Selective Alcohol Oxidation with Molecular Oxygen Catalyzed by Os−Cr and Ru−Cr Complexes

The heterobimetallic complexes [Y][M(N)R2(μ-O)2CrO2] (where Y is either N(n-Bu)4+ or PPh4+; M is either Ru or Os; and R is an alkyl or aryl ligand) catalyze the selective oxidation of alcohols with molecular oxygen. The rate of the reaction is higher with a ruthenium-containing complex than with an analogous osmium-containing catalyst. The rate decreases with steric bulk in either the catalyst or substrate. Single-crystal X-ray diffraction studies of [N(n-Bu)4][Ru(N)(CH2SiMe3)2(μ-O)2CrO2] and [PPh4][Os(N)Me(CH2SiMe3)(μ-O)2CrO2] show the chromate group coordinated to each nitrido(dialkyl)metal center through two oxo ligands. The oxidation of benzyl alcohol by [N(n-Bu)4][Os(N)(CH2SiMe3)2(μ-O)2CrO2] was examinined in detail. It is first order in alcohol and substrate, and, when oxygen partial pressure is low, the rate depends directly on the O2 partial pressure. A mechanism in which alcohol coordinates to the osmium center and is oxidized by β-hydrogen elimination is consistent with the data. [N(n-Bu)4][Os(N)(CH2SiMe3)2(μ-O)2CrO2] catalyzes the oxidation of dppe with O2 through a different pathway.