Electronic structure and reactivity studies of a nonsymmetric one-electron oxidized Cu^II bis-phenoxide complex

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 ¹H NMR studies, coupled with theoretical calculations, showed that [2^[rad]]⁺ 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^[rad]]⁺ afforded [2H]⁺, with a protonated phenol bonded to Cu^II, and an additional H-bonding interaction with the SbF₆⁻ counterion. Stoichiometric reaction of dilute solutions of [2^[rad]]⁺ 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.