ELECTRON-TRANSFER .117. COPPER CATALYSIS IN THIOL REDUCTION OF BOUND SUPEROXIDE - A 2ND MECHANISTIC VARIATION

Reduction of the binuclear superoxo complex of cobalt(III) [Co2(O2)(NH 3)10]5+ by 2-aminoethanethiol is specifically catalyzed by dissolved C u(II). Each superoxide cation oxidizes one RSH, yielding the disulfide R2S2 and the peroxo complex, [Co2(O2)(NH3)10]4+. Although some kineti c profiles for this reaction, measured with the reductant (RedH-2+) in excess, are nearly exponential, most exhibit a prominent nearly linea r component during the initial stages of reaction. Curves for 17 runs, carried out at pH 2.9-4.3 with [thiol] = (5-40) X 10(-4) M and [Cu2+] = (7-200) X 10(-7) M are consistent with a sequence (eqs 2-8 in text) featuring the generation of Cu(I) by internal electron transfer (k = 12 s-1, 25-degrees-C) within a Cu(II)-thiyl precursor, followed by a r apid reduction of the superoxide (k = 6 X 10(4) M-1 s-1) by a Cu(I)-th iyl complex. Detailed rate dependencies on [thiol] and pH indicate the precursor to be Cu(II)(Red-)(RedH) and the reactive intermediate to b e Cu(I)(Red-)(RedH). The system is characterized by several very large association constants (10(15)-10(24)) for both oxidation states of co pper with chelating thiyl ligands. Reductions of the superoxo complex by thiyl-ligated Cu(I), by Cr2+, by Eu2+, and by V2+ appear to be oute r-sphere processes. Comparison of the rate constant for the Cu(I)-supe roxide reaction with the (very nearly equal) value for reduction by Eu 2+, in conjunction with the Marcus model for outer-sphere reactions, l eads to an estimated self-exchange rate of 10(2)-10(3) M-1 s-1 for Cu( I,II) in our system, a value slightly below that for S4-ligated copper centers but over 10(11) as great as that for aqua-substituted ions. T he present system differs from copper-catalyzed reductions, using both ascorbate and organic thiols, of [Co2(O2)(CN)10]5-, for which past ev idence supports inner-sphere attack by Cu(I). Unlike thiol reductions of the cyano-substituted oxidant, the present system exhibits no inhib ition at high concentrations of the reductant.