Synthesis, spectroscopy and redox chemistry of bis(n-aryl-3,5-di-tert-butylsalicylaldiminato)copper(II) complexes

A series of new bis(N-aryl-3,5-di-tert-butylsalicylaldiminato)copper(II) complexes (I), prepared from 3,5-di-tert-butylsalicylaldehyde and mono-substituted anilines, X-C6H4NH2(X = H, o-F, Cl, Br, CH3, CH3O; p-F, Cl, Br, CH3, CH3O, t-Bu and 5,6-benzo), and their spectroscopic properties as well as electrochemistry and reactivity toward PPh3 are described. Comparison of g ‖ (2.223–2.249), A ‖ (150–170 G) tensors and the axial symmetry parameter, g ‖/A ‖, (140–155 cm) of I with those of model Cu(II) complexes reveals that the coordination site is distorted from square planar toward tetrahedral geometry. The di-tert-butyl groups on the salicylaldehyde moiety unlike those introduced on the aniline rings do not facilitate reactivity toward PPh3. Electrochemical studies of I in acetonitrile solutions reveal highly irreversible behavior, due to chemical or stereochemical changes subsequent to electron transfer. In the oxidation of some complexes with (NH4)2Ce(NO3)6 in acetonitrile at 300 K, along with decreasing ESR signals of Cu(II), the appearance of coordinated phenoxyl radical signals centered at g = 2.004 were detected. The CV studies on acetonitrile solutions of I revealed well-resolved quasi-reversible redox waves at = 0.61–0.99 V and = 1.09–1.23 V (vs. Ag/AgCl) attributable to ligand-centered oxidation processes to yield corresponding coordinated phenoxyl radical species such as Cu(II)(phenoxyl)(phenolate) and Cu(II)(phenoxyl)2.