(N-Benzyl-bis-N',N''-salicylidene)-cis-1,3,5-triaminocyclohexane copper(II): a novel catalyst for the aerobic oxidation of benzyl alcohol

Reaction of Cu(BF4)2·6H2O with the N3O2 donor ligand H2L (where H2L = N-benzyl-N,N-di-tert-butyl-disalicyl-triaminocyclohexane) results in the formation of a novel CuIIL complex, 1. X-Ray crystallography of 1 shows the CuII centre coordinated by two phenolate oxygens and two imine nitrogens in a distorted square plane with an elongated bond to the amine nitrogen (2.512 Å) in the axial position. EPR spectroscopy of 1 gives g values of g1 = 2.277, g2 = 2.100, g3 = 2.025, and A1 = 15.6 mT which are consistent with the distorted square pyramidal coordination environment determined from the X-ray structure. UV/visible and electrochemical analysis of 1 shows that it undergoes two reversible processes assigned to the successive oxidation of the phenolate oxygens to phenoxyl radicals, the first at E½ = 0.89 V (E = 81 mV, vs. Ag/AgCl) and the second at E½ = 1.13V (E = 84 mV, vs. Ag/AgCl). Chemical oxidation of 1 results in the formation of a species, assigned as [ 1]+ which is EPR silent due to antiferromagnetic coupling between the CuII centre and the bound phenoxyl radical. The oxidised species catalyses the oxidation of benzyl alcohol to benzaldehyde.