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

A series of complexes {[NBu4][LCuII(O2CR)] (R = -C6F5, -C6H4(NO2), -C6H5,-C6H4(OMe), -CH3, and-C6H4(Pr-i)(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(Pr-i)(3), which exhibits N,N',O-coordination. For the N,N',N-bound complexes, redox properties, UV vis ligand-tometal charge transfer (LMCT) features, and rates of hydrogen atom abstraction from 2,4,6,-tri-t-butylphenol using the oxidized, formally Cu(III) compounds LCuIII(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 [Cun(III)(O2CR)](2+) core.