Substrate Oxidation by Copper−Dioxygen Adducts: Mechanistic Considerations

A series of copper−dioxygen adducts [{CuII(MePY2)R}2(O2)](B(C6F5)4)2 (1R), systematically varying in their electronic properties via ligand pyridyl donor substituents (R = H, MeO, and Me2N), oxidize a variety of substrates with varying C−H or O−H bond dissociation enthalpies. Detailed mechanistic studies have been carried out, including investigation of 1R thermodynamic redox properties, 1R tetrahydrofuran (THF) and N,N‘-dimethylaniline (DMA) oxidation kinetics (including analyses of substrate dicopper binding equilibria), and application of mechanistic probes (N-cyclopropyl-N-methylaniline (CMA) and (p-methoxyphenyl)-2,2-dimethylpropanol (MDP)), which can distinguish if proton-coupled electron-transfer (PCET) processes proceed through concerted electron-transfer proton-transfer (ETPT) or consecutive electron-transfer proton-transfer (ET/PT) pathways. The results are consistent with those of previous complementary studies; at low thermodynamic driving force for substrate oxidation, an ET/PT is operable, but once ET (i.e., substrate one-electron oxidation) becomes prohibitively uphill, the ETPT pathway occurs. Possible differences in coordination structures about 1Me2N/1MeO compared to those of 1H are also used to rationalize some of the observations.