| 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 studies concerning the reactions between silametallacyclopropane and substrates with E-H single bonds (E= O, N, P). To our knowledge, few theoretical studies were performed for such kinds of ring-opening reactions. Peculiar bonding features of three-membered silametallacycles prompt us to find out the nature of the reactions. Thus, it will be very interesting and of guiding meaning to make study on silametallacycles complexes.In this thesis we studied the mechanisms on reactions and bonding properties of three-membered cyclic compounds with ring constituents of silicon and transition metal atoms such as Fe, Ir, Mo and other atoms, respectively. From the experiments we know that there exists cleavage of different bonds when they react with MeOH. We chose representative reactions of three-membered silametallacycles with MeOH for studying their bonding features and mechanisms involved using density functional theory (DFT). Our goal is to find out the varying tendency of structural parameters with ring-opening, and give detailed discussion on the energetics involved in the reaction mechanisms.Chapter 1 gives brief introduction to the work in this thesis. Chapter 2 briefly describes the computational methods used in this thesis. It was found that the DFT method is the best and was used throughout our work.In chapter 3, we chose a representative reaction of Cp~*(CO)Fe{K~2(Si,P)-SiMe2PPh2} with MeOH to investigate the reaction mechanism and bonding features. The mechanism for the model reaction of Cp*(CO)Fe{K2(iS"/,P)-SiMe2PH2} with MeOH has been completely studied by using density functional theory (DFT). It was found that the methanol oxygen atom first to attack the silicon atom to open the three-membered ring, and then the hydrogen atom migrates to the P atom from H-OMe. The low reaction barrier calculated (6.17 kcal/mol) corresponds to the mild experimental reaction condition. The reaction energy (-31.34 kcal/mol) indicates that the reaction is much favorable thermodynamically for the release of ring strain and the formation of P-H and Si-0 a bonds. Our calculations revealed that, from TSl to P (see Fig. 4), the bond distance of Fe-P decreases while that of Fe-Si increases sequently. The lone pair of electrons on the PH2 ligand and the hybridization modes of Si atom are responsible for the different variational trends.Chapter 4 studies the mechanisms on the model reaction of Cp*(PMe3)Ir{K2(Si,C)-SiH2CH2}, derived from Cp*(PMe3)Ir{K2(Si,Q-SiPh2CH2}, with MeOH. Our results of calculations reveal that the cleavage of Ir-Si bond is favored kinetically and unfavorable thermodynamically over that of Si-C bond. Based on the experimental results, we predict that the reaction is dominated kinetically rather than thermodynamically, affording the product obtained from the Ir-Si bond cleavage.In chapter 5, the reaction of Cp2Mo{K2(Si,Si)-SiMe2SiMe2} with MeOH gives different products because of different ring-opening modes. The mechanisms for both ring-opening reactions (break Mo-Si and Si-Si bonds, respectively) have been investigated. Calculated results predict that the amount of the products, obtained from the cleavage of Si-Si and Mo-Si bonds, are dependent upon their stability. The product obtained from the cleavage of Si-Si bond is more stable in energy than that from the cleavage of Mo-Si bond by 2.11 kcal/mol, which is in agreement with the experimental observations (the ratio of the former product to the latter one is 8:2). |
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