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

Abstract

Kinetic studies of the reactions of two previously characterized copper(III)-hydroxide complexes (LCuOH and ^NO₂LCuOH, where L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and ^NO₂L = N,N′-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of para substituted phenols (^XArOH 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 < ^NO₂LCuOH. 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 ^NO₂ArOH 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 ^NO₂ArO⁻ as a reaction product, observation of an intermediate [LCu(OH₂)]⁺ species derived from proton transfer from ^NO₂ArOH to LCuOH, and thermodynamic arguments indicating that initial PT should be competitive with CPET.