Selective alcohol oxidation with molecular oxygen catalyzed by Os-Cr and Ru-Cr complexes

The heterobimetallic complexes [Y] [M(N)R-2(mu-O)(2)CrO2] (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)(mu-O)(2)CrO2] and [PPh4] [Os(N)Me(CH2SiMe3)(mu-O)(2)CrO2] 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)(mu-O)(2)CrO2] 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 O-2 partial pressure. A mechanism in which alcohol coordinates to the osmium center and is oxidized by P-hydrogen elimination is consistent with the data. [N(n-Bu)(4)][Os(N)(CH2SiMe3)(2)(mu-O)(2)CrO2] catalyzes the oxidation of dppe with Oz through a different pathway.