Kinetics of oxidation of hydrogen peroxide and ascorbic acid by a tribridged manganese(IV,IV) dimer in feebly acidic media

Abstract

In weakly acidic, aqueous buffer (MeCO₂⁻+ bipy), the complex ion [Mn₂^IV(µ-O)₂(µ-MeCO₂)(bipy)₂(H₂O)₂]³⁺, 1 (bipy = 2,2prime-bipyridine), coexists in rapid equilibrium with its hydrolytic derivatives, [Mn₂^IV(µ-O)₂(bipy)₂(H₂O)₄]⁴⁺, 2, and [Mn2IV(µ-O)₂(µ-MeCO₂)(bipy)(H₂O)₄]³⁺, 3. The solution quantitatively oxidizes hydrogen peroxide to oxygen and ascorbic acid to dehydroascorbic acid, itself being reduced to Mn^II. In the presence of excess reductant, the reactions follow simple first-order kinetics with no evidence for the accumulation of a significant amount of any intermediate manganese complex. The ascorbate anion shows overwhelming kinetic dominance over ascorbic acid, but no evidence is available for deprotonation of hydrogen peroxide. The preferred intimate mechanism for hydrogen peroxide is inner sphere but that for ascorbic acid is uncertain. For both reductants, increased extent of aquation leads to increased kinetic activity in the order: 1 < 2 < 3.Key words: kinetics, manganese, ascorbic acid, hydrogen peroxide, 2,2prime-bipyridine.