Structural and Spectroscopic Characterization of Copper(III) Complexes and Subsequent One-Electron Oxidation Reaction and Reactivity Studies
M. Bera, Simarjeet Kaur, Kritika Keshari, Aakash Santra, Dooshaye Moonshiram, Sayantan Paria
Abstract
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.