ELECTRON-TRANSFER .113. REDUCTION OF BOUND SUPEROXIDE WITH THIOLS - CATALYSIS BY COPPER(II)

The binuclear superoxo complex of cobalt(III) [Co2(O2)(CN)10]5- is reduced exceedingly slowly by the mercapto derivatives 2-aminoethanethiol and cysteine at pH 4.5-7.2. Reaction is catalyzed dramatically by dissolved copper but by none of the other usual transition metal centers. Each superoxide anion oxidizes one RSH, yielding the disulfide R2S2 and the binuclear peroxo complex [Co2(O2)(CN)10]6-, which rapidly decomposes to [Co(CN)s(H2O)]2-and H2O2. All reactions are first order in copper with the unipositive state the active reductant. Transformations are markedly inhibited by high concentrations of thiol, pointing to partial conversion of the Cu(I) center to a much less reactive thiol adduct. With 2-aminoethanethiol, conversion of added Cu(II) to Cu(I) appears to be complete within a few seconds of mixing. Reductions with cysteine in deficiency give linear decay profiles with slopes independent of both [RSH] and [oxidant]. Under these conditions, rates are fixed by the unimolecular generation of Cu(I) from Cu(II)(SR). As [RSH] is raised, progressive conversion of the more active to the less active Cul reductant is reflected in a change in the rate-determining step, and reactions become first order in [oxidant]. Integration of differential equations based upon the proposed sequence (eqs 7-13 in text) reproduces the observed profiles, not only in the linear and exponential regions but also in an intermediate region where decay curves are of mixed order. Dissimilarities between the reductive actions of the two thiols are related to the differing stabilities of the Cu(II)(SR) intermediates. The catalytic effectiveness of Cu(I) in thiol systems, in comparison to Fe(II), is attributed both to the more negative potential of Cu(I,II) and to the lesser substitution lability of thiol-bound Fe(II).