Kinetics and Mechanism of the Oxidation of Hydroquinones by a trans-Dioxoruthenium(VI) Complex

The kinetics of the oxidation of hydroquinone (H2Q) and its derivatives (H2Q-X) by trans-[RuVI(tmc)(O)2]2+ (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) have been studied in aqueous acidic solutions and in acetonitrile. In H2O, the oxidation of H2Q has the following stoichiometry: trans-[RuVI(tmc)(O)2]2+ + H2Q → trans-[RuIV(tmc)(O)(OH2)]2+ + Q. The reaction is first order in both RuVI and H2Q, and parallel pathways involving the oxidation of H2Q and HQ- are involved. The kinetic isotope effects are k(H2O)/k(D2O) = 4.9 and 1.2 at pH = 1.79 and 4.60, respectively. In CH3CN, the reaction occurs in two steps, the reduction of trans-[RuVI(tmc)(O)2]2+ by 1 equiv of H2Q to trans-[RuIV(tmc)(O)(CH3CN)]2+, followed by further reduction by another 1 equiv of H2Q to trans-[RuII(tmc)(CH3CN)2]2+. Linear correlations between log(rate constant) at 298.0 K and the O−H bond dissociation energy of H2Q−X were obtained for reactions in both H2O and CH3CN, consistent with a H-atom transfer (HAT) mechanism. Plots of log(rate constant) against log(equilibrium constant) were also linear for these HAT reactions.