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 1 H 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.