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

摘要

The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO2] (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)-Ar-x (X = para substituent NO2, CF3, Cl, H, Me, Bu-t, OMe, or NMe2) 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 H2O2 upon treatment with protic acid, and independent preparation from reaction of [NBu4] [LCuOH] with H2O2 (K-eq = 0.022 +/- 0.007 for the reverse reaction). The reactions with (ArOH)-Ar-x also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett sigma parameter, O-H bond dissociation free energy, pK(a), E-1/2) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO2, CF3, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe2 (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 [CuO2](+) complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.