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

摘要

[Mn-3(IV)(mu-O)(4)(phen)(4)(H2O)(2)](4+) (1, phen = 1,10-phenanthroline) equilibrates with its conjugate base [Mn-3(mu-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 reversible arrow 2 + H+, pK(a) = 4.00 (+/- 0.15) at 25.0 degrees 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-3(IV) to Mn-II transition in the present observation proceeds through the intermediate generation of the spectrally characterised mixed-valent (MnMnIV)-Mn-III 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.