Kinetic and mechanistic studies on the oxidation of hydroxylamine by a tri-bridged manganese(iv,iv) dimer in weakly acidic media

Abstract

The complex ion, [Mn^IV₂(μ-O)₂(μ-MeCO₂)(bipy)₂(H₂O)₂]³⁺ (1³⁺) (bipy = 2,2′-bipyridine) and its aqua derivatives [Mn₂^IV(μ-O)₂(bipy)₂(H₂O)₄]⁴⁺ (2⁴⁺) and [Mn₂^IV(μ-O)₂(μ-MeCO₂)(bipy)(H₂O)₄]³⁺ (3³⁺) coexists in rapid equilibria in aqueous buffer in the presence of excess of bipy and MeCO₂⁻ in the range pH 4.00–5.30. The solutions are reasonably stable up to pH 5.50 and react quantitatively with hydroxylamine to produce manganese(II) and N₂O. The reactions follow simple first-order kinetics in the presence of excess hydroxylamine. There is UV–vis spectral evidence for the intermediate Mn^III,IV complex, [(bipy)₂Mn^III(μ-O)₂Mn^IV(bipy)₂]³⁺, in the presence of excess bipy and deficit hydroxylamine which supports that 1³⁺ and its hydrolytic derivatives are reduced by one-electron steps. Increased extent of aquation at the manganese(IV) centre leads to increased kinetic activity in the order: 1³⁺ < 2⁴⁺ < 3³⁺. The rate of reduction increased with an increase in the concentration of hydroxylamine but decreased with increase in c_bipy and c_OAc. The mild oxidising character of the complex ion along with major structural changes associated with one-electron oxidation of hydroxylamine disfavours an outer-sphere pathway. The overall first-order rate constants decrease linearly with increased mol% of D₂O suggesting proton-coupled electron transfer pathways.