Ruthenium(IV)–oxo complexes: a novel hydrophobic effect of tertiary phosphine ligands on the selective oxidation of alcohols

Abstract

Kinetic and mechanistic studies on the oxidation of a series of alcohols by [Ru^IVO(bipy)₂(ER₃)]²⁺(bipy = 2,2′-bipyridine and ER₃ is a tertiary phosphine or arsine ligand) have been conducted in both aqueous and non-aqueous solutions. Under all conditions, the rate law shows a first-order dependence on both the alcohol and the ruthenium(IV)–oxo species. The rate of alcohol oxidation in aqueous solution is dependent on the hydrophobic nature of the target alcohol, where the second-order rate constant increases in the following order: methanol < ethanol < propan-1-ol < 2-methylpropan-1-ol < butan-1-ol < 3-methylbutan-1-ol < pentan-1-ol. The rate of alcohol oxidation is also strongly dependent on the tertiary phosphine ligand, where the rate of aqueous oxidation of allyl alcohol by [Ru^IVO(bipy)₂(PPh₃)]²⁺[k=(1.68 ± 0.02)× 10^–1 dm³ mol^–1 s^–1] is 250 times faster than the rate of oxidation by [Ru^IVO(bipy)₂(PEt₃)]²⁺[k=(6.7 ± 0.3)× 10^–4 dm³ mol^–1 s^–1]. A primary isotope effect of k_H/k_D= 10, and secondary isotope effects of 1.4 per methyl group and 1.2 for the hydroxy group were observed for the aqueous oxidation of propan-2-ol by [Ru^IVO(bipy)₂(PPh₃)]²⁺, suggesting a concerted outer-sphere redox mechanism where a hydride ion is transferred from the target alcohol to the oxo ligand of the Ru^IVO moiety. To account for the hydrophobic selectivity of alcohol oxidation by pnictogen–ruthenium(IV)–oxo complexes, a mechanism involving a preassociation of target alcohol and co-ordinated phosphine ligand prior to hydride transfer is proposed.