Cuprous complexes and dioxygen. Part 11. Concomitant one‐ and two‐electron reduction of O2 by the Cu ion

The autoxidation of CuI in aqueous MeCN has been studied using a Clark oxygen electrode in the presence and absence of Cu11. The reaction is inhibited by Cu11 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 CuI under the influence of Cu2+ ion. Detailed kinetics analysis of a total of 275 measurements has shown that an unstable primary adduct CuO+2 decomoses to give .O or HO , depnding on pH, and also reacts directly with a second Cu+ ion, avoiding one‐electrton reduction of O2 by this path. Reaction of HO is faster with CuI than with Cu11 by a factor of 20, and single‐electron transfer within CuO+2 to Cu2+ and .O predominates over reaction with a second copper ion for [CuItot] < 2. 10−3M in the absence of Cu2+. The most likely value for the reaction of .O with CuI is 5.3 · 108 M−1S−1, but even this high rate constant is at the limit of significance. All secondary reactions followinfg the initial formation of CuO+2 are shown to be very fast, a fact that should be properly considered in the discussion of mechanisms of copper‐catalyzed oxidations and oxygenations.