Theoretical study of mechanism of extraction reaction between germylene carbene and its derivatives and thiirane

The mechanism of the sulfur extraction reaction between singlet germylene carbene and its derivatives [X₂Ge?C: (X = H, F, Cl, CH₃)] and thiirane has been investigated with density functional theory, including geometry optimization and vibrational analyses for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by B3LYP/6‐311G(d,p) method. From the potential energy profile, it can be predicted that the reaction pathway of this kind consists two steps: (1) the two reactants firstly form an intermediate (INT) through a barrier‐free exothermic reaction; (2) the INT then isomerizes to a product via a transition state (TS). This kind reaction has similar mechanism, when the germylene carbene and its derivatives [X₂Ge?C: (X = H, F, Cl, CH₃)] and thiirane get close to each other, the shift of 3p lone electron pair of S in thiirane to the 2p unoccupied orbital of C in X₂Ge = C: gives a p → p donor–acceptor bond, leading to the formation of INT. As the p → p donor–acceptor bond continues to strengthen (that is the C? S bond continues to shorten), the INT generates product (P + C₂H₄) via TS. It is the substituent electronegativity that mainly affects the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Theoretical study on the mechanism of extraction reaction between silylene carbene and its derivatives and thiirane

The mechanism of the sulfur extraction reaction between singlet silylene carbine and its derivatives and thiirane has been investigated with density functional theory (DFT), including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by B3LYP/6-311G(d, p) method. From the potential energy profile, it can be predicted that the reaction pathway of this kind consists in two steps: (1) the two reactants firstly form an intermediate through a barrier-free exothermic reaction; (2) the intermediate then isomerizes to a product via a transition state. This kind of reactions has similar mechanism: when the silylene carbene and its derivatives [X₂Si=C: (X = H, F, Cl, CH₃)] and thiirane approach each other, the shift of 3p lone electron pair of S in thiirane to the 2p unoccupied orbital of C in X₂Si=C: gives a p → p donor-acceptor bond, thereby leading to the formation of intermediate (INT). As the p → p donor-acceptor bond continues to strengthen (that is the C-S bond continues to shorten), the intermediate (INT) generates product (P + C₂H₄) via transition state (TS). It is the substituent electronegativity that mainly affect the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater.     

Ab initio Study on Mechanism of Forming a Silicic Bis-Heterocyclic Compound Between Dimethylmethylenesilylene and Ethene

The mechanism of the cycloaddition reaction of forming a silicic bis-heterocyclic compound between singlet dimethylmethylenesilylene (Me₂C=Si:) and ethene has been investigated with the CCSD(T)//MP2/6-31G^* method. From the potential energy profile, it can be predicted that, this reaction has one dominant channel. The presented rule of this dominant channel: the 3p unoccupied orbital of Si in dimethylmethylenesilylene and the π orbital of ethene forming the π→p donor-acceptor bond, resulting in the formation of three-membered ring intermediate (INT1); INT1 then isomerizes to a four-membered ring silylene (P2), which is driven by ring-enlargement effect; due to sp³ hybridization of Si atom in P2, P2 further combines with ethene to form a silicic bis-heterocyclic compound.     

Ab initio Study on Formation Mechanism of Spiro-Si-Heterocyclic Ring Compound Involving Ge from H2Ge=Si: and Formaldehyde

H₂Ge=Si: and its derivatives (X₂Ge=Si:, X=H, Me, F, Cl, Br, Ph, Ar,…) are new species. Its cycloaddition reactions are new area for the study of silylene chemistry. The cycloaddition reaction mechanism of singlet H₂Ge=Si: and formaldehyde has been investigated with the MP2/aug-cc-pVDZ method. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rule is that two reactants firstly form a four-membered Ge-heterocyclic ring silylene through the [2+2] cycloaddition reaction. Because of the 3p unoccupied orbital of Si: atom in the four-membered Ge-heterocyclic ring silylene and the π orbital of formaldehyde forming a π→p donor-acceptor bond, the four-membered Ge-heterocyclic ring silylene further combines with formaldehyde to form an intermediate. Because the Si: atom in the intermediate undergoes sp³ hybridization after transition state, then the intermediate isomerizes to a spiro-Si-heterocyclic ring compound involving Ge via a transition state. The result indicates the laws of cycloaddition reaction between H₂Ge=Si: or its derivatives (X₂Ge=Si:, X=H, Me, F, Cl, Br, Ph, Ar,…) and asymmetric π-bonded compounds are significant for the synthesis of small-ring involving Si and Ge and spiro-Si-heterocyclic ring compounds involving Ge.     

Cycloaddition Chemistry of a Silylene-Nickel Complex toward Organic π-Systems: From Reversibility to C−H Activation

The versatile cycloaddition chemistry of the Si−Ni multiple bond in the acyclic (amido)(chloro)silylene→Ni⁰ complex 1 , [(^TMSL)ClSi→Ni(NHC)₂] (^TMSL=N(SiMe₃)Dipp; Dipp=2,6-iPr₂C₆H₄; NHC=C[(iPr)NC(Me)]₂), toward unsaturated organic substrates is reported, which is both reminiscent of and expanding on the reactivity patterns of classical Fischer and Schrock carbene–metal complexes. Thus, 1:1 reaction of 1 with aldehydes, imines, alkynes, and even alkenes proceed to yield [2+2] cycloaddition products, leading to a range of four-membered metallasilacycles. This cycloaddition is in fact reversible for ethylene, whereas addition of an excess of this olefin leads to quantitative sp²-CH bond activation, via a 1-nickela-4-silacyclohexane intermediate. These results have been supported by DFT calculations giving insights into key mechanistic aspects.     

Ab initio study of mechanism of forming a silicic <Emphasis Type="Italic">bis</Emphasis>-heterocyclic compound between (CH<Subscript>3</Subscript>)<Subscript>2</Subscript>Si=Si: and ethane

The mechanism of the cycloaddition reaction of forming a silicic bis-heterocyclic compound between singlet state (CH₃)₂Si=Si: and ethene has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has one dominant reaction pathway. The presented rule of this dominant reaction pathway that the 3p unoccupied orbital of Si: in (CH₃)₂Si=Si: and the π orbital of ethane forming a π → p donor-acceptor bond, resulting in the formation of a three-membered ring intermediate (INT1); Then, INT1 isomerizes to a four-membered ring silylene (P1), which driven by ring-enlargement effect; Due to sp ³ hybridization of Si: atom in the four-membered ring silylene (P1), P1 further combines with ethene to form a silicic bis-heterocyclic compound (P2).     

Effect of the Electronic Structure of Substituents at the Silicon Atom on the Structure and Bond Energies of Methylhydrosilanes and Silenes

Ab initiocalculations with full geometry optimization were performed for methylhydrosilanes R₂HSiCH₃, dimethylsilanes C₂Si(CH₃)₂, and silenes R₂Si = CH₂ (R = H, CH₃, SiH₃, CH₃O, NH₂, Cl, F). The enthalpies of dehydrogenation methylhydrosilanes into silenes and of dehydrocondesation of methylhydrosilanes into dimethylsilanes were calculated. The enthalpies of dehydrogenation and dehydrocondensation increase with the electronegativity of substituent R. The Si-C and Si = C bond energies were calculated. As the electronegativity of the substituent increases, the Si-C bond shortens and strengthens, while the Si = C bond shortens and weakens.     

Ab Initio Study on the Mechanism of Cycloaddition Reaction between Silylene Carbene （H_2Si=C：） and Acetone

The mechanism of cycloaddition reaction between singlet silylene carbene and acetone has been investigated with CCSD(T)//MP2/6-31G method. From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. One consists of two steps: (1) the two reactants (R1, R2) firstly form a four-membered ring intermediate (INT4) through a barrier-free exothermic reaction of 585.9 kJ/mol; (2) Then intermediate (INT4) isomerizes to CH3-transfer product (P4.1) via a transition state (TS4.1) with energy barrier of 5.3 kJ/mol. The other is as follows: on the basis of intermediate (INT4) created between R1 and R2, intermediate (INT4) further reacts with acetone (R2) to form the intermediate (INT5) through a barrier-free exothermic reaction of 166.3 kJ/mol; Then, intermediate (INT5) isomerizes to a silicic bis-heterocyclic product (P5) via a transition state (TS5), for which the barrier is 54.9 kJ/mol. The presented rule of this reaction: the [2+2] cycloaddition effect between the π orbital of silylene carbene and the π orbital of π-bonded compounds leads to the formation of a four-membered ring intermediate (INT4); The unsaturated property of C atom from carbene in the four-membered ring intermediate (INT4) results in the generation of CH3-transfer product (P4.1) and silicic bis-heterocyclic compound (P5).     

Ab initio study of mechanism of forming a Spiro-Si-heterocyclic ring compound

The mechanism of the cycloaddition reaction of forming a spiro-Si-heterocyclic ring compound between singlet dichloroalkylidenesilylene (Cl₂C=Si:) and ethene has been investigated with CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has one dominant reaction pathway. The presented rule of this reaction is that the 3p unoccupied orbital of Si in dichloroalkylidene and the π orbital of ethene forming the π → p donor-acceptor bond, resulting in the formation of a three-membered ring intermediate. Ring-enlargement effect make the three-membered ring intermediate isomerizes to a four-membered ring silylene. Due to sp ³ hybridization of Si atom in the four-membered ring silylene, the four-membered ring silylene further combines with ethene to form a spiro-Si-heterocyclic ring compound.     

An N‐Heterocyclic Silylene‐Stabilized Digermanium(0) Complex

The synthesis of an N‐heterocyclic silylene‐stabilized digermanium(0) complex is described. The reaction of the amidinate‐stabilized silicon(II) amide [LSiN(SiMe₃)₂] ( 1 ; L=PhC(NtBu)₂) with GeCl₂?dioxane in toluene afforded the Si^II–Ge^II adduct [L{(Me₃Si)₂N}Si→GeCl₂] ( 2 ). Reaction of the adduct with two equivalents of KC₈ in toluene at room temperature afforded the N‐heterocyclic carbene silylene‐stabilized digermanium(0) complex [L{(Me₃Si)₂N}Si→ Ge?Ge←Si{N(SiMe₃)₂}L] ( 3 ). X‐ray crystallography and theoretical studies show conclusively that the N‐heterocyclic silylenes stabilize the singlet digermanium(0) moiety by a weak synergic donor–acceptor interaction.     

Theoretical study of mechanism of cycloaddition reaction between singlet dichlorosilylene carbene (Cl<Subscript>2</Subscript>Si=C:) and formaldehyde

The mechanism of the cycloaddition reaction between singlet dichlorosilylene carbene (Cl₂Si=C:) and formaldehyde has been investigated with MP2/6-31G* method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by Zero-point energy and CCSD (T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. The first dominant reaction pathway consists of two steps: (1) the two reactants (R1, R2) firstly form a four-membered ring intermediate (INT4) through a barrier-free exothermic reaction of 387.9 kJ/mol; (2) intermediate (INT4) then isomerizes to H-transfer product (P4.2) via a transition state (TS4.2) with energy barrier of 4.7 kJ/mol. The second dominant reaction pathway as follows: on the basis of intermediate (INT4) created between R1 and R2, intermediate (INT4) further reacts with formaldehyde (R2) to form the intermediate (INT5) through a barrier-free exothermic reaction of 158.3 kJ/mol. Then, intermediate (INT5) isomerizes to a silicic bis-heterocyclic product (P5) via a transition state (TS5), for which the barrier is 40.1 kJ/mol.     

Synthesis and Rearrangement of Stable NHC→Silylene Adducts and Their Unique Reactivity towards Cyclohexylisocyanide

The syntheses and reactivity of the two N‐heterocyclic carbene (NHC)→ silylene complexes 2 and 4 have been investigated. The latter are easily accessible by reaction of the zwitterionic, N‐heterocyclic silylene LSi: 1 [L=Ar‐N‐C(=CH₂)CH?C(Me)‐N‐Ar, Ar=2,6‐iPr₂C₆H₃] with 1,3,4,5‐tetramethylimidazol‐2‐ylidene and 1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene, respectively. While compound 2 undergoes facile rearrangement above ?20 °C to give the unsymmetrical N‐heterocyclic silylcarbene 3 , the derivative 4 remains unchanged even after boiling in benzene. The remarkable reactivity of 3 and 4 towards cyclohexylisocyanide has been examined which leads in a unique series of C? H, Si? H, and C? N bond activations to the new triaminosilanes 5 and 6 , respectively. The novel compounds 3 , 4 , 5 , and 6 were fully characterized by ¹H, ¹³C, and ²⁹Si NMR spectroscopy, EI‐MS, elemental analysis, and single‐crystal X‐ray diffraction.     

Reaction of an N‐Heterocyclic Carbene‐Stabilized Silicon(II) Monohydride with Alkynes: [2+2+1] Cycloaddition versus Hydrogen Abstraction

An in depth study of the reactivity of an N‐heterocyclic carbene (NHC)‐stabilized silylene monohydride with alkynes is reported. The reaction of silylene monohydride 1 , tBu₃Si(H)Si←NHC, with diphenylacetylene afforded silole 2 , tBu₃Si(H)Si(C₄Ph₄). The density functional theory (DFT) calculations for the reaction mechanism of the [2+2+1] cycloaddition revealed that the NHC played a major part stabilizing zwitterionic transition states and intermediates to assist the cyclization pathway. A significantly different outcome was observed, when silylene monohydride 1 was treated with phenylacetylene, which gave rise to supersilyl substituted 1‐alkenyl‐1‐alkynylsilane 3 , tBu₃Si(H)Si(CH?CHPh)(C?CPh). Mechanistic investigations using an isotope labelling technique and DFT calculations suggest that this reaction occurs through a similar zwitterionic intermediate and subsequent hydrogen abstraction from a second molecule of phenylacetylene.     

The crystal structures of m-trifluoromethylphenylsilatranone and p-fluorophenylsilatranone

The crystal structures of two silatranone derivatives are reported. The close N → Si approach (2.106(3) Å in m-trifluoromethylphenyl-, and 2.129(3) Å in p-fluorophenyl-silatranone) indicates strong dative acceptor bonds. For various silatrane derivatives and inverse relation has been revealed between the mean group electronegativity of the substituent R attached to silicon and the N → Si dative bond distance. In both structures there are long (1.72 Å) SiO bonds in the SiOCO moiety. The m-trifluoromethylphenyl derivative contains a disordered CF₃ group.     

A Disilene Base Adduct with a Dative Si–Si Single Bond

An experimental and theoretical study of the base‐stabilized disilene 1 is reported, which forms at low temperatures in the disproportionation reaction of Si₂Cl₆ or neo‐Si₅Cl₁₂ with equimolar amounts of NMe₂Et. Single‐crystal X‐ray diffraction and quantum‐chemical bonding analysis disclose an unprecedented structure in silicon chemistry featuring a dative Si→Si single bond between two silylene moieties, Me₂EtN→SiCl₂→Si(SiCl₃)₂. The central ambiphilic SiCl₂ group is linked by dative bonds to the amine donor and the bis(trichlorosilyl)silylene acceptor, which leads to push–pull stabilization. Based on experimental and theoretical examinations a formation mechanism is presented that involves an autocatalytic reaction of the intermediately formed anion Si(SiCl₃)₃^? with neo‐Si₅Cl₁₂ to yield 1 .     

Vinylic selenides

The ¹H, ¹³C and ²⁹Si NMR spectra of 1,3-dioxa-6-aza-2-silacyclooctanes (I) containing a transannular N → Si bond have been studied at different temperatures in solution. The sterically less crowded equatorial location of the Si substituent at the trigonal bipyramid (TBP) corresponds to the energetically unfavourable axial position in the eight-membered heterocycle (C₈). For this reason, the preferred orientation of the Si substituent and the conformational equilibrium of C₈ strongly depend on the energy of the N → Si bond. The C₈ of I in solution has the boat-chair (BC) and/or chair-chair (CC) conformations. The weakening of the N → Si bond leads to an increase in dihedral angle ?_ON and shifts the conformational equilibrium of C₈ to the CC form. The exchange between the Si substituent positions at the TBP, observed at low temperatures proceeds through the inversion of C₈ and requires dissociation of the N → S bond.     

Ab initio study of mechanism of forming a spiro-heterocyclic ring compound involving Si from dichlorosilylenesilylene (Cl2Si=Si:)→Cl2Si=Si: and aldehyde

The mechanism of the cycloaddition reaction between singlet dichlorosilylenesilylene (Cl₂Si=Si:)→Cl₂Si=Si: and aldehyde has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rules presented is that the two reactants firstly form a four-membered ring silylene through the [2+2] cycloaddition reaction. Because of the 3p unoccupied orbital of Si: atom in the four-membered ring silylene and the π orbital of aldehyde forming a π → p donor–acceptor bond, the four-membered ring silylene further combines with aldehyde to form an intermediate. Because the Si: atom in the intermediate happens sp ³ hybridization after transition state, then the intermediate isomerizes to a spiro-heterocyclic ring compound involving Si via a transition state.     

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.     

Theoretical insights into the relative bonding of normal and abnormal N‐heterocyclic carbenes in [PdCl2(NHCR)2] and [PdCl2(NHCR)(aNHCR)] (R = H,Ph, Mes)

Quantum chemical insights into normal Pd‐C2(NHC^R) and abnormal Pd‐C5(aNHC^R) bonding, dominated by dispersion interactions in N‐hetereocyclic carbene complexes [PdCl₂(NHC^R)₂] ( I , R = H; II , R = Ph; III , R = Mes (2,4,6‐trimethyl)phenyl)) and [PdCl₂(NHC^R)(aNHC^R] ( IV , R = H; V , R = Ph; VI , R = Mes) have been investigated at DFT and DFT‐D3(BJ) level of theory with particular emphasis on the effects of the noncovalent interactions on the structures and the nature of Pd‐C2(NHC^R) and Pd‐C5(aNHC^R) bonds. The optimized geometries are good agreement with the experimental values. The Pd‐C bonds are essentially single bond. Hirshfeld charge distributions indicate that the abnormal aNHC^R carbene ligand is relatively better electron donor than the normal NHC^R carbene ligand. The C2 atom has larger %s contribution along Pd‐C2 bond than the C5 atom along Pd‐C5 bond. As a consequence the Pd‐C2(NHC^R) bonds are relative stronger than the Pd‐C5(aNHC^R) bonds. Thus, the results of natural hybrid orbital analysis support the key point of the present study. Calculations predict that for bulky substituent (R = Ph, Mes) at carbene, the Pd‐C2(NHC^R) bond is stronger than Pd‐C5(aNHC^R) bond due to large dispersion energy in [PdCl₂(NHC^R)₂] than in [PdCl₂(NHC^R)(aNHC^R)]. However, in case of non‐bulky substituent with small and almost equal contribution of dispersion energy, the Pd‐C2(NHC^R) bond is relative weaker than Pd‐C5(aNHC^R) bond. The bond dissociation energies are dependent on the R substituent, the DFT functional and the inclusion of dispersion interactions. Major point of this study is that the abnormal aNHCs are not always strongly bonded with metal center than the normal NHCs. Effects of dispersion interaction of substituent at nitrogen atoms of carbene ligand are found to play a crucial role on estimation of relative bonding strengths of the normal and abnormal aNHCs with metal center. © 2016 Wiley Periodicals, Inc.     

Silylene–Nickel Promoted Cleavage of B−O Bonds: From Catechol Borane to the Hydroborylene Ligand

The first 16 valence electron [bis(NHC)](silylene)Ni⁰ complex 1 , [(^TMSL)ClSi:→Ni(NHC)₂], bearing the acyclic amido-chlorosilylene (^TMSL)ClSi: (^TMSL=N(SiMe₃)Dipp; Dipp=2,6-Prⁱ₂C₆H₄) and two NHC ligands (N-heterocyclic carbene=:C[(Prⁱ)NC(Me)]₂) was synthesized in high yield and structurally characterized. Compound 1 is capable of facile dihydrogen activation under ambient conditions to give the corresponding HSi-NiH complex 2 . Most notably, 1 reacts with catechol borane to afford the unprecedented hydroborylene-coordinated (chloro)(silyl)nickel(II) complex 3 , {[cat(^TMSL)Si](Cl)Ni←:BH(NHC)₂}, via the cleavage of two B−O bonds and simultaneous formation of two Si−O bonds. The mechanism for the formation of 3 was rationalized by means of DFT calculations, which highlight the powerful synergistic effects of the Si:→Ni moiety in the breaking of incredibly strong B−O bonds.