Hydrosilylation vs. [2+2]-cycloaddition:: A theoretical study with iron and ruthenium complexes

Recently an exciting new mechanism of hydrosilylation had been found in experiments with the ruthenium-silylene complex [Cp*(i-Pr3P)Ru(H)(2)Si(H)Ph center dot OEt2][B(C6F5)(4)]by Glaser and Tilley. The mechanism of the hydrosilylation and possible alternative pathways are investigated with quantum chemical methods utilizing the B3LYP method, a double zeta pseudopotential basis set for iron and ruthenium and the 6-31G* basis set for all other elements. Starting from the model complex [Cp(H3P)Ru(H)(2)Si(H)Ph](+) the coordination of ethene at the silicon atom leads preferably to the hydrosilylation of a terminal Si-H-bond. The analysis of the electron density distribution of the catalytic active complex shows surprising bond features between Ru and Si. The Ru-Si bond is bridged by two hydrogen atoms. The [2 + 2]-cycloaddition of the alkene to the Ru-Si-bond, which would be a reasonable alternative reaction pathway, was not observed. It is necessary to make drastic changes in the ligand environment of the transition metal-silicone complex to observe cyclo-addition reactions. With complexes of the type (OC)(4)M=Si(H)Ph (M = Ru, Fe) the cycloaddition could be a serious alternative to the hydrosilylation. (c) 2006 Elsevier B.V. All rights reserved.