan S., Merino G., Chattaraj P.K.
Editorial, Frontiers in Chemistry, 2021, DOI Link
Carbodicarbene Bismaalkene Cations: Unravelling the Complexities of Carbene versus Carbone in Heavy Pnictogen Chemistry
Walley J.E., Warring L.S., Wang G., Dickie D.A., Pan S., Frenking G., Gilliard R.J.
Article, Angewandte Chemie - International Edition, 2021, DOI Link
View abstract ⏷
We report a combined experimental and theoretical study on the first examples of carbodicarbene (CDC)-stabilized bismuth complexes, which feature low-coordinate cationic bismuth centers with C=Bi multiple-bond character. Monocations [(CDC)Bi(Ph)Cl][SbF6] (8) and [(CDC)BiBr2(THF)2][SbF6] (11), dications [(CDC)Bi(Ph)][SbF6]2 (9) and [(CDC)BiBr(THF)3][NTf2]2 (12), and trication [(CDC)2Bi][NTf2]3 (13) have been synthesized via sequential halide abstractions from (CDC)Bi(Ph)Cl2 (7) and (CDC)BiBr3 (10). Notably, the dications and trication exhibit C (Formula presented.) Bi double dative bonds and thus represent unprecedented bismaalkene cations. The synthesis of these species highlights a unique non-reductive route to C−Bi π-bonding character. The CDC-[Bi] complexes (7–13) were compared with related NHC-[Bi] complexes (1, 3–6) and show substantially different structural properties. Indeed, the CDC ligand has a remarkable influence on the overall stability of the resulting low-coordinate Bi complexes, suggesting that CDC is a superior ligand to NHC in heavy pnictogen chemistry.
Generation and Characterization of the C3O2− Anion with an Unexpected Unsymmetrical Structure
Wang L., Pan S., Lu B., Dong X., Li H., Deng G., Zeng X., Zhou M., Frenking G.
Article, Angewandte Chemie - International Edition, 2021, DOI Link
View abstract ⏷
The carbon suboxide anion C3O2− is generated in solid neon matrix. It is characterized by infrared absorption spectroscopy as well as quantum chemical calculations to have a planar Cs structure where two CO groups with significantly different bond lengths and angles are attached in a zigzag fashion to the central carbon atom. Bonding analysis indicates that it is best described by the bonding interactions between a neutral CO in a triplet excited state and a doublet excited state of CCO−.
CO-Induced Dinitrogen Fixation and Cleavage Mediated by Boron
Deng G., Pan S., Dong X., Wang G., Zhao L., Zhou M., Frenking G.
Article, Chemistry - A European Journal, 2021, DOI Link
View abstract ⏷
The boron atoms react with carbon monoxide and dinitrogen forming the end-on bonded NNBCO complex in solid neon or in nitrogen matrices. The NNBCO complex rearranges to the (η2-N2)BCO isomer with a more activated side-on bonded dinitrogen ligand upon visible light excitation. (η2-N2)BCO and its weakly CO-coordinated complexes further isomerize to the NBNCO and B(NCO)2 molecules with N−N bond being completely cleaved under UV light irradiation. The geometries, energies and vibrational spectra of the molecules are calculated with quantum chemical methods and the electronic structures are analyzed with charge- and energy-partitioning methods.
Revisiting the Bonding Scenario of Two Donor Ligand Stabilized C2Species
Gorantla S.M.N.V.T., Pan S., Mondal K.C., Frenking G.
Article, Journal of Physical Chemistry A, 2021, DOI Link
View abstract ⏷
Quantum chemical calculations using density functional methods were performed for complexes of type L2C2 with L = NHCMe (1), SNHCMe (2) (S = saturated), cAACMe (3), and diamidocarbene (DACMe) (4). The equilibrium structures of 1-4 possess almost linear C4 cores. A high thermochemical stability of the complexes with respect to dissociation, L2C2 → C2 + 2L, is indicated by the large bond dissociation energy following the order 3 > 4 > 2 > 1. The results show that the use of SNHCMe and DACMe as ligands is preferable over NHCMe. The bonding analysis using charge and energy decomposition methods reveals that (cAACMe)2C2 and (DACMe)2C2 possess genuine cumulene C4 moieties, which results from the electron-sharing bonding between quintet L2 and quintet C2 fragments. In contrast, the bonding in (NHCMe)2C2 and (SNHCMe)2C2 comes from a combination of dative and electron-sharing interactions between doublet L2+ and doublet C2- fragments.
Generation and Identification of the Linear OCBNO and OBNCO Molecules with 24 Valence Electrons
Deng G., Pan S., Jin J., Wang G., Zhao L., Zhou M., Frenking G.
Article, Chemistry - A European Journal, 2021, DOI Link
View abstract ⏷
Two structural isomers containing five second-row element atoms with 24 valence electrons were generated and identified by matrix-isolation IR spectroscopy and quantum chemical calculations. The OCBNO complex, which is produced by the reaction of boron atoms with mixtures of carbon monoxide and nitric oxide in solid neon, rearranges to the more stable OBNCO isomer on UV excitation. Bonding analysis indicates that the OCBNO complex is best described by the bonding interactions between a triplet-state boron cation with an electron configuration of (2s)0(2pσ)0(2pπ)2 and the CO/NO− ligands in the triplet state forming two degenerate electron-sharing π bonds and two ligand-to-boron dative σ bonds.
Intriguing structural, bonding and reactivity features in some beryllium containing complexes
Pan S., Jana G., Saha R., Zhao L., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2020, DOI Link
View abstract ⏷
Although the toxicity of beryllium compounds causes impediments in experiments involving them, beryllium chemistry has seen a recent upsurge of interest and considerable progress. Computations play a very important complementary role in analyzing the structure, stability and bonding of these compounds. In this perspective article, we highlighted our contribution to beryllium chemistry which is either completely through theoretical results or sometimes supported by experimental findings. It starts with the smallest 2π aromatic system, Be32-, which also exhibits rare bond-stretch isomerism. Furthermore, its reactivity towards different transformations is mentioned. Because of the ability of beryllium to attain a high ionic potential, the beryllium center in an appropriate situation can act as an excellent Lewis acid which is utilized to bind noble gas (Ng) atoms, carbon monoxide and dinitrogen through donor-Acceptor types of interactions. We made several efforts to have strong Ng-Be bonds which led us to NgBeNCN that is recorded to have the strongest Ng-Be bond among the neutral Ng-Be complexes reported so far. Significant dinitrogen activation was also achieved in (NN)2Be(η2-N2) and OCBeNN complexes. In the latter case, a complete cleavage of the N-N bond producing the most stable NBeNCO molecule has occurred. We also found viable M2(NHBMe)2 (M = Be, Mg) complexes having unusual bonding where the interacting fragments are best described as the neutral M2 and (NHBMe)2 but M2 still possesses a single bond. We finally discussed the complex comprising an unusual Be(i) oxidation state, [BeI(cAACAr)2]+ and di-ortho-beryllated carbodiphosphorane exhibiting BeaC double dative bonds. This journal is
Group 6 Hexacarbonyls as Ligands for the Silver Cation: Syntheses, Characterization, and Analysis of the Bonding Compared with the Isoelectronic Group 5 Hexacarbonylates
Bohnenberger J., Kratzert D., Gorantla S.M.N.V.T., Pan S., Frenking G., Krossing I.
Article, Chemistry - A European Journal, 2020, DOI Link
View abstract ⏷
The syntheses of the two novel complexes [Ag{Mo/W(CO)6}2]+[F-{Al(ORF)3}2]− (RF=C(CF3)3) are reported along with their structural and spectroscopic characterization. The X-ray structure shows that three carbonyl ligands from each M(CO)6 fragment bend towards the silver atom within binding Ag−C distance range. DFT calculations of the free cations [Ag{M(CO)6}2]+ (M=Cr, Mo, W) in the electronic singlet state give equilibrium structures with C2 symmetry with two bridging carbonyl groups from each hexacarbonyl ligand. Similar structures with C2 symmetry (M=Nb) and D2 symmetry (M=V, Ta) are calculated for the isoelectronic group 5 anions [Ag{M(CO)6}2]− (M=V, Nb, Ta). The electronic structure of the cations is analyzed with the QTAIM and EDA-NOCV methods, which provide detailed information about the nature of the chemical bonds between Ag+ and the {M(CO)6}2q (q = −2, M = V, Nb, Ta; q = 0, M = Cr, Mo, W) ligands.
A diradical based on odd-electron σ-bonds
Yang W., Zhang L., Xiao D., Feng R., Wang W., Pan S., Zhao Y., Zhao L., Frenking G., Wang X.
Article, Nature Communications, 2020, DOI Link
View abstract ⏷
The concept of odd-electron σ–bond was first proposed by Linus Pauling. Species containing such a bond have been recognized as important intermediates encountered in many fields. A number of radicals with a one-electron or three-electron σ-bond have been isolated, however, no example of a diradical based odd-electron σ-bonds has been reported. So far all stable diradicals are based on two s/p-localized or π-delocalized unpaired electrons (radicals). Here, we report a dication diradical that is based on two Se∴Se three-electron σ–bonds. In contrast, the dication of sulfur analogue does not display diradical character but exhibits a closed-shell singlet.
Quadruple bonding of bare group-13 atoms in transition metal complexes
Pan S., Manoj S., Frenking G.
Article, Dalton Transactions, 2020, DOI Link
View abstract ⏷
Density functional theory calculations at the M06-D3/def2-TZVPPD level of the group-13 anion complexes EFe(CO)3- (E = B-Tl) and the isoelectronic neutral and charged boron adducts BTM(CO)3q (TMq = Fe-, Ru-, Os-, Co, Rh, Ir, Ni+, Pd+, Pt+) give tetrahedral (C3v) geometries in the 1A1 electronic ground state as equilibrium structures. The analysis of the bonding situation with the energy decomposition analysis in combination with natural orbital for chemical valence method suggests that the E-TM(CO)3q bonds possess four bonding components: (a) one strong electron-sharing σ bond E-TM(CO)3q; (b) two π backdonations ETM(CO)3q and (c) one weak σ donation E→TM(CO)3q. The relative strength of the four bonding components depends on the charge of the system, the transition metal TM and the group-13 atom E. The σ donation E→TM(CO)3q is in all systems rather weak while the associated charge migration is not negligible. A similar situation of the bonding of terminal group-13 atoms Ga and In is found in Ga-TM(GaCp)4+ and E-Pt(PMe3)3+ (TM = Ni, Pd, Pt; E = Ga, In), which are model compounds for the stable complexes GaTM(GaCp∗)4+ (TM = Ni, Pt) and InPt(PPh3)3+. The quadruple bonds E→TML2 are hybrids of electron-sharing and dative bonds. This journal is
Stabilization of Linear C3 by Two Donor Ligands: A Theoretical Study of L-C3-L (L=PPh3, NHCMe, cAACMe)**
Gorantla S.M.N.V.T., Pan S., Mondal K.C., Frenking G.
Article, Chemistry - A European Journal, 2020, DOI Link
View abstract ⏷
Quantum chemical studies using density functional theory and ab initio methods have been carried out for the molecules L-C3-L with L=PPh3 (1), NHCMe (2, NHC=N-heterocyclic carbene), and cAACMe (3, cAAC=cyclic (alkyl)(amino) carbene). The calculations predict that 1 and 2 have equilibrium geometries where the ligands are bonded with rather acute bonding angles at the linear C3 moiety. The phosphine adduct 1 has a synclinal (gauche) conformation whereas 2 exhibits a trans conformation of the ligands. In contrast, the compound 3 possesses a nearly linear arrangement of the carbene ligands at the C3 fragment. The bond dissociation energies of the ligands have the order 1<2<3. The bonding analysis using charge and energy decomposition methods suggests that 3 is best described as a cumulene with electron-sharing double bonds between neutral fragments (cAACMe)2 and C3 in the respective electronic quintet state yielding (cAACMe)=C3=(cAACMe). In contrast, 1 and 2 possess electron-sharing and dative bonds between positively charged ligands [(PPh3)2]+ or [(NHCMe)2]+ and negatively charged [C3]− fragments in the respective doublet state.
Alkaline Earth Metals Activate N2 and CO in Cubic Complexes Just Like Transition Metals: A Conceptual Density Functional Theory and Energy Decomposition Analysis Study
Bettens T., Pan S., De Proft F., Frenking G., Geerlings P.
Article, Chemistry - A European Journal, 2020, DOI Link
View abstract ⏷
Following the recent discovery of stable octa-coordinated alkaline earth metals with N2 and CO, the role of group II metals in the catalytic reduction of these ligands by means of density functional theory (DFT) calculations and conceptual DFT-based reactivity indices is investigated. Cubic group IV and octahedral group VI transition metal complexes as well as the free ligands are computed for reference. The outer and most accessible atoms of N2 and CO become much more nucleophilic and electrophilic in all complexes, relevant for N2 fixation, as probed by the Fukui function and local softness. Within one row of the periodic table, the alkaline earth complexes often show the strongest activation. On the contrary, the electrostatic character is found to be virtually unaffected by complexation. Trends in the soft frontier orbital and hard electrostatic character are in agreement with calculated proton affinities and energy decomposition analyses of the protonated structures, demonstrating the dominance of the soft (HOMO–LUMO) orbital interactions.
Beryllium Atom Mediated Dinitrogen Activation via Coupling with Carbon Monoxide
Deng G., Pan S., Wang G., Zhao L., Zhou M., Frenking G.
Article, Angewandte Chemie - International Edition, 2020, DOI Link
View abstract ⏷
The reactions of laser-ablated beryllium atoms with dinitrogen and carbon monoxide mixtures form the end-on bonded NNBeCO and side-on bonded (η2-N2)BeCO isomers in solid argon, which are predicted by quantum chemical calculations to be almost isoenergetic. The end-on bonded complex has a triplet ground state while the side-on bonded isomer has a singlet electronic ground state. The complexes rearrange to the energetically lowest lying NBeNCO isomer upon visible light excitation, which is characterized to be an isocyanate complex of a nitrene derivative with a triplet electronic ground state. A bonding analysis using a charge- and energy decomposition procedure reveals that the electronic reference state of Be in the NNBeCO isomers has an 2s02p2 excited configuration and that the metal-ligand bonds can be described in terms of N2→Be←CO σ donation and concomitant N2←Be→CO π backdonation. The results demonstrate that the activation of N2 with the N−N bond being completely cleaved can be achieved via coupling with carbon monoxide mediated by a main group atom.
Di- ortho-beryllated Carbodiphosphorane: A Compound with a Metal-Carbon Double Bond to an Element of the s-Block
Buchner M.R., Pan S., Poggel C., Spang N., Muller M., Frenking G., Sundermeyer J.
Article, Organometallics, 2020, DOI Link
View abstract ⏷
Double bonds have been realized for a wide variety of elements in the p-, d-, and f-blocks. However, no s-block metal complexes with a double bond have been identified. Here we report the synthesis and characterization of a di-ortho-beryllated carbodiphosphorane, which exhibits a double dative Be═C bond. This species shows an unprecedented bonding situation at the metal center, which was extensively analyzed by experimental and computational means.
Filling a Gap: The Coordinatively Saturated Group 4 Carbonyl Complexes TM(CO)8 (TM=Zr, Hf) and Ti(CO)7
Deng G., Lei S., Pan S., Jin J., Wang G., Zhao L., Zhou M., Frenking G.
Article, Chemistry - A European Journal, 2020, DOI Link
View abstract ⏷
Homoleptic Group 4 metal carbonyl cation and neutral complexes were prepared in the gas phase and/or in solid neon matrix. Infrared spectroscopy studies reveal that both zirconium and hafnium form eight-coordinate carbonyl neutral and cation complexes. In contrast, titanium forms only the six-coordinate Ti(CO)6+ and seven-coordinate Ti(CO)7. Titanium octacarbonyl Ti(CO)8 is unstable as a result of steric repulsion between the CO ligands. The 20-electron Zr(CO)8 and Hf(CO)8 complexes represent the first experimentally observed homoleptic octacarbonyl neutral complexes of transition metals. The molecules still fulfill the 18-electron rule, because one doubly occupied valence orbital does not mix with any of the metal valence atomic orbitals. Zr(CO)8 and Hf(CO)8 are stable against the loss of one CO because the CO ligands encounter less steric repulsion than Zr(CO)7 and Hf(CO)7. The heptacarbonyl complexes have shorter metal−CO bonds than that of the octacarbonyl complexes due to stronger electrostatic and covalent bonding, but the significantly smaller repulsive Pauli term makes the octacarbonyl complexes stable.
d–d Dative Bonding Between Iron and the Alkaline-Earth Metals Calcium, Strontium, and Barium
Stegner P., Farber C., Oetzel J., Siemeling U., Wiesinger M., Langer J., Pan S., Holzmann N., Frenking G., Albold U., Sarkar B., Harder S.
Article, Angewandte Chemie - International Edition, 2020, DOI Link
View abstract ⏷
Double deprotonation of the diamine 1,1′-(tBuCH2NH)-ferrocene (1-H2) by alkaline-earth (Ae) or EuII metal reagents gave the complexes 1-Ae (Ae=Mg, Ca, Sr, Ba) and 1-Eu. 1-Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1-Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1-Ca, 1-Sr, and 1-Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1H NMR chemical-shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1-Ae complexes shows that the heavier species 1-Ca, 1-Sr, and 1-Ba possess genuine Fe→Ae bonds which involve vacant d-orbitals of the alkaline-earth atoms and partially filled d-orbitals on Fe. In 1-Mg, a weak Fe→Mg donation into vacant p-orbitals of the Mg atom is observed.
Noble gas endohedral fullerenes
Jalife S., Arcudia J., Pan S., Merino G.
Review, Chemical Science, 2020, DOI Link
View abstract ⏷
This review focuses on the available experimental and theoretical investigations on noble gas (Ng) endohedral fullerenes, addressing essential questions related to the mutual effects that confinement of one or more Ng atoms induces on the electronic structure, bonding, and different properties of fullerenes. It also summarizes the different contributions to the mechanisms of formation and decomplexation, the reactivity towards Diels-Alder cycloaddition reactions, the chemical bonding situation of Ng endohedral fullerenes, and the interactions that dominate within these systems.
Synthesis and characterization of heterometallic complexes involving coinage metals and isoelectronic Fe(CO)5, [Mn(CO)5]-and [Fe(CO)4CN]-ligands
Ponduru T.T., Wang G., Manoj S., Pan S., Zhao L., Frenking G., Frenking G., Dias H.V.R.
Article, Dalton Transactions, 2020, DOI Link
View abstract ⏷
The chemistry of coinage metal ions with Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- has been explored using Mes3P and N-heterocyclic carbene supporting ligands. A comparison of [(SIPr)Au-Fe(CO)5][SbF6], [(Et2CAAC)Au-Fe(CO)5][SbF6] and [(Mes3P)Au-Fe(CO)5][SbF6] shows that the ligand donor strength towards Au(i) follows the order Mes3P > Et2CAAC > SIPr. These Fe(CO)5 complexes show significant blue shifts in CO bands relative to those observed for free Fe(CO)5 as a result of it serving as a net electron donor to Au(i). Au(i) is a much stronger acceptor in (SIPr)Au-Mn(CO)5 compared to Ag(i) in (SIPr)Ag-Mn(CO)5. The structural details of Mes3PAu-Mn(CO)5 are also presented. [Fe(CO)4CN]- afforded CN bridged coinage metal complexes with (IPr∗)Au+, (SIPr)Ag+ and (SIPr)Cu+ moieties, rather than molecules with direct Fe/coinage metal bonds. The computed total interaction energies indicate that both [Mn(CO)5]- and [Fe(CO)4CN]- are stronger donors toward Au(i) than Fe(CO)5. A detailed analysis of the bonding interactions between the coinage metal ions and Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- suggests that the largest contribution comes from electrostatic attraction, while the covalent component follows the Dewar-Chatt-Duncanson model. The σ-donor interactions of these organometallic ligands with coinage metal ions are considerably stronger than the π-backbonding from the coinage metal ions.
Side-On Bonded Beryllium Dinitrogen Complexes
Deng G., Pan S., Wang G., Zhao L., Zhou M., Frenking G.
Article, Angewandte Chemie - International Edition, 2020, DOI Link
View abstract ⏷
The preparation and spectroscopic identification of the complexes NNBe(η2-N2) and (NN)2Be(η2-N2) and the energetically higher lying isomers Be(NN)2 and Be(NN)3 are reported. NNBe(η2-N2) and (NN)2Be(η2-N2) are the first examples of covalently side-on bonded N2 adducts of a main-group element. The analysis of the electronic structure using modern methods of quantum chemistry suggests that NNBe(η2-N2) and (NN)2Be(η2-N2) should be classified as π complexes rather than metalladiazirines.
Comment on “Realization of Lewis Basic Sodium Anion in the NaBH3− Cluster”
Pan S., Frenking G.
Letter, Angewandte Chemie - International Edition, 2020, DOI Link
View abstract ⏷
We challenge the interpretation of the chemical bond in NaBH3− proposed by Liu et al. We argue that NaBH3− has an electron-sharing Na−BH3− covalent bond rather than a dative bond Na−→BH3.
A Stable, Crystalline Beryllium Radical Cation
Wang G., Walley J.E., Dickie D.A., Pan S., Frenking G., Gilliard R.J.
Article, Journal of the American Chemical Society, 2020, DOI Link
View abstract ⏷
The alkaline-earth elements (Be, Mg, Ca, Sr, and Ba) strongly favor the formation of diamagnetic compounds in the +2 oxidation state. Herein we report a paramagnetic beryllium radical cation, [(CAAC)2Be]+⢠(2) [CAAC = cyclic (alkyl)(amino)carbene], prepared by oxidation of a zero-valent beryllium complex with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). Compound 2 was characterized by EPR spectroscopy, elemental analysis, X-ray crystallography, and DFT calculations. Notably, the isolation of 2 represents the first s-block charged radical and the first crystalline beryllium radical.
Comment on “revisiting π backbonding: The influence of d orbitals on metal-CO bonds and ligand red shifts” by D. Koch, Y. Chen, P. Golub and S. Manzhos,: Phys. Chem. Chem. Phys., 2019, 21, 20814
Pan S., Frenking G.
Article, Physical Chemistry Chemical Physics, 2020, DOI Link
View abstract ⏷
We challenge the statement of Koch et al. that the M → CO charge transfer and the decrease of the CO stretching frequency in metal carbonyl complexes do not depend on the metal d orbitals. The approach of the authors is severely flawed and leads to misleading conclusions.
Persistent Borafluorene Radicals
Yang W., Krantz K.E., Freeman L.A., Dickie D.A., Molino A., Frenking G., Pan S., Wilson D.J.D., Gilliard R.J.
Article, Angewandte Chemie - International Edition, 2020, DOI Link
View abstract ⏷
N-Heterocyclic carbene (NHC)- and cyclic (alkyl)(amino)carbene (CAAC)-stabilized borafluorene radicals have been isolated and characterized by elemental analysis, single-crystal X-ray diffraction, UV/Vis absorption, cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and theoretical studies. Both the CAAC–borafluorene radical (2) and the NHC–borafluorene radical (4) have a considerable amount of spin density localized on the boron atoms (0.322 for 2 and 0.369 for 4). In compound 2, the unpaired electron is also partly delocalized over the CAAC ligand carbeneC and N atoms. However, the unpaired electron in compound 4 mainly resides throughout the borafluorene π-system, with significantly less delocalization over the NHC ligand. These results highlight the Lewis base dependent electrostructural tuning of materials-relevant radicals. Notably, this is the first report of crystalline borafluorene radicals, and these species exhibit remarkable solid-state and solution stability.
Bonding Analysis of the Shortest Bond between Two Atoms Heavier than Hydrogen and Helium: O2 2+
Fu M., Pan S., Zhao L., Frenking G.
Article, Journal of Physical Chemistry A, 2020, DOI Link
View abstract ⏷
Quantum chemical calculations using ab initio methods at the CCSD(T) level with large basis sets and DFT calculations using the BP86 functional have been carried out for O2 2+ and N2. An energy decomposition analysis of the chemical bonds suggests that the shorter bond in O2 2+ compared with isoelectronic N2 is due to the weaker Pauli repulsion in the dication, which overcompensates the weakening of attractive interactions that are operative in N2. At the equilibrium distance of N2, the orbital (covalent) bonding in O2 2+ is weaker than in N2, and the attractive Coulomb interactions in the neutral diatomic system become repulsive in the dication, but the weakening of the Pauli repulsion caused by the shrinking of the orbitals in O2 2+ compensates for these forces and leads to a shortening of the bond. The results also show that the bond dissociation energy is not a reliable indicator for the strength of bond, which is more faithfully given by the (local) force constant.
Embedding a Planar Hypercoordinate Carbon Atom into a [4n+2] π-System
Yanez O., Baez-Grez R., Garza J., Pan S., Barroso J., Vasquez-Espinal A., Merino G., Tiznado W.
Article, ChemPhysChem, 2020, DOI Link
View abstract ⏷
Through delicate tuning of the electronic structure, we report herein a rational design of seventeen new putative global minimum energy structures containing a planar tetra- or pentacoordinate carbon atom embedded in an aromatic hydrocarbon. These structures are the result of replacing three consecutive hydrogen atoms of an aromatic hydrocarbon by less electronegative groups, forming a multicenter σ-bond with the planar hypercoordinate carbon atom and participating in the π-electron delocalization. This strategy that maximizes both mechanical and electronic effects through aromatic architectures can be extended to several molecular combinations to achieve new and diverse compounds containing planar hypercoordinate carbon centers.
Filling the void: Controlled donor-acceptor interaction facilitates the formation of an M-M single bond in the zero oxidation state of M (M = Zn, Cd, Hg)
Saha R., Pan S., Chattaraj P.K., Merino G.
Article, Dalton Transactions, 2020, DOI Link
View abstract ⏷
The intriguing question of whether it is possible to form a genuine M0-M0 single bond for the M2 species (M = Zn, Cd, Hg) is addressed here. So far, all the bonds reported in the literature are exclusively MI-MI. Herein, we present viable M2(NHBMe)2 (M = Zn, Cd, Hg; NHBMe = (HCNMe)2B) complexes in which the controlled donor-acceptor interaction leads to an M0-M0 single bond. In these complexes, M2 in the 1∑g ground state with the (nσg+)2(nσu+)2 (n = 7, 10 and 14 for M = Zn, Cd and Hg, respectively) valence electron configuration forms donor-acceptor bonding with singlet 2NHBMe ligands where a combined effect of dominant (+,-) σ-backdonation from the antibonding (nσu+)2 orbital of M2 to the 2NHBMe ligands and a somewhat weaker (+,+) σ-donation from the 2NHBMe ligands to the bonding (n + 1)σg+ orbital leads to the unorthodox bonding situation of forming an M-M single bond in the zero oxidation state by eventually nullifying one effect by another. This is an unprecedented situation in the sense that the NHBMe ligand acts as a strong σ-acceptor and a weaker σ-donor. A comparison with the experimentally reported M2(PhDipp)2 complexes reveals the uniqueness of the NHBMe ligand in exhibiting such a bonding scenario. The M2(NHBMe)2 complex is thermochemically viable with respect to possible dissociation channels at room temperature, except for metal extrusion processes, M2(NHBMe)2 → M + M(NHBMe)2 and M2(NHBMe)2 → M2 + (NHBMe)2. Although the latter two processes are exergonic, they are kinetically protected by a high free energy barrier of 26.5-39.5 kcal mol-1. The experimental characterization of M2(PhDipp)2 despite similar exergonic channels reveals such kinetic stability to be enough for the viability of the M2(NHBMe)2 complexes. Furthermore, the ligand exchange reaction considering M2(PhMe)2 as the starting material also turned out to be feasible. Therefore, the M2(NHBMe)2 complexes are the first cases that feature a neutral M2 moiety with a single M0-M0 covalent bond, where M is a Group 12 metal.
Donor-Acceptor vs Electron-Shared Bonding: Triatomic SinC3-n (n ≤ 3) Clusters Stabilized by Cyclic Alkyl(amino) Carbene
Ghara M., Pan S., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2019, DOI Link
View abstract ⏷
SinC3-n (n ≤ 3) clusters are interstellar species that are transient in nature at ambient conditions. Herein, the structure, stability, and nature of bonding in cyclic alkyl(amino) carbene (cAAC) protected SinC3-n (n ≤ 3) clusters are studied in silico. The Si3(cAAC)3 complex was previously reported to be synthesized in large scale. The present results indicate that because the C-CcAAC bond is stronger than the Si-CcAAC bond, C3(cAAC)3 and SiC2(cAAC)3 complexes have significantly larger stability with respect to ligand dissociation than the Si3(cAAC)3 complex, while Si2C(cAAC)3 has almost the same stability as in the latter complex. Moreover, considering the Si3(cAAC)3 complex as a precursor, the hypothetical successive single Si substitution process by a single C atom in Si3(cAAC)3 complex is exergonic in nature. The bonding situation is analyzed by employing natural bond orbital (NBO), electron density, and energy decomposition analyses in combination with the natural orbital for chemical valence theory. These studies show that the nature of bonding in C-CcAAC and Si-CcAAC bonds differs significantly from each other. The former bonds are best described as an electron-shared double bond, whereas the latter bonds are of donor-acceptor type consisting of two components, SiCcAAC σ-donation and Si→CcAAC-back-donation. Nevertheless, in the former bonds, covalent character is larger than the ionic one but in the latter bonds the reverse is true. For some Si-CcAAC bonds, the natural orbital cannot be located by the NBO method, presumably because of slightly lower occupancy than the cutoff values, but the electron density analysis confirms that different Si-CcAAC bonds in a given complex are almost equivalent in terms of electron density distribution. This paper reports an interesting change in bonding pattern when one replaces Si by a C atom in triatomic silicon carbide clusters stabilized by a ligand.
Octa-coordinated alkaline earth metal–dinitrogen complexes M(N2)8 (M=Ca, Sr, Ba)
Wang Q., Pan S., Lei S., Jin J., Deng G., Wang G., Zhao L., Zhou M., Frenking G.
Article, Nature Communications, 2019, DOI Link
View abstract ⏷
We report the isolation and spectroscopic identification of the eight-coordinated alkaline earth metal–dinitrogen complexes M(N2)8 (M=Ca, Sr, Ba) possessing cubic (Oh) symmetry in a low-temperature neon matrix. The analysis of the electronic structure reveals that the metal-N2 bonds are mainly due to [M(dπ)]→(N2)8 π backdonation, which explains the observed large red-shift in N-N stretching frequencies. The adducts M(N2)8 have a triplet (3A1g) electronic ground state and exhibit typical bonding features of transition metal complexes obeying the 18-electron rule. We also report the isolation and bonding analysis of the charged dinitrogen complexes [M(N2)8]+ (M=Ca, Sr).
Transition-Metal Chemistry of Alkaline-Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr, Ba)
Wang Q., Pan S., Wu Y.-B., Deng G., Bian J.-H., Wang G., Zhao L., Zhou M., Frenking G.
Review, Angewandte Chemie - International Edition, 2019, DOI Link
View abstract ⏷
We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)3 (M=Sr, Ba) in low-temperature Ne matrix. Both complexes are characterized by a D3 symmetric structure involving three equivalent η6-bound benzene ligands and a closed-shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal–ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n−1)d AOs of M and strong backdonation from the occupied (n−1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20-electron complexes have 18 effective valence electrons, and, thus, fulfill the 18-electron rule if only the metal–ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.
Fluxional Boron Clusters: From Theory to Reality
Pan S., Barroso J., Jalife S., Heine T., Asmis K.R., Merino G.
Article, Accounts of Chemical Research, 2019, DOI Link
View abstract ⏷
ConspectusIsolated boron clusters exhibit many intriguing properties, which have only recently been unfolding with the hand-in-hand advancement of state-of-the-art experimental and theoretical methods for the analyses of their electronic structure, chemical reactivity, and nuclear dynamics. A fascinating property that a number of these clusters display is fluxionality, a dynamical phenomenon associated with the delocalized nature of the chemical bonding and related to the continuous exchange between interatomic neighbors. The electron-deficient nature of boron is the driving force behind its extraordinary ability to form multicenter bonds, and this in turn leads to fluxional behavior only when an appropriate combination of topology and bonding is present. The first instance of fluxionality in boron clusters, the quasi-planar anion B19 -, was reported in 2010. The rotational barrier of the inner B6 unit spinning within the peripheral B13 ring can be overcome even at low temperature, mimicking the characteristic motion of a rotary internal combustion engine, and hence, B19 - was entitled a boron-based molecular Wankel engine. Shortly after that, it was found that other quasi-planar boron clusters, like B13 + and B18 2-, also exhibit an almost barrier-free rotation of internal planar moieties. The case of the B13 + cation is special because, on the one hand, it was chosen to examine the way to initiate, control, and direct the internal rotation using circularly polarized laser radiation, and on the other hand, the experimental manifestation of fluxionality was first established for this system through infrared experiments. Nevertheless, fluxional behavior is not limited to planar or pure boron clusters. Larger boron clusters, such as the fullerene-analogue borospherenes B40 and B39 -, are also predicted to show pronounced dynamical behavior that is related to the interconversion between six- and seven-membered rings. Be6B11 -, a triple-layer cluster, is another particularly interesting system since it exhibits multifold fluxionality consisting of the revolution of the outer boron ring around the Be6 core and the spinning of the two Be3 rings with respect to each other. The essential criteria for dynamical behavior in boron clusters are (1) the absence of a localized two-center, two-electron (2c-2e) bond between two molecular regions that tend to rotate with respect to each other, (2) the absence of steric hindrances for rotation and reorganization, and (3) retention of the delocalized electronic structure throughout the rotation/reorganization process. The fulfillment of the above three conditions ensures that low energy barriers will be associated with the rotation or reorganization of molecular moieties. The first two points can be illustrated from the facts that a single localized C-B σ bond in CB18 raises the rotational barrier by 27.0 kcal·mol-1 and the expansion of the outer ring by a single boron atom in moving from B12 + to B13 + lowers the rotational barrier by 7.5 kcal·mol-1. Alternatively, it is also possible to make a rigid boron cluster fluxional through doping, where the geometric and electronic changes caused by a suitable dopant, as in MB12 - (M = Co, Rh, Ir) and B10Ca, reduce the corresponding rotational barriers enough to achieve fluxionality. At present, there are 13 pure boron clusters (B11 -/0/+, B13 +/0/-, B15 +/0/-, B18 2-, B19 -, and B20 -/2-) and eight metal-doped boron clusters (B10Ca, NiB11 -, [B2-Ta@B18]-, Be6B11 -, Be6B10 2-, and MB18 - (M = K, Rb, Cs)) that have sufficiently small rotational barriers (less than ∼1.5 kcal·mol-1) to exhibit fluxional behavior at low temperature. Some of the other reported boron clusters show more sizable barriers, and their dynamical behavior is manifested only at elevated temperatures. The research on such systems is driven by the notion that it ultimately will pave the way for the development of light-harvesting boron-based nanomotors/machines and robots, a reality that may not be that far away!.
Octacarbonyl Ion Complexes of Actinides [An(CO)8]+/− (An=Th, U) and the Role of f Orbitals in Metal–Ligand Bonding
Chi C., Pan S., Jin J., Meng L., Luo M., Zhao L., Zhou M., Frenking G.
Article, Chemistry - A European Journal, 2019, DOI Link
View abstract ⏷
The octacarbonyl cation and anion complexes of actinide metals [An(CO)8]+/− (An=Th, U) are prepared in the gas phase and are studied by mass-selected infrared photodissociation spectroscopy. Both the octacarbonyl cations and anions have been characterized to be saturated coordinated complexes. Quantum chemical calculations by using density functional theory show that the [Th(CO)8]+ and [Th(CO)8]− complexes have a distorted octahedral (D4h) equilibrium geometry and a doublet electronic ground state. Both the [U(CO)8]+ cation and the [U(CO)8]− anion exhibit cubic structures (Oh) with a 6A1g ground state for the cation and a 4A1g ground state for the anion. The neutral species [Th(CO)8] (Oh; 1A1g) and [U(CO)8] (D4h; 5B1u) have also been calculated. Analysis of their electronic structures with the help on an energy decomposition method reveals that, along with the dominating 6d valence orbitals, there are significant 5f orbital participation in both the [An]←CO σ donation and [An]→CO π back donation interactions in the cations and anions, for which the electronic reference state of An has both occupied and vacant 5f AOs. The trend of the valence orbital contribution to the metal–CO bonds has the order of 6d≫5f>7s≈7p, with the 5f orbitals of uranium being more important than the 5f orbitals of thorium.
How far can one push the noble gases towards bonding?: A personal account
Saha R., Jana G., Pan S., Merino G., Chattaraj P.K.
Review, Molecules, 2019, DOI Link
View abstract ⏷
Noble gases (Ngs) are the least reactive elements in the periodic table towards chemical bond formation when compared with other elements because of their completely filled valence electronic configuration. Very often, extreme conditions like low temperatures, high pressures and very reactive reagents are required for them to form meaningful chemical bonds with other elements. In this personal account, we summarize our works to date on Ng complexes where we attempted to theoretically predict viable Ng complexes having strong bonding to synthesize them under close to ambient conditions. Our works cover three different types of Ng complexes, viz., non-insertion of NgXY type, insertion of XNgY type and Ng encapsulated cage complexes where X and Y can represent any atom or group of atoms. While the first category of Ng complexes can be thermochemically stable at a certain temperature depending on the strength of the Ng-X bond, the latter two categories are kinetically stable, and therefore, their viability and the corresponding conditions depend on the size of the activation barrier associated with the release of Ng atom(s). Our major focus was devoted to understand the bonding situation in these complexes by employing the available state-of-the-art theoretic tools like natural bond orbital, electron density, and energy decomposition analyses in combination with the natural orbital for chemical valence theory. Intriguingly, these three types of complexes represent three different types of bonding scenarios. In NgXY, the strength of the donor-acceptor Ng→XY interaction depends on the polarizing power of binding the X center to draw the rather rigid electron density of Ng towards itself, and sometimes involvement of such orbitals becomes large enough, particularly for heavier Ng elements, to consider them as covalent bonds. On the other hand, in most of the XNgY cases, Ng forms an electron-shared covalent bond with X while interacting electrostatically with Y representing itself as [XNg]+Y−. Nevertheless, in some of the rare cases like NCNgNSi, both the C-Ng and Ng-N bonds can be represented as electron-shared covalent bonds. On the other hand, a cage host is an excellent moiety to examine the limits that can be pushed to attain bonding between two Ng atoms (even for He) at high pressure. The confinement effect by a small cage-like B12N12 can even induce some covalent interaction within two He atoms in the He2@B12N12 complex.
Response to Comment on “Observation of alkaline earth complexes M(CO)8 (M = Ca, Sr, or Ba) that mimic transition metals”
Zhao L., Pan S., Zhou M., Frenking G.
Review, Science, 2019, DOI Link
View abstract ⏷
Landis et al. claim in their comment that Ca does not bind like a transition metal in Ca(CO)8. We reject their statement, which is based on a misconception of bonding models and misleading application and interpretation of quantum chemical methods for analyzing chemical bonds.
Chemical Bonding and Bonding Models of Main-Group Compounds
Zhao L., Pan S., Holzmann N., Schwerdtfeger P., Frenking G.
Review, Chemical Reviews, 2019, DOI Link
View abstract ⏷
The focus of this review is the presentation of the most important aspects of chemical bonding in molecules of the main group atoms according to the current state of knowledge. Special attention is given to the difference between the physical mechanism of covalent bond formation and its description with chemical bonding models, which are often confused. This is partly due to historical reasons, since until the development of quantum theory there was no physical basis for understanding the chemical bond. In the absence of such a basis, chemists developed heuristic models that proved extremely valuable for understanding and predicting experimental studies. The great success of these simple models and the associated rules led to the fact that the model conceptions were regarded as real images of physical reality. The complicated world of quantum theory, which eludes human imagination, made it difficult to link heuristic models of chemical bonding with quantum chemical knowledge. In the early days of quantum chemistry, some suggestions were made which have since proved untenable. In recent decades, there has been a stormy development of quantum chemical methods, which are not limited to the quantitative accuracy of the calculated properties. Also, methods have been developed where the experimentally developed models can be quantitatively expressed and visually represented using mathematically well-defined terms that are derived from quantum chemical calculations. The calculated numbers may however not be measurable values. Nevertheless, as orientation data for the interpretation and classification of experimental findings as well as a guideline for new experiments, they form a coordinate system that defines the multidimensional world of chemistry, which corresponds to the Hilbert space formalism of physics. The nonmeasurability of model values is not a weakness of chemistry but a characteristic by which the infinite complexity of the material world becomes scientifically accessible and very useful for chemical research. This review examines the basis of the commonly used quantum chemical methods for calculating molecules and for analyzing their electronic structure. The bonding situation in selected representative molecules of main-group atoms is discussed. The results are compared with textbook knowledge of common chemistry.
Modified Particle Swarm Optimization Algorithms for the Generation of Stable Structures of Carbon Clusters, Cn (n = 3–6, 10)
Jana G., Mitra A., Pan S., Sural S., Chattaraj P.K.
Article, Frontiers in Chemistry, 2019, DOI Link
View abstract ⏷
Particle Swarm Optimization (PSO), a population based technique for stochastic search in a multidimensional space, has so far been employed successfully for solving a variety of optimization problems including many multifaceted problems, where other popular methods like steepest descent, gradient descent, conjugate gradient, Newton method, etc. do not give satisfactory results. Herein, we propose a modified PSO algorithm for unbiased global minima search by integrating with density functional theory which turns out to be superior to the other evolutionary methods such as simulated annealing, basin hopping and genetic algorithm. The present PSO code combines evolutionary algorithm with a variational optimization technique through interfacing of PSO with the Gaussian software, where the latter is used for single point energy calculation in each iteration step of PSO. Pure carbon and carbon containing systems have been of great interest for several decades due to their important role in the evolution of life as well as wide applications in various research fields. Our study shows how arbitrary and randomly generated small Cn clusters (n = 3–6, 10) can be transformed into the corresponding global minimum structure. The detailed results signify that the proposed technique is quite promising in finding the best global solution for small population size clusters.
Unprecedented Bonding Situation in Viable E2(NHBMe)2 (E=Be, Mg; NHBMe=(HCNMe)2B) Complexes: Neutral E2 Forms a Single E−E Covalent Bond
Saha R., Pan S., Merino G., Chattaraj P.K.
Article, Angewandte Chemie - International Edition, 2019, DOI Link
View abstract ⏷
Is it possible to facilitate the formation of a genuine Be−Be or Mg−Mg single bond for the E2 species while it is in its neutral state? So far, (NHCR)Be−Be(NHCR) (R=H, Me, Ph) have been reported where Be2 is in 1Δg excited state imposing a formal Be−Be bond order of two. Herein, we present the formation of a single E−E (E=Be, Mg) covalent bond in E2(NHBMe)2 (E=Be, Mg; NHBMe=(HCNMe)2B) complexes where E2 is in 3∑u+ excited state having (nσg+)2(nσu+)1((n+1)σg+)1 (n=2 for Be and n=4 for Mg) valence electron configuration and it forms electron-shared bonding with two NHBMe radicals. The effects of bonding with nσu+ and (n+1)σg+ orbitals will cancel each other, providing the former E−E bond order as one. Be2(NHBMe)2 complex is thermochemically stable with respect to possible dissociation channels at room temperature, whereas the two exergonic channels, Mg2(NHBMe)2 → Mg + Mg(NHBMe)2 and Mg2(NHBMe)2 → Mg2 + (NHBMe)2, are kinetically inhibited by a free energy barrier of 15.7 and 18.7 kcal mol−1, respectively, which would likely to be further enhanced in cases of bulkier substituents attached to the NHB ligands. Therefore, the title complexes are first viable systems which feature a neutral E2 moiety with a single E−E covalent bond.
Microsolvation of lithium–phosphorus double helix: a DFT study
Jana G., Jha R., Pan S., Chattaraj P.K.
Article, Theoretical Chemistry Accounts, 2019, DOI Link
View abstract ⏷
The chemistry of complexes becomes interesting due to their structural diversity in different environments like in aqueous phase, in gas-phase or in the interior of a host. In the last few decades, powerful tools for the determination of gas-phase have been developed. In this context, the microsolvation approach of Li 7 P 7 helix, where the passage from the bare double-strand helix to the hydrated denatured helix, has been addressed through successive attachment of water molecules using density functional theory. The stability of helical structure of the small clusters has been analyzed on the basis of polar bonding interaction between oxygen end of water molecule and Li centers of the Li 7 P 7 helix. The Li 7 P 7 helix is favored when associated with zero to eight water molecules, but the binding of the ninth water molecule brings a drastic change in the structure. Our results suggest that the natural charges on some sites in Li 7 P 7 are large enough to induce partial and eventually total dissociation of water molecules. We shed light on the bonding situation through natural bond orbital, quantum theory of atoms in molecules and energy decomposition analyses which suggest dominant electrostatic interaction between Li centers of Li 7 P 7 and O centers of water molecules (accounting for 60–64% of total bonding attraction). Nevertheless, 31–36% of total attraction is also originated from the orbital interaction. Variation in reactivity on microhydration is also analyzed. In order to check the site selectivity, we have computed conceptual density functional theory-based local reactivity descriptors such as dual descriptor based on the Fukui function, Δf(r), and multiphilic descriptor based on the philicity, Δω(r).
Chemical bonding in the hexamethylbenzene–SO 2+ dication
Pecher L., Pan S., Frenking G.
Article, Theoretical Chemistry Accounts, 2019, DOI Link
View abstract ⏷
A thorough bonding analysis is performed on the dication [C 6 (CH 3 ) 6 SO] 2+ . The results show that the molecule is best described in terms of covalent interactions between the cations C 6 (CH3) 6 + and SO + , whereby the bonding consists of two dominating contributions. The strongest bonding comes from dative interaction from the HOMO of C 6 (CH 3 ) 6 + to the LUMO of SO + , which has overall σ symmetry. The second significant component is due to electron-sharing bonding between the singly occupied orbitals of the two fragments. The bonding situation may be sketched with the formula [C6(CH)6-→SO]2+. The bare dication is thermodynamically unstable with regard to dissociation into two cations. It is kinetically stable due to the activation barrier, and it is further stabilized by counterions.
Octacarbonyl Anion Complexes of the Late Lanthanides Ln(CO) 8 − (Ln=Tm, Yb, Lu) and the 32-Electron Rule
Jin J., Pan S., Jin X., Lei S., Zhao L., Frenking G., Zhou M.
Article, Chemistry - A European Journal, 2019, DOI Link
View abstract ⏷
The lanthanide octacarbonyl anion complexes Ln(CO) 8 − (Ln=Tm, Yb, Lu) were produced in the gas phase and detected by mass-selected infrared photodissociation spectroscopy in the carbonyl stretching-frequency region. By comparison of the experimental CO-stretching frequencies with calculated data, which are strongly red-shifted with respect to free CO, the Yb(CO) 8 − and Lu(CO) 8 − complexes were determined to possess octahedral (O h ) symmetry and a doublet X 2 A 2u (Yb) and singlet X 1 A 1g (Lu) electronic ground state, whereas Tm(CO) 8 − exhibits a D 4h equilibrium geometry and a triplet X 3 B 1g ground state. The analysis of the electronic structures revealed that the metal-CO attractive forces come mainly from covalent orbital interactions, which are dominated by [Ln(d)]→(CO) 8 π backdonation and [Ln(d)]←(CO) 8 σ donation (contributes ≈77 and 16 % to covalent bonding, respectively). The metal f orbitals play a very minor role in the bonding. The electronic structure of all three lanthanide complexes obeys the 32-electron rule if only those electrons that occupy the valence orbitals of the metal are considered.
Alkali Metal Covalent Bonding in Nickel Carbonyl Complexes ENi(CO) 3 −
Chi C., Pan S., Meng L., Luo M., Zhao L., Zhou M., Frenking G.
Article, Angewandte Chemie - International Edition, 2019, DOI Link
View abstract ⏷
The alkali metal-nickel carbonyl anions ENi(CO) 3 − with E=Li, Na, K, Rb, Cs have been produced and characterized by mass-selected infrared photodissociation spectroscopy in the gas phase. The molecules are the first examples of 18-electron transition metal complexes with alkali atoms as covalently bonded ligands. The calculated equilibrium structures possess C 3v geometry, where the alkali atom is located above a nearly planar Ni(CO) 3 − fragment. The analysis of the electronic structure reveals a peculiar bonding situation where the alkali atom is covalently bonded not only to Ni but also to the carbon atoms.
Noble-Noble Strong Union: Gold at Its Best to Make a Bond with a Noble Gas Atom
Pan S., Jana G., Merino G., Chattaraj P.K.
Review, ChemistryOpen, 2019, DOI Link
View abstract ⏷
This Review presents the current status of the noble gas (Ng)-noble metal chemistry, which began in 1977 with the detection of AuNe+ through mass spectroscopy and then grew from 2000 onwards; currently, the field is in a somewhat matured state. On one side, modern quantum chemistry is very effective in providing important insights into the structure, stability, and barrier for the decomposition of Ng compounds and, as a result, a plethora of viable Ng compounds have been predicted. On the other hand. experimental achievement also goes beyond microscopic detection and characterization through spectroscopic techniques and crystal structures at ambient temperature; for example, (AuXe4)2+(Sb2F11−)2 have also been obtained. The bonding between two noble elements of the periodic table can even reach the covalent limit. The relativistic effect makes gold a very special candidate to form a strong bond with Ng in comparison to copper and silver. Insertion compounds, which are metastable in nature, depending on their kinetic stability, display an even more fascinating bonding situation. The degree of covalency in Ng–M (M=noble metal) bonds of insertion compounds is far larger than that in non-insertion compounds. In fact, in MNgCN (M=Cu, Ag, Au) molecules, the M−Ng and Ng−C bonds might be represented as classical 2c–2e σ bonds. Therefore, noble metals, particularly gold, provide the opportunity for experimental chemists to obtain sufficiently stable complexes with Ng at room temperature in order to characterize them by using experimental techniques and, with the intriguing bonding situation, to explore them with various computational tools from a theoretical perspective. This field is relatively young and, in the coming years, a lot of advancement is expected experimentally as well as theoretically.
Eyringpy: A program for computing rate constants in the gas phase and in solution
Dzib E., Cabellos J.L., Ortiz-Chi F., Pan S., Galano A., Merino G.
Article, International Journal of Quantum Chemistry, 2019, DOI Link
View abstract ⏷
Eyringpy is a modular program for calculating thermochemical properties and rate constants for reactions in the gas phase and in solution. The code is written in Python and it has a user-friendly interface and a simple input format. Unimolecular and bimolecular reactions with one and two products are supported. Thermochemical properties are estimated through canonical ensemble and rate constants are computed according to the transition state theory. One-dimensional Wigner and Eckart tunneling corrections are also available. Rate constants of bimolecular reactions involving the formation of pre-reactive complexes are also estimated. To compute rate constants in solution, Eyringpy uses the Collins–Kimball theory to include the diffusion-limit, the Marcus theory for electron transfer processes, and the molar fractions to account for the solvent pH effect.
A theoretical investigation on boron-ligand cooperation to activate molecular hydrogen by a frustrated Lewis pair and subsequent reduction of carbon dioxide
Ghara M., Pan S., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2019, DOI Link
View abstract ⏷
The role of boron-ligand cooperation in activating molecular hydrogen by a set of six frustrated Lewis pair (FLP) systems is explored using density functional theory. The results obtained from thermochemical calculations show that all the studied FLP systems are capable of activating molecular hydrogen as the activation free energies are not too high (17.6-25.6 kcal mol-1). Sulphur based FLP 6 is the most promising one as it results in the smallest activation barrier among the studied sets. For a given FLP, the introduction of an electron donating -NMe2 group at the para position of the pyridine ring somewhat lowers the barrier and enhances the B-X (X = O, N, S) interaction. The B-X bond present within the FLPs plays a crucial role in facilitating the H2 activation process where it gets changed from the B+-X- type of interaction in the FLP to the B ← X dative bond upon H2 activation as understood from the energy decomposition analysis in combination with the natural orbital for chemical valence theory (EDA-NOCV). This mode of operation is termed as boron-ligand cooperation in analogy with the metal-ligand cooperation in transition metal complexes. The EDA-NOCV results obtained at the TS also support an electron transfer model where simultaneous electron transfer takes place from the Lewis basic center (N) of the FLP to σ∗(H2) and from σ(H2) to the Lewis acidic center (B) of the FLP, resulting in a weakened H-H bond. The change in the aromaticity of the pyridine rings during the course of H2 activation is also monitored by nucleus independent chemical shift calculations. Finally, the ability of the studied FLP systems to act as hydrogenation catalysts is elucidated by studying the hydrogenation of CO2 to yield formic acid.
Cerium-carbon dative interactions supported by carbodiphosphorane
Su W., Pan S., Sun X., Zhao L., Frenking G., Zhu C.
Article, Dalton Transactions, 2019, DOI Link
View abstract ⏷
A set of complexes containing dative interactions between a rare-earth metal and carbon are reported. Complex 2, Br3Ce(CDP)(THF), with a Ce←C bond was synthesized by the reaction of CeBr3 with a carbon(0) ligand, carbodiphosphorane (CDP). More significantly, a trivalent cerium complex 3, [BrCe(CDP)2](BPh4)2, with two σ dative interactions C→Ce←C was also isolated, which represents an unusual example of two dative interactions formed with the same atom in a molecule. Furthermore, π donation by the second lone-pair electrons of the CDP ligand is rather weak. Single-crystal X-ray diffraction shows that the Ce-C bond lengths in these complexes are comparable with those in cerium(iii)-carbene species. Density functional theory calculations support the dative interaction formation in these complexes and the strength of σ-donation in 3 is stronger than that in 2.
Reply to the ‘Comment on “exploiting electronic strategies to stabilize a planar tetracoordinate carbon in cyclic aromatic hydrocarbons”‘ by V. S. Thimmakondu,: Chem. Commun., 2019, DOI: 10.1039/c9cc04639a
Yanez O., Vasquez-Espinal A., Pino-Rios R., Ferraro F., Pan S., Osorio E., Merino G., Tiznado W.
Article, Chemical Communications, 2019, DOI Link
View abstract ⏷
The effectiveness of our proposed approach to stabilize a planar tetracoordinate carbon (ptC) in cyclic aromatic hydrocarbons, introduced in the title article, is unquestionable as our exhaustive searches on the singlet and triplet potential energy surfaces of the new ptC molecules identified as viable species are reproducible. Besides, the T1 diagnostic value for the Si2C5H2 system reported in the comment seems to be the T1 amplitudes. We recomputed the T1 diagnostic value using different software (Gaussian and ORCA), which gave similar values to that reported in our communication. Additionally, a multiconfigurational (complete active space SCF) calculation fully confirms the mono-configurational character of the questioned Si2C5H2 ptC structure. We accept that the linear isomer for the C7H2 system, in the triplet electronic state, is competitive with the isomer reported in our article, in the singlet electronic state, as mentioned in the title comment. However, this is a minor correction that does not affect the primary goal and main conclusions of our communication.
Adsorption of Molecular Hydrogen on Lithium-Phosphorus Double-Helices
Jana G., Pan S., Rodriguez-Kessler P.L., Merino G., Chattaraj P.K.
Article, Journal of Physical Chemistry C, 2018, DOI Link
View abstract ⏷
The possible interaction of the unprecedented but recently predicted inorganic double-helices made up of lithium and phosphorous (Li n P n ; n = 7-9) with dihydrogen (H 2 ) molecules is explored via density functional theory-based computations. Because of the large amount of Li → P electron transfer, the Li chain carries a high positive charge, which can be utilized to interact with quite less-reactive elements such as H 2 . Despite low polarizability of the target species to be bound, these double-helices are found to interact with H 2 molecules, having binding energies within a range of 1.7-3.2 kcal/mol per H 2 molecule. Further, the periodic calculation with the LiP helix reveals that each Li center binds with two H 2 molecules with an average binding energy of 2.5 kcal/mol per H 2 , and this leads to a 9.6 wt % of H 2 uptake. The interactions in Li···H 2 are mainly originating from both orbital and electrostatic contributions as reflected in the energy decomposition analysis. However, a global minimum search for H 2 @Li 7 P 7 by a modified kick algorithm reveals that the lowest energy isomer is a significantly distorted structure from a helix, and having two P-H bonds. Therefore, chemisorption should be preferable over the interaction in molecular form. However, for that purpose, the rupture of the H-H bond in the H 2 molecule is essential, which needs at least an activation energy barrier of 14.9 kcal/mol to overcome. Given the fact that the H 2 storage in Li-decorated clusters would only be achieved at low temperature, the chemisorption is not likely to take place. Further, their interaction with noble gases (Ar-Rn) is also studied herein. Moreover, an inspection of their band gap structures indicates that the LiP helix could exhibit wide band gap semiconducting properties with a direct band gap value of 2.64 eV.
Double dative bond between divalent carbon(0) and uranium
Su W., Pan S., Sun X., Wang S., Zhao L., Frenking G., Zhu C.
Article, Nature Communications, 2018, DOI Link
View abstract ⏷
Dative bonds between p- and d-block atoms are common but species containing a double dative bond, which donate two-electron pairs to the same acceptor, are far less common. The synthesis of complexes between UCl4 and carbodiphosphoranes (CDP), which formally possess double dative bonds Cl4U⇇CDP, is reported in this paper. Single-crystal X-ray diffraction shows that the uranium−carbon distances are in the range of bond lengths for uranium−carbon double bonds. A bonding analysis suggests that the molecules are uranium−carbone complexes featuring divalent carbon(0) ligands rather than uranium−carbene species. The complexes represent rare examples with a double dative bond in f-block chemistry. Our study not only introduces the concept of double dative bonds between carbones and f-block elements but also opens an avenue for the construction of other complexes with double dative bonds, thus providing new opportunities for the applications of f-block compounds.
Revisiting the Formation Mechanism of 1,3,4-Oxadiazole-2(3 H)-ones from Hydrazonyl Chloride and Carbon Dioxide
Murillo F., Barroso J., De Los Santos M.G., Avila G., Pan S., Fernandez-Herrera M.A., Merino G.
Article, Journal of Organic Chemistry, 2018, DOI Link
View abstract ⏷
The reaction mechanism for the synthesis of 1,3,4-oxadiazole-2(3H)-ones from hydrazonyl chloride and CO2 in the presence of CsF/18-crown-6 and toluene, is revisited via density functional theory computations. Although this reaction was earlier classified as a 1,3-dipolar cycloaddition, we found some competing pathways involved therein. The mechanisms including the (F-CO2)- anion and the nitrile imine intermediate are some options. The dimerization of nitrile imine is another competing mechanism in this reaction. Our results show that the most favorable mechanism proceeds via a stepwise pathway without involving any nitrile imine intermediate or the (F-CO2)- anion. The F- anion, resulting from the formation of a complex between 18-crown-6 and Cs+ cation, deprotonates the nitrile imine precursor easily, which acts then as a nucleophilic anion, enhancing the reactivity of CO2 toward it. The mechanism for the reaction with COS, an isoelectronic analogue of CO2, is also explored.
Improvement in hydrogen binding ability of closo-dicarboranes via functionalization and designing of extended frameworks
Pan S., Zhao L., Merino G.
Article, Journal of Molecular Modeling, 2018, DOI Link
View abstract ⏷
Neutral closo-dicarboboranes are reported to have very low H2 binding ability. Herein, we report an improvement in H2 binding energy (Eb) of C2B4H6 by substituting H atoms with different functional groups like X = F, Cl, Br, and XY = BO, CN and NC via quantum-chemical density functional theory based computations. In going from B6H6 2− to C2B4H6, the Eb value is reduced from 14.6 kJ mol−1 to 2.7 kJ mol−1. C2B4X6 and C2B4(XY)6 systems, which can bind a total of eight H2 molecules, with one H2 molecule occupying at each B-B-C face, possess an Eb value per H2 in the range of 4.5 kJ mol−1 for X = F, 3.9 kJ mol−1 for X = Cl, 5.9 kJ mol−1 for X = Br, 6.8 kJ mol−1 for XY = BO, 5.8 kJ mol−1 for XY = CN and 5.2 kJ mol−1 for XY = NC. The improvement in Eb value is found to be the highest in case of C2B4(BO)6, which has the ability to bind 6.6 gravimetric wt% of H2. The situation can be made more favorable by applying an external electric field. Energy decomposition analysis reveals that although the dispersion interaction (ca. 55–65%) has significant role in binding H2 with such types of molecules, contribution from electrostatic and orbital interaction is also considerable. Further, we modeled an extended system by linking C2B4(BO)n through ‘C ≡ C’ units for H2 storage purpose. The energy difference between the highest occupied and the lowest unoccupied molecular orbitals gradually lessens with the increase in molecular length. Therefore, it can be tuned gradually by controlling the chain length, which may further open up their potency in the field of electronics. [Figure not available: see fulltext.].
Modulation of an Anagostic Interaction in SiPSi-Type Pincer Platinum Complexes
Zamora-Moreno J., Murillo F., Munoz-Hernandez M.A., Grellier M., Pan S., Jalife S., Merino G., Sabo-Etienne S., Montiel-Palma V.
Article, Organometallics, 2018, DOI Link
View abstract ⏷
The reactivities of tris(benzyldimethylsilyl)phosphine [P(o-C6H4-CH2SiMe2H)3] (1) and tris(benzyldiphenylsilyl)phosphine [P(o-C6H4-CH2SiPh2H)3] (6) toward the same platinum precursor [Pt(PPh3)3] are strikingly different. The reaction with 1 renders the trans disilyl platinum(II) complex [Pt{P(o-C6H4-CH2SiMe2)2(o-C6H4-CHSiMe2)}PPh3] (2) in which the ligand coordinates in a tridentate fashion while a new Si-C bond is formed from the third Si moiety. The most prominent feature is an anagostic interaction that is established at the apical position. In contrast, the reaction of [Pt(PPh3)3] with 6 yields the hexacoordinated hydrido trisilyl platinum(IV) complex [PtH{P(o-C6H4-CH2SiPh2)3}PPh3] (7). We have studied the effect of the variation of the monodentate ligand in 2 by simple substitution reactions. We found a systematic variation of the chemical shift of the anagostic hydrogen in the 1H nuclear magnetic resonance spectrum of the corresponding PMe3, P(OPh)3, and CO complexes that can in principle be ascribed to a varying degree of the π acceptor character of the ancillary ligand. However, theoretical calculations at the density functional theory level show only slight changes in the frontier orbitals in line with predominantly closed-shell electrostatic interactions.
Stabilization of Boron-Boron Triple Bonds by Mesoionic Carbenes
Saha R., Pan S., Chattaraj P.K.
Article, ACS Omega, 2018, DOI Link
View abstract ⏷
Density functional theory-based computations are carried out to analyze the electronic structure and stability of B2(MIC)2 complexes, where MIC is a mesoionic carbene, viz., imidazolin-4-ylidenes, pyrazolin-4-ylidene, 1,2,3-triazol-5-ylidene, tetrazol-5-ylidene, and isoxazol-4-ylidene. The structure, stability, and the nature of bonding of these complexes are further compared to those of the previously reported B2(NHC)2 and B2(cAAC)2. A thorough bonding analysis via natural bond order, molecular orbital, and energy decomposition analyses (EDA) in combination with natural orbital for chemical valence (NOCV) reveals that MICs are suitable ligands to stabilize B2 species in its (3)1-g + excited state, resulting in an effective B-B bond order of 3. Their high dissociation energy and endergonicity at 298 K for the dissociations L-BB-L → 2 B-L and L-BB-L → BB + 2 L (L = Ligand) indicate their viability at ambient condition. The donor property of MICs is comparable to that of NHCMe. The orbital interaction plays a greater role than the coulombic interaction in forming the B-L bonds. The EDA-NOCV results show that the sum of the orbital energies associated with the (+, +) and (+, -) L → [B2] L σ-donations is far larger than that of L [B2]→L π-back donation. It also reveals that cAACMe possesses the largest σ-donation and π-back donation abilities among the studied ligands, and the σ-donation and π-back donation abilities of MICs are comparable to those of NHCMe. Therefore, the present study shows that MICs would also be an excellent choice as ligands to experimentally realize new compounds having a strong B-B triple bond.
Noble Gas Inserted Metal Acetylides (Metal = Cu, Ag, Au)
Jana G., Pan S., Merino G., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2018, DOI Link
View abstract ⏷
Metal acetylides (MCCH, M = Cu, Ag, Au) were already experimentally detected in molecular form. Herein, we investigate the possibility of noble gas (Ng) insertion within the C-H bond of MCCH and their stability is compared with those of the reported MNgCCH and HCCNgH molecules. Our coupled-cluster-level computations show that MCCNgH (Ng = Kr, Xe, Rn) systems are local minima on the corresponding potential energy surfaces, whereas their lighter analogues do not remain in the chemically bound form. Further, their stability is analyzed with respect to all possible dissociation channels. The most favorable dissociation channel leads to the formation of free Ng and MCCH. However, there exists a high free energy barrier (29.3-46.9 kcal/mol) to hinder the dissociation. The other competitive processes against their stability include two-body and three-body neutral dissociation channels, MCCNgH � MCC + NgH and MCCNgH � MCC + Ng + H, respectively, which are slightly exergonic in nature at 298 K for Ng = Kr, Xe and M = Cu, Ag, and for AuCCKrH. However, the Xe analogues for Cu and Ag and AuCCKrH would be viable at a lower temperature. AuCCNgH (Ng = Kr-Rn) molecules are the best candidates for experimental realization, since they have higher dissociation energy values and higher kinetic protection in the case of feasible dissociation channels compared to the Cu and Ag systems. A detailed bonding analysis indicates that the Ng-H bonds are genuine covalent bonds and there is also a substantial covalent character in Ng-C contacts of these molecules. Moreover, the possibility of insertion of two Xe atoms in AuCCH resulting in AuXeCCXeH and the stability of XeAuXeCCXeH are also tested herein.
Observation of alkaline earth complexes M(CO)8 (M = Ca, Sr, or Ba) that mimic transition metals
Wu X., Zhao L., Jin J., Pan S., Li W., Jin X., Wang G., Zhou M., Frenking G.
Article, Science, 2018, DOI Link
View abstract ⏷
The alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) typically engage in chemical bonding as classical main-group elements through their ns and np valence orbitals, where n is the principal quantum number. Here we report the isolation and spectroscopic characterization of eight-coordinate carbonyl complexes M(CO)8 (where M = Ca, Sr, or Ba) in a low-temperature neon matrix. Analysis of the electronic structure of these cubic Oh-symmetric complexes reveals that the metal–carbon monoxide (CO) bonds arise mainly from [M(dp)] → (CO)8 p backdonation, which explains the strong observed red shift of the C-O stretching frequencies. The corresponding radical cation complexes were also prepared in gas phase and characterized by mass-selected infrared photodissociation spectroscopy, confirming adherence to the 18-electron rule more conventionally associated with transition metal chemistry.
Bonding and Mobility of Alkali Metals in Helicenes
Barroso J., Murillo F., Martinez-Guajardo G., Ortiz-Chi F., Pan S., Fernandez-Herrera M.A., Merino G.
Article, Chemistry - A European Journal, 2018, DOI Link
View abstract ⏷
In this work, we analyze the interactions of alkali metal cations with [6]- and [14]helicene and the cation mobility of therein. We found that the distortion of the carbon skeleton is the reason that some of the structures which are local minima for the smallest cations are not energetically stable for K+, Rb+, and Cs+. Also, the most favorable complexes are those where the cation is interacting with two rings forming a metallocene-like structure, except for the largest cation Cs+, where the distortion provoked by the size of the cation destabilizes the complex. As far as mobility is concerned, the smallest cations, particularly Na+, are the ones that can move most efficiently. In [6]helicene, the mobility is limited by the capture of the cation forming the metallocene-like structure. In larger helicenes, the energy barriers for the cation to move are similar both inside and outside the helix. However, complexes with the cation between two layers are more energetically favored so that the movement will be preferred in that region. The bonding analysis reveals that interactions with no less than 50 % of orbital contribution are taking place for the series of E+-[6]helicene. Particularly, the complexes of Li+ show remarkable orbital character (72.5–81.6 %).
E5M7 + (E=C–Pb, M=Li–Cs): A Source of Viable Star-Shaped Clusters
Vasquez-Espinal A., Palacio-Rodriguez K., Ravell E., Orozco-Ic M., Barroso J., Pan S., Tiznado W., Merino G.
Article, Chemistry - An Asian Journal, 2018, DOI Link
View abstract ⏷
Herein we report the systematic exploration of the potential energy surfaces of a series of clusters with formula E5M7 + (E=C-Pb and M=Li-Cs). Fifteen of these combinations adopt a D5h three-dimensional seven-pointed star-like structure in a singlet state, where M atoms interact electrostatically with the E5 ring. The determining factors in the relative preference of having the D5h structure over the most competitive isomer or vice-versa are analyzed. These star-shaped systems satisfy the 4n+2 Hückel's rule and exhibit a strong diatropic (σ and π) response to an external magnetic field.
Bonding in Binuclear Carbonyl Complexes M2(CO)9 (M = Fe, Ru, Os)
Pan S., Zhao L., Dias H.V.R., Frenking G.
Article, Inorganic Chemistry, 2018, DOI Link
View abstract ⏷
Quantum-chemical density functional theory calculations using the BP86 functional in conjunction with a triple-ζ basis set and dispersion correction by Grimme with Becke-Johnson damping D3(BJ) were performed for the title molecules. The nature of the bonding was examined with the quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) methods and with the energy decomposition analysis in conjunction with the natural orbital for chemical valence (EDA-NOCV) analysis. The energetically lowest-lying form of Fe2(CO)9 is the triply bridged D3h structure, whereas the most stable structures of Ru2(CO)9 and Os2(CO)9 are singly bridged C2 species. The calculated reaction energies for the formation of the cyclic trinuclear carbonyls M3(CO)12 from the dinuclear carbonyls M2(CO)9 are in agreement with experiment, as the iron complex Fe2(CO)9 is thermodynamically stable in these reactions, but the heavier homologues Ru2(CO)9 and Os2(CO)9 are not. The metal-CO bond to the bridging CO ligands is stronger than the bonds to the terminal CO ligands. This holds for the triply bridged D3h structures as well as for the singly bridged C2 or C2v species. The analysis of the orbital interactions with the help of the EDA-NOCV method suggests that the overall M→CO π backdonation is always stronger than the M→CO σ donation. The bridging carbonyls are more strongly bonded than the terminal CO ligands, and they are engaged in stronger σ donation and backdonation, but the formation of bridging carbonyls requires reorganization energy, which may or may not be compensated by the stronger metal-ligand interactions. The lower-lying D3h form of Fe2(CO)9 and C2 structures of Ru2(CO)9 and Os2(CO)9 are due to a delicate balance of several forces.
Structure and Bonding in CE5 − (E=Al–Tl) Clusters: Planar Tetracoordinate Carbon versus Pentacoordinate Carbon
Ravell E., Jalife S., Barroso J., Orozco-Ic M., Hernandez-Juarez G., Ortiz-Chi F., Pan S., Cabellos J.L., Merino G.
Article, Chemistry - An Asian Journal, 2018, DOI Link
View abstract ⏷
The structure, bonding, and stability of clusters with the empirical formula CE5 − (E=Al–Tl) have been analyzed by means of high-level computations. The results indicate that, whereas aluminum and gallium clusters have C2v structures with a planar tetracoordinate carbon (ptC), their heavier homologues prefer three-dimensional C4v forms with a pentacoordinate carbon center over the ptC one. The reason for such a preference is a delicate balance between the interaction energy of the fifth E atom with CE4 and the distortion energy. Moreover, bonding analysis shows that the ptC systems can be better described as CE4 −, with 17-valence electrons interacting with E. The ptC core in these systems exhibits double aromatic (both σ and π) behavior, but the σ contribution is dominating.
Li2B12 and Li3B12: Prediction of the Smallest Tubular and Cage-like Boron Structures
Dong X., Jalife S., Vasquez-Espinal A., Ravell E., Pan S., Cabellos J.L., Liang W.-Y., Cui Z.-H., Merino G.
Article, Angewandte Chemie - International Edition, 2018, DOI Link
View abstract ⏷
An intriguing structural transition from the quasi-planar form of B12 cluster upon the interaction with lithium atoms is reported. High-level computations show that the lowest energy structures of LiB12, Li2B12, and Li3B12 have quasi-planar (Cs), tubular (D6d), and cage-like (Cs) geometries, respectively. The energetic cost of distorting the B12 quasi-planar fragment is overcompensated by an enhanced electrostatic interaction between the Li cations and the tubular or cage-like B12 fragments, which is the main reason of such drastic structural changes, resulting in the smallest tubular (Li2B12) and cage-like (Li3B12) boron structures reported to date.
Boron Nanowheels with Axles Containing Noble Gas Atoms: Viable Noble Gas Bound M©B10 − Clusters (M=Nb, Ta)
Pan S., Kar S., Saha R., Osorio E., Zarate X., Zhao L., Merino G., Chattaraj P.K.
Article, Chemistry - A European Journal, 2018, DOI Link
View abstract ⏷
The viability of noble gas axled boron nanowheels NgnM©B10 − (Ng=Ar–Rn; M=Nb, Ta; n=1, 2) is explored by ab initio computations. In the resulting Ng2–M complexes, the Ng-M-Ng nanorod passes through the center of the B10 − ring, providing them with an inverse sandwich-like structure. While in the singly Ng bound analogue, the Ng binding enthalpy Hb at 298 K ranges from 2.5 to 10.6 kcal mol−1, in doubly Ng bound cases it becomes very low for the Ng2M©B10 −→Ng+NgM©B10 − dissociation channel, except for the case of Rn, for which the corresponding Hb values are 3.4 (Nb) and 4.0 kcal mol−1 (Ta). For a given Ng, Ta has slightly higher Ng-binding ability than Nb. The corresponding free-energy changes indicate that these systems, particularly the Xe and Rn complexes, are good candidates for experimental realization in a low-temperature matrix. The Ng−M bonds were found to be covalent in nature, as reflected in their large Wiberg bond indices, formation of a 2c–2e σ orbital between Ng and M centers in natural bond orbital and adaptive natural density partitioning (AdNDP) analyses, and the short Ng−M distances. Energy decomposition analysis and a study on the natural orbitals for chemical valence show that the Ng−M contact is supported mainly by the orbital and electrostatic interactions, with almost equal contributions. Although both the Ng→M σ donation and Ng←M π backdonation play roles in the origin of orbital interaction, the former is significantly dominant over the latter. Further, AdNDP analysis indicates that the doubly aromatic character (both σ and π) in MB10 − clusters is not perturbed by the interaction with Ng atoms.
Stable NCNgNSi (Ng=Kr, Xe, Rn) Compounds with Covalently Bound C-Ng-N Unit: Possible Isomerization of NCNSi through the Release of the Noble Gas Atom
Pan S., Jana G., Ravell E., Zarate X., Osorio E., Merino G., Chattaraj P.K.
Article, Chemistry - A European Journal, 2018, DOI Link
View abstract ⏷
Although the noble gas (Ng) compounds with either Ng−C or Ng−N bonds have been reported in the literature, compounds containing both bonds are not known. The first set of systems having a C-Ng-N bonding unit is predicted herein through the analysis of stability and bonding in the NCNgNSi (Ng=Kr–Rn) family. While the Xe and Rn inserted analogues are thermochemically stable with respect to all dissociation channels, but for the one producing CNSiN and free Ng, NCKrNSi has another additional three-body dissociation channel, NCKrNSi→CN+Kr+NSi, which is exergonic by −9.8 kcal mol−1 at 298 K. This latter dissociation can be hindered by lowering the temperature. Moreover, the NCNgNSi→Ng+CNSiN dissociation is also kinetically prohibited by a quite high free energy barrier ranging from 25.2 to 39.3 kcal mol−1, with a gradual increase in going from Kr to Rn. Therefore, these compounds are appropriate candidates for experimental realization. A detailed bonding analysis by employing natural bond orbital, electron density, energy decomposition, and adaptive natural density partitioning analyses indicates that both Ng−N and C−Ng bonds in the title compounds are covalent in nature. In fact, the latter analysis indicates the presence of delocalized 3c–3e σ-bond within the C-Ng-N moiety and a totally delocalized 5c–2e σ-bond in these compounds. This is an unprecedented bonding characteristic in the sense that the bonding pattern in Ng inserted compounds is generally represented as the presence of covalent bond in one side of Ng, and the ionic interaction in the other side. Further, the dissociation of Ng from NCNgNSi facilitates the formation of a higher energy isomer of NCNSi, CNSiN, which cannot be formed from bare NCNSi as such, because of the very high free energy barrier associated with the isomeric transformation. Therefore, in the presence of Ng atoms it might be possible to detect the high energy isomer.
Hydrogen storage in all-metal and nonmetal aromatic clusters
Saha R., Pan S., Chattaraj P.K.
Book chapter, Emerging Materials for Energy Conversion and Storage, 2018, DOI Link
View abstract ⏷
Although aromaticity is widely used in explaining the “extra stability” of a particular class of organic compounds, it has not been properly defined. It is subsequently extended to several other systems including inorganic and all-metal systems. Hence, an aromatic moiety with extraordinary stability can act as a promising building block for various nanomaterials. In this chapter, we present aromaticity in various all-metal and nonmetal systems, and the hydrogen (H2) storage potential of different novel molecular templates composed of aromatic units. A thorough analysis is carried out to understand the effect of H2 binding on the aromaticity of the template and vice versa. Whereas aromaticity is assessed through the study of various energetic, geometrical, magnetic, and reactivity criteria, H2 binding ability is evaluated by computing the related binding energy. The construction of temperature-pressure (T-P) phase diagrams for various systems highlights T-P regions where the adsorption or desorption of H2 would be favorable. Furthermore, the effect of an external electric field on improving the H2 binding ability of a template is explored.
Planar pentacoordinate carbons
Vassilev-Galindo V., Pan S., Donald K.J., Merino G.
Review, Nature Reviews Chemistry, 2018, DOI Link
View abstract ⏷
Carbon centres in typical organic molecules have a coordination number that can reach a maximum of four, in which case the bonded atoms are situated at the vertices of a tetrahedron. Exceptions to those two structural rules have been posited and examined for decades, and planar tetracoordinate carbon (ptC) species are notable molecules that violate the second rule. There is continued interest in experimental and theoretical studies of ptCs, as well as emerging molecules that contain planar pentacoordinate carbon (ppC) and planar hexacoordinate carbon (phC) atoms, species that violate both structural rules. This Review describes recent progress in the theoretical prediction of viable entities that contain ppC centres. The first such molecule reported, the D5h-symmetric ppC species CAl5+, was followed by a series of predicted ppC species that could be obtained by substituting the Al centres for other heteroatoms. More complicated ppC systems have also been suggested, including metallocene-stabilized ppCs and quasi-ppCs embedded within cage structures or 2D materials. To date, computational studies have identified at least 65 local and 39 global minimum energy structures that contain ppCs or quasi-ppCs. The general design principles for ptC-centred candidate structures include delocalization of the central C 2pz lone electron pair, ensuring an 18 valence electron count and allowing for strong electron delocalization. These principles have been extended to ppC systems with some success. It is hard to predict the extent to which the coordination number of planar C can be increased because it depends not only on the valence and size of C but also on the size of the atoms bonded to it and the mode of bonding. Although a few energetically low-lying planar hexacoordinate and heptacoordinate C species have been identified computationally, none have been observed experimentally.
Cyanide-isocyanide isomerization: stability and bonding in noble gas inserted metal cyanides (metal = Cu, Ag, Au)
Jana G., Pan S., Osorio E., Zhao L., Merino G., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2018, DOI Link
View abstract ⏷
The internal isomerization, MNC ↔ MCN (M = Cu, Ag, Au), is investigated through quantum chemical computations. CuNC and AgNC are shown to be neither thermochemically nor kinetically stable against transformation to MCN. The free energy barrier (ΔG‡) for AuNC is somewhat considerable (7.1 kcal mol-1), indicating its viability, particularly at low temperature. Further, the Ng inserted analogues, MNgCN (M = Cu, Ag, Au; Ng = Xe, Rn) turn out to be thermochemically stable with respect to all possible dissociation channels but for two two-body dissociation channels, viz., MNgCN → Ng + MCN and MNgCN → Ng + MNC, which are connected to the internal isomerization processes, MNgCN → NgMCN and MNgCN → NgMNC, respectively. However, they are kinetically protected by substantial ΔG‡ values (11.8-15.4 kcal mol-1 for Cu, 9.8-13.6 kcal mol-1 for Ag, and 19.7-24.7 kcal mol-1 for Au). The pathways for such internal conversion are explored in detail. A thorough inspection of the bonding situation of the studied molecules, employing natural bond order, electron density, adaptive natural density partitioning, and energy decomposition analyses indicates that the M-Ng bonds in MNgCN and Ng-C bonds in AuNgCN can be represented as an electron-shared covalent bond. For the other Ng-C bonds, although an ionic description is better suited, the degree of covalent character is also substantial therein.
Planar pentacoordinate carbon in CGa5+ derivatives
Pan S., Cabellos J.L., Orozco-Ic M., Chattaraj P.K., Zhao L., Merino G.
Article, Physical Chemistry Chemical Physics, 2018, DOI Link
View abstract ⏷
We report a family of systems having a planar pentacoordinate carbon (ppC) based on the next heavier analogue of CAl5+, the ppC system par excellence. Although because of the larger size of Ga, the ppC isomer is not even a local minimum in CGa5+, a single isoelectronic substitution of Ga by smaller sized Be maximizes the bonding in the ppC form. Retaining the 18 valence electron rule, the global minimum structures of CGa4Be, CGa3Be2-, CGa2Be32-, and CGaBe43- clusters and their corresponding lithium salts have a ppC.
Noble gas encapsulated B40 cage
Pan S., Ghara M., Kar S., Zarate X., Merino G., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2018, DOI Link
View abstract ⏷
The efficacy of B40 borospherene to act as a host for noble gas atoms is explored via density functional theory based computations. Although the Ng@B40 complexes are thermochemically unstable with respect to dissociation into free Ng and B40, it does not rule out their viability as all the systems possess a high activation free energy barrier (84.7-206.3 kcal mol-1). Therefore, once they are formed, it is hard to take out the Ng atom. Two Ng atoms can also be incorporated within B40 for the lighter Ng atoms (He and Ne). In fact, the destabilization offered by the encapsulation of one and two He atoms and one Ne atom inside B40 is significantly less than that in experimentally synthesized He@C20H20, highlighting their greater possibility for synthesis. Although Ar2 and Kr2 encapsulated B40 systems are very much destabilized by the repulsive interaction between Ng2 and B40, an inspection of the bonding situation reveals that the confinement can even induce some degree of covalent interaction between two otherwise non-bonded Ng atoms. Ng atoms transfer electrons towards B40 which is smaller for lighter Ng atoms and gradually increases along He to Rn. Even if the electrostatic interaction between Ng and B40 is the most predominant term in these systems, the extent of the orbital interaction is also considerable. However, the very large Pauli repulsion counterbalances the attractive interaction, eventually turning the interaction repulsive in nature. Ng@B40 also shows dynamical behaviour involving continuous exchange between hexagonal and heptagonal holes, similar to the host cage, as understood from the very little variation in the activation barrier because of the Ng encapsulation. Furthermore, sandwich complexes like [(η5-C5Me5)Fe(η6-B40)]+ and [(η5-C5Me5)Fe(η7-B40)]+ are noted to be viable with the latter being slightly more stable than the former. The encapsulation of Xe slightly improves the dissociation energy associated with the decomposition into Xe@B40 and [Fe(η5-C5Me5)]+ compared to that in the bare one.
Structural Evolution of the Rhodium-Doped Silver Clusters AgnRh (n ≤ 15) and Their Reactivity toward NO
Rodriguez-Kessler P.L., Pan S., Florez E., Cabellos J.L., Merino G.
Article, Journal of Physical Chemistry C, 2017, DOI Link
View abstract ⏷
Structural properties of AgnRh (n ≤ 15) clusters are investigated using a successive growth algorithm coupled with density functional theory computations. The structures of the clusters are revisited, including a detailed discussion of their electronic properties. In contrast to these previous contributions, the lowest energy structures of the clusters are planar for n = 3-6, while three-dimensional for n = 7 onward. Our present searches identify new lowest energy structures for n = 3-6 and 9-13. The most stable isomers are selected to study the adsorption of NO. The size-dependent reactivity of the clusters indicates that Rh atom acts as a more effective adsorption site for NO than Ag. Since the transition from Rh-exposed to Rh-encapsulated structures occurs at n = 9, the reactivity toward NO for AgnRh clusters with n ≤ 8 is considerably higher than that for the larger homologues. Further, the results show that doping Agn clusters with Rh increases the reactivity toward NO adsorption.
Kekulene: Structure, stability and nature of H•••H interactions in large PAHs
Poater J., Paauwe J., Pan S., Merino G., Guerra C.F., Bickelhaupt F.M.
Article, Molecular Astrophysics, 2017, DOI Link
View abstract ⏷
We have quantum chemically analyzed how the stability of small and larger polycyclic aromatic hydrocarbons (PAHs) is determined by characteristic patterns in their structure using density functional theory at the BLYP/TZ2P level. In particular, we focus on the effect of the nonbonded H•••H interactions that occur in the bay region of kinked (or armchair) PAHs, but not in straight (or zigzag) PAHs. Model systems comprise anthracene, phenanthrene, and kekulene as well as derivatives thereof. Our main goals are: (1) to explore how nonbonded H•••H interactions in armchair configurations of kinked PAHs affect the geometry and stability of PAHs and how their effect changes as the number of such interactions in a PAH increases; (2) to understand the extent of stabilization upon the substitution of a bay C[sbnd]H fragment by either C• or N; and (3) to examine the origin of such stabilizing/destabilizing interactions.
MNgCCH (M = Cu, Ag, Au; Ng = Xe, Rn): The First Set of Compounds with M-Ng-C Bonding Motif
Jana G., Pan S., Merino G., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2017, DOI Link
View abstract ⏷
Although Ng-M (M = Cu, Ag, Au; Ng = noble gas) and Ng-C bonds are known to exist in different viable species, we report here a series of systems with formula MNgCCH (Ng = Xe, Rn) in which both bonds coexist. These compounds possess reasonably high kinetic stability (free energy barrier, δG‡ of 14.0-34.8 kcal/mol) along an exergonic isomerization channel, MNgCCH → NgMCCH. For a given M, the δG‡ associated with this channel increases from Xe to Rn, whereas for a given Ng, it increases along Ag < Cu < Au. No other possible dissociation channel is feasible at standard condition, except for the Ag-Xe analogue, where one three-body neutral dissociation channel, AgXeCCH → Ag + Xe + CCH, is slightly exergonic by 2.4 kcal/mol. Examination of the thermochemical stability of the Ng-M bonds in noninserted compounds against the dissociation, NgMCCH → Ng + MCCH reveals that Kr-Rn bound Cu and Au analogues, and Xe and Rn bound Ag analogues would be viable at 298 K. The natural bond order analysis indicates the formation of M-Ng covalent bond and Ng-C ionic bonds in these compounds having an ionic representation of (MNg)+(CCH)-. Energy decomposition analysis reveals a significant contribution of the electrostatic term in the M-Ng covalent bonds.
E3M3 + (E=C–Pb, M=Li–Cs) Clusters: The Smallest Molecular Stars
Contreras M., Pan S., Orozco-Ic M., Cabellos J.L., Merino G.
Article, Chemistry - A European Journal, 2017, DOI Link
View abstract ⏷
Extensive potential energy surface explorations of twenty-five clusters with the formula E3M3 + (E=Group 14 element and M=Group 1 element) through density functional theory and high-level ab initio computations reveal that the lowest-energy isomer for all these systems corresponds to a non-classical D3h star-like structure in the singlet state, where three M atoms interact electrostatically with the triangular E3 core, occupying three bridging positions around it. More than 18 200 calculations were done in the search for the minima structures, starting with a first phase at the PBE0/LANL2DZ level and ending with an analysis of the most representative clusters at the CCSD(T)/def2-TZVP//PBE0/def2-TZVP level. The title clusters represent the smallest molecular stars with three planar tetracoordinate E atoms (E=Group 14 element). All these E3M3 + clusters behave like superalkali cations with small vertical electron affinities (smaller than Cs), large vertical electron detachment energies, and HOMO–LUMO energy gaps. Their energetics, bonding, and electron delocalization are discussed in detail. The high stability of these clusters is reflected from the large dissociation energy needed for different dissociation channels. The electron delocalization is confirmed by the presence of two delocalized π electrons over the E3 core and strong diatropic responses.
Coaxial Triple-Layered versus Helical Be6B11− Clusters: Dual Structural Fluxionality and Multifold Aromaticity
Guo J.-C., Feng L.-Y., Wang Y.-J., Jalife S., Vasquez-Espinal A., Cabellos J.L., Pan S., Merino G., Zhai H.-J.
Article, Angewandte Chemie - International Edition, 2017, DOI Link
View abstract ⏷
Two low-lying structures are unveiled for the Be6B11− nanocluster system that are virtually isoenergetic. The first, triple-layered cluster has a peripheral B11 ring as central layer, being sandwiched by two Be3 rings in a coaxial fashion, albeit with no discernible interlayer Be−Be bonding. The B11 ring revolves like a flexible chain even at room temperature, gliding freely around the Be6 prism. At elevated temperatures (1000 K), the Be6 core itself also rotates; that is, two Be3 rings undergo relative rotation or twisting with respect to each other. Bonding analyses suggest four-fold (π and σ) aromaticity, offering a dilute and fluxional electron cloud that lubricates the dynamics. The second, helix-type cluster contains a B11 helical skeleton encompassing a distorted Be6 prism. It is chiral and is the first nanosystem with a boron helix. Molecular dynamics also shows that at high temperature the helix cluster readily converts into the triple-layered one.
Modeling of 1-D Nanowires and analyzing their Hydrogen and Noble Gas Binding Ability
Pan S., Saha R., Gupta A., Chattaraj P.K.
Article, Journal of Chemical Sciences, 2017, DOI Link
View abstract ⏷
The theoretical calculation at the M05-2X/6-311+G(d,p) level reveals that the B–B bond length in [N4-B2-N4]2− system (1.506 Å) is slightly smaller than that of typical B=B bond in B2H2 (1.518 Å). These systems interact with each M+ (M = Li, Na, K) ion very strongly with a binding energy of 213.5 (Li), 195.2 (Na) and 180.3 (K) kcal/mol. Additionally, the relief of the Coulomb repulsion due to the presence of counter-ion, M+, the B–B bond contracts to 1.484–1.488 Å in [N4-B2-N4]M2. We have further extended our study to [N4-B2-N4-B2-N4]4− and [N4-B2-N4-B2-N4-B2-N4]6− systems. The B–B bond length is found to be 1.496 Å in the former case, whereas the same is found to be 1.493 Å and 1.508 Å, respectively, for the two B–B bonds present in the latter one. The M + counter-ions stabilize such negatively charged systems and thus, create a possibility to design a long 1-D nanowire. Their utilities as probable hydrogen and noble gas (Ng) binding templates are explored taking [N4-B2-N4-B2-N4]Li4 system as a reference. It is found that each Li center binds with three H2 molecules with an average binding energy of 2.1 kcal/mol, whereas each Ng (Ar–Rn) atom interacts with Li center having a binding energy of 1.8–2.1 kcal/mol. The H2 molecules interact with Li centers mainly through equal contribution from orbital and electrostatic interaction, whereas the orbital interaction is found to be major term (ca. 51–58%) in Ng-Li interaction followed by dispersion (ca. 24–27%) and electrostatic interaction (ca. 17–24%). [Figure not available: see fulltext.].
Ligand-Supported E3 Clusters (E=Si–Sn)
Pan S., Saha R., Osorio E., Chattaraj P.K., Frenking G., Merino G.
Article, Chemistry - A European Journal, 2017, DOI Link
View abstract ⏷
The interaction among E3 (E=Si, Ge, Sn) clusters and different ligands (L) encompassing five carbon-based donors (cyclic (alkyl)(amino)carbene (cAAC), N-heterocyclic carbene (NHC), saturated NHC (SNHC), mesoionic carbenes (MIC1, and MIC2)), two nitrogen-based donors (trimethylamine and pyridine), and two phosphorous-based donors (phosphinine and trimethylphosphine) in E3(L)3 complexes is explored through DFT computations. Although all carbenes form very strong bonds with E3 clusters, cAAC makes the strongest bond with Si3 and Ge3 clusters, and MIC1 with the Sn3 cluster. Nevertheless, other ligand-bound complexes are also viable at room temperature. This finding indicates that experimentalists may make use of them to synthesize the desired clusters based on precursor availability. The nature of the interaction in E−L bonds is analyzed through natural bond orbital analysis; energy decomposition analysis, in combination with the natural orbital for chemical valence; and adaptive natural density partitioning analysis. The L→E σ-donation and L←E π-back-donation play important roles in making contacts between L and E3 clusters favorable; where the former is significantly more dominant over the latter.
Importance of Dispersion on the Stability of the Concave-Bound CpM (M = Fe, Ru, Os) Complexes of Sumanene
Martinez S.H., Pan S., Cabellos J.L., Dzib E., Fernandez-Herrera M.A., Merino G.
Article, Organometallics, 2017, DOI Link
View abstract ⏷
The preference for concave mode binding of the CpM unit with sumanene in CpM(η6-sumanene)+ (M = Fe, Ru, Os) over the convex mode is analyzed by various density functional theory based methods including (or not) dispersion and solvent effects. In the case of the iron complex, the concave-bound isomer becomes energetically more favorable than the convex form only after the proper inclusion of dispersion effects, highlighting the importance of such contributions to stabilize the former arrangement. For the ruthenium complex, both the dispersion and solvent effects should be taken into account to provide a correct trend. The noncovalent interaction index corroborates the role of dispersion in concave selectivity. Our computations also show that the presence of the counterion is not relevant for this selectivity, discarding the previously reported argument made by Okumura et al.
Binding of Small Gas Molecules by Metal-Bipyridyl Monocationic Complexes (Metal = Cu, Ag, Au) and Possible Bond Activations Therein
Jana G., Pan S., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2017, DOI Link
View abstract ⏷
The viability of a series of small gas molecules (H2, N2, CO, CO2, H2O, H2S, C2H2, CH4, CH3Cl, C2H4, and C2H6) bound [M-(bipy)]+ (bipy = bipyridyl; M = Cu, Ag, Au) complexes is investigated at the PBE0/cc-pVTZ/cc-pVTZ-PP level with a special emphasis on the possible bond activation within the bound ligands. While the bond dissociation energy, enthalpy change, and free energy change are computed to show the stability of the complexes with respect to the dissociation into [M-(bipy)]+ and free gas molecule (L), natural bond orbital, electron density, and energy decomposition analyses in conjunction with natural orbitals for chemical valence are carried out to characterize the nature of L-M bonds. For a given L, the L binding ability is the highest for Au followed by Cu and Ag complexes, except for quite loosely bound CO2. For all ligand cases, the dissociation processes from the respective bound complexes are endergonic in nature at room temperature, except for the H2-, CH4-, and C2H6-bound Ag complexes and CO2-bound Ag and Au complexes. The interaction between L and M centers is supported by orbital and ionic interactions with latter being more dominant over the former. The delocalization index and local energy density values support the covalent character in L-M bonds in most of the cases. These M centers can act as a mild bond activation agent for L, Au being the best candidate in this series for this purpose. Particularly, the H-H bond in H2, C=C bond in C2H4, C≡C bond in C2H2, and C-H bonds in CH4 and C2H6 (the last two are for Au) are elongated along with a significant red-shift in the corresponding stretching frequency, compared to those in free molecules. These can be explained by the significant π-back-donation populating the lowest unoccupied antibonding molecular orbital of L in these complexes.
NgMCp+: Noble Gas Bound Half-Sandwich Complexes (Ng = He-Rn, M = Be-Ba, and Cp = η5-C5H5)
Saha R., Pan S., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2017, DOI Link
View abstract ⏷
Structures, bonding, and stability of half-sandwich complexes with general formula, NgMCp+ (Ng = He-Rn, M = Be-Ba, Cp = η5-C5H5) are analyzed through ab initio computation. MCp+ complexes possess remarkable Ng binding ability, particularly for M = Be and Mg. While for Ar-Rn bound analogues the bond dissociation energy in the former complex ranges within 17.5-28.0 kcal mol-1, it becomes 10.4-18.7 kcal mol-1 in the latter complex. In fact, BeCp+ is able to form a strong bond with the two most inert elements, He and Ne. Although the Ng binding ability of MCp+ gradually diminishes in moving from Be to Ba, the corresponding free energy change values show that Kr-Rn bound complexes involving the heavier congeners of Mg would remain in the bound state avoiding dissociation into Ng and MCp+. The nature of the Ng-M bond is characterized by natural bond orbital, electron density and energy decomposition analyses in conjunction with the natural orbital for chemical valence (EDA-NOCV) analysis. While the electron density analysis reveals that Ng-Be (Ng = Kr, Xe, Rn) and Ng-Mg (Ng = Xe, Rn) bonds are partly covalent in nature, the orbital interaction (ΔEorb) is found to be the most important term in the Ng-M attractive energy as revealed by the EDA-NOCV. For all Ngs, the major contribution toward the ΔEorb energy term originates from Ng→MCp+ σ-donation. Additionally, CpBeNgF (Ng = Xe, Rn) and CpNgF (Ng = Kr-Rn) are found to be viable systems with kinetic protection for the exergonic dissociation channels, CpBeNgF → Ng + CpBeF and CpNgF → Ng + CpF, respectively, where the activation free energy barrier in the latter systems (24.1-34.7 kcal mol-1) is significantly larger than that in the former ones (6.6-8.9 kcal mol-1). CpNgF (Ng = Kr-Rn) complexes are predicted to be stable even above 300 K, whereas CpBeNgF (Ng = Xe, Rn) would be viable up to ∼100 K. While the F-Ng bonds are ionic in nature, the Ng-Be and Ng-C bonds in these complexes have significant covalent character.
A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB12- Clusters (M = Co, Rh, Ir)
Saha R., Kar S., Pan S., Martinez-Guajardo G., Merino G., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2017, DOI Link
View abstract ⏷
Strong binding of carbon monoxide (CO) and dinitrogen (N2) by MB12- (M = Co, Rh, Ir) clusters results in a spinning umbrella-like structure. For OCMB12- and NNMB12- complexes, the bond dissociation energy values range within 50.3-67.7 kcal/mol and 25.9-35.7 kcal/mol, respectively, with the maximum value obtained in Ir followed by that in Co and Rh analogues. COMB12- complex is significantly less stable than the corresponding C-side bonded isomer. The associated dissociation processes for OCMB12- and NNMB12- into CO or N2 and MB12- are highly endergonic in nature at 298 K, implying their high thermochemical stability with respect to dissociation. In OCMB12- and NNMB12- complexes, the C-O and N-N bonds are found to be elongated by 0.022-0.035 Å along with a large red-shift in the corresponding stretching frequencies, highlighting the occurrence of bond activation therein toward further reactivity due to complexation. The obtained red-shift is explained by the dominance of L←M π-back-donation (L = CO, OC, NN) over L→M σ-donation. The binding of L enhances the energy barrier for the rotation of the inner B3 unit within the outer B9 ring by 0.4-1.8 kcal/mol, which can be explained by a reduction in the distance of the longest bond between inner B3 and outer B9 rings upon complexation. A good correlation is found between the change in rotational barrier relative to that in MB12- and the energy associated with the L→M σ-donation. Born-Oppenheimer molecular dynamics simulations further support that the M-L bonds in the studied systems are kinetically stable enough to retain the original forms during the internal rotation of inner B3 unit.
Structure and Bonding of Alkali-Metal Pentalenides
Barroso J., Mondal S., Cabellos J.L., Osorio E., Pan S., Merino G.
Article, Organometallics, 2017, DOI Link
View abstract ⏷
The lowest energy isomers of alkali-metal pentalenides, E2C8H6 (E = Li, Na, K, Rb, Cs), are inverted sandwiches. Along Li to Cs, the location of the E atoms shifts toward the points over the center of the pentalene moiety even in the presence of solvent molecules such as dimethoxyethane. Adaptive natural density partitioning analysis reveals the equivalent 10 π-bonding frameworks in the C8H62- and E2C8H6 systems. The stability of these complexes practically originates from the electrostatic interaction (84-92%) between C8H62- and [E···E]2+. While the sharp drop in interaction energy in Na complex, in comparison to that in the Li analogue, is due to the lower contribution from both electrostatic (by 31.6 kcal mol-1) and orbitalic (by 48.1 kcal mol-1) terms, for the rest of the complexes the obtained trend of interaction energy originates from the reduced ionic contacts. Although the orbital interaction is less important in these complexes, it plays an important role in deciding their geometries. The obtained geometrical change along Li to Cs is a consequence of the participation of the d orbitals in the heavier analogues.
The strongest CO binding and the highest C-O stretching frequency
Saha R., Pan S., Frenking G., Chattaraj P.K., Merino G.
Article, Physical Chemistry Chemical Physics, 2017, DOI Link
View abstract ⏷
A coupled-cluster study is performed on CO bound BeY complexes (Y = O, CO3, SO4, NH, NCN, and NBO) to understand the effect of attached ligands (Y) on the CO binding ability and C-O stretching frequency (νCO). Herein, we report that BeNCN has the highest CO binding ability (via both C- and O-side binding) among the studied neutral Be-based clusters, whereas OCBeSO4 has the highest νCO among the neutral carbonyls. The nature and extent of shift in νCO compared to free CO are explained in terms of change in polarization in the bonding orbitals of CO and relative contribution from OC→BeY or CO→BeY σ-donation, and OC←BeY or CO←BeY π-back-donation. The largest blue-shift in OCBeSO4 and the largest red-shift in COBeNH are consequences of the smallest OC←BeSO4 π-back-donation and the largest CO←BeNH π-back-donation, respectively.
Endohedral gas adsorption by cucurbit[7]uril: A theoretical study
Pan S., Jana G., Gupta A., Merino G., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2017, DOI Link
View abstract ⏷
The selectivity of cucurbit[7]uril (CB[7]) towards adsorbing a series of 14 molecules encompassing four hydrocarbons (C2H2, C2H4, C2H6, and CH4), diatomic molecules of halogens (F2 and Cl2), nitrogen oxides (NO2 and NO), carbon oxides (CO2 and CO), SO2, H2S, N2, and H2 is explored via a density functional theory based study. CB[7] is noted to have high selectivity towards adsorbing SO2 over the other considered molecules, highlighting its probable utility to separate SO2 from flue gas or other gas mixtures containing these molecules. The nature of bonding is deciphered via the computations of non-covalent interaction indices and energy decomposition analysis. Although in all cases the dispersion interaction turns out to be the most dominating contributor in stabilizing these complexes, the electrostatic contribution is also considerable. In fact, the combined effect of these two energy terms in SO2@CB[7] is responsible for the obtained selectivity.
Revisiting the racemization mechanism of helicenes
Barroso J., Cabellos J.L., Pan S., Murillo F., Zarate X., Fernandez-Herrera M.A., Merino G.
Article, Chemical Communications, 2017, DOI Link
View abstract ⏷
Herein we propose a general mechanism for the racemization of [n]helicenes up to n = 24. It is a concerted process for n = 4-7, but a multi-step mechanism is followed for n ≥ 8, involving 2n - 14 intermediates. The changes in the barriers are a delicate consequence of the steric hindrance and the π-interactions.
Does H4SO5 exist?
Murillo F., Vargas-Caamal A., Pan S., Cabellos J.L., Mora-Fonz M.J., Munoz-Castro A., Restrepo A., Merino G.
Article, Physical Chemistry Chemical Physics, 2017, DOI Link
View abstract ⏷
The possible existence of H4SO5 in aqueous sulfuric acid is analyzed in detail. For bare H4SO5, the computed free energy barrier for the exergonic transformation of H4SO5 into the H2SO4⋯H2O complex is only 3.8 kcal mol-1. The presence of water or sulfuric acid catalyzes the dehydration to such an extent that it becomes almost a barrierless process. In the gas phase, dehydration of H4SO5 is an autocatalytic reaction as the water molecule produced by the decomposition of one H4SO5 molecule induces further dissociation. Thus, in solution, the surrounding water molecules make the para-sulfuric acid a very vulnerable species to exist. The simulated Raman spectra also corroborate the absence of H4SO5 in solution.
Exploiting electronic strategies to stabilize a planar tetracoordinate carbon in cyclic aromatic hydrocarbons
Yanez O., Vasquez-Espinal A., Pino-Rios R., Ferraro F., Pan S., Osorio E., Merino G., Tiznado W.
Article, Chemical Communications, 2017, DOI Link
View abstract ⏷
A new approach to stabilize compounds containing a planar tetracoordinate carbon (ptC), embedded in aromatic hydrocarbons, is presented herein. This is achieved by using ligands that promote the formation of a 3c-2e σ-bond with the ptC under two conditions: without altering the sp2 hybridization of the aromatic carbons; and containing empty orbitals perpendicular to the aromatic ring to participate in the aromatic π-electronic delocalization.
Planar pentacoordinate carbon atoms embedded in a metallocene framework
Cui Z.-H., Vassilev-Galindo V., Luis Cabellos J., Osorio E., Orozco M., Pan S., Ding Y.-H., Merino G.
Article, Chemical Communications, 2017, DOI Link
View abstract ⏷
Viable planar pentacoordinate carbon (ppC) systems with a ppC bonded to a transition metal and embedded in a metallocene framework are reported. Our detailed global minima search shows that CAl4MX2 (M = Zr and Hf; X = F-I and C5H5) clusters with ppCs are appropriate candidates for experimental realization in the gas phase. The fulfillment of the 18 electron rule and electron delocalization is found to be crucial for the stabilization of these ppC arrangements.
Quantitative structure-activity/property/toxicity relationships through conceptual density functional theory-based reactivity descriptors
Pan S., Gupta A., Subramanian V., Chattaraj P.K.
Book chapter, Pharmaceutical Sciences: Breakthroughs in Research and Practice, 2016, DOI Link
View abstract ⏷
Developing effective structure-activity/property/toxicity relationships (QSAR/QSPR/QSTR) is very helpfulin predicting biological activity, property, and toxicity of a given set of molecules. Regular change inthese properties with the structural alteration is the main reason to obtain QSAR/QSPR/QSTR models.The advancement in making different QSAR/QSPR/QSTR models to describe activity, property, andtoxicity of various groups of molecules is reviewed in this chapter. The successful implementation ofConceptual Density Functional Theory (CDFT)-based global as well as local reactivity descriptors inmodeling effective QSAR/QSPR/QSTR is highlighted.
Statistical significance of the maximum hardness principle applied to some selected chemical reactions
Saha R., Pan S., Chattaraj P.K.
Article, Molecules, 2016, DOI Link
View abstract ⏷
The validity of the maximum hardness principle (MHP) is tested in the cases of 50 chemical reactions, most of which are organic in nature and exhibit anomeric effect. To explore the effect of the level of theory on the validity of MHP in an exothermic reaction, B3LYP/6-311++G(2df,3pd) and LC-BLYP/6-311++G(2df,3pd) (def2-QZVP for iodine and mercury) levels are employed. Different approximations like the geometric mean of hardness and combined hardness are considered in case there are multiple reactants and/or products. It is observed that, based on the geometric mean of hardness, while 82% of the studied reactions obey the MHP at the B3LYP level, 84% of the reactions follow this rule at the LC-BLYP level. Most of the reactions possess the hardest species on the product side. A 50% null hypothesis is rejected at a 1% level of significance.
Noble Gas Binding Ability of Metal-Bipyridine Monocationic Complexes (Metal=Cu, Ag, Au): A Computational Study
Jana G., Saha R., Pan S., Kumar A., Merino G., Chattaraj P.K.
Article, ChemistrySelect, 2016, DOI Link
View abstract ⏷
Noble gas (Ng) binding ability of monocationic M-bipyridine (M=Cu, Ag, Au) complexes is investigated at the MPW1B95/cc-pVTZ/cc-pVTZ-PP level. While the bond dissociation energy, enthalpy change, and free energy change for the dissociation process are computed to assess the efficacy of the Ng binding ability of these complexes, topological analysis of electron density, natural bond orbital, and energy decomposition analyses are carried out to characterize the nature of Ng−M bonds. The range of Ng−M dissociation energy values is within 5.8-13.7 kcal/mol for Cu, 4.0-12.0 kcal/mol for Ag, and 5.5-19.7 kcal/mol for Au complexes with gradual increase in moving from Ar to Rn. For a given Ng, the Ng binding ability is highest for Au followed by Cu and Ag complexes, except for the Ar case. In all the cases, the Kr−Rn dissociation processes from the respective bound complexes are endergonic in nature at room temperature. The interaction between Ng and M centers are supported dominantly by orbital and ionic interactions with almost equal contribution. The partial covalent nature of Ng−M bonds is also reflected in the topological analysis of electron density.
Dynamical behavior of boron clusters
Jalife S., Liu L., Pan S., Cabellos J.L., Osorio E., Lu C., Heine T., Donald K.J., Merino G.
Article, Nanoscale, 2016, DOI Link
View abstract ⏷
Several of the lowest energy structures of small and medium sized boron clusters are two-dimensional systems made up of a pair of concentric rings. In some cases, the barriers to the rotation of one of those rings relative to the other are remarkably low. We find that a combination of electronic and geometrical factors, including apparently the relative sizes and symmetries of the inner and outer rings, are decisive for the diminished barriers to in-plane rotation in these two dimensional clusters. A sufficiently large outer ring is important; for instance, expansion of the outer ring by a single atom may reduce the barrier significantly. A crucial factor for an apparent rotation is that the σ-skeleton of the individual rings remains essentially intact during the rotation. Finally, the transition state for the rotation of the inner ring comprises the transformation of a square into a diamond, which may be linked to a mechanism suggested decades ago for the isomerization of carboranes and boranes.
A computational study on structure, stability and bonding in Noble Gas bound metal Nitrates, Sulfates and Carbonates (Metal = Cu, Ag, Au)
Ghara M., Pan S., Deb J., Kumar A., Sarkar U., Chattaraj P.K.
Article, Journal of Chemical Sciences, 2016, DOI Link
View abstract ⏷
A density functional theory based study is performed to investigate the noble gas (Ng = Ar-Rn) binding ability of nitrates, sulfates and carbonates of noble metal (M). Their ability to bind Ng atoms is assessed through bond dissociation energy and thermochemical parameters like dissociation enthalpy and dissociation free energy change corresponding to the dissociation of Ng bound compound producing Ng and the respective salt. The zero-point energy corrected dissociation energy values per Ng atom for the dissociation process producing Ng atom(s) and the corresponding salts range within 6.0–13.1 kcal/mol in NgCuNO3, 3.1–9.8 kcal/mol in NgAgNO3, 6.0–13.2 kcal/mol in NgCuSO4, 3.2–10.1 kcal/mol in NgAgSO4, 5.1–11.7 kcal/mol in Ng2Cu2SO4, 2.5–8.6 kcal/mol in Ng2Ag2SO4, 8.1–19.9 kcal/mol in Ng2Au2SO4, 5.7–12.4 kcal/mol in NgCuCO3, 2.3–8.0 kcal/mol in Ng2Ag2CO3 and 7.3–18.2 kcal/mol in Ng2Au2CO3, with a gradual increase in moving from Ar to Rn. For a given type of system, the stability of Ng bound analogues follows the order as Au > Cu > Ag. All dissociation processes are endothermic in nature whereas they become endergonic as well in most of the cases of Kr-Rn bound analogues at 298 K. Natural population analysis along with the computation of Wiberg bond indices, and electron density analyses provide insights into the nature of the Ng-M bonds. The Ng-M bonds can be represented as partial covalent bonds as supported by the different electron density descriptors. [Figure not available: see fulltext.]
Why CpAl–Cr(CO)5 is linear while CpIn–Cr(CO)5 is not? Understanding the structure and bonding of the CpE–Cr(CO)5 (E = Group 13 element) complexes
Mondal S., Osorio E., Pan S., Cabellos J.L., Martinez S., Florez E., Merino G.
Article, Theoretical Chemistry Accounts, 2016, DOI Link
View abstract ⏷
Density functional theory computations at the BP86-D3/def2-TZVP level are reported for the CpE–Cr(CO)5 complexes (E = Group 13 element). In principle, we have answered two important facts: first the nature and trend of the E–Cr bonding along B to Tl complexes; second, the deviation of Cp (centroid)-E–Cr angle in In and Tl from linearity. The bonding situation in the complexes is examined via the natural bond orbital, adaptive natural density partitioning, and energy decomposition analysis schemes. Our results reveal that the E–Cr bonding in the lighter compounds is mainly ionic, while this bonding in the In and Tl complexes is dominated by an orbitalic contribution. We also clarify the origin of deviation of Cp (centroid)-E–Cr angle for the In and Tl complexes using simple molecular orbital arguments and find that the repulsive intermolecular contacts in the crystals are not the real source of this deviation as was claimed.
Selectivity in Gas Adsorption by Molecular Cucurbit[6]uril
Pan S., Saha R., Mandal S., Mondal S., Gupta A., Fernandez-Herrera M.A., Merino G., Chattaraj P.K.
Article, Journal of Physical Chemistry C, 2016, DOI Link
View abstract ⏷
The relative preference in adsorption among 19 common gas molecules, namely, C2H2, C2H4, C2H6, CH4, X2, HX (X = F, Cl, Br), CO2, CS2, CO, H2, H2O, H2S, N2, NO2, and NO within the cavity of cucurbit[6]uril (CB[6]) is investigated via density functional theory computations. Energies associated with the dissociation of gas@CB[6] producing CB[6] and gas molecules show the order of the efficacy to be encapsulated within CB[6], C2H2@CB[6] being the most viable system. However, the dissociation free energy change implies that CB[6] is most efficient in accommodating Cl2 followed by C2H2 among the considered gas molecules. In general, guest molecules having large surface contact with the host and/or high polarizability and/or having acidic hydrogen to make hydrogen bond with >C=O show larger propensity to be encapsulated within CB[6] cavitand. Functionalized CB[6] are better candidates for gas adsorption than CB[6]. However, the nature of functionalization needed to improve the adsorption ability varies with the change in the guest molecule. While full -C2H5 substitution improves C2H2 and CO2 adsorption ability of CB[6] the most, the -CN functionalized CB[6] is the best candidate to encapsulate C2H4 and C2H6 among the studied -OH, -C2H5, and -CN substituted analogues. The interaction is mostly of van der Waals type, except in the cases of C2H2, H2O, H2S, and HX (X = F, Cl, Br), in which both the electrostatic and dispersion contributions are important owing to the interaction between acidic hydrogen of these guest molecules and oxygen centers of the host moiety.
A noble interaction: An assessment of noble gas binding ability of metal oxides (metal = Cu, Ag, Au)
Pan S., Saha R., Kumar A., Gupta A., Merino G., Chattaraj P.K.
Article, International Journal of Quantum Chemistry, 2016, DOI Link
View abstract ⏷
An in silico study is performed on the structure and the stability of noble gas (Ng) bound MO complexes (M = Cu, Ag, Au). To understand the stability of these Ng bound complexes, dissociation energies, dissociation enthalpy, and dissociation free energy change are computed. The stability of NgMO is also compared with that of the experimentally detected NgMX (X= F, Cl, Br). It is found that MO has lower Ng binding ability than that of MX. All the dissociation processes producing Ng and MO are endothermic in nature and for the Kr-Rn bound MO (M = Cu, Au), and Xe and Rn bound AgO cases, the corresponding dissociation processes are turned out to be endergonic in nature at standard state. The Wiberg bond indices of Ng M bonds and Ng→M electron transfer gradually increase from Ar to Rn and for the same Ng they follow the order of NgAuO > NgCuO > NgAgO. Energy decomposition analysis shows that the Ng M bonds in NgMO are partly covalent and partly electrostatic in nature. Electron density analysis further highlights the partial covalent character in Ng M bonds.
Breaking the Isolated Pentagon Rule by Encapsulating Xe2 in C60: The Guest Defines the Shape of the Host
Jalife S., Mondal S., Cabellos J.L., Pan S., Mendez-Rojas M.A., Fernandez I., Frenking G., Merino G.
Article, ChemistrySelect, 2016, DOI Link
View abstract ⏷
While many fullerenes obeying the isolated pentagon rule (IPR) are experimentally known, isomers which violate this rule may become accessible via endohedral encapsulation of a guest molecule. Density functional theory computations predict a lower energy of non-IPR endohedral noble gas fullerenes over IPR analogues, specifically when C60 encapsulates a Xe dimer! So, the guest defines the shape of the carbon fullerene.
Encapsulation of small gas molecules and rare gas atoms inside the octa acid cavitand
Chakraborty D., Pan S., Chattaraj P.K.
Article, Theoretical Chemistry Accounts, 2016, DOI Link
View abstract ⏷
The potential for gas storage (C2H2, C2H4, C2H6, CO2, CO, H2, N2, NO2, NO) molecules and rare gas (Rg) atoms (Hen–Xen, where n = 1, 2) within the recently synthesized octa acid (OA) moiety is assessed through density functional theory-based computations. It is shown that C2H2, C2H4, C2H6, N2, Kr, and Xe atoms/molecules bind with octa acid in a thermodynamically favorable way. Wiberg bond indices, non-covalent interaction indices, and energy decomposition analyses are used to explore the nature of the interaction between guest atoms and octa acid. The nature of the interaction in between either two guest atoms (in the cases of Rg atoms) or guest and cage atoms is mostly of non-covalent type in nature. An ab initio molecular dynamics simulation carried out at 50 and 298 K temperatures reveal that many of the studied systems particularly concerning polar and π electron cloud containing guest molecules show good dynamical stability at both temperature regimes. Except for the case of Ne-encapsulated octa acid, all other rare gases tend to get liberated from the host at room temperature although they remain inside the host at low temperature, thereby showing good dynamical stability of the Rg-encapsulated octa acid complexes up to 500 fs. In order to reaffirm the dynamical stability, Ne2@OA and CO@OA are studied at 50 and 298 K up to 600 fs as test cases.
Application of conceptual density functional theory in developing QSAR models and their usefulness in the prediction of biological activity and toxicity of molecules
Pan S., Gupta A., Roy D.R., Sharma R.K., Subramanian V., Mitra A., Chattaraj P.K.
Book chapter, Chemometrics Applications and Research: QSAR in Medicinal Chemistry, 2016,
View abstract ⏷
The modeling of quantitative structure-activity relationships (QSAR) is a very useful approach in establishing a direct relationship between the physico-chemical properties and the biological activities of the studied species. They, therefore, act as trustworthy statistical tools in predicting the biological property of new species. The structural alteration, which causes the variation in biological properties, is the main driving force in building QSAR. In this chapter, we have reviewed the different approaches in constructing QSAR and their successful application in predicting biological activity and toxicity of different class of molecules. Their scope of applicability in medicinal chemistry toward drug design and the limitations therein have also been highlighted. Special attention has been drawn to represent the effective modeling of QSAR based on different global and local reactivity descriptors of conceptual density functional theory.
Structure and stability of noble gas bound EX 3 + compounds (E = C, Ge, Sn, Pb; X = H, F, Cl, Br)
Pan S., Moreno D., Ghosh S., Chattaraj P.K., Merino G.
Article, Journal of Computational Chemistry, 2016, DOI Link
View abstract ⏷
It has been analyzed at the MP2/def2-QZVPPD level whether EX3+ (E = C-Pb; X = H, F-Br) can bind noble gas atoms. Geometrical and electronic structures, dissociation energy values, thermochemical parameters, natural bond order, electron density, and energy decomposition analyses highlight the possibility of such noble gas bound EX3+ compounds. Except He and Ne, the other heavier congeners of this family make quite strong bonds with E. In fact, the dissociations of Ar-Rn bound analogues turn out to be endergonic in nature at 298 K, except in the cases of ArGe Cl3+, Ar/KrGeBr3+, and ArSnBr3+. GeH3+ and EF3+ (E = Ge-Pb) can even bind two Ng atoms with reasonably high dissociation energy. As the pz orbital of the E center in EX3+ plays a crucial role in its binding with the noble gas atoms, the effect of the π back-bonding causing X → E electron transfer ought to be properly understood. Due to the larger back-donation, the Ng binding ability of EX3+ gradually decreases along F to Br. EH2+ and the global minimum HE+...H2 (E = Sn, Pb) complexes are also able to bind Ar-Rn atoms quite effectively. The Ng-E bonds in Ar-Rn bound CH3+, GeH3+, and EF3+ (E = Ge-Pb) and Xe/Rn-E bonds in NgECl3+ and NgEBr3+ (E = Ge, Sn) are mainly of covalent type.
σ-Aromatic cyclic M3+ (M = Cu, Ag, Au) clusters and their complexation with dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gases and carbon monoxide
Pan S., Saha R., Mandal S., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2016, DOI Link
View abstract ⏷
The σ-aromaticity of M3+ (M = Cu, Ag, Au) is analyzed and compared with that of Li3+ and a prototype σ-aromatic system, H3+. Ligands (L) like dimethyl imidazol-2-ylidene, pyridine, isoxazole and furan are employed to stabilize these monocationic M3+ clusters. They all bind M3+ with favorable interaction energy. Dimethyl imidazol-2-ylidene forms the strongest bond with M3+ followed by pyridine, isoxazole and furan. Electrostatic contribution is considerably more than that of orbital contribution in these M-L bonds. The orbital interaction arises from both L → M σ donation and L ← M back donation. M3+ clusters also bind noble gas atoms and carbon monoxide effectively. In general, among the studied systems Au3+ binds a given L most strongly followed by Cu3+ and Ag3+. Computation of the nucleus-independent chemical shift (NICS) and its different extensions like the NICS-rate and NICS in-plane component vs. NICS out-of-plane component shows that the σ-aromaticity in L bound M3+ increases compared to that of bare clusters. The aromaticity in pyridine, isoxazole and furan bound Au3+ complexes is quite comparable with that in the recently synthesized Zn3(C5(CH3)5)3+. The energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital also increases upon binding with L. The blue-shift and red-shift in the C-O stretching frequency of M3(CO)3+ and M3(OC)3+, respectively, are analyzed through reverse polarization of the σ- and π-orbitals of CO as well as the relative amount of OC → M σ donation and M → CO π back donation. The electron density analysis is also performed to gain further insight into the nature of interaction.
Noble gas bound beryllium chromate and beryllium hydrogen phosphate: A comparison with noble gas bound beryllium oxide
Pan S., Ghara M., Ghosh S., Chattaraj P.K.
Article, RSC Advances, 2016, DOI Link
View abstract ⏷
A comparative study is made on the noble gas (Ng) binding ability of beryllium hydrogen phosphate (BeHPO4), beryllium chromate (BeCrO4), and beryllium oxide (BeO) via density functional theory and ab initio calculations. BeO serves as a prototype example of a Be based Lewis acid with remarkable Ng binding capability. Although NgBeHPO4 and NgBeCrO4 have lower Ng-Be bond dissociation energy by 1.4-4.6 and 2.4-6.3 kcal mol-1, respectively, than NgBeO, the corresponding free energy changes at the standard state show that Ar-Rn analogues may be viable even at an ambient condition. The nature of bonding in all these Ng bound complexes is exactly the same, being exclusively a donor-acceptor type of interaction as indicated by the natural bond orbital, electron density and energy decomposition analyses (EDA) in conjunction with natural orbitals for chemical valence calculations. The negative local energy density values at the bond critical points of Ng-Be bonds involving Kr-Rn imply the covalent nature of the bonding which is further supported by the dominant orbital contribution (80-88%) towards the total stabilization as obtained from the EDA. In fact, the variation in the orbital term is responsible for the observed trend of their Ng binding ability in changing either the Ng atoms or the Be system. Further, Ng → BeY (Y = HPO4, CrO4, O) σ-donation is the key contributor (70-82%) of the orbital term, whereas Ng ← BeY π-back donation is responsible only for 15-21% of the total orbital interaction.
Structure, stability, and nature of bonding in carbon monoxide bound EX3+ complexes (E = group 14 element; X = H, F, Cl, Br, I)
Ghara M., Pan S., Kumar A., Merino G., Chattaraj P.K.
Article, Journal of Computational Chemistry, 2016, DOI Link
View abstract ⏷
A density functional theory study is performed to predict the structures and stability of carbon monoxide (CO) bound (Formula presented.) (E = C, Si, Ge, Sn, Pb; X = H, F, Cl, Br, I) complexes. The possibility of bonding through both C- and O-sides of CO is considered. Thermochemical analysis reveals that all the dissociation processes producing CO and (Formula presented.) are endothermic in nature whereas most of the dissociation reactions are endergonic in nature at room temperature. The nature of bonding in EC/O bonds is analyzed via Wiberg bond index, natural population analysis, electron density, and energy decomposition analyses in conjunction with natural orbitals for chemical valence scheme. In comparison to CO stretching frequency ((Formula presented.)) in free CO, while a red shift is noted in O-side binding, the C-side binding results in a blue shift in (Formula presented.). The relative change in (Formula presented.) values in CO bound (Formula presented.) complexes on changing either E or X is scrutinized and possible explanation is provided in terms of polarization in the σ- and π-orbitals and the relative strength of C→E or O→E σ-donation and E→C or E→O π-back-donation. © 2016 Wiley Periodicals, Inc.
Noble gas supported B3+ cluster: Formation of strong covalent noble gas-boron bonds
Saha R., Pan S., Mandal S., Orozco M., Merino G., Chattaraj P.K.
Article, RSC Advances, 2016, DOI Link
View abstract ⏷
The stability of noble gas (Ng) bound B3+ clusters is assessed via an in silico study, highlighting their structure and the nature of the Ng-B bonds. Ar to Rn atoms are found to form exceptionally strong bonds with B3+ having each Ng-B bond dissociation energy in the range of 15.1-34.8 kcal mol-1 in B3Ng3+ complexes with a gradual increase in moving from Ar to Rn. The computed thermochemical parameters like enthalpy and free energy changes for the Ng dissociation processes from B3Ng3+ also support the stability of Ar to Rn analogues for which the corresponding dissociation processes are endergonic in nature even at room temperature. The covalent nature of the Ng-B bonds is indicated by the localized natural Ng-B bond orbitals and high Wiberg bond indices (0.57-0.78) for Ng-B bonds. Electron density analysis also supports the covalency of these Ng-B bonds where the electron density is accumulated in between Ng and B centres. The orbital interaction energy is the main contributor (ca. 63.0-64.4%) of the total attraction energy in Ng-B bonds. Furthermore, the Ng-B bonding can be explained in terms of a donor-acceptor model where the Ng (HOMO) → B3Ng2+ (LUMO) σ-donation has the major contribution.
Back to basics: Identification of reaction intermediates in the mechanism of a classic ligand substitution reaction on Vaska’s complex
Durango-Garcia C.J., Jalife S., Cabellos J.L., Martinez S.H., Jimenez-Halla J.O.C., Pan S., Merino G., Montiel-Palma V.
Article, RSC Advances, 2016, DOI Link
View abstract ⏷
The mechanism of methylation of Vaska's complex trans-[ClIr(CO)(PPh3)2] by trimethylgallium was studied and the identification of the spectroscopically detected intermediates was achieved with the aid of computational methods. The reaction pathway, computed by means of density functional theory (M05-2X-D3/def2-SVP), involves the initial formation of a chloride-bridged adduct trans-[(Cl·GaMe3)Ir(CO)(PPh3)2] to then proceeds to a transition state [(μ2-Cl,C-ClMeGaMe2)Ir(CO)(PPh3)2]. This transition state subsequently evolves to the methylated adduct [MeIr(CO)(PPh3)2·(GaMe2Cl)] to finally release the alkylated product trans-[MeIr(CO)(PPh3)2] together with GaMe2Cl.
Structure and bonding of IrB12-: Converting a rigid boron B12 platelet to a Wankel motor
Liu L., Moreno D., Osorio E., Castro A.C., Pan S., Chattaraj P.K., Heine T., Merino G.
Article, RSC Advances, 2016, DOI Link
View abstract ⏷
The global minimum of IrB12- is a C3v symmetric bowl-like structure in which the Ir atom is located on the concave side of the bowl, similar to its lighter congeners, CoB12- and RhB12- clusters. Although all these MB12- (M = Co, Rh, Ir) clusters show dynamical behaviour, analogous to that of the so-called 'Wankel motors', the energy barrier for the rotation of the inner B3 ring within the peripheral B9 ring is the lowest in the IrB12- case (5.0 kcal mol-1 only). The geometrical feature along with the lower interaction energy between B3 and MB9 moieties are responsible for a smaller rotational energy barrier in IrB12- than those in CoB12- and RhB12- clusters.
10-π-Electron arenes: À la carte: Structure and bonding of the [E-(CnHn)-E]n-6 (E = Ca, Sr, Ba; N = 6-8) complexes
Mondal S., Cabellos J.L., Pan S., Osorio E., Torres-Vega J.J., Tiznado W., Restrepo A., Merino G.
Article, Physical Chemistry Chemical Physics, 2016, DOI Link
View abstract ⏷
In this paper, we provide solid evidence to show that among an overwhelming structural diversity, alkaline earth metals (Ca, Sr, Ba) have the ability to form inverted sandwich compounds with C6H6, C7H7+, and C8H82+ of Dnh symmetry and general formula [E-(CnHn)-E]n-6 (n = 6-8) with planar 10-π-electron aromatic cores by virtue of transferring two electrons per metal atom to the ring. However, the origin of the orbital interaction between the metals and the carbon ring is quite different; while [E-(C6H6)-E] complexes are dominated by δ-interactions, both π- and δ-interactions are important in [E-(C7H7)-E]+ and [E-(C8H8)-E]2+ complexes.
How strong are the metallocene-metallocene interactions? Cases of ferrocene, ruthenocene, and osmocene
Vargas-Caamal A., Pan S., Ortiz-Chi F., Cabellos J.L., Boto R.A., Contreras-Garcia J., Restrepo A., Chattaraj P.K., Merino G.
Article, Physical Chemistry Chemical Physics, 2016, DOI Link
View abstract ⏷
An exhaustive exploration of the potential energy surfaces of ferrocene, ruthenocene and osmocene dimers has been performed. Our computations involving dispersion show that only four different isomers are present in each metallocene dimer. The collective action of small interaction energies of dispersive nature leads to a dissociation energy of 7.5 kcal mol-1 for the ferrocene dimer. Dispersion has strong effects on the geometrical parameters, reducing the M⋯M distances by almost 1 Å. Our results also reveal that inclusion of entropic factors modifies the relative stability of the complexes. The nature of bonding is examined using the energy decomposition analysis and the non-covalent interaction index. Both analyses indicate that dispersion is the major contributing factor in stabilizing a metallocene dimer.
A coupled-cluster study on the noble gas binding ability of metal cyanides versus metal halides (metal = Cu, Ag, Au)
Pan S., Gupta A., Saha R., Merino G., Chattaraj P.K.
Article, Journal of Computational Chemistry, 2015, DOI Link
View abstract ⏷
A coupled-cluster study is carried out to investigate the efficacy of metal(I) cyanide (MCN; M = Cu, Ag, Au) compounds to bind with noble gas (Ng) atoms. The M£Ng bond dissociation energy, enthalpy change, and Gibbs free energy change for the dissociation processes producing Ng and MCN are computed to assess the stability of NgMCN compounds. The Ng binding ability of MCN is then compared with the experimentally detected NgMX (X = F, Cl, Br) compounds. While CuCN and AgCN have larger Ng binding ability than those of MCl and MBr (M = Cu, Ag), AuCN shows larger efficacy toward bond formation with Ng than that of AuBr. Natural bond orbital analysis, energy decomposition analysis in conjunction with the natural orbital for chemical valence theory, and the topological analysis of the electron density are performed to understand the nature of interaction occurring in between Ng and MCN. The Ng-M bonds in NgMCN are found comprise an almost equal contribution from covalent and electrostatic types of interactions. The different electron density descriptors also reveal the partial covalent character in the concerned bonds.
Three-dimensional networks containing rectangular Sr4 and Ba4 units: Synthesis, structure, bonding, and potential application for Ne gas separation
Mandal S., Pan S., Deb D., Giri S., Duley S., Radenkovic S., Cooper D.L., Bultinck P., Anoop A., Bhattacharjee M., Chattaraj P.K.
Article, International Journal of Quantum Chemistry, 2015, DOI Link
View abstract ⏷
New porous three-dimensional metal-organic frameworks are synthesized that contain infinite chains of Srn and Ban rectangles. Their structures are elucidated by means of spectroscopic techniques such as nuclear magnetic resonance and Fourier transform infrared, and the respective crystal structures are determined. The electronic structure of basic units of the crystals are computed using density functional theory at the B3LYP/6-31G(d,p)/def2-TZVP level, and the bonding and reactivity are analyzed using natural bond orbital analysis, the quantum theory of atoms in molecules, and conceptual density functional theory. The possibilities of noble gas (Ng) storage inside the crystal structures are explored through modeling a Ng atom inside the frozen geometry of the crystal. It was found that a neon atom can fit into a cavity in the Sr and Ba crystal structures whereas other Ngs (He, Ar, Kr) exhibit repulsive interactions with the crystal structure. Ab initio molecular dynamics simulations for up to 500 fs at 77 and 298 K suggest that the structures incorporating a neon atom are kinetically stable.
Cucurbit[6]uril: A Possible Host for Noble Gas Atoms
Pan S., Mandal S., Chattaraj P.K.
Article, Journal of Physical Chemistry B, 2015, DOI Link
View abstract ⏷
Density functional and ab initio molecular dynamics studies are carried out to investigate the stability of noble gas encapsulated cucurbit[6]uril (CB[6]) systems. Interaction energy, dissociation energy and dissociation enthalpy are calculated to understand the efficacy of CB[6] in encapsulating noble gas atoms. CB[6] could encapsulate up to three Ne atoms having dissociation energy (zero-point energy corrected) in the range of 3.4-4.1 kcal/mol, whereas due to larger size, only one Ar or Kr atom encapsulated analogues would be viable. The dissociation energy value for the second Ar atom is only 1.0 kcal/mol. On the other hand, the same for the second Kr is -0.5 kcal/mol, implying the instability of the system. The noble gas dissociation processes are endothermic in nature, which increases gradually along Ne to Kr. Kr encapsulated analogue is found to be viable at room temperature. However, low temperature is needed for Ne and Ar encapsulated analogues. The temperature-pressure phase diagram highlights the region in which association and dissociation processes of Kr@CB[6] would be favorable. At ambient temperature and pressure, CB[6] may be used as an effective noble gas carrier. Wiberg bond indices, noncovalent interaction indices, electron density, and energy decomposition analyses are used to explore the nature of interaction between noble gas atoms and CB[6]. Dispersion interaction is found to be the most important term in the attraction energy. Ne and Ar atoms in one Ng entrapped analogue are found to stay inside the cavity of CB[6] throughout the simulation at 298 K. However, during simulation Ng<inf>2</inf> units in Ng<inf>2</inf>@CB[6] flip toward the open faces of CB[6]. After 1 ps, one Ne atom of Ne<inf>3</inf>@CB[6] almost reaches the open face keeping other two Ne atoms inside. At lower temperature (77 K), all the Ng atoms in Ng<inf>n</inf>@CB[6] remain well inside the cavity of CB[6] throughout the simulation time (1 ps).
Analyzing torquoselectivity in electrocyclic ring opening reactions of trans-3,4-dimethylcyclobutene and 3-formylcyclobutene through electronic structure principles
Morales-Bayuelo A., Pan S., Caballero J., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2015, DOI Link
View abstract ⏷
The validity of maximum hardness, minimum electrophilicity and minimum polarizability principles is assessed to explain the phenomenon of torquoselectivity (inward and outward preference) in the conrotatory ring opening reactions of trans-3,4-dimethylcyclobutene into Z,Z- and E,E-butadienes and 3-formylcyclobutene into E- and Z-2,4-pentadienals. The hardness, average polarizability and electrophilicity profiles are computed along the intrinsic reaction coordinate and divided into three relevant stages. The transition states involved in the unfavorable inward conrotation of trans-3,4-dimethylcyclobutene and in the unfavorable outward conrotation of 3-formylcyclobutene are found to be higher in energy, softer, more electrophilic and more polarizable than the transition states corresponding to the torquoselective outward and inward conrotations, respectively. These observations are in conformity with the maximum hardness, minimum electrophilicity and minimum polarizability principles. The sharp changes in the local reactivity descriptors are also observed around the transition states in their respective profiles.
Comparative Study on the Noble-Gas Binding Ability of BeX Clusters (X = SO4, CO3, O)
Saha R., Pan S., Merino G., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2015, DOI Link
View abstract ⏷
Ab initio computations are carried out to assess the noble gas (Ng) binding capability of BeSO4 cluster. We have further compared the stability of NgBeSO4 with that of the recently detected NgBeCO3 cluster. The Ng-Be bond in NgBeCO3 is somewhat weaker than that in NgBeO cluster. In NgBeSO4, the Ng-Be bond is found to be stronger compared with not only the Ng-Be bond in NgBeCO3 but also that in NgBeO, except the He case. The Ar-Rn-bound BeSO4 analogues are viable even at room temperature. The Wiberg bond indices of Be-Ng bonds and the degree of electron transfer from Ng to Be are somewhat larger in NgBeSO4 than those in NgBeCO3 and NgBeO. Electron density and energy decomposition analyses are performed in search of the nature of interaction in the Be-Ng bond in NgBeSO4. The orbital energy term (ΔEorb) contributes the maximum (ca. 80-90%) to the total attraction energy. The Ar/Kr/Xe/Rn-Be bonds in NgBeSO4 could be of partial covalent type with a gradual increase in covalency along Ar to Rn.
On the stability of noble gas bound 1-tris(pyrazolyl)borate beryllium and magnesium complexes
Pan S., Saha R., Chattaraj P.K.
Article, New Journal of Chemistry, 2015, DOI Link
View abstract ⏷
An in silico study is performed to assess the noble gas (Ng) binding ability of 1-tris(pyrazolyl)borate beryllium and magnesium cationic complexes (TpBe+ and TpMg+). The Be and Mg centers in these complexes are found to bind heavier Ng atoms quite effectively. Both the zero point energy and basis set superposition error corrected dissociation energy values for the bonds between Ar-Rn and metal atoms range within 5.8-10.2 kcal mol-1 for Be and within 5.2-9.9 kcal mol-1 for Mg. The dissociation of the Kr-Rn bound analogues of TpBe+ and Ar-Rn bound analogues of TpMg+ into the individual Ng atoms and TpBe+ or TpMg+ complexes is endergonic in nature at room temperature. The remaining lighter Ng bound complexes would be stable at lower temperatures. The nature of Be-Ng or Mg-Ng bonds is explored via Wiberg bond indices computation, atoms-in-molecules and energy decomposition analyses. The degree of covalent character in the Be/Mg-Ng bonds increases gradually in moving from He to its heavier congeners. The Be-Xe/Rn and Mg-Xe/Rn bonds could be categorized as being of the partial covalent type. The contribution from the orbital term is at the maximum towards the total attraction. The magnitude of this term becomes gradually larger from He to Rn, implying a larger degree of covalent character for heavier Ng atoms.
Dynamical behavior of Borospherene: A Nanobubble
Martinez-Guajardo G., Cabellos J.L., Diaz-Celaya A., Pan S., Islas R., Chattaraj P.K., Heine T., Merino G.
Article, Scientific Reports, 2015, DOI Link
View abstract ⏷
The global minimum structure of borospherene (B<inf>40</inf>) is a cage, comprising two hexagonal and four heptagonal rings. Born-Oppenheimer Molecular Dynamics simulations show that continuous conversions in between six and seven membered rings take place. The activation energy barrier for such a transformation is found to be 14.3 kcal·mol<sup>-1</sup>. The completely delocalized σ - and π-frameworks, as well as the conservation of the bonding pattern during rearrangement, facilitate the dynamical behavior of B<inf>40</inf>. B<inf>40</inf> is predicted to act as a support-free spherical two-dimensional liquid at moderate temperature. In other words, B<inf>40</inf> could be called as a nanobubble.
Exploring the nature of silicon-noble gas bonds in H3SiNgNSi and HSiNgNSi compounds (Ng = Xe, Rn)
Pan S., Saha R., Chattaraj P.K.
Article, International Journal of Molecular Sciences, 2015, DOI Link
View abstract ⏷
Ab initio and density functional theory-based computations are performed to investigate the structure and stability of H3SiNgNSi and HSiNgNSi compounds (Ng = Xe, Rn). They are thermochemically unstable with respect to the dissociation channel producing Ng and H3SiNSi or HSiNSi. However, they are kinetically stable with respect to this dissociation channel having activation free energy barriers of 19.3 and 23.3 kcal/mol for H3SiXeNSi and H3SiRnNSi, respectively, and 9.2 and 12.8 kcal/mol for HSiXeNSi and HSiRnNSi, respectively. The rest of the possible dissociation channels are endergonic in nature at room temperature for Rn analogues. However, one three-body dissociation channel for H3SiXeNSi and one two-body and one three-body dissociation channels for HSiXeNSi are slightly exergonic in nature at room temperature. They become endergonic at slightly lower temperature. The nature of bonding between Ng and Si/N is analyzed by natural bond order, electron density and energy decomposition analyses. Natural population analysis indicates that they could be best represented as (H3SiNg)+(NSi)− and (HSiNg)+(NSi)−. Energy decomposition analysis further reveals that the contribution from the orbital term (ΔEorb) is dominant (ca. 67%–75%) towards the total attraction energy associated with the Si-Ng bond, whereas the electrostatic term (ΔEelstat) contributes the maximum (ca. 66%–68%) for the same in the Ng–N bond, implying the covalent nature of the former bond and the ionic nature of the latter.
Quantitative structure-activity/property/toxicity relationships through conceptual density functional theory-based reactivity descriptors
Pan S., Gupta A., Subramanian V., Chattaraj P.K.
Book chapter, Quantitative Structure-Activity Relationships in Drug Design, Predictive Toxicology, and Risk Assessment, 2015, DOI Link
View abstract ⏷
Developing effective structure-activity/property/toxicity relationships (QSAR/QSPR/QSTR) is very helpful in predicting biological activity, property, and toxicity of a given set of molecules. Regular change in these properties with the structural alteration is the main reason to obtain QSAR/QSPR/QSTR models. The advancement in making different QSAR/QSPR/QSTR models to describe activity, property, and toxicity of various groups of molecules is reviewed in this chapter. The successful implementation of Conceptual Density Functional Theory (CDFT)-based global as well as local reactivity descriptors in modeling effective QSAR/QSPR/QSTR is highlighted.
Metastable behavior of noble gas inserted tin and lead fluorides
Pan S., Gupta A., Mandal S., Moreno D., Merino G., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2015, DOI Link
View abstract ⏷
Ab initio computations are carried out to explore the structure and stability of FNgEF3 and FNgEF (E = Sn, Pb; Ng = Kr-Rn) compounds. They are the first reported systems to possess Ng-Sn and Ng-Pb bonds. Except for FKrEF3, the dissociations of FNgSnF3 and FNgEF, producing Ng and SnF4 or EF2, are only exergonic in nature at room temperature, whereas FNgPbF3 has a thermochemical instability with respect to two two-body dissociation channels. However, they are kinetically stable, having positive activation barriers (ranging from 2.2 to 49.9 kcal mol-1) with respect to those dissociations. The kinetic stability gradually improves in moving from the Kr to Rn analogues. The remaining possible dissociation channels for these compounds are found to be endergonic in nature. The nature of the bonding is analyzed by natural bond order, electron density, and energy decomposition analyses. Particularly, the natural population analysis reveals that they are best represented as F-(NgEF3)+ and F-(NgEF)+. All the Xe/Rn-E bonds in FNgEF3 and FNgEF are covalent in nature.
Conceptual density functional theory (DFT) approach to all-metal aromaticity and hydrogen storage
Das R., Chakraborty A., Pan S., Chattaraj P.K.
Book chapter, Compendium of Hydrogen Energy: Hydrogen Storage, Distribution and Infrastructure: Volume 2, 2015, DOI Link
View abstract ⏷
The efficacy of different conceptual density functional theory based reactivity descriptors and nucleus independent chemical shift in analyzing the hydrogen trapping potential of a wide variety of systems is reviewed in this chapter. The influence of aromaticity on the stability/reactivity of hydrogen storage material as well as structural and bonding aspects of those materials are explored. Charges on the different active sites in a molecule play a crucial role in their hydrogen-trapping ability. Temperature−pressure diagrams highlighting the ΔG<0 region to identify the region of the thermodynamically favorable hydrogen adsorption process are given. The applied electric field also improves the hydrogen-binding capability.
Stability of noble-gas-bound SiH3+ clusters
Pan S., Moreno D., Merino G., Chattaraj P.K.
Article, ChemPhysChem, 2014, DOI Link
View abstract ⏷
The stability of noble gas (Ng)-bound SiH3+ clusters is explored by ab initio computations. Owing to a high positive charge (+1.53 e-), the Si center of SiH3+ can bind two Ng atoms. However, the Si-Ng dissociation energy for the first Ng atom is considerably larger than that for the second one. As we go down group 18, the dissociation energy gradually increases, and the largest value is observed for the case of Rn. For NgSiH3+ clusters, the Ar-Rn dissociation processes are ender-gonic at room temperature. For He and Ne, a much lower temperature is required for it to be viable. The formation of Ng2SiH3+ clusters is also feasible, particularly for the heavier members and at low temperature. To shed light on the nature of Si-Ng bonding, natural population analysis, Wiberg bond indices computations, electron-density analysis, and energy-decomposition analysis were performed. Electron transfer from the Ng centers to the electropositive Si center occurs only to a small extent for the lighter Ng atoms and to a somewhat greater extent for the heavier analogues. The Si-Xe/Rn bonds can be termed covalent bonds, whereas the Si-He/Ne bonds are noncovalent. The Si-Ar/Kr bonds possess some degree of covalent character, as they are borderline cases. Contributions from polarization and charge transfer and exchange are key terms in forming Si-Ng bonds. We also studied the effect of substituting the H atoms of SiH3+ by halide groups (-X) on the Ng binding ability. SiF3+ showed enhanced Ng binding ability, whereas SiCl3+ and SiBr3+ showed a lower ability to bind Ng than SiH3+. A compromise originates from the dual play of the inductive effect of the - X groups and X→Si π backbonding (pz-pz interaction).
Movement of Ng2 molecules confined in a C60 cage: An ab initio molecular dynamics study
Khatua M., Pan S., Chattaraj P.K.
Article, Chemical Physics Letters, 2014, DOI Link
View abstract ⏷
An ab initio molecular dynamics study on Ng2@C60 (Ng = HeKr) systems is performed to analyze the movement of Ng2 molecules inside a C60 cage. Within 500 fs time window, the He2 undergoes precession encompassing translation, vibration and rotation readily whereas other Ng2 molecules show usual vibration but the degrees of translation and rotation decrease with an increase in size of the Ng atoms. Increase in interaction between the Ng centers and cage carbons and an increased distortion of cage in moving from He to Kr seem to be responsible for this. During the movement, the Ng2 units behave as single entity. © 2014 Elsevier B.V.
The inorganic analogues of carbo-benzene
Jalife S., Audiffred M., Islas R., Escalante S., Pan S., Chattaraj P.K., Merino G.
Article, Chemical Physics Letters, 2014, DOI Link
View abstract ⏷
Inspired by carbo-benzene, we have analyzed in silico the stability of carbo-borazine (C12B3N3H6) and the iminobora-mer of borazine (B9N9H6). Both systems may be regarded as the inorganic analogues of carbo-benzene, being B9N9H6 the perfect case. Unlike aromatic carbo-benzene, C12B3N3H6 and B 9N9H6 can be classified as almost nonaromatic systems as indicated by the computed induced magnetic field. All these systems undergo dimerization very readily; therefore, they cannot be synthesized as such. However, akin to substituted carbo-benzene, the substitution of the hydrogen atom of C12B3N3H6 and B9N9H6 by other groups could stabilize them. © 2014 Elsevier B.V. All rights reserved.
Ab initio study on the stability of NgnBe2N 2, NgnBe3N2 and NgBeSiN2 clusters
Pan S., Moreno D., Cabellos J.L., Merino G., Chattaraj P.K.
Article, ChemPhysChem, 2014, DOI Link
View abstract ⏷
The global minima of Be2N2, Be3N 2 and BeSiN2 clusters are identified using a modified stochastic kick methodology. The structure, stability and bonding nature of these clusters bound to noble gas (Ng) atoms are studied at the MP2/def2-QZVPPD level of theory. Positive Be-Ng bond dissociation energy, which gradually increases down Group 18 from He to Rn, indicates the bound nature of Ng atoms. All of the Ng-binding processes are exothermic in nature. The Xe and Rn binding to Be2N2 and Be3N2 clusters and Ar-Rn binding to BeSiN2 are exergonic processes at room temperature; however, for the lighter Ng atoms, lower temperatures are needed. Natural population analysis, Wiberg bond index computations, electron density analysis, and energy decomposition analysis are performed to better understand the nature of Be-Ng bonds. Noble bonds: An ab initio study shows that the positively charged Be centers in experimentally accessible Be2N2, Be3N2, and BeSiN2 clusters can bind noble gas (Ng) atoms. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
B182−: Ax quasi-planar bowl member of the Wankel motor family
Moreno D., Pan S., Zeonjuk L.L., Islas R., Osorio E., Guajardo G.-M., Chattaraj P.K., Heine T., Merino G.
Article, Chemical Communications, 2014, DOI Link
View abstract ⏷
A quasi-planar member of the so-called ‘Wankel motor’ family, B182−, is found. This boron cluster is an electronically stable dianion and a concentric doubly σ- and π-aromatic system. The inner B6 unit in B182− undergoes quasi-free rotation inside the perimeter of the B12 ring. The absence of any localized σ-bond between the inner ring and the peripheral boron atoms makes the system fluxional. © The Partner Organisations 2014.
Carbo-cages: A computational study
Azpiroz J.M., Islas R., Moreno D., Fernandez-Herrera M.A., Pan S., Chattaraj P.K., Martinez-Guajardo G., Ugalde J.M., Merino G.
Article, Journal of Organic Chemistry, 2014, DOI Link
View abstract ⏷
Inspired by their geometrical perfection, intrinsic beauty, and particular properties of polyhedranes, a series of carbo-cages is proposed in silico via density functional theory computations. The insertion of alkynyl units into the C-C bonds of polyhedranes results in a drastic lowering of the structural strain. The induced magnetic field shows a significant delocalization around the three-membered rings. For larger rings, the response is paratropic or close to zero, suggesting a nonaromatic behavior. In the carbo-counterparts, the values of the magnetic response are shifted with respect to their parent compounds, but the aromatic/nonaromatic character remains unaltered. Finally, Born-Oppenheimer molecular dynamics simulations at 900 K do not show any drastic structural changes up to 10 ps. In the particular case of a carbo-prismane, no structural change is perceived until 2400 K. Therefore, although carbo-cages have enthalpies of formation 1 order of magnitude higher than those of their parent compounds, their future preparation and isolation should not be discarded, because the systems are kinetically stable, explaining why the similar systems like carbo-cubane have already been synthesized. © 2014 American Chemical Society.
Confinement induced binding of noble gas atoms
Khatua M., Pan S., Chattaraj P.K.
Article, Journal of Chemical Physics, 2014, DOI Link
View abstract ⏷
The stability of Ngn@B12N12 and Ng n@B16N16 systems is assessed through a density functional study and ab initio simulation. Although they are found to be thermodynamically unstable with respect to the dissociation of individual Ng atoms and parent cages, ab initio simulation reveals that except Ne 2@B12N12 they are kinetically stable to retain their structures intact throughout the simulation time (500 fs) at 298 K. The Ne2@B12N12 cage dissociates and the Ne atoms get separated as the simulation proceeds at this temperature but at a lower temperature (77 K) it is also found to be kinetically stable. He-He unit undergoes translation, rotation and vibration inside the cavity of B 12N12 and B16N16 cages. Electron density analysis shows that the He-He interaction in He2@B 16N16 is of closed-shell type whereas for the same in He2@B12N12 there may have some degree of covalent character. In few cases, especially for the heavier Ng atoms, the Ng-N/B bonds are also found to have some degree of covalent character. But the Wiberg bond indices show zero bond order in He-He bond and very low bond order in cases of Ng-N/B bonds. The energy decomposition analysis further shows that the Eorb term contributes 40.9% and 37.3% towards the total attraction in the He2 dimers having the same distances as in He 2@B12N12 and He2@B 16N16, respectively. Therefore, confinement causes some type of orbital interaction between two He atoms, which akins to some degree of covalent character. © 2014 AIP Publishing LLC.
In quest of strong Be-Ng bonds among the neutral Ng-Be complexes
Pan S., Moreno D., Cabellos J.L., Romero J., Reyes A., Merino G., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2014, DOI Link
View abstract ⏷
The global minimum geometries of BeCN2 and BeNBO are linear BeN-CN and BeN-BO, respectively. The Be center of BeCN2 binds He with the highest Be-He dissociation energy among the studied neutral He-Be complexes. In addition, BeCN2 can be further tuned as a better noble gas trapper by attaching it with any electron-withdrawing group. Taking BeO, BeS, BeNH, BeNBO, and BeCN2 systems, the study at the CCSD(T)/def2-TZVP level of theory also shows that both BeCN2 and BeNBO systems have higher noble gas binding ability than those related reported systems. ΔG values for the formation of NgBeCN2/NgBeNBO (Ng = Ar-Rn) are negative at room temperature (298 K), whereas the same becomes negative at low temperature for Ng = He and Ne. The polarization plus the charge transfer is the dominating term in the interaction energy. © 2013 American Chemical Society.
DFT study on the ground state and excited state intramolecular proton transfer of propargyl arm containing Schiff bases in solution and gas phases
Annaraj B., Pan S., Neelakantan M.A., Chattaraj P.K.
Article, Computational and Theoretical Chemistry, 2014, DOI Link
View abstract ⏷
Electronic structure calculations on 6,6'-(1E,1'E)-1,1'-(propane-1,3-diylbis(azan-1-yl-1-ylidene))bis(ethan-1-yl-1-ylidene)bis(3-(prop-2-ynyloxy)phenol) (L1) and (E)-2-(1-(2-hydroxyethylimino)ethyl)-5-(prop-2-ynyloxy)phenol (L2) compounds are carried out at B3LYP/6-311. +. G(d,p) level of theory. The enol forms are found to be more stable than the corresponding keto forms in gas phase, whereas in solvent phase the reverse is true. The computed vibrational frequencies of L1 and L2 are compared with the available experimental data. Major orbital contributions for each electronic transition are assigned with the help of time-dependent density functional theory (TD-DFT). The UV-Visible spectral data of L1 and L2 coincide with the theoretical data of keto forms, which reveal that the compounds L1 and L2 exist mostly in keto forms rather than in enol forms in solution. Potential energy curves for the intramolecular proton transfer in the ground (GSIPT) and excited (ESIPT) states are generated in gas and solution (solvent is dimethyl sulfoxide) phases. GSIPT for both L1 and L2 goes through a low activation barrier, whereas in case of ESIPT, barrierless proton transfer occurs. © 2013 Elsevier B.V.
Confinement of (HF)2in Cn (n = 60, 70, 80, 90) cages
Khatua M., Pan S., Chattaraj P.K.
Article, Chemical Physics Letters, 2014, DOI Link
View abstract ⏷
Density functional theory calculations are performed to assess the influence of con.nement on the strength of H⋯F hydrogen bond in (HF)2@Cn (n = 60, 70, 80, 90). The (HF)2 entrapping process into C60 cage is thermodynamically unfavorable whereas it is favorable in other cages. The hydrogen bond is shorter in confined cages than that in free dimer. The interaction energy between two HF units is maximum in C80 whereas the bond is the shortest in C70. It appears that in confined situation a shorter bond does not necessarily mean a stronger bond. Energy decomposition analysis and electron density analysis are performed to explain the results.
On the nature of CH62+
Jalife S., Grande-Aztatzi R., Moreno D., Fernandez-Herrera M.A., Osorio E., Pan S., Von Rague Schleyer P., Martinez-Guajardo G., Merino G.
Article, Indian Journal of Chemistry - Section A Inorganic, Physical, Theoretical and Analytical Chemistry, 2014,
View abstract ⏷
The meta-stability of the hexacoordinate CH62+ dication in the gas phase is confirmed by a detailed computational exploration of its potential energy surface, using a modified "Kick" heuristic methodology and by Born-Oppenheimer Molecular-Dynamics simulations to assess its kinetic persistence. The transition states for deprotonation, decomposition into CH3+ and H3+, hydrogen scrambling, and H-H rotation are found. In addition, a nearly perfect correlation between the protonation affinities and their coordination number is obtained.
Structure and stability of (NG)nCN3Be 3+clusters and comparison with (NG)BeY0/+
Pan S., Jalife S., Kumar R.M., Subramanian V., Merino G., Chattaraj P.K.
Article, ChemPhysChem, 2013, DOI Link
View abstract ⏷
The noble gas binding ability of CN3Be3+ clusters was assessed both by ab intio and density functional studies. The global minimum structure of the CN3Be3+ cluster binds with four noble-gas (NG) atoms, in which the Be atoms are acting as active centers. The electron transfer from the noble gas to the Be atom plays a key role in binding. The dissociation energy of the Be-NG bond gradually increases from He to Rn, maintaining the periodic trend. The HOMO-LUMO gap, an indicator for stability, gives additional insight into these NG-bound clusters. The temperature at which the NG-binding process is thermodynamically feasible was identified. In addition, we investigated the stability of two new neutral NG compounds, (NG)BeSe and (NG)BeTe, and found them to be suitable candidates to be detected experimentally such as (NG)BeO and (NG)BeS. The dissociation energies of the Be-NG bond in monocationic analogues of (NG)BeY (Y=O, S, Se, Te) were found to be larger than in the corresponding neutral counter-parts. Finally, the higher the positive charge on the Be atoms, the higher the dissociation energy for the Be-NG bond becomes. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Attractive Xe-Li interaction in Li-decorated clusters
Pan S., Jalife S., Romero J., Reyes A., Merino G., Chattaraj P.K.
Article, Computational and Theoretical Chemistry, 2013, DOI Link
View abstract ⏷
Xe-binding ability of star-shaped C5Li7+ cluster and O2Li5+ super-alkali cluster is studied using the MP2 method. Both C5Li7+ and O2Li5+ clusters are found to bind with maximum twelve Xe atoms. We have also studied a series of Li decorated clusters for Xe-binding. All these clusters show good Xe-binding ability. Generally, monocationic clusters have greater binding ability with Xe atoms than the neutral clusters. In addition, a charged Li center binds Xe atoms with better dissociation energy and enthalpy than those with He through Kr. The electron transfer from Xe atoms to Li centers plays a crucial role in binding. The relative contribution of different interaction energy terms towards total interaction energy is analyzed via energy decomposition analysis (EDA). The stability of these Xe-loaded clusters is analyzed in terms of the dissociation energies and reaction enthalpies. © 2013 Elsevier B.V.
On the validity of the maximum hardness principle and the minimum electrophilicity principle during chemical reactions
Pan S., Sola M., Chattaraj P.K.
Article, Journal of Physical Chemistry A, 2013, DOI Link
View abstract ⏷
Hardness and electrophilicity values for several molecules involved in different chemical reactions are calculated at various levels of theory and by using different basis sets. Effects of these aspects as well as different approximations to the calculation of those values vis-à-vis the validity of the maximum hardness and minimum electrophilicity principles are analyzed in the cases of some representative reactions. Among 101 studied exothermic reactions, 61.4% and 69.3% of the reactions are found to obey the maximum hardness and minimum electrophilicity principles, respectively, when hardness of products and reactants is expressed in terms of their geometric means. However, when we use arithmetic mean, the percentage reduces to some extent. When we express the hardness in terms of scaled hardness, the percentage obeying maximum hardness principle improves. We have observed that maximum hardness principle is more likely to fail in the cases of very hard species like F-, H2, CH4, N2, and OH appearing in the reactant side and in most cases of the association reactions. Most of the association reactions obey the minimum electrophilicity principle nicely. The best results (69.3%) for the maximum hardness and minimum electrophilicity principles reject the 50% null hypothesis at the 2% level of significance. © 2013 American Chemical Society.
C5Li7+ and O2Li 5+ as noble-gas-trapping agents
Pan S., Contreras M., Romero J., Reyes A., Chattaraj P.K., Merino G.
Article, Chemistry - A European Journal, 2013, DOI Link
View abstract ⏷
The noble-gas-trapping ability of the star-shaped C5Li 7+ cluster and O2Li5+ super-alkali cluster is studied by using ab initio and density functional theory (DFT) at the MP2 and M05-2X levels with 6-311+G(d,p) and 6-311+G(d) basis sets. These clusters are shown to be effective noble-gas-trapping agents. The stability of noble-gas-loaded clusters is analyzed in terms of dissociation energies, reaction enthalpies, and conceptual DFT-based reactivity descriptors. The presence of an external electric field improves the dissociation energy. Caught in a trap: Ab initio and density functional studies reveal that the Li centers of star-shaped C5Li7+ clusters and O2Li5+ super-alkali clusters can bind noble gas (Ng) atoms effectively (see figure). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Favorable direction in a chemical reaction through the maximum hardness principle
Pan S., Chattaraj P.K.
Article, Journal of the Mexican Chemical Society, 2013,
View abstract ⏷
Recently, an assessment regarding the validity of maximum hardness principle has been done taking 34 exothermic chemical reactions (Poater, J.; Swart, M.; Solà, M. J. Mex. Chem. Soc. 2012, 56, 311) in which only 46% and 53% of the total reactions have greater hardness for the products and the reactants than those for the reactants and the transition states, respectively. They have also mentioned that a larger set of reactions should be studied to draw a general conclusion regarding the validity of maximum hardness principle. We have noticed that the reactions having fewer number of reactants than that of products and / or very hard atoms like H, N, O, F or very hard molecules like H2, N2, HF, HCN, CH4, etc. appearing in the reactant side, are more likely to disobey maximum hardness principle. In addition, dependence of hardness values on level of theory, basis sets, definitions, formulas, approximations should be kept in mind before criticising the validity of maximum hardness principle. Since these electronic structure principles are qualitative in nature, one should not expect them to be valid in all cases. © 2013, Sociedad Química de México.
Cucurbiturils as promising hydrogen storage materials: A case study of cucurbit[7]uril
Pan S., Mondal S., Chattaraj P.K.
Article, New Journal of Chemistry, 2013, DOI Link
View abstract ⏷
We have assessed the hydrogen storage capability of cucurbiturils that are experimentally available. For this purpose, first we have investigated the hydrogen binding ability of the repeating unit and prompted by an encouraging result, we have studied the hydrogen storage capacity of cucurbit[7]uril, as a representative of the cucurbituril family, at the ωB97X-D/6-31G(d,p) level of theory. Cucurbit[7]uril is found to interact with hydrogen in both exohedral and endohedral fashion. A total of 52 hydrogen molecules are found to be stored in cucurbit[7]uril, in which five hydrogens remain in the cavity of the cage and the remaining hydrogens prefer to bind exohedrally, leading to 8.3 gravimetric wt% of hydrogen. The N and O centers act as the active sites for the exohedral hydrogen binding. Each hydrogen in 52H2@cucurbit[7]uril interacts with cucurbit[7]uril having average binding energy value of 7.8 kJ mol-1. BSSE correction reduces the binding energy to some extent. The variation of binding energy per H2 molecule is also explored when H2 molecules are adsorbed in a sequence. All probable hydrogen binding processes are found to be exothermic in nature. The effect of an external electric field in improving binding energy and its consequence on structures and different bonding parameters are explored. © 2013 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
Biological activity and toxicity: A conceptual DFT approach
Chakraborty A., Pan S., Chattaraj P.K.
Article, Structure and Bonding, 2013, DOI Link
View abstract ⏷
Quantitative structure - activity relationship (QSAR) models are generated for biological activity and toxicity in terms of global and local reactivity descriptors within a conceptual density functional theory framework. Possible anticancer activity of two new metal - borane clusters is analyzed. © Springer-Verlag Berlin Heidelberg 2013.
Aromaticity in polyacenes and their structural analogues
Das R., Chakraborty A., Pan S., Chattaraj P.K.
Article, Current Organic Chemistry, 2013, DOI Link
View abstract ⏷
The successful synthesis of different polyacenes including theoretical assessment on the stability of larger acenes are discussed. The existence of favorable aromaticity criterion in polyacenes is understood in terms of different aromaticity indicators like nucleus independent chemical shift (NICS), harmonic oscillator model of aromaticity (HOMA), bond resonance energy (BRE). Clar's π-sextet rule is also very much effective in explaining their aromaticity. By virtue of low HOMO-LUMO gap, the probable application of polyacenes in the field of organic electronics is also highlighted. The polyacene analogues of inorganic ring compounds, viz., BN-acenes, CN-acenes, BO-acenes, BS-acenes, AlN-acenes and of alkali ring compounds, viz., Na-acenes and K-acenes also have polyacene-like aromaticity although in few cases the origin of aromaticity and qualitative nature of aromaticity differ significantly. © 2013 Bentham Science Publishers.
Designing of some novel molecular templates suitable for hydrogen storage applications: A theoretical approach
Mondal S., Chakraborty A., Pan S., Chattaraj P.K.
Book chapter, Nanoscience and Computational Chemistry: Research Progress, 2013, DOI Link
View abstract ⏷
Modeling of new molecular networks and aggregates - one of the most “sought after” topics in current chemical research is investigated on the basis of the theoretical paradigm of conceptual density functional theory and its various reactivity variants. The utility of these molecular materials as plausible storage templates for hydrogen gas is also investigated. The stability of these molecules and their hydrogen-loaded analogs is assessed through the dual perspectives of a charge analysis on the active atomic centers of the given systems as well as a comparison of the nucleus-independent chemical shift (NICS) values. Effects of the application of an external electric field and construction of relevant T-P phase diagrams reveal a thermodynamically spontaneous hydrogen binding process for many template moieties with a conspicuous increase in loading potential with an increase in the field gradient. Ab initio as well as classical molecular dynamics simulations are also carried out for few systems to assess their bulk properties as well as hydrogen trapping potentials.
Some novel molecular frameworks involving representative elements
Chakraborty A., Bandaru S., Das R., Duley S., Giri S., Goswami K., Mondal S., Pan S., Sen S., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2012, DOI Link
View abstract ⏷
Several new molecular frameworks with interesting structures, based on clusters of main group elements have been studied at different levels of theory with various basis sets. Conceptual density functional theory based reactivity descriptors and nucleus independent chemical shift provide important insights into their bonding, reactivity, stability and aromaticity. This journal is © 2012 the Owner Societies.
The hydrogen trapping potential of some Li-doped star-like clusters and super-alkali systems
Pan S., Merino G., Chattaraj P.K.
Article, Physical Chemistry Chemical Physics, 2012, DOI Link
View abstract ⏷
Prompted by the stability of some lithium decorated star-like clusters and super-alkali systems, their hydrogen trapping potential is assessed at the M06/6-311+G(d,p) and the M052X/6-311+G(d) levels, respectively. The effect of an applied electric field is also analyzed. Most of these systems are found to have the potential to become effective hydrogen storage materials with high gravimetric weight percent owing to the charges on the Li centers. The presence of an external electric field improves the situation. © 2012 The Owner Societies.
Role of Lithium Decoration on Hydrogen Storage Potential
Pan S., Banerjee S., Chattaraj P.K.
Article, Journal of the Mexican Chemical Society, 2012,
View abstract ⏷
Hydrogen storage potential of two sets of lithium containing systems, viz., Li-doped borazine derivatives and various bondstretch isomers of Li 3Al 4 - is studied at the B3LYP/6-311+G(d) level of theory occasionally supplemented by the results from the associated MP2/6-31+G(d) calculations. Negative values of interaction energy, reaction enthalpy, reaction electrophilicity, and desorption energies for the gradual hydrogen-trapping processes justify the efficacy of these systems as the hydrogen storage material. Presence of Li as well as aromaticity improves the situation. Various conceptual density functional theory based reactivity descriptors like electronegativity, hardness, and electrophilicity and the associated electronic structure principles such as the principles of maximum hardness and minimum electrophilicity lend additional support. © 2012, Sociedad Química de México.
A computational study on the hydrogen adsorption capacity of various lithium-Doped boron hydrides
Pan S., Giri S., Chattaraj P.K.
Article, Journal of Computational Chemistry, 2012, DOI Link
View abstract ⏷
An aromatic boron hydride B 3H 3 2- and its various Li/Li + doped isomers have been studied at the B3LYP/6-311+G(d) and M06/6-311+G(d) levels of theory to assess their hydrogen storage potential. Different types of interaction energies, reaction enthalpies and reaction electrophilicities associated with the hydrogen adsorption process suggest that B 3H 3 2- itself and some of its Li-decorated analogues may turn out to be effective hydrogen storage material. Nucleus independent chemical shift and conceptual density functional theory based reactivity descriptors lend additional support. The temperature-pressure phase diagram identifies the temperature-pressure zone where the reaction Gibbs free energy for the hydrogen adsorption is negative making it a thermodynamically feasible process. Copyright © 2011 Wiley Periodicals, Inc.
