3c 2e Research Articles

Planar Tetracoordinate Silicon in Si3Cu3− Cluster

AbstractPhotoelectron spectroscopy and theoretical calculations have identified the global minimum structure of the 16‐valence electron Si3Cu3− cluster, which features a planar tetracoordinate silicon (ptSi) in a rhombic arrangement. The Si3 and Cu3 triangles are interconnected by a Si2/Cu2 edge, forming an ordered chain‐like structure. Besides the conventional 2c–2e σ‐bond connecting Si3 and Cu3, the stability of this cluster is reinforced by a delocalized 3c–2e σ‐bond in Cu3 and a π‐bond in Si3. Our study provides experimental confirmation of a planar hypercoordinate heavier Group 14 element, opening possibilities for exploring similar structures in two‐dimensional materials.

On the Bonding Nature in the Crystalline Tri-Thorium Cluster: Core-Shell Syngenetic σ-Aromaticity.

A unique thorium‐thorium bond was observed in the crystalline tri‐thorium cluster [{Th(η8‐C8H8)(μ3‐Cl)2}3{K(THF)2}2]∞, though the claim of σ‐aromaticity for Th3 bond has been questioned. Herein, a new type of core–shell syngenetic bonding model is proposed to describe the stability of this tri‐thorium cluster. The model involves a 3c–2e bond in the Th3 core and a multicentered (ThCl2)3 charge‐shift bond with 12 electrons scattering along the outer shell. To differentiate the strengths of the 3c–2e bond and the charge‐shift bond, the block‐localized wavefunction (BLW) method which falls into the ab initio valence bond (VB) theory is employed to construct a strictly core/shell localized state and its contributing covalent resonance structure for the Th3 core bond. By comparing with the σ‐aromatic H3 + and nonaromatic Li3 +, the computed resonance energies and extra cyclic resonance energies confirm that this Th3 core bond is truly delocalized and σ‐aromatic.

On the Bonding Nature in the Crystalline Tri‐Thorium Cluster: Core‐Shell Syngenetic σ‐Aromaticity

AbstractA unique thorium‐thorium bond was observed in the crystalline tri‐thorium cluster [{Th(η8‐C8H8)(μ3‐Cl)2}3{K(THF)2}2]∞, though the claim of σ‐aromaticity for Th3 bond has been questioned. Herein, a new type of core–shell syngenetic bonding model is proposed to describe the stability of this tri‐thorium cluster. The model involves a 3c–2e bond in the Th3 core and a multicentered (ThCl2)3 charge‐shift bond with 12 electrons scattering along the outer shell. To differentiate the strengths of the 3c–2e bond and the charge‐shift bond, the block‐localized wavefunction (BLW) method which falls into the ab initio valence bond (VB) theory is employed to construct a strictly core/shell localized state and its contributing covalent resonance structure for the Th3 core bond. By comparing with the σ‐aromatic H3+ and nonaromatic Li3+, the computed resonance energies and extra cyclic resonance energies confirm that this Th3 core bond is truly delocalized and σ‐aromatic.

Formation of an air-stable diborane via a stepwise BH3 addition of pyrido[1,2-a]isoindole with H2 evolution.

A novel and air-stable organo(hydro)diborane featuring a five-membered aryl ring supported bridging B-C-B three-centre-two-electron (3c-2e) bond has been reported. Pyrido[1,2-a]isoindole was found to undergo a stepwise BH3 addition reaction, during which a mono-BH3 adduct was formed from a electrophilic addition at the Cγ in pyrido[1,2-a]isoindole. A molecule of hydrogen was eliminated throughout the second step of addition reaction. DFT calculations indicate that the H2 evolution is concerted to the second BH3 addition rather than forming BC before the second BH3 attack.

Computational Investigation of the Mechanism of FLP Catalyzed H2Activation and Lewis Base Assisted Proton Transfer

AbstractDensity functional calculations have been performed to elucidate the mechanism of formation ofN‐phenylbenzylamine (Am) from alkylation of aniline (An) with benzaldehyde (Bz), with the intermediacy of the imine,N‐benzylideneaniline (Im), catalyzed by the triaryl borane (BAr3), B(2,6‐Cl2C6H3)(p‐HC6F4)2, and utilizing H2as the reductant. Our computational investigation reveals that H2activation byBAr3/LB(LB=Lewis Base;An,Im,THForBz) frustrated Lewis pairs (FLPs) proceeds through a stepwise mechanism consisting of initial H2capture followed by heterolytic H−H cleavage, via a 3c–2e H2coordinated borane species. H2activation subsequently leads to a H2dissociated ion pair, overcoming energy barriers (31 kcal/mol at M06‐2X/6‐311++G(d,p)) accessible at the experimental temperature (100 °C). Consequently, the iminium hydridoborate ion pair, [BAr3‐H]−[Im‐H]+, undergoes hydride transfer from boron (−BH) to the imine carbon (−C=N−) and develops the targeted amine. Furthermore, we also identify the crucial role of an unbound aniline and imine which actively participates in proton shuttle mechanism from a coordinated aniline to −C=O of benzaldehyde to generate the desired imine, along with the release of water.

A novel quinoxalinedione-bicapped tri-ruthenium carbonyl cluster [Ru3(μ-H)2(CO)6(μ3-HDCQX)2]: synthesis, characterization, anticancer activity and theoretical investigation of Ru–Ru and Ru–Ligand bonding interactions

A new tri-ruthenium dihydrido cluster, [Ru3(μ-H)2(CO)6(HDCQX)2] (1), in which two HDCQX (H2DCQX = 6,7-dichloroquinoxaline-2,3-dione) ligands cap both faces of the tri-ruthenium triangle similarly through oxygen atoms, has been synthesized and characterized by mass, IR, NMR (1H and 13C) and electronic spectroscopy. DFT calculations, in both gas and solution (CPCM/DMSO) phases, were performed to analyze the electronic structure of the cluster and interpret its vibrational and NMR spectra. The presence of non-equivalent bridging hydrides was assigned by 1H NMR as two singlet resonances at δ −16.85 and −17.67 ppm and confirmed by theoretical DFT-GIAO calculations at δ −12.92 and −14.48 ppm. Time-dependent density functional theory (TD-DFT) calculations, performed to characterize the frontier orbitals and interpret the electronic spectrum, indicated that the lowest-energy transition of 1 originates from a highly delocalized HOMO with Ru–Ru (σ), Ru–CO (π) and Ru–CO (σ*) contributions and ends in the LUMO with Ru1–Ru2–Ru3 (σ*) and QX/QX′ (π*) anti-bonding character. The natural transition orbital (NTO) corresponding to this transition revealed a decrease of the electron density on the Ru1Ru2(CO)4 fragment and an increase of the electron density on the Ru1–Ru2–Ru3 unit and the HDCQX ligands. The Ru–Ru and Ru–ligand bonding was studied by the quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) methods. Both confirmed the presence of unique bonding between the Ru1 and Ru2 atoms and not between Ru3 and the other Ru atoms. While the QTAIM assigned the Ru1–Ru2 bonding as a transit between pure closed-shell and pure shared-shell interactions, the ELF analysis suggested that this bond is dominated by a fluctuation of electron density. The covariance analysis indicated that the core basin populations C(Ru1) and C(Ru2) account for 76% of the V(Ru1, Ru2) variance. Natural bond orbital (NBO) analysis of the Ru3(µ-H)2O4(CO)6 unit indicated a 2c–2e bonding type for the Ru–O, Ru–C and Ru–Ru bonds with orbital occupations of 1.88–1.95, 1.95–1.96, and 1.57 e, respectively, and a 3c–2e bonding for the Ru–H–Ru bonds with an orbital occupation of ca. 1.7 e. NBO second-order perturbative energy analysis was performed to estimate the strength of the Ru–CO π-back-donation and to establish a correlation between the stabilization energy (E(2)) and the spectroscopic data. Although the calculated stabilization energy (E(2)total) follows the expected trend of decreasing the CO stretching frequency (νCO) and increasing the 13CO chemical shift with the extent of the Ru–CO π-back-donation, the latter parameter was more consistent in estimating the strength of such interactions. The in vitro anticancer activities of the title compound and cisplatin were determined against three common types of human cancer cell lines, HCT-116 (colon), HELA (cervix) and MCF-7 (breast). The IC50 values of 3.18 ± 0.40, 4.21 ± 0.29 and 5.39 ± 0.52 μM, respectively, represent 4- and 2-times improvement in potency relative to cisplatin (12.06 ± 1.28, 10.34 ± 2.08 and 6.29 ± 1.68 μM, respectively) against the HCT-116 and HeLa cancer cell lines.

Electronic structure of triangular M3 (M = B, Al, Ga): nonclassical three-center two electron π bond and σ delocalization.

The small molecule clusters have received more and more attention due to their widespread applications in chemical insulators, explosives, semiconductors and the high energy density materials industry. The electron deficiency of group IIIA elements endows their clusters with interesting properties. In this work, the electronic structures of M3 (M = B, Al, Ga) have been investigated by means of a complete active space self-consistent field (CASSCF) method. The nature of the chemical bond has been analyzed using the quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) analyses. The following conclusions can be drawn: in M3 (M = B, Al, Ga) clusters, two π electrons are shared by three atoms forming a 3c-2e delocalization π bond. Going from B3 to Al3 to Ga3, more and more electrons move from the bond pair to the outside of the M atom, which leads to a gradual enhancement of the delocalization of σ electrons. Aromaticity and the adaptive natural density partitioning (AdNDP) analyses reveal the existence of the 3c-2e π bond and delocalization of σ electrons in the studied systems.

Representation of Three-Center–Two-Electron Bonds in Covalent Molecules with Bridging Hydrogen Atoms

Many compounds can be represented well in terms of the two-center–two-electron (2c–2e) bond model. However, it is well-known that this approach has limitations; for example, certain compounds require the use of three-center–two-electron (3c–2e) bonds to provide an adequate description of the bonding. Although a classic example of a compound that features a 3c–2e bond is provided by diborane, B2H6, 3c–2e interactions also feature prominently in transition metal chemistry, as exemplified by bridging hydride compounds, agostic compounds, dihydrogen complexes, and hydrocarbon and silane σ-complexes. In addition to being able to identify the different types of bonds (2c–2e and 3c–2e) present in a molecule, it is essential to be able to utilize these models to evaluate the chemical reasonableness of a molecule by applying the octet and 18-electron rules; however, to do so requires determination of the electron counts of atoms in molecules. Although this is easily achieved for molecules that possess only 2c–2e bonds, the situation is more complex for those that possess 3c–2e bonds. Therefore, this article describes a convenient approach for representing 3c–2e interactions in a manner that facilitates the electron counting procedure for such compounds. In particular, specific attention is devoted to the use of the half-arrow formalism to represent 3c–2e interactions in compounds with bridging hydrogen atoms.

Fluxional Bonds in Tubular Molecular Rotors B3-[Ta@B18] and B4-[Ta@B18]+ in 18-Electron Configurations

Boron and metal-doped boron nanoclusters possess unique fluxional behaviors in dynamics. Detailed bonding analyses performed in this work indicate that, similar to the experimentally observed B2-[Ta@B18]− (1), the theoretically predicted tubular molecular rotors B3-[Ta@B18] (2) and B4-[Ta@B18]+ (3) possess typical fluxional 4c–2e and 3c–2e σ-bonds atop the Ta-centered [Ta@B18] double-ring tube between the Bn unit (n = 3, 4) and upper B9 ring, unveiling the fluxional bonding nature of the Bn-[Ta@B18]q complex series (n = 2–4, q = n − 3) which follow the 18-electron rule in different charge states. Chiral conversions via pseudo-rotations are observed in the fluxional processes between the Bn unit (n = 3, 4) and Ta-centered [Ta@B18] double-ring tube.

Multicenter Bonding and the Electron Deficient Molecules with Special Emphasis to Boron and Aluminium Compounds

Compounds that contain fewer valence electrons than valence orbitals are referred to as electron deficient. In order to engage all of their valence orbitals in bonding, the atoms in electron deficient compounds sometimes form three-center two-electron (3c–2e) bonds. In a 3c–2e bond, the two electrons are shared by three atoms with each atom contributing one orbital to the formation of the bond. Essentially, this notion of a 3c–2e bond, with two electrons delocalized over three atoms, can be thought of in terms of resonance. Though the bonding pattern of these types of bond has been discovered, many questions remain unresolved. In this work, we are trying to explore computationally the actual bonding description in these types of complexes. Here we are trying to compare the extent of interaction between the two central atoms either directly or through bridge. As well as we are trying to examine the difference of reactivity of the bridgehead and terminal atom due to their different bonding pattern, we are trying to address all these questions for hydrido, halo and methyl compounds of two non-metals viz. B and Al.

Elucidating metal hydride reactivity using late transition metal boryl and borane hydrides: 2c–2e terminal hydride, 3c–2e bridging hydride, and 3c–4e bridging hydride

A general trend for the hydrogenation reactivity of metal hydride(s): 3c–4e bridging hydride > 2c–2e terminal hydride > 3c–2e bridging hydride.

Global minimum beryllium hydride sheet with novel negative Poisson's ratio: first-principles calculations.

As one of the most prominent metal-hydrides, beryllium hydride has received much attention over the past several decades, since 1978, and is considered as an important hydrogen storage material. By reducing the dimensionality from 3 to 2, the beryllium hydride monolayer is isoelectronic with graphene; thus the existence of its two-dimensional (2D) form is theoretically feasible and experimentally expected. However, little is known about its 2D form. In this work, by a global minimum search with the particle swarm optimization method via density functional theory computations, we predicted two new stable structures for the beryllium hydride sheets, named α–BeH2 and β–BeH2 monolayers. Both structures have more favorable thermodynamic stability than the recently reported planar square form (Nanoscale, 2017, 9, 8740), due to the forming of multicenter delocalized Be–H bonds. Utilizing the recently developed SSAdNDP method, we revealed that three-center-two-electron (3c–2e) delocalized Be–H bonds are formed in the α–BeH2 monolayer, while for the β–BeH2 monolayer, novel four-center-two-electron (4c–2e) delocalized bonds are observed in the 2D system for the first time. These unique multicenter chemical bonds endow both α– and β–BeH2 with high structural stabilities, which are further confirmed by the absence of imaginary modes in their phonon spectra, the favorable formation energies comparable to bulk and cluster beryllium hydride, and the high mechanical strength. These results indicate the potential for experimental synthesis. Furthermore, both α– and β–BeH2 are wide-bandgap semiconductors, in which the α–BeH2 has unusual mechanical properties with a negative Poisson's ratio of −0.19. If synthesized, it would attract interest both in experiment and theory, and be a new member of the 2D family isoelectronic with graphene.

A Dimeric Gallium Hydrazide as an Active Lewis Pair – Complexation and Activation of Me2GaH and Various Heterocumulenes

Treatment of the dimeric gallium hydrazide [Me2Ga‐N(2‐Ad)‐NC5H10]2 (5) with Me2GaH resulted in the formation of an adduct 6 by Ga–N bond cleavage and coordination of the metal hydride via a Ga–N and a 3c–2e Ga–H–Ga bond. This reaction reflects the typical behavior of frustrated Lewis pairs. Reactions with heterocumulenes R–N=C=X (R = Ph, CMe3, Dipp, X = O; R = Ph, X = S) or X=C=X (X = O, S) resulted in the formation of the cyclic Ga–N insertion products Me2Ga–N(R)C(O)N(2‐Ad)‐NC5H10 (7a–c), Me2GaS2C‐N(2‐Ad)‐NC5H10 (8) or Me2GaX2C‐N(2‐Ad)‐NC5H10 [X = O (9); S (10)] in moderate to good yields. Three different structural motifs were observed in the solid state: Five‐membered GaNCN2 heterocycles with exocyclic C=O bonds for compounds 7a–c, four‐membered GaSCN or GaSCS heterocycles for compounds 8 and 9 (chelating coordination of the Ga atoms by SCN and CS2 ligands) and an eight‐membered (GaOCO)2 heterocycle for the dimeric CO2 insertion product 10. Treatment of 5 with PhCN or Ph2CO resulted in a completely different reaction and afforded a dimeric Ga imide 11a or an alcoholate 11b. These reactions may start by retro‐hydrogallation with the formation of H10C5N–N=C(C9H14) and Me2GaH and proceed by addition of the metal hydride to the polar multiple bonds of the nitrile or ketone.

Deciphering chemical bonding in BnHn2−(n = 2–17): flexible multicenter bonding

Deciphering flexible multicenter bonding incloso-borane dianions BnHn2−.

High-pressure behavior of α-boron studied on single crystals by X-ray diffraction, Raman and IR spectroscopy

In the present study single crystals of rhombohedral α-B were investigated under pressure to 60GPa by means of single-crystal X-ray diffraction. The bulk modulus of α-B was found to be K=224(7)GPa (K′=3.0(3)). Measurements of interatomic distances as a function of pressure revealed that the intericosahedral two-center two-electron (2c–2e) bonds are almost as stiff as some of intraicosahedral ones. The three-center two-electron (3c–2e) intericosahedral bonds show much higher compliance compared to other bonds in α-B. The vibrational properties of α-B under pressure were investigated by Raman spectroscopy at pressures up to 160GPa and IR spectroscopy at pressures up to 53GPa.

Silicon Tetrahydroborate and Silylene Dihydroborate with Interelement B–H–Si and B═Si Bonds

Reactions of laser-ablated boron atoms with SiH4 molecules gave silylene dihydroborate (H2BSiH2) in excess argon, which rearranged to silicon tetrahydroborate [Si(μ-H)2BH2] upon 300–350 nm irradiation. These reaction products were identified by functional group infrared frequencies, isotopic shifts, and comparison to infrared frequencies calculated by using density functional theory. It is found that H2BSiH2 has planar doublet ground state with electron-deficient B–Si double-bond character. Bonding analysis for Si(μ-H)2BH2 shows that 3c–2e B–H–Si bond is formed by accepting electrons donated from B–H σ bond.

Reactivity of a Monomeric Aluminium Hydrazide towards Isocyanates and Isothiocyanates: Active Lewis Pair Behaviour versus Classical Insertion Reactions

Hydroalumination of H10C5N–N=C(Ad) with HAliBu2 yielded the hydrazide H10C5N–N(AliBu2)Ad (4; Ad = adamantyl, NC5H10 = piperidinyl). Compound 4 is monomeric and contains a highly strained AlN2 heterocycle formed by a donor–acceptor interaction between the Al atom and the β‐N atom of the hydrazine. Two equivalents of the hydride afforded an adduct with an HAliBu2 molecule coordinated to 4 by an Al–N and a 3c–2e Al–H–Al bond (5). tBuN=C=O did not insert into the relatively weak Al←N donor–acceptor bond of 4; instead, insertion into the shorter Al–N(amide) bond yielded five‐membered AlN3C heterocycle 6 with an exocyclic C=O group. PhN=C=O showed a similar reaction, but the product 7 was dimeric in the solid state with two Al–O bonds. Reaction in a 1:2 ratio afforded a unique linear dimer of phenyl isocyanate, which is stabilised by the aluminium hydrazide (8). Isothiocyanates gave thiosemicarbazides 9–11, which have Al atoms coordinated in a chelating manner by formation of AlSCN heterocycles and show fascinating differences in their bonding situation.

Anagostic Interactions under Pressure: Attractive or Repulsive?

Square-planar d(8)-ML4 complexes might display subtle but noticeable local Lewis acidic sites in axial direction in the valence shell of the metal atom. These sites of local charge depletion provide the electronic prerequisites to establish weakly attractive 3c-2e M⋅⋅⋅H-C agostic interactions, in contrast to earlier assumptions. Furthermore, we show that the use of the sign of the (1)H NMR shifts as major criterion to classify M⋅⋅⋅H-C interactions as attractive (agostic) or repulsive (anagostic) can be dubious. We therefore suggest a new characterization method to probe the response of these M⋅⋅⋅H-C interactions under pressure by combined high pressure IR and diffraction studies.

Anagostic Interactions under Pressure: Attractive or Repulsive?

AbstractSquare‐planar d8‐ML4 complexes might display subtle but noticeable local Lewis acidic sites in axial direction in the valence shell of the metal atom. These sites of local charge depletion provide the electronic prerequisites to establish weakly attractive 3c–2e M⋅⋅⋅HC agostic interactions, in contrast to earlier assumptions. Furthermore, we show that the use of the sign of the 1H NMR shifts as major criterion to classify M⋅⋅⋅HC interactions as attractive (agostic) or repulsive (anagostic) can be dubious. We therefore suggest a new characterization method to probe the response of these M⋅⋅⋅HC interactions under pressure by combined high pressure IR and diffraction studies.

Supertetrahedral B80H20, C80H20, and Al80H20 analogs of dodecahedrane and their substituted molecules

Stability of the new B80H20, C80H20, and Al80H20 frame complexes containing tetrahedral B4H, C4H, and Al4H fragments instead of the C–H fragments at the vertices of the dodecahedron scaffold, respectively, is predicted based on the DFT formalism (B3LYP/6-311+G(d, p)). AdNDP, NBO, and Elf analyses have shown that the chemical bonding in C80H20 can be described in terms of classical 2c–2e C–C σ bonds, while electron-deficient B80H20 and Al80H20 analogs are shown to exhibit both 2c–2e and 3c–2e σ bonds, responsible for the bonding between and within tetrahedral fragments, respectively.