Kinetics and mechanisms of the oxidation of phenols by a trans-Dioxoruthenium(VI) complex

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-diaza5,8-dioxacyclopentadedane) have been studied in aqueous acidic solutions and in acetonitrile. In H2O 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 0.1 M and pH = 2.98, the kinetic isotope effects are k(H2O)/k(D2O) = 4.8 and 0.74 for k(x) and k(y), respectively, and k(f)(C6H5OH)/k(f)(C6D5OH) = 1.1. It is proposed that the k(x) step occurs by a hydrogen atom abstraction mechanism, while the ky 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,. A similar mechanism is proposed for reactions in CH3CN. A plot of log kx 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 BIDE but no 2,6-tert-butyl groups. This is as expected if hydrogen atom abstraction but not electron transfer is occurring.