Structural and Spectroscopic Characterization of Copper(III) Complexes and Subsequent One-Electron Oxidation Reaction and Reactivity Studies

The formation of Cu­(III) species are often invoked as the key intermediate in Cu-catalyzed organic transformation reactions. In this study, we synthesized Cu­(II) (1) and Cu­(III) (3) complexes supported by a bisamidate–bisalkoxide ligand consisting of an ortho-phenylenediamine (o-PDA) scaffold and characterized them through an array of spectroscopic techniques, including UV–visible, electron paramagnetic resonance, X-ray crystallography, and 1H nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy. The Cu–N/O bond distances in 3 are ∼0.1 Å reduced compared to 1, implying a significant increase in 3’s overall effective nuclear charge. Further, a Cu­(III) complex (4) of a bisamidate–bisalkoxide ligand containing a trans-cyclohexane-1,2-diamine moiety exhibits nearly identical Cu–N/O bond distances to that of 3, inferring that the redox-active o-PDA backbone is not oxidized upon one-electron oxidation of the Cu­(II) complex (1). In addition, a considerable difference in the 1s → 4p and 1s → 3d transition energy was observed in the X-ray absorption near-edge structure data of 3 vs 1, which is typical for the metal-centered oxidation process. Electrochemical measurements of the Cu­(II) complex (1) in acetonitrile exhibited two consecutive redox couples at −0.9 and 0.4 V vs the Fc+/Fc reference electrode. One-electron oxidation reaction of 3 further resulted in the formation of a ligand-oxidized Cu complex (3a), which was characterized in depth. Reactivity studies of species 3 and 3a were explored toward the activation of the C–H/O–H bonds. A bond dissociation free energy (BDFE) value of ∼69 kcal/mol was estimated for the O–H bond of the Cu­(II) complex formed upon transfer of hydrogen atom to 3. The study represents a thorough spectroscopic characterization of high-valent Cu complexes and sheds light on the PCET reactivity studies of Cu­(III) complexes.