Electronic structure and reactivity studies of a nonsymmetric one-electron oxidized CuII bis-phenoxide complex

Abstract The tetradentate mixed imino/amino phenoxide ligand (N-(3,5-di- tert -butylsalicylidene)-N′-(2-hydroxyl-3,5-di-tert-butylbenzyl))- trans -1,2-cyclohexanediamine (salalen) was complexed with Cu II , and the resulting Cu complex ( 2 ) was characterized by a number of experimental techniques and theoretical calculations. Two quasi-reversible redox processes for 2 , as observed by cyclic voltammetry, demonstrated the potential stability of oxidized forms, and also the increased electron-donating ability of the salalen ligand in comparison to the salen analogue. The electronic structure of the one-electron oxidized [2] + was studied in detail, and Cu K-edge X-ray Absorption Spectroscopy (XAS) measurements confirmed a Cu II -phenoxyl radical complex in solution. Resonance Raman (rR) and variable temperature 1 H NMR studies, coupled with theoretical calculations, showed that [ 2 ] + is a triplet ( S  = 1) Cu II -phenoxyl radical species, with localization of the radical on the more electron-rich aminophenoxide. Attempted isolation of X-ray quality crystals of [ 2 ] + afforded [ 2 H] + , with a protonated phenol bonded to Cu II , and an additional H-bonding interaction with the SbF 6 − counterion. Stoichiometric reaction of dilute solutions of [ 2 ] + with benzyl alcohol showed that the complex reacts in a similar manner as the oxidized Cu II -salen analogue, and does not exhibit a substrate-binding pre-equilibrium as observed for the oxidized bis-aminophenoxide Cu II -salan derivative.