A computational study of the reactivities of four-membered heavy carbene systems

The potential energy surfaces for the chemical reactions of four-membered N-heterocyclic group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ). Five four-membered group 14 heavy carbene species, (i-Pr)(2) NP(NR)(2) E:, in which E = C, Si, Ge, Sn, and Pb, were chosen as the model reactants in this work. Also, four kinds of chemical reactions, C-H bond insertion, water addition, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these group 14 four-membered N-heterocyclic carbene species. Basically, our present theoretical work predicts that the larger the ∠NEN bond angle of the four-membered group 14 heavy carbene species, the smaller the singlet-triplet splitting, the lower the activation barrier, and, in turn, the more rapid, its chemical reactions to various chemical species. Moreover, our theoretical investigations suggest that the relative carbenic reactivity decreases in the order: C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is its four-membered carbene toward chemical reactions. As a result, our results predict that the four-membered group 14 heavy carbene species (E = Si, Ge, Sn, and Pb) should be more kinetically stable than the observed carbene species and, thus, can be also readily synthesized and isolated at room temperature. Furthermore, the singlet-triplet energy splitting of the four-membered group 14 carbene species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.     

Theoretical investigations of the reactivities of cationic six-membered carbene analogues of group 14 elements

The potential energy surfaces for the chemical reactions of cationic six-membered group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ) and CCSD (CCSD/LANL2DZ//B3LYP/LANL2DZ) methods. Five six-membered group 14 cationic heavy carbene species, [HC(CMeNPh)2E:](+), where E = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reaction, C-H bond insertion, multiple bond cycloaddition, dimerization, and O-H bond insertion, have been used to study the chemical reactivities of these group 14 cationic carbene species. Basically, our present theoretical work predicts that the larger the angle NEN bond angle and the smaller the singlet-triplet splitting of the carbene, the lower its activation barriers will be and, in turn, the more rapid are its chemical reactions with other species. Moreover, the theoretical investigations suggest that the relative carbenic reactivity decreases in the order C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is its cationic carbene toward chemical reaction. As a result, we predict that the cationic six-membered group 14 carbene species (E = C, Si, Ge, Sn, and Pb) should be stable, readily synthesized, and isolated at room temperature. Our computational results are in good agreement with the available experimental observations. Furthermore, the singlet-triplet energy splitting of the carbene, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict its reactivities. The results obtained allow a number of predictions to be made.     

Reactivity for boryl(phosphino)carbenyl carbene analogues with group 14 elements (C, Si, Ge, Sb, and Pb) as a heteroatom: a theoretical study

The potential energy surfaces for the chemical reactions of group 14 carbenes have been studied using density functional theory (B3LYP/LANL2DZ). Five boryl(phosphino)-based carbene (B-?-P) species, where ? = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reactions; intramolecular 1,2-migration, water insertion, alkene cycloaddition, and intermolecular dimerization, have been used to study the chemical reactivities of these group 14 carbenes. The present theoretical investigations suggest that the relative carbenic reactivity decreases in the order C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is the boryl(phosphino)-based B-?-P species towards chemical reactions. Our theoretical findings thus demonstrate that all boryl(phosphino)-based carbenes are isolable at room temperature because they are quite inert to chemical reactions, except that they are also moisture-sensitive molecules. Furthermore, the singlet-triplet energy splitting of the B-?-P, as described in the configuration mixing model attributed to the work of Pross and Shaik, can serve as a diagnostic tool for a better understanding and predicting of their chemical reactivities, kinetically and thermodynamically. The results obtained allow a number of predictions to be made.     

Theoretical investigation of the mechanisms for the reaction of fused tricyclic dimetallenes containing highly strained E═E (E = C, Si, Ge, Sn, and Pb) double bonds

The potential energy surfaces for the reactions of fused tricyclic dimetallenes that feature a highly strained E═E double bond, Rea-E═E, where E = C, Si, Ge, Sn, and Pb, were studied using density functional theory (B3LYP/LANL2DZ). Three types of chemical reactions (i.e., a self-isomerization reaction, a [2 + 2] cycloaddition with a ketone and a methanol 1,2-addition reaction) were used to determine the reactivity of the Rea-E═E molecules. The theoretical findings reveal that the smaller the singlet-triplet splitting of the Rea-E═E, the lower are its activation barriers and, in turn, the more rapid are its chemical reactions with other chemical molecules. Theoretical observations suggest that the relative reactivity increases in the following order: C═C ? Si═Si < Ge═Ge < Sn═Sn < Pb═Pb. Namely, the smaller the atomic weight of the group 14 atom (E), the smaller is the atomic radius of E and the more stable is its fused tricyclic Rea-E═E to chemical reaction. It is thus predicted that the fused tricyclic Rea-C═C and Rea-Si═Si molecules should be stable and readily synthesized and isolated at room temperature. The computational results show good agreement with the available experimental observations. The theoretical results obtained from this work allow a number of predictions to be made.     

Theoretical study of addition reactions of heavy carbenes to carbon and boron nitride nanotubes

In an effort to gain insight into the activation energies and reaction enthalpies of the chemical functionalization of carbon and boron nitride nanotubes, calculations using density functional theory have been carried out for the cycloaddition of a heavy carbene to a single-walled carbon (SWCNT; C(130)H(20)) and a boron nitride (SWBNNT; B(65)N(65)H(20)) nanotube. The (CH(3))(2)X + SWCNT and (CH(3))(2)X + SWBNNT (X = C, Si, Ge, Sn, and Pb) reactions are the subject of the present study. All the stationary points were determined at the B3LYP/LANL2DZ level of theory. The major conclusions that can be drawn from this work are as follows: (i) Considering both the activation barrier and reaction enthalpy based on the model calculations presented here, it is found that the order of (CH(3))(2)X reactivity is X = C > Si > Ge > Sn > Pb, irrespective of whether cycloaddition is to a SWCNT or a SWBNNT sidewall. That is to say, (CH(3))(2)C and (CH(3))(2)Si can readily add to the sidewalls of SWCNT and SWBNNT, whereas (CH(3))(2)Ge, (CH(3))(2)Sn, and (CH(3))(2)Pb are unreactive. (ii) Since the chemical reactivities of SWCNT and SWBNNT sidewalls closely resemble those of the small C(16)H(10) and B(8)N(8)H(10) molecules, at least in a qualitative sense, the use of the above small molecules as models is sufficient to provide qualitatively correct results. (iii) Our theoretical observations indicate that all the (5,5) SWCNT and SWBNNT cycloadducts favor opened rather than closed three-membered ring structures. (iv) The theoretical investigations demonstrate that the singlet-triplet splitting of the carbene species (R(2)X) as well as that of the small model molecules can be used as a diagnostic tool to predict the addition reactivities of carbene analogues and sidewalls of various nanotubes, respectively. Moreover, the results obtained in this work allow a number of predictions to be made.     

Complexes of an anionic gallium(I) N-heterocyclic carbene analogue with group 14 element(II) fragments: synthetic, structural and theoretical studies

The reactions of the anionic gallium(I) N-heterocyclic carbene (NHC) analogue, [K(tmeda)][:Ga{[N(Ar)C(H)]2}], Ar = C6H3Pri2-2,6, with the heavier group 14 alkene analogues, R2E=ER2, E = Ge or Sn, R = -CH(SiMe3)2, have been carried out. In 2:1 stoichiometries, these lead to the ionic [K(tmeda)][R2EGa{[N(Ar)C(H)]2}] complexes which exhibit long E-Ga bonds. The nature of these bonds has been probed by DFT calculations, and the complexes have been compared to neutral NHC adducts of group 14 dialkyls. The 4:1 reaction of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] with R2Sn=SnR2 leads to the digallyl stannate complex, [K(tmeda)][RSn[Ga{[N(Ar)C(H)]2}]2], presumably via elimination of KR. In contrast, the reaction of the gallium heterocycle with PbR2 affords the digallane4, [Ga{[N(Ar)C(H)]2}]2, via an oxidative coupling reaction. For sake of comparison, the reactions of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] with Ar'2E=EAr'2, E = Ge, Sn or Pb, Ar' = C6H2Pri3-2,4,6, were carried out and led to either no reaction (E = Ge), the formation of [K(tmeda)][Ar'2SnGa{[N(Ar)C(H)]2}] (E = Sn), or the gallium(III) heterocycle, [Ar'Ga{[N(Ar)C(H)]2}] (E = Pb). Salt elimination reactions between [K(tmeda)][:Ga{[N(Ar)C(H)]2}] and the guanidinato group 14 complexes [(Giso)ECl], E = Ge or Sn, Giso = [Pri2NC{N(Ar)}2]-, gave the neutral [(Giso)EGa{[N(Ar)C(H)]2}] complexes. All complexes have been characterized by NMR spectroscopy and X-ray crystallographic studies.     

Probing the aromaticity of unsaturated N-heterocyclic carbenes and their heavy analogues with the EDDB criterion

Russian Chemical Bulletin - Three series of N-heterocyclic carbene analogues including Arduengo-type systems (HCNBut)2E (1; E = C, Si, Ge, Sn), their benzannulated derivatives C6H4(NCH2But)2E (2; E...     

Probing the electronic structure,chemical bonding,and excitation spectra of [CuE]+/0/− (E = 14 group element) diatomics employing DFT and ab initio methods

The electronic structure, chemical bonding, and excitation spectra of neutral, cationic, and anionic diatomic molecules of Cu and 14 group elements formulated as [CuE]^+/0/? (E = C, Si, Ge, Sn, Pb) were investigated by density functional theory (DFT) and time‐dependent (TD)‐DFT methods. The electronic and bonding properties of the diatomics analyzed by natural bond orbital (NBO) analysis approch revealed a clear picture of the chemical bonding in these species. The spatial organization of the bonding between Cu and E atoms in the [CuE]^+/0/? (E = Si, Ge, Sn, Pb) molecules can easily be recognized by the cut‐plane electron localization function representations. Particular emphasis was given on the absorption spectra of the [CuE]^+/0/? which were simulated using the results of TD‐DFT calculations employing the hybrid Coulomb‐attenuating CAM‐B3LYP functional. The absorption bands have thoroughly been analyzed and assignments of the contributing principal electronic transitions associated to individual excitations have been made. © 2012 Wiley Periodicals, Inc.     

Infrared spectra of group 14 hydrides in solid hydrogen: experimental observation of PbH4, Pb2H2, and Pb2H4

Laser-ablated Si, Ge, Sn, and Pb atoms have been co-deposited with pure hydrogen at 3.5 K to form the group 14 hydrides. The initial SiH(2) product reacts completely to SiH(4), whereas substantial proportions of GeH(2), SnH(2), and PbH(2) are trapped in solid hydrogen. Further hydrogen atom reactions form the trihydride radicals and tetrahydrides of Ge, Sn, and Pb. The observation of PbH(4) at 1815 cm(-)(1) and PbD(4) at 1302 cm(-)(1) is in agreement with the prediction of quantum chemical calculations for these unstable tetrahydride analogues of methane. In addition, new absorptions are observed for Pb(2)H(2) and Pb(2)H(4), which have dibridged structures based on quantum chemical calculations.     

Theoretical study on the reactivities of stannylene and plumbylene and the origin of their activation barriers

The potential energy surfaces corresponding to the reactions of heavy carbenes with various molecules were investigated by employing computations at the B3LYP and CCSD(T) levels of theory. To understand the origin of barrier heights and reactivities, the model system (CH3)2X+Y (X=C, Si, Ge, Sn, and Pb; Y=CH4, SiH4, GeH4, CH3OH, C2H6, C2H4, and C2H2) was chosen for the present study. All reactions involve initial formation of a precursor complex, followed by a high-energy transition state, and then a final product. My theoretical investigations suggest that the heavier the X center, the larger the activation barrier, and the less exothermic (or the more endothermic) the chemical reaction. In particular, the computational results show that (CH3)2Sn does not insert readily into C-H, Si-H, C-H, Ge-H, or C-C bonds. It is also unreactive towards C=C bonds, but is reactive towards C identical with C and O-H bonds. My theoretical findings are in good agreement with experimental observations. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. It is demonstrated that the singlet-triplet splitting of a heavy carbene (CH3)2X plays a decisive role in determining its chemical reactivity. The results obtained allow a number of predictions to be made.     

Are N-heterocyclic carbenes "better" ligands than phosphines in main group chemistry? A theoretical case study of ligand-stabilized E2 molecules, L-E-E-L (L = NHC, phosphine; E = C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi)

A theoretical examination of the L-E-E-L class of molecules has been carried out (E = group 14, group 15 element; L = N-heterocyclic carbene, phosphine), for which Si, Ge, P, and As-NHC complexes have recently been synthesized. The focus of this study is to predict whether it is possible to stabilize the elusive E(2) molecule via formation of L-E-E-L beyond the few known examples, and if the ligand set for this class of compounds can be extended from the NHC to the phosphine class of ligands. It is predicted that thermodynamically stable L-E-E-L complexes are possible for all group 14 and 15 elements, with the exception of nitrogen. The unknown ligand-stabilized Sn(2) and Pb(2) complexes may be considered attractive synthetic targets. In all cases the NHC complexes are more stable than the phosphines, however several of the phosphine derivatives may be isolable. The root of the extra stability conferred by the NHC ligands over the phosphines is determined to be a combination of the NHCs greater donating ability, and for the group 15 complexes, superior π acceptor capability from the E-E core. This later factor is the opposite as to what is normally observed in transition metal chemistry when comparing NHC and phosphine ligands, and may be an important consideration in the ongoing "renaissance" of low-valent main group compounds supported by ligands.     

Intramolecular cyclization and subsequent rearrangements of alkyne-tethered N-heterocyclic carbenes

Alkyne-tethered imidazole and 1,2,4-triazole-based N-heterocyclic carbene precursors have been prepared and studies of the intramolecular reactions of carbenes are performed. Products consistent with intramolecular cyclizations and subsequent rearrangements were observed. Mechanistic studies using crossover experiments showed that the products did arise from intramolecular carbene additions. The reactions are proposed to go through vinylogous diaminocarbene intermediates similar to vinylogous dialkoxycarbenes formed during Boger cycloaddition reactions. Imidazole substituted dienes were observed to be the major products of tandem cyclization and elimination reactions that were observed for imidazole-based N-heterocyclic carbenes.     

Carbodiphosphorane analogues E(PPh3)2 with E=C-Pb: a theoretical study with implications for ligand design

Quantum-chemical calculations at the BP86/TZVPP level have been carried out for the heavy Group 14 homologues of carbodiphosphorane E(PPh(3))(2), where E=Si, Ge, Sn, Pb, which are experimentally unknown so far. The results of the theoretical investigation suggest that the tetrelediphosphoranes E(PPh(3))(2) (1E) are stable compounds that could become isolated in a condensed phase. The molecules possess donor-acceptor bonds Ph(3)P→E←PPh(3) to a bare tetrele atom E, which retains its four valence electrons as two electron lone pairs. The analysis of the bonding situation and the calculation of the chemical reactivity indicate that the molecules 1E belong to the class of divalent E(0) compounds (ylidones). All molecules 1C-1Pb have very large first but also very large second proton affinities, which distinguishes them from the N-heterocyclic carbene homologues, in which the donor atom is a divalent E(II) species that possesses only one electron lone pair. Compounds 1E are powerful double donors that strongly bind Lewis acids such as BH(3) and AuCl in the complexes 1E(BH(3))(n) and 1E(AuCl)(n) (n=1, 2). The bond dissociation energies (BDEs) of the second BH(3) and AuCl molecules are only slightly less than the BDE of the first BH(3) and AuCl. The results of this work are a challenge for experimentalists.     

Quantum chemical study of interactions of carbenes and their analogs of the EH2 and EHX types (E = Si, Ge, Sn; X = F, Cl, Br) with HX and H2, respectively: the insertion and substituent exchange reactions

Interactions of carbenes and carbene analogs EH₂ and EHX with HX and H₂ (E = C, Si, Ge, Sn; X = F, Cl, Br), respectively, were studied by quantum chemical methods. Theoretical analysis of the carbene and silylene systems was carried out at the G3 level of theory using the MP2(full)/6?C31G(d) calculated geometries and vibrational frequencies. The stannylene systems were examined at the MP2 level using a modified LANL2DZ basis set for the Sn atoms and the 6?C31+G(d,p) basis sets for other atoms. Transformations in the germylene systems were studied within the framework of both approaches, which gave similar results. This allowed one to compare the reaction pathways and their energy profiles for the whole series of systems. In addition to the insertions into the H-X and H-H bonds, the exchange reactions resulting in interconversions of EH₂ and EHX can proceed in the systems under consideration. The effects of the nature of the E and X atoms on the reaction barriers and exothermicity of both the insertion and exchange reactions are analyzed. Possible role of radical processes in these systems is assessed.     

Chemistry of Complexes of Group 14 Elements Based on Redox-Active Ligands of the <Emphasis Type="Italic">o</Emphasis>-Iminoquinone Type

Specific features of the synthesis and structures of the complexes of Group 14 elements (Si, Ge, Sn, Pb) with o-iminoquinone ligands are discussed. The chemical reactions of the above indicated compounds accompanied by the transformation of the redox-active ligand in the coordination sphere of the complex- forming agent are considered.     

Mechanism of abstraction reactions of dimetallenes (R2X=XR2; X = C, Si, Ge, Sn, Pb) with halocarbons: a theoretical study

Potential energy surfaces for the abstraction reactions of dimetallenes with halocarbons have been studied using density functional theory (B3LYP). Five dimetallene species, (SiH(3))(2)X=X(SiH(3))(2), where X = C, Si, Ge, Sn, and Pb, have been chosen in this work as model reactants. The present theoretical investigations suggest that the relative dimetallenic reactivity increases in the order C=C < Si=Si < Ge=Ge < Sn=Sn < Pb=Pb. That is to say, for halocarbon abstractions there is a very clear trend toward lower activation barriers and more exothermic reactions on going from C to Pb. Moreover, for a given dimetallene, the overall barrier heights are determined to be in the order CF(4) > CCl(4) > CBr(4) > CI(4). That is, the heavier the halogen atom (Y), the more facile its abstraction from CY(4). Halogen abstraction is always predicted to be much faster than the abstraction of a CY(3) group irrespective of the dimetallene or halocarbon involved. Our model conclusions are consistent with some available experimental findings. Furthermore, both a configuration mixing model based on the work of Pross and Shaik and bonding dissociation energies can be used to rationalize the computational results. These results allow a number of predictions to be made.     

Preparation of Stable Low‐Oxidation‐State Group 14 Element Amidohydrides and Hydride‐Mediated Ring‐Expansion Chemistry of N‐Heterocyclic Carbenes

Various low oxidation state (+2) group 14 element amidohydride adducts, IPr ? EH(BH₃)NHDipp (E=Si or Ge; IPr=[(HCNDipp)₂C:], Dipp=2,6‐iPr₂C₆H₃), were synthesized. Thermolysis of the reported adducts was investigated as a potential route to Si‐ and Ge‐based clusters; however, unexpected transmetallation chemistry occurred to yield the carbene–borane adduct, IPr ? BH₂NHDipp. When a solution of IPr ? BH₂NHDipp in toluene was heated to 100 °C, a rare C? N bond‐activation/ring‐expansion reaction involving the bound N‐heterocyclic carbene donor (IPr) transpired.     

Theoretical study of cycloaddition reactions of heavy carbenes with C60

The potential energy surfaces for the cycloaddition reaction Me2X:+C60-->Me2X(C60) (X=C, Si, Ge, Sn, and Pb) have been studied at the B3LYP/LANL2DZ level of theory. It has been found that there are two competing pathways in these reactions, which can be classified as a [6,5]-attack (path 1) and a [6,6]-attack (path 2). It was found that, given the same reaction conditions, the cycloaddition reaction of C60 via a [6,6]-attack is more favorable than that via a [6,5]-attack, both kinetically and thermodynamically. A qualitative model that is based on the theory of Pross and Shaik has been used to develop an explanation for the reaction barrier heights. As a result, our theoretical investigations suggest that the singlet-triplet splitting DeltaEst(=Etriplet-Esinglet) of the 6 valence electron Me2X: and C60 species can be used as a guide to predict their reactivity toward cycloaddition reactions. Our model results demonstrate that the reactivity of heavy carbene cycloaddition to C60 decreases in the order Me2C:>Me2Si:>Me2Ge>Me2Sn:>Me2Pb:. As a consequence, we show that electronic effects play a decisive role in determining the energy barriers as well as the reaction enthalpy.     

Complexes of carbene analogs with Lewis bases: matrix IR spectroscopy and quantum-chemical studies

The results of studies on complexes of carbene analogs ER₂ (E = Si, Ge, Sn, Pb) with Lewis bases by matrix IR spectroscopy and quantum chemistry methods are considered. Trends in changes in the spectral characteristics, structures, and stability of the complexes are outlined.