Reactivity of the copper(iii)-hydroxide unit with phenols

Kinetic studies of the reactions of two previously characterized copper(iii)-hydroxide complexes (LCuOH and LCuOH, where L = , '-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and L = , '-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of substituted phenols ( ArOH where X = NMe , OMe, Me, H, Cl, NO , or CF ) were performed using low temperature stopped-flow UV-vis spectroscopy. Second-order rate constants ( ) were determined from pseudo first-order and stoichiometric experiments, and follow the trends CF < NO < Cl < H < Me < OMe < NMe and LCuOH < LCuOH. The data support a concerted proton-electron transfer (CPET) mechanism for all but the most acidic phenols (X = NO and CF ), for which a more complicated mechanism is proposed. For the case of the reactions between 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 ArO as a reaction product, observation of an intermediate [LCu(OH )] species derived from proton transfer from ArOH to LCuOH, and thermodynamic arguments indicating that initial PT should be competitive with CPET.