Kinetics and mechanism of the oxidation of hydroxylamine by a {Mn3O4}4+ core in aqueous acidic media

Abstract

In this work we report the kinetics of oxidation of hydroxylamine by a trinuclear Mn(IV) oxidant, [Mn₃(μ-O)₄(phen)₄(H₂O)₂]⁴⁺ (1, phen = 1,10-phenanthroline), in aqueous solution over a pH range 2.0–4.0. The trinuclear Mn(IV) species (1) deprotonates in aqueous solution at physiological pH: 1 ⇌ 2 + H⁺; pK₁ = 4.00 (± 0.15) at 25.0 °C, I = 1.0 (M) NaNO₃. Both 1 and 2 are reactive oxidants reacting with the conjugate acid of hydroxylamine, viz.NH₃OH⁺ where the deprotonated oxidant 2 reacts faster. This finding is in contrast to a common observation and belief that protonated oxidants react quicker than their deprotonated analogues. Mn^IV₃ to Mn^II transition in the present reaction proceeds through the intervention of a spectrally detected mixed-valent Mn^IIIMn^IV dimer that quickly collapses to Mn^II. The rate of the reaction was found to be lowered in D₂O-enriched media in comparison to that in pure H₂O media. An initial one electron one proton transfer to Mn^IV₃ (electroprotic; 1e, 1H⁺) could be mechanistically conceived as the rate step. We also demonstrate by means of high level DFT studies that, among the two sets of Mn(IV) atoms in the trinuclear oxidant, the unique one that is coordinated with two phen ligands and two oxo-bridges is reduced to Mn(III) at the rate step. This is explained based on energetic and spin density calculations. Moreover, this result agrees with the charge distribution on the Mn atoms of the trinuclear complex.