Reaction of hydrogen peroxide with coordinated superoxide in [(NH3)5CoIII(μ-O2)CoIII(NH3)5]5+: a mechanistic study

Abstract

In aqueous acetate buffer media, hydrogen peroxide reduces the bridging superoxide in [(NH₃)₅Co^III(μ-O₂)Co^III(NH₃)₅]⁵⁺ (1) to the corresponding peroxide in the complex, [(NH₃)₅Co^III(μ-O₂H)Co^III(NH₂)(NH₃)₄]⁴⁺ (2), itself being oxidized to HO₂˙. The complex 2 thus produced decomposes rapidly to the final products, Co^II, NH₃, etc. instead of reacting with a second molecule of hydrogen peroxide. In the presence of excess [H₂O₂] over (1), the reaction obeyed first-order kinetics and exhibited inverse proton dependence. [(NH₃)₅Co^III(μ-O₂)Co^III((NH₂)(NH₃)₄]⁴⁺ (3), a conjugate base of 1, seems to be the kinetically reactive species and the cause for the observed inverse proton dependence. Kinetics is little affected when one of the hydrogen atoms from hydrogen peroxide is replaced with an alkyl group, as in tert-butyl hydroperoxide. But replacement of both the H atoms with alkyl groups halts the reaction as seen with di-tert-butyl peroxides, and peroxodisulfate ion. The reaction rate with hydrogen peroxide significantly decreases with increasing proportion of D₂O replacing water in the solvent and the rate-limiting step seems to be an H-atom transfer.