Silicon Transition Metal Three-membered Ring And Theoretical Study Of Small-molecule Reaction

Unsaturated silicon complexes are important intermediates in new catalytic reactions of organosilicon compounds, including new routes to organosilicon polymers. Recently, the role of the small membered silametallacycles, particularly three-membered ring compounds, has attracted considerable interest because of their strained molecular structures, peculiar bonding modes, and novel reactivities.Although several silametallacycles have been prepared and their reactivity toward various substrates has been investigated, there have been only limited theoretical studies concerning the reactions. Thus, theoretical investigation on the structures, bonding and reaction mechanisms of silametallacycle transition metal complexes toward some small molecules will be very interesting and of guiding meaning to experimentalists.In this thesis we first introduced the origin and development of organometallic chemistry, the development of computational chemistry and its application in the field of organometallic chemistry, the choice of computational method in this work. Then, we studied the reaction mechanisms, structures and bonding properties for the reactions of silametallacycle complexes including two Cp rings with small substrates. The structures were optimized and the frequencies and energies of the compounds involved in the reactions were computed by using the B3LYP method of density functional theory (DFT).We first studied the reactions of Cp₂Zr(η²-SiMe₂=N^tBu)(PMe₃) with H₂, H₂C=CH₂ O=CH₂, respectively. The reaction mechanisms are confirmed theoretically to undergo two steps, dissociation of PMe₃ from the transition metal Zr and the insertion of H₂, CH₂=CH₂ and CH₂=O into Zr-Si bond. Theoretical findings are found as follows. The relative stabilities of the three insertion products with respect to the reactant, Cp₂Zr(η² -SiMe₂ =N^tBu)(PMe₃), are in the order H₂ < CH₂=CH₂ < CH₂=O. H₂ insertion and CH₂=CH₂ insertion are the rate-determining steps for the former two reactions, while PMe₃ dissociation is the rate-determining step for the third reaction. Only the precursor formed by coordination of O=CH₂ to Cp₂Zr(η²-SiMe₂=N^tBu) is located, while those formed by coordination of H₂ and CH₂=CH₂ to Cp₂Zr(η²-SiMe₂=N^tBu) are not found. The H2-insertion product, Cp₂Zr(N^tBuSiMe₂H)H, was structurally discussed. It was found the Zr H-Si agostic interaction is present. The bonding features involved in Cp₂Zr(N^tBuSiMe₂H)H were analyzed in comparison to the designed model complex, Cp₂Zr(NH₂)H. The geometrical structural difference between Cp₂Zr(N^tBuSiMe₂H)H and Cp₂Zr(NH₂)H results from their different bonding properties and steric environments. We also predicted the H-2-insertion product is different from the one obtained from the reaction of [Cp₂ZrHCl]_n with LiN^tBuSiMe₂H·THF. Our results of calculations also confirmed the CH₂=O insertion product contains the Zr-O bond while not the Zr-C one due to the stronger interaction of Zr with O atom versus with C atom.In addition, we have theoretically investigated three reactions, Cp₂W(η²-Me₂Si=CH₂) toward H₂ CH₂=CH₂ and MeOH, respectively. For the reaction of Cp₂W(η²-Me₂Si=CH₂) with H₂, three possible pathways were proposed. They involve, respectively ,silicon group migration, Cp ring slipage and σ-bond metathesis. Our theoretical findings indicate the first pathway is the most favored kinetically. For the reaction of Cp₂W(η²-Me₂Si=CH₂) with CH₂=CH₂,' two possible pathways were proposed. They are also related to the silicon group migration and Cp ring slipage, respectively. It was found the first pathway involving silicon group migration is more favored. For the last reaction, Cp₂W(η²- Me₂Si=CH₂) with MeOH, only one pathway was proposed, in which σ-bond metathesis was considered. This proposed pathway was theoretically confirmed to be available under the experimental conditions.