Mechanistic Dichotomy in Proton-Coupled Electron-Transfer Reactions of Phenols with a Copper Superoxide Complex.

The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, L = a bis(arylcarboxamido)pyridine ligand) with 2,2,6,6-tetramethylpiperdine- N-hydroxide (TEMPOH) and the para-substituted phenols ArOH (X = para substituent NO, CF, Cl, H, Me, Bu, OMe, or NMe) at low temperatures were studied. The reaction with TEMPOH occurs rapidly ( k = 35.4 ± 0.3 M s) by second-order kinetics to yield TEMPO and [LCuOOH] on the basis of electron paramagnetic resonance spectroscopy, the production of HO upon treatment with protic acid, and independent preparation from reaction of [NBu][LCuOH] with HO ( K = 0.022 ± 0.007 for the reverse reaction). The reactions with ArOH also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett σ parameter, O-H bond dissociation free energy, p K, E) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO, CF, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe (data for X = H, Me, Bu are intermediate between the extremes). Thermodynamic analysis and comparisons to previous results for LCuOH, a different copper-oxygen intermediate with the same supporting ligand, and literature for other [CuO] complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.