Debanjan Dhar, Gereon M. Yee, Todd F. Markle, James M. Mayer, William B. Tolman

Kinetic studies of the reactions of two previously characterized copper(iii)-hydroxide complexes (LCuOH and (NO2) LCuOH, where L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and (NO2) L = N,N'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of para substituted phenols ((X)ArOH where X = NMe2, OMe, Me, H, Cl, NO2, or CF3) were performed using low temperature stopped-flow UV-vis spectroscopy. Second-order rate constants (k) were determined from pseudo first-order and stoichiometric experiments, and follow the trends CF3 < NO2 < Cl < H < Me < OMe < NMe2 and LCuOH < (NO2) LCuOH. The data support a concerted proton-electron transfer (CPET) mechanism for all but the most acidic phenols (X = NO2 and CF3), for which a more complicated mechanism is proposed. For the case of the reactions between (NO2) ArOH and LCuOH in particular, competition between a CPET pathway and one involving initial proton transfer followed by electron transfer (PT/ET) is supported by multiwavelength global analysis of the kinetic data, formation of the phenoxide (NO2) ArO(-) as a reaction product, observation of an intermediate [LCu(OH2)](+) species derived from proton transfer from (NO2) ArOH to LCuOH, and thermodynamic arguments indicating that initial PT should be competitive with CPET.