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 CoIII-bound superoxo ligand in {(μ-superoxo-bis[pentaamminecobalt(III)]}5+ (1) to its corresponding hydroperoxo complex, {μ-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.

The work addresses how the cyclic nitroxyl radicals react with coordinated superoxide via an acid-catalyzed path.Figure optionsDownload as PowerPoint slideHighlights
► Stronger reductant TEMPO reduces the superoxo complex faster than weaker one, 4-oxo-TEMPO.
► Reactions show prominent acid catalysis.
► Solvent effects suggest a pre-equilibrium protonation on the superoxo complex.