Mechanistic studies on oxidation of hydrogen peroxide and hydrazine by a metal-bound superoxide

In aqueous acetate buffer, hydrogen peroxide and hydrazine reduce 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). In the presence of excess [H2O2] and [N2H5+] over [1], both the reactions obeyed first-order kinetics and exhibited inverse proton dependence. 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 and that accounts for the observed inverse proton dependence on rate. Reaction rates significantly decrease 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.