Electron Transfer. 129. Copper Catalysis in the Thiol Reduction of Oxime-Bound Nickel(IV)(1)

Solutions of the Ni(IV) complex of the dianion of 2,6-diacetylpyridine dioxime (chelate II in text) are reduced very slowly by 2-aminoethanethiol at pH 2.3-3.0, but this reaction is catalyzed dramatically and specifically by dissolved copper, with Cu(I) the active reductant. When the [thiol]/[Ni(IV)] ratio exceeds 1.6, each Ni(IV) oxidizes two molecules of thiol, forming Ni(II) and R(2)S(2). At low concentrations of catalyst and reductant, reaction profiles are almost exponential, but at higher concentrations of either, curves become progressively more nearly linear. Reactions are sharply retarded by increases in acidity. Profiles for 14 runs, carried out with [H(+)] = 0.001-0.0040 M, [Ni(IV)] = (0.94-1.2) x 10(-)(5) M, [thiol] = (2.0-32) x 10(-)(4) M, and [Cu(2+)] = (2.5-80) x 10(-)(6)M, are consistent with a reaction sequence (eqs 2-10 in text) in which Cu(I) is generated in competing homolyses of the complexes Cu(II)(SRH) and Cu(II)(SRH)(2). Reduction of Ni(IV) appears then to proceed through a Ni(IV)Cu(I) adduct, which can undergo electron transfer (yielding Ni(III) and Cu(II)), either in a unimolecular fashion or, alternatively, as a result of attack by a second Cu(I) species. The Ni(IV)Cu(I) + Cu(I) process is reflected in approach to second-order dependences on [Cu(II)] and [thiol] (which generate Cu(I)) at high concentrations of these reagents. Reductions of the Ni(III) intermediate are taken to be much more rapid than those of Ni(IV). Kinetic trends in the present system stand in contrast to the more familiar catalytic patterns such as those seen when the same combination of thiol and catalyst is used to reduce superoxo complexes of cobalt(III). With the latter reactions, decay profiles for the oxidant tend to be exponential at high reagent concentrations but approach linearity at low.