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 CuII , 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 then studied in detail, and Cu K-edge X-ray Absorption Spectroscopy (XAS) measurements confirmed a CuII -phenoxyl radical complex in solution. Subsequent resonance Raman (rR) and variable temperature 1 H NMR studies, coupled with theoretical calculations, showed that [ 2• ]+ is a triplet ( S = 1) CuII -phenoxyl radical species, with localization of the radical on the more electron-rich aminophenoxide. Attempted isolation of X-ray quality crystals of [ 2• ]+ afforded [ 2H ]+ , with a protonated phenol bonded to CuII , and an additional H-bonding interaction with the SbF6 - counterion. Stoichiometric reaction of dilute solutions of [ 2• ]+ with benzyl alcohol showed that the complex reacted in a similar manner as the oxidized CuII -salen analogue, and does not exhibit a substrate-binding pre-equilibrium as observed for the oxidized bisaminophenoxide CuII -salan derivative.