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

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^IV=O 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.