Carboxylate Structural Effects on the Properties and Proton-Coupled Electron Transfer Reactivity of [CuO₂CR]²⁺ Cores

A series of complexes {[NBu₄][LCu^II(O₂CR)] (R = -C₆F₅, -C₆H₄(NO₂), -C₆H₅, -C₆H₄(OMe), -CH₃, and -C₆H₂(ⁱPr)₃)} were characterized (with the complex R = -C₆H₄(m-Cl) having been published elsewhere (Mandal et al. J. Am. Chem. Soc. 2019, 141, 17236)). All feature N,N′,N″-coordination of the supporting L^2- ligand, except for the complex with R = -C₆H₂(ⁱPr)₃, which exhibits N,N′,O-coordination. For the N,N′,N″-bound complexes, redox properties, UV-vis ligand-to-metal charge transfer (LMCT) features, and rates of hydrogen atom abstraction from 2,4,6,-tri-t-butylphenol using the oxidized, formally Cu(III) compounds LCu^III(O₂CR) correlated well with the electron donating nature of R as measured both experimentally and computationally. Specifically, the greater the electron donation, the lower is the energy for LMCT and the slower is the reaction rate. The results are interpreted to support an oxidatively asynchronous proton-coupled electron transfer mechanism that is sensitive to the oxidative power of the [Cu^III(O₂CR)]²⁺ core.