Reduction mechanism of a coordinated superoxide by thiols in acidic media

Abstract

In weakly acidic media ([H⁺], 0.01–0.06 M), 2-mercaptoethanol (mercap, RSH), thioglycolic acid (tga, R′SH) and L-cysteine (cys, R′′SH) reduce the superoxo ligand of the complex ion, {μ-amido-μ-superoxo-bis[tetraamminecobalt(III)]}⁴⁺ (1) to its corresponding hydroperoxo complex, {μ-amido-μ-hydroperoxo-bis[tetraamminecobalt(III)]}³⁺ (2). During this act, RSH and R′SH are quantitatively oxidized to their respective disulfides. However, cysteine (R′′SH) is converted to a mixture of ∼80% of the disulfide, cystine and ∼20% to cystine sulfinic acid. Cystine itself is not a source of the sulfinic acid. Dissolved copper, even at the impurity level, dramatically catalyzes the reaction such that the direct reactions are inaccessible. Nevertheless, the catalyzed path can be masked completely with 0.20 mM dipicolinic acid and it can be determined for the first time that, the direct reactions are first-order in [1], in [total thiol] and in basicity. Rate decreases linearly with increasing mol% of D₂O in the solvent. H-atom (H⁺ + e) transfer from thiols to superoxide in 1 seems logical for the conversion of 1 to 2.