Kinetics of oxidation of nitroxyl radicals by a coordinated superoxide

In aqueous acetate buffer media (pH 4.5-5.9), nitroxyl radicals (X) TEMPO (X = H and 4-oxo) reduce Co-III-bound superoxo ligand in {(mu-superoxo-bis[pentaamminecobalt(III)]}(5+) (1) to its corresponding hydroperoxo complex, {mu-hydroperoxo-bis[pentaamminecobalt(III)]}(5+) (2), which decomposes rapidly in solution. In the presence of excess [(X) TEMPO] over [1], the reactions obeyed acid catalyzed first-order kinetics in [1]. The stronger reducing agent TEMPO (E = 0.73 V) reduces 1 faster than the weaker reductant, 4-oxo-TEMPO (E = 0.91 V). The observed rates of reactions of (X) TEMPO with 1 were found to increase in media enriched with D2O in comparison to that in H2O suggesting a pre-equilibrium protonation on 1 prior to electron transfer. (C) 2012 Elsevier B. V. All rights reserved.