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)Co^III(O₂)Co^III(en)(dien)](ClO₄)₅ (1) to the corresponding hydroperoxo complex [(en)(dien)Co^III(μ-O₂H)Co^III(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)Co^III(μ-O₂H)Co^III(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.