CUPROUS COMPLEXES AND DIOXYGEN .2. CONCOMITANT ONE-ELECTRON AND 2-ELECTRON REDUCTION OF O-2 BY THE CU-AQ+ ION

The autoxidation of Cu(I) in aqueous MeCN has been studied using a Clark oxygen electrode in the presence and absence of Cu(II). The reaction is inhibited by Cu(II) in the pH range of 0.5 to 5.0, reaching a lower limiting value at the highest concentrations. The reaction order changes from 1 to 2 with respect to Cu(I) under the influence of Cu2+ ion. Detailed kinetics analysis of a total of 275 measurements has shown that an unstable primary adduct CuO2+ decomposes to give .O2- or HO2., depending on pH, and also reacts directly with a second Cu+ ion, avoiding one-electron reduction of O2 by this path. Reaction of HO2. is faster with Cu(I) than with Cu(II) by a factor of 20, and single-electron transfer within CuO2+ to Cu2+ and .O2- predominates over reaction with a second copper ion for [Cu(tot)I] value for the reaction of .O2- with Cu(I) is 5.3.10(8) M-1 S-1, but even this high rate constant is at the limit of significance. All secondary reactions following the initial formation of CuO2+ are shown to be very fast, a fact that should be properly considered in the discussion of mechanisms of copper-catalyzed oxidations and oxygenations.