OXIDATION OF PHENOL BY BRIDGING SUPEROXIDE LIGAND IN A BINUCLEAR CoIII COMPLEX CONTAINING HETEROLEPTIC LIGANDS: A KINETIC AND MECHANISTIC STUDY

Abstract

PhOH is a good scavenger of superoxide ions as well as very good free radical scavenger (FRS). In aqueous acetate buffer (pH:3.4-4.2), PhOH reduces the bridging superoxide in [(en)(dien)CoIII(O2)CoIII(en)(dien)](ClO4)5(1) to the corresponding hydroperoxo complex, [(en)(dien)CoIII(μ-O2H)CoIII(en)(dien)]]5+(2) and itself gets oxidized, following both proton coupled electron transfer (PCET) and electron transfer(EC) path. In the presence of excess [PhOH] over [1], the reaction obeys first-order kinetics and rate of the reaction decreases with increase in [H+]. Since de-protonation of PhOH(pKa = 10.0) is not possible in the working pH, so protonation of 1 at equilibrium generates [(en)(dien)CoIII(μ- O2H)CoIII(en)(dien)]]6+(1H), the conjugate acid from 1, which appears to be a kinetic dead-end species and that accounts for the observed inverse proton dependence on rate. Reaction rate significantly decreases with increasing proportion of D2O replacing H2O in the solvent and an H-atom transfer (HAT) from the reducing species to the bridging superoxide in 1 seems reasonable at the rate step.