Kinetics of oxidation of nitrogen(III) by an oxo-bridged dinuclear manganese(III,IV) complex in weakly acidic media

Abstract

The heterovalent oxo-bridged dinuclear manganese(III,IV) complex [Mn₂O₂(phen)₄]³⁺1 aquates to [Mn₂O₂(phen)₃(H₂O)₂]³⁺2(phen = 1,10-phenanthroline) in phen–Hphen⁺ buffer (pH 4–5). The equilibrium constant for the reaction was kinetically evaluated as K_1H= 35 ± 10. Nitrite ion formed adducts 1N and 2N with 1 and 2 respectively. Their respective association constants, K_1N= 14.1 ± 2 dm³ mol^–1 and K_2N= 50 ± 10 dm³ mol^–1, are small and indicate the outer-sphere nature of the adducts. Adduct 2N is reduced via one-electron steps, ultimately to Mn^II with a rate constant k_2N=(3.33 ± 0.6)× 10^–2 s^–1 at 30.0 °C and I= 0.10 mol dm^–3 for the first step. No kinetic activity of 1N was detectable under the experimental condition employed. Added Mn²⁺ has been found to scavenge phen from the reaction media and thus promotes the formation of 2N and hence the reaction rate.