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)2Si(H)Ph · 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)2Si(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 cycloaddition reactions. With complexes of the type (OC)4Mdouble bond; length as m-dashSi(H)Ph (M = Ru, Fe) the cycloaddition could be a serious alternative to the hydrosilylation.