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

Abstract

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 Cu^IIL complex, 1. X-Ray crystallography of 1 shows the Cu^II 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 E_½ = 0.89 V (ΔE = 81 mV, vs. Ag/AgCl) and the second at E_½ = 1.13V (ΔE = 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 Cu^II centre and the bound phenoxyl radical. The oxidised species catalyses the oxidation of benzyl alcohol to benzaldehyde.