Theoretical Study of the Cp_2Zr-Catalyzed Hydrosilylation of Ethylene. Reaction Mechanism Including New σ-Bond Activation

摘要

The Cp_2Zr-catalyzed hydrosilylation of ethylene was theoretically investigated with DFT and MP2-MP4(SDQ) methods, to clarify the reaction mechanism and the characteristic features of this reaction. Although ethylene insertion into the Zr-SiH_3 bond of Cp_2Zr(H)(SiH_3) needs a very large activation barrier of 41.0 (42.3) kcal/mol, ethylene is easily inserted into the Zr-H bond with a very small activation barrier of 2.1 (2.8) kcal/mol, where the activation barrier and the energy of reaction calculated with the DFT(B3LYP) method are given and in parentheses are those values which have been corrected for the zero-point energy, hereafter. Not only this ethylene insertion reaction but also the coupling reaction between Cp_2Zr(C_2H_4) and SiH_4 easily takes place to afford Cp_2Zr(H)(CH_2CH_2SiH_3) and Cp_2Zr(CH_2CH_3)(SiH_3) with activation barriers of 0.3 (0.7) and 5.0 (5.4) kcal/mol, respectively. This coupling reaction involves a new type of Si-H -bond activation which is similar to metathesis. The important interaction in the coupling reaction is the bonding overlap between the d_π-π~* bonding orbital of Cp_2Zr(C_2H_4) and the Si-H σ~* orbital. The final step is neither direct C-H nor Si-C reductive elimination, because both reductive eliminations occur with a very large activation barrier and significantly large endothermicity. This is because the d orbital of Cp_2Zr is at a high energy. On the other hand, ethylene-assisted C-H reductive elimination easily occurs with a small activation barrier, 5.0 (7.5) kcal/mol, and considerably large exothermicity, -10.6 (-7.1) kcal/mol. Also, ethylene-assisted Si-C reductive elimination and metatheses of Cp_2Zr(H)(CH_2CH_2SiH_3) and Cp_2Zr(CH_2CH_3)(SiH_3) with SiH4 take place with moderate activation barriers, 26.5 (30.7), 18.4 (20.5), and 28.3 (31.5) kcal/mol, respectively. From these results, it is clearly concluded that the most favorable catalytic cycle of the Cp_2Zr-catalyzed hydrosilylation of ethylene consists of the coupling reaction of Cp_2Zr(C_2H_4) with SiH4 followed by the ethylene-assisted C-H reductive elimination.