Charge-shift Bond Research Articles

Double-boron heterocyclic carbenes: a computational study of Diels-Alder reactions.

An aromatic boron-containing organic compound, C2B2H2, with an unusual CC bond was experimentally synthesized in 2017. Here we investigate the structure and bonding nature of C2B2H2 and its derivatives C2B2R2 using DFT and VB theory. Although the CC bond in C2B2R2 consists of a π bond and a charge-shift (CS) bond, C2B2F2 has the lowest LUMO energy and its LUMO is similar to that of ethylene, suggesting that C2B2F2 can be an ideal dienophile for the Diels-Alder reaction. Subsequently, the mechanism and stereoselectivity of the Diels-Alder reaction of C2B2F2 with 5-substituted cyclopentadienes are studied. Computations demonstrate that these Diels-Alder reactions are feasible thermodynamically and kinetically. The stereoselectivity and distortion angles of C2B2R2 exhibit linear correlations with the electronegativity difference between the two substituents bonded to the C(sp3) of cyclopentadiene, suggesting that the stereoselectivity of related Diels-Alder reaction products can be modulated by the substitution of cyclopentadiene. Considering the current interest in boron neutron capture therapy (BNCT), we design six BNCT drugs through the Diels-Alder reaction of C2B2F2 with dienes containing peptide fragments. Thus, we demonstrate a new method for designing three-in-one BNCT drugs via the facile Diels-Alder reaction.

Re‐establishing the Lost Covalency of the Inverted Bond in [1.1.1]Propellane

AbstractThe nature of the bridging C−C bond in [1.1.1]propellane has been a topic of much debate, and finally it has been established to be a charge‐shift bond. But the question remains: Can we restore the “lost” covalency of this charge shift bond? Herein we have attempted to investigate the nature of the bridging C−C bond, by replacing the CH2 groups at the wing positions with NH, O, and S. Detailed analyses using atoms in molecules (AIM) and electron localization function (ELF) reveal that the bridging bond emerges as a covalent bond upon substitution with S.

Actinide inverse trans influence versus cooperative pushing from below and multi-center bonding

Actinide-ligand bonds with high multiplicities remain poorly understood. Decades ago, an effect known as 6p pushing from below (PFB) was proposed to enhance actinide covalency. A related effect—also poorly understood—is inverse trans influence (ITI). The present computational study of actinide-ligand covalent interactions with high bond multiplicities quantifies the energetic contributions from PFB and identifies a hitherto overlooked fourth bonding interaction for 2nd-row ligands in the studied organometallic systems. The latter are best described by a terminal O/N ligand exhibiting quadruple bonding interactions with the actinide. The 4th interaction may be characterized as a multi-center or charge-shift bond involving the trans ligand. It is shown in this work that the 4th bonding interaction is a manifestation of ITI, assisted by PFB, and provides a long-sought missing piece in the understanding of actinide chemistry.

Synthesis of Bicyclo[1.1.1]pentane (BCP)-Based Straight-Shaped Diphosphine Ligands.

[1.1.1]Propellane, which is structurally simple and compact, exhibits promising potential for the synthesis of disubstituted straight-shaped bicyclo[1.1.1]pentane (BCP) compounds by manipulation of its highly reactive internal C-C bond. BCPs are considered to be isosteres of 1,4-disubstituted benzenes, which have found broad applications in the areas of functional molecules and drug discovery. The internal C-C single bond of [1.1.1]propellane is regarded as a charge-shift bond, which can be readily cleaved by radical means to construct BCPs. We herein report a novel synthetic method for (un)symmetric diphosphines based on the BCP motif, which can be interpreted as isosteres of 1,4-bis(diphenylphosphino)benzenes. The obtained BCP-diphosphine derivatives were used to generate a straight-shaped Au complex and an Eu-based coordination polymer.

An atoms-in-molecules characterization of the nature of the OO bond in peroxides and nitroxide dimers.

The quantum theory of atoms-in-molecules (QTAIM) method is used to examine the OO bond in peroxides (RO-OR) and nitroxide dimers (R2 NO-ONR2 ), including Fremy's salt. The electron density (ρ), electron kinetic energy density [K(ρ)], and Laplacian of the electron density (∇2 ρ) at bond critical points characterize the nature of the OO bond. The data distinguish OO bonding of two kinds. Large values of ρ and positive ∇2 ρ and K(ρ) suggest that simple peroxides have charge-shift bonds. Nitroxide dimers, with smaller ρ, positive ∇2 ρ, and near-zero K(ρ), show a lack of shared electron density, suggesting there is no conventional OO bonding in these molecules. QTAIM analysis at the B3LYP/6-311+G(d,p) level of theory gives results in agreement with valence-bond theory and X-ray diffraction characterizations of peroxide OO bonds as charge-shift bonds. In contrast, CCSD/cc-pVDZ calculations fail to agree with previous results because of an insufficient, single-determinant treatment of the charge-shift bond.

Gold(I)···Lanthanide(III) Bonds in Discrete Heterobimetallic Compounds: A Combined Computational and Topological Study.

The chemical nature of the ligand-unsupported gold(I)-lanthanide(III) bond in the proposed [LnIII(η5-Cp)2][AuIPh2] (Ln-Au; LnIII = LaIII, EuIII, or LuIII; Cp = cyclopentadienide; Ph = phenyl) models is examined from a theoretical viewpoint. The covalent bond-like Au-Ln distances (Au-La, 2.95 Å; Au-Eu, 2.85 Å; Au-Lu, 2.78 Å) result from a strong interaction between the oppositely charged fragments (ΔEintMP2 > 600 kJ mol-1), including the aforementioned metal-metal bond and additional LnIII-Cipso and C-H···π interactions. The Au-Ln bond has been characterized as a chemical bond rather than a strong metallophilic interaction with the aid of energy decomposition analysis, interaction region indicator, and quantum theory of atoms in molecules topological tools. The chemical nature of the Au-Ln bond cannot be fully ascribed to a covalent or an ionic model; an intermediate situation or a charge shift bond is proposed. The [AuIPh2]- anion has also been identified as a suitable lanthanide(III) emission sensitizer for La-Au and Lu-Au.

Exploring the nature of electron-pair bonds: an energy decomposition analysis perspective

In this paper, the nature of electron-pair bonds is explored from an energy decomposition perspective. The recently developed valence bond energy decomposition analysis (VB-EDA) scheme is extended for the classification of electron-pair bonds, which divides the bond dissociation energy into frozen, reference state switch, quasi-resonance and polarization terms. VB-EDA investigations are devoted to a series of electron-pair bonds, including the covalent bonds (H–H, H3C–CH3, H3C–H, and H2N–NH2), the ionic bonds (Na–Cl, Li–F), the charge-shift (CS) bonds (HO–OH, F–F, Cl–Cl, Br–Br, H–F, F–Cl, H3Si–F and H3Si–Cl), and the inverted central carbon–carbon bond in [1.1.1] propallene. It is shown that the VB-EDA approach at the VBSCF level is capable of predicting the characters of the electron-pair bonds. The perspective from VB-EDA illustrates that a relatively high value of quasi-resonance term indicates a CS bond while a large portion of polarization term suggests a classical covalent bond.

Partial Double Metal-Carbon Bonding Model in Transition Metal Methyl Compounds.

The chemical bond between a transition metal and a methyl group (M-CH3) is typically defined as a single covalent bond, which is of fundamental significance and general interest in understanding the structural properties and reactivity of transition metal alkyl compounds. Herein, we demonstrate that the M-CH3 bonding involves varying σ and π components and thus should be best described in terms of the partial double M═CH3 bond. The often-neglected π bonding stems from an occupied π-symmetric orbital of the methyl group comprising all three C-H σ bonds (but one C-H' contributes more than the other two) and a vacant low-lying metal d(π) orbital, and is associated with the intramolecular C-H'···M agostic effect (i.e., an acute M-C-H' angle and a short H'···M distance), whose origin is still controversial. We quantify the geometric and energetic impacts of the π interaction involved in the M-CH3 bond by explicitly computing the intramolecular πCH' → dM interaction with the ab initio valence bond (VB) theory. Our computations of the ligand-free [TiCH3]3+ and a series of metallocene catalysts provide a direct proof for the presence of the π bonding in M-CH3 bonds, which is the cause for the agostic effect. The partial double M═CH3 bonding model is not only validated by a range of bonding analyses including VB self-consistent field (VBSCF)-based energy decomposition and quantum theory of atoms in molecules (QTAIM) but also authenticated by the specific activity of double M═CH3 bonds in the C-H activation and olefin insertion. More importantly, the σ bond gradually switches from a classical covalent bond to a novel charge-shift bond with the π bonding becoming increasingly significant. We anticipate that the recognition of the π interaction between electrophilic metal centers and C-H bonds can benefit the understanding of the nature of metal-carbon bonds in transition metal ethyl, alkyl, and carbene compounds.

HSH-C10: A new quasi-2D carbon allotrope with a honeycomb-star-honeycomb lattice

Two-dimensional (2D) materials with honeycomb, kagome or star lattice have been intensively studied because electrons in such lattices could give rise to exotic quantum effects. In order to improve structural diversity of 2D materials to achieve unique properties, here we propose a new quasi-2D honeycomb-star-honeycomb (HSH) lattice based on first-principles calculations. A carbon allotrope named HSH-C10 is designed with the HSH lattice, and its mechanical properties have been intensively investigated through total energy, phonon dispersion, ab initio molecular dynamic simulations, as well as elastic constants calculations. Besides the classical covalent bonds, there is an interesting charge-shift bond in this material from the chemical bonding analysis. Additionally, through the analysis of electronic structure, HSH-C10 is predicted to be a semiconductor with a direct band gap of 2.89 eV, which could combine the desirable characteristics of honeycomb and star lattice. Importantly, by modulating coupling strength, a flat band near the Fermi level can be obtained in compounds HSH-C6Si4 and HSH-C6Ge4, which have potential applications in superconductivity. Insight into such mixed lattice would inspire new materials with properties we have yet to imagine.

Nature of the Trigger Linkage in Explosive Materials Is a Charge-Shift Bond.

Explosion begins by rupture of a specific bond, in the explosive, called a trigger linkage. We characterize this bond in nitro-containing explosives. Valence-bond (VB) investigations of X-NO2 linkages in alkyl nitrates, nitramines, and nitro esters establish the existence of Pauli repulsion that destabilizes the covalent structure of these bonds. The trigger linkages are mainly stabilized by covalent-ionic resonance and are therefore charge-shift bonds (CSBs). The source of Pauli repulsion in nitro explosives is unique. It is traced to the hyperconjugative interaction from the oxygen lone pairs of NO2 into the σ(X-N)* orbital, which thereby weakens the X-NO2 bond, and depletes its electron density as X becomes more electronegative. Weaker trigger bonds have higher CSB characters. In turn, weaker bonds increase the sensitivity of the explosive to impacts/shocks which lead to detonation. Application of the analysis to realistic explosives supports the CSB character of their X-NO2 bonds by independent criteria. Furthermore, other families of explosives also involve CSBs as trigger linkages (O-O, N-O, Cl-O, N-I, etc. bonds). In all of these, detonation is initiated selectively at the CSB of the molecule. A connection is made between the CSB bond-weakening and the surface-electrostatic potential diagnosis in the trigger bonds.

Relating Bond Strength and Nature to the Thermodynamic Stability of Hypervalent Togni-Type Iodine Compounds.

The bond strength and nature of a set of 32 Togni-like reagents have been investigated at the M062X/def2-TZVP(D) level of theory in acetonitrile described with the SMD continuum solvent model, to rationalize the main factors responsible for their thermodynamic stability in different conformations, and trifluoromethylation capabilities. For the assessment of bond strength, we utilized local stretching force constants and associated bond strength orders, complemented with local features of the electron density to access the nature of the bonds. Bond dissociation energies varied from 31.6 to 79.9 kcal/mol depending on the polarizing power of the ligand trans to CF3 . Based on the analysis of the Laplacian of the density, we propose that the charge-shift bond character plays an important role in the stability of the molecules studied, especially for those containing I-O bonds. New insights on the trans influence and on possible ways to fine-tune the stability of these reagents are provided.

Theoretical Investigation on the Elusive Reaction Mechanism of Spirooxindole Formation Mediated by Cytochrome P450s: A Nascent Feasible Charge-Shift C-O Bond Makes a Difference.

Spirooxindoles are pivotal biofunctional groups widely distributed in natural products and clinic drugs. However, construction of such subtle chiral skeletons is a long-standing challenge to both organic and bioengineering scientists. The knowledge of enzymatic spirooxindole formation in nature may inspire rational design of new catalysts. To this end, we presented a theoretical investigation on the elusive mechanism of the spiro-ring formation at the 3-position of oxindole mediated by cytochrome P450 enzymes (P450). Our calculated results demonstrated that the electrophilic attack of CpdI, the active species of P450, to the substrate, shows regioselectivity, i.e., the attack at the C9 position forms a tetrahedral intermediate involving an unusual feasible charge-shift C9δ+-Oδ- bond, while the attack at the C1 position forms an epoxide intermediate. The predominant route is the first route with the charge-shift bonding intermediate due to holding a relatively lower barrier by >5 kcal mol-1 than the epoxide route, which fits the experimental observations. Such a delocalized charge-shift bond facilitates the formation of a spiro-ring mainly through elongation of the C1-C9 bond to eliminate the aromatization of the tricyclic beta-carboline. Our theoretical results shed profound mechanistic insights for the first time into the elusive spirooxindole formation mediated by P450s.

The electronic structure of carbones revealed: insights from valence bond theory.

In this contribution, we studied the OC-C bond in carbon suboxide and related allene compounds using the valence bond method. The nature of this bond has been the subject of debate, whether it is a regular, electron sharing bond or a dative bond. We compared the nature of this bond in carbon suboxide with the gold-CO bond in Au(CO)2+, which is a typical dative bond, and we studied its charge-shift bond character. We found that the C-CO bond in carbon suboxide is unique in the sense that it cannot be assigned as either a dative or electron sharing bond, but it is an admixture of electron sharing and dative components, together with a high contribution of ionic character. These findings provide a clear basis for distinguishing the commonly found dative bonds between ligands and transition metals and the present case of what may be described as coordinative bonding to carbon.

Charge-Shift Bonding in Xenon Hydrides: An NBO/NRT Investigation on HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CCH, CN) via H-Xe Blue-Shift Phenomena.

Noble-gas bonding represents curiosity. Some xenon hydrides, such as HXeY (Y = Cl, Br, I) and their hydrogen-bonded complexes HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CN, CCH), have been identified in matrixes by observing H-Xe frequencies or its monomer-to-complex blue shifts. However, the H-Xe bonding in HXeY is not yet completely understood. Previous theoretical studies provide two answers. The first one holds that it is a classical covalent bond, based on a single ionic structure H-Xe+ Y−. The second one holds that it is resonance bonding between H-Xe+ Y− and H− Xe+-Y. This study investigates the H-Xe bonding, via unusual blue-shifted phenomena, combined with some NBO/NRT calculations for chosen hydrogen-bonded complexes HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CN, CCH). This study provides new insights into the H-Xe bonding in HXeY. The H-Xe bond in HXeY is not a classical covalent bond. It is a charge-shift (CS) bond, a new class of electron-pair bonds, which is proposed by Shaik and Hiberty et al. The unusual blue shift in studied hydrogen-bonded complexes is its H-Xe CS bonding character in IR spectroscopy. It is expected that these studies on the H-Xe bonding and its IR spectroscopic property might assist the chemical community in accepting this new-class electron-pair bond concept.

Laplacian of the Hamiltonian Kinetic Energy Density as an Indicator of Binding and Weak Interactions.

The kinetic energy is the center of a controversy between two opposite points of view about its role in the formation of a chemical bond. One school states that a lowering of the kinetic energy associated with electron delocalization is the key stabilization mechanism of covalent bonding. In contrast, the opposite school holds that a chemical bond is formed by a decrease in the potential energy due to a concentration of electron density within the binding region. In this work, a topographic analysis of the Hamiltonian Kinetic Energy Density (KED) and its laplacian is presented to gain more insight into the role of the kinetic energy within chemical interactions. This study is focused on atoms, diatomic and organic molecules, along with their dimers. In addition, it is shown that the laplacian of the Hamiltonian KED exhibits a shell structure in atoms and that their outermost shell merge when a molecule is formed. A covalent bond is characterized by a concentration of kinetic energy, potential energy and electron densities along the internuclear axis, whereas a charge-shift bond is characterized by a fusion of external concentration shells and a depletion in the bonding region. In the case of weak intermolecular interactions, the external shell of the molecules merge into each other resulting in an intermolecular surface comparable to that obtained by the Non-covalent interaction (NCI) analysis.

Switchable dual bonding nature in silabicyclo[1.1.0]butanes that exhibit bond stretch isomerism

AbstractSilicon analogues of bicyclo[1.1.0]butanes are known to exhibit a bond stretch isomerism where the bridgehead SiSi bond changes in both length and geometry. The bridgehead silicon atom of long bond (lb) isomers exhibits an unusual geometry where all the bonds are located on the same hemisphere, making the geometry of the bridgehead SiSi bond the so‐called inverted σ bond in contrast with the typical σ bond geometry found in the short bond (sb) isomers. Herein, the bonding nature of bridgehead SiSi bonds was theoretically investigated in detail using valence bond theory calculation at SL‐BOVB/6‐31G(d) level of theory. Our calculation demonstrates that the bond type of the bridgehead bond depends on the geometry around the bridgehead atoms with those in sb and lb isomers classified as covalent (COV) and charge‐shift (CS) bonds, respectively. The different bond types between two bond stretch isomer indicate that the bonding nature is switchable in one molecule. The detailed examination of the bond stretch isomerization in parent 1,3‐disilabicyclo[1.1.0]butane demonstrated that the enhanced CS bond nature in the lb isomer is mainly attributed to the geometry inversion of the σ bond rather than the bond elongation. This switchable dual bonding nature provides an important platform to compare covalent and CS bond on equal footing for both theoretical and experimental investigations.

Giant piezoelectricity in B/N doped 4,12,2-graphyne

The effects of boron (B) and nitrogen (N) substitutions in 4,12,2-graphyne on its geometric structure and mechanical as well as electronic properties have been systematically investigated with the aid of density functional theory (DFT). The trend in the elastic properties of the substituted systems is determined by the doping positions and the type of the dopants. The Bader charge analysis reveals that the N dopant at the sp-site destroys the acetylenic linkage in 4,12,2-graphyne, but instead tends to form a polar bond, or even possibly a charge-shift bond. In particular, an obvious in-plane piezoelectricity is induced by foreign atom substitutions owing to the deformation of the pristine square symmetry. The electron and electronic thermal conductivities are also estimated by performing the Boltzmann's semiclassical transport calculations.

The directional bonding of [1.1.1]propellane with next generation QTAIM

We investigated the [1.1.1]propellane molecule using the 3-D bond-path framework set B and the stress tensor Bσ within the quantum theory of atoms in molecules (QTAIM). The controversial axial bond was determined to be a charge-shift bond comprising significant bond metallicity that correlated with values of the bond stiffness S < 1. The influence of the axial bond on the neighboring bonding is quantified in terms of unexpectedly low bond stiffness S values and a new measure of charge-shift bonding, the polarizability P. Consistency of these results was found at the MP2, CCSD and B3LYP theory levels.

Structures, metallophilic interactions and electronic excitation energy of linear metal chain complexes PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3, a theoretical investigation

The structures, metallophilic interactions and electronic excitation energy of linear metal chain complexes PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 (m + n = 2–5) (1–18) have been investigated by using MP2 and density functional theory (DFT) methods. Calculated results indicated that geometries of PdmPtn(PH2CH2PH2)3 (m + n = 2) (1–3) optimized by SVWN5 method are well consistent with those by MP2 method and experiment. Then, SVWN5 functional was used to the investigation of the large system of PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 complexes. The lowest energy structures of complexes 1–18 optimized by SVWN5 method are singlet. The lengths of Pd∙∙∙Pd and Pt∙∙∙Pt of Pdm[PH2(CH2PH)m-2CH2PH2]3 (m > 2) and Ptn[PH2(CH2PH)n-2CH2PH2]3 (n > 2) complexes at SVWN5 level are shorter than those of Pd2(PH2CH2PH2)3 and Pt2(PH2CH2PH2)3, indicating the interactions of Pd∙∙∙Pd and Pt∙∙∙Pt increase with the increase of the size of Pdm[PH2(CH2PH)m-2CH2PH2]3 and Ptn[PH2(CH2PH)n-2CH2PH2]3 complexes. Pd atoms in the mixed PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 complexes are favorable to locate at the ends of the metal chain. The lengths of Pd∙∙∙Pd and Pt∙∙∙Pt bonds in the mixed metal chain of PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 complexes have no significantly variation compared with those of the Pdm+n[PH2(CH2PH)m+n-2CH2PH2]3 and Ptm+n[PH2(CH2PH)m+n-2CH2PH2]3. But, the distances of Pd∙∙∙Pt of the mixed metal chain of PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 (m + n > 2) complexes are shorter than those of PdPt(PH2CH2PH2)3, showing the interaction between Pd and Pt increases as the increase of m + n. The WBIs of Pd∙∙∙Pd, Pd∙∙∙Pt and Pt∙∙∙Pt of complexes 1–18 are in the range of 0.22–0.25, 0.27–0.29 and 0.31–0.35, respectively, thus, there are weak interactions between adjacent transition metals in complexes 1–18. QTAIM analysis indicates that the interactions of Pd∙∙∙Pd, Pd∙∙∙Pt and Pt∙∙∙Pt of complexes 1–18 are not covalent, ionic or charge-shift bond. The gradient isosurfaces of complexes 1–18 show that the interaction between the adjacent transition metals is metallophilic interactions. The first electronic excitation energy of complexes 1–18 associated with HOMO → LUMO transition nonlinearly increases with m+n. In addition, the first electronic excitation energies of PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 complexes follow the order of Pdm+n[PH2(CH2PH)m+n-2CH2PH2]3 < PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 < Ptm+n[PH2(CH2PH) m+n-2CH2PH2]3.

Experimental observation of charge-shift bond in fluorite CaF2.

On the basis of a multipole refinement of single-crystal X-ray diffraction data collected using an Ag source at 90 K to a resolution of 1.63 Å-1, a quantitative experimental charge density distribution has been obtained for fluorite (CaF2). The atoms-in-molecules integrated experimental charges for Ca2+ and F- ions are +1.40 e and -0.70 e, respectively. The derived electron-density distribution, maximum electron-density paths, interaction lines and bond critical points along Ca2+...F- and F-...F- contacts revealed the character of these interactions. The Ca2+...F- interaction is clearly a closed shell and ionic in character. However, the F-...F- interaction has properties associated with the recently recognized type of interaction referred to as `charge-shift' bonding. This conclusion is supported by the topology of the electron localization function and analysis of the quantum theory of atoms in molecules and crystals topological parameters. The Ca2+...F- bonded radii - measured as distances from the centre of the ion to the critical point - are 1.21 Å for the Ca2+ cation and 1.15 Å for the F- anion. These values are in a good agreement with the corresponding Shannon ionic radii. The F-...F- bond path and bond critical point is also found in the CaF2 crystal structure. According to the quantum theory of atoms in molecules and crystals, this interaction is attractive in character. This is additionally supported by the topology of non-covalent interactions based on the reduced density gradient.