The kinetics of the oxidation of phenols by trans-[Ru(VI)(L)(O)(2)](2+) (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in aqueous acidic solutions and in acetonitrile. In H(2)O the oxidation of phenol produces the unstable 4,4'- biphenoquinone, as evidenced by a rapid increase and then a slow decrease in absorbance at 398 nm. The first step is first-order in both Ru(VI) and phenol, and rate constants are dependent on [H(+)] according to the relationship k(f) = k(x) + (k(y)K(a)/[H(+)]), where k(x) and k(y) are the rate constants for the oxidation of PhOH and PhO(-), respectively. At 298 K and I = 0.1 M, k(x) = 12.5 M(-1) s(-1) and k(y) = 8.0 x 10(8) M(-1) s(-1). At I = 0.1 M and pH = 2.98, the kinetic isotope effects are k(H(2)O)/k(D(2)O) = 4.8 and 0.74 for k(x) and k(y), respectively, and k(f)(C(6)H(5)OH)/k(f)(C(6)D(5)OH) = 1.1. It is proposed that the k(x) step occurs by a hydrogen atom abstraction mechanism, while the k(y) step occurs by an electron-transfer mechanism. In both steps the phenoxy radical is produced, which then undergoes two rapid concurrent reactions. The first is a further three-electron oxidation by Ru(VI) and Ru(V) to give p-benzoquinone and other organic products. The second is a coupling and oxidation process to give 4,4'-biphenoquinone, followed by the decay step, k(s). A similar mechanism is proposed for reactions in CH(3)CN. A plot of log k(x) vs O-H bond dissociation enthalpies (BDE) of the phenols separates those phenols with bulky tert-butyl substituents in the ortho positions from those with no 2,6-di-tert-butyl groups into two separate lines. This arises because there is steric crowding of the hydroxylic groups in 2,6-di-tert-butyl phenols, which react more slowly than phenols of similar O-H BDE but no 2,6-tert-butyl groups. This is as expected if hydrogen atom abstraction but not electron transfer is occurring.

The kinetics of the oxidation of hydroquinone (H(2)Q) and its derivatives (H(2)Q-X) by trans-[Ru(VI)(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 H(2)O, the oxidation of H(2)Q has the following stoichiometry: trans-[Ru(VI)(tmc)(O)(2)](2+) + H(2)Q --> trans-[Ru(IV)(tmc)(O)(OH(2))](2+) + Q. The reaction is first order in both Ru(VI) and H(2)Q, and parallel pathways involving the oxidation of H(2)Q and HQ(-) are involved. The kinetic isotope effects are k(H(2)O)/k(D(2)O) = 4.9 and 1.2 at pH = 1.79 and 4.60, respectively. In CH(3)CN, the reaction occurs in two steps, the reduction of trans-[Ru(VI)(tmc)(O)(2)](2+) by 1 equiv of H(2)Q to trans-[Ru(IV)(tmc)(O)(CH(3)CN)](2+), followed by further reduction by another 1 equiv of H(2)Q to trans-[Ru(II)(tmc)(CH(3)CN)(2)](2+). Linear correlations between log(rate constant) at 298.0 K and the O-H bond dissociation energy of H(2)Q-X were obtained for reactions in both H(2)O and CH(3)CN, consistent with a H-atom transfer (HAT) mechanism. Plots of log(rate constant) against log(equilibrium constant) were also linear for these HAT reactions.