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 2 ] (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 X ArOH (X = para substituent NO 2, CF 3, Cl, H, Me, t Bu, OMe, or NMe 2 ) at low temperatures were studied. The reaction with TEMPOH occurs rapidly ( k = 35.4 ± 0.3 M –1 s –1 ) by second-order kinetics to yield TEMPO • and [LCuOOH] − on the basis of electron paramagnetic resonance spectroscopy, the production of H 2 O 2 upon treatment with protic acid, and independent preparation from reaction of [NBu 4 ][LCuOH] with H 2 O 2 ( K eq = 0.022 ± 0.007 for the reverse reaction). The reactions with X 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 a, E 1/2 ) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO 2, CF 3, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe 2 (data for X = H, Me, t 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 2 ] + complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.