Mechanistic studies on oxidation of nitrite by a {Mn3O4}4+ core in aqueous acidic media

Abstract

[Mn^IV₃(µ-O)₄(phen)₄(H₂O)₂]⁴⁺ (1, phen = 1,10-phenanthroline) equilibrates with its conjugate base [Mn₃(µ-O)₄(phen)₄(H₂O)(OH)]³⁺ (2) in aqueous solution. Among the several synthetic multinuclear oxo- and/or carboxylato bridged manganese complexes known to date containing metal-bound water, to the best of our knowledge, only 1 deprotonates (1 ⇌ 2 + H⁺, pK_a = 4.00 (±0.15) at 25.0 °C, I = 1.0 M, maintained with NaNO₃) at physiological pH. An aqueous solution of 1 quantitatively oxidises N(III) (HNO₂ and NO₂⁻) to NO₃⁻ within pH 2.3–4.1, the end manganese state being Mn^II. Both 1 and 2 are reactive oxidants in the title redox. In contrast to a common observation that anions react quicker than their conjugate acids in reducing metal centred oxidants, HNO₂ reacts faster than NO₂⁻ in reducing 1 or 2. The observed rates of nitrite oxidation do not depend on the variation of 1,10-phenanthroline content of the solution indicating that the Mn^IV-bound phen ligands do not dissociate in solution under experimental conditions. Also, there was no kinetic evidence for any kind of pre-equilibrium replacement of Mn^IV-bound water by nitrite prior to electron transfer which indicates the substitution-inert nature of the Mn^IV-bound waters and the 1,10-phenanthroline ligands. The Mn^IV₃ to Mn^II transition in the present observation proceeds through the intermediate generation of the spectrally characterised mixed-valent Mn^IIIMn^IV dimer that quickly produces Mn^II. The reaction rates are substantially lowered when solvent H₂O is replaced by D₂O and a rate determining 1e, 1H⁺ electroprotic mechanism is proposed.