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Experimental and theoretical investigations on the catalytic hydrosilylation of carbon dioxide with ruthenium nitrile complexes.

Ruthenium complexes, mer-[RuX(3)(MeCN)(3)] and cis/trans-[RuX(2)(MeCN)(4)] with X=Br, Cl, were investigated as precatalysts in homogeneously catalyzed hydrosilylation of CO(2). The oxidation state of ruthenium and nature of the halide in the precatalysts were found to influence the catalytic activity in the conversion of Me(2)PhSiH to the formoxysilane Me(2)PhSiOCHO, with Ru(III) having chloride ligands being most active. Monitoring the reactions by in-situ IR spectroscopy in MeCN as the solvent indicates an interaction of the precatalyst with the silane prior to activation of CO(2). In the absence of CO(2), hydrosilylation of the MeCN solvent occurs. Catalytic activity in CO(2) hydrosilylation is enhanced by Me(2)PhSiCl, generated during reduction of Ru(III) in mer-[RuX(3)(MeCN)(3)] to Ru(II) or, when added as promoter to Ru(II) precatalysts. The reaction mechanism for the catalytic cycle has been calculated by DFT methods for the reaction of Me(3)SiH. The key steps are: Transfer of the Me(3)Si moiety to a coordinated halide ligand, resulting in an L(n)RuH(XSiMe(3)) intermediate --> CO(2) coordination --> Me(3)Si transfer to CO(2) --> reductive elimination of formoxysilane product. This reaction sequence is more favorable energetically for chloride complexes than for the analogous bromide complexes, which accounts for their differences in catalytic activity. Calculations also explain the rate increase observed experimentally in the presence of Me(2)PhSiCl. A parallel reaction pathway leads to (Me(3)Si)(2)O as a minor byproduct which arises from the condensation of two initially formed Me(3)SiOH molecules.