Electronic structures of copper(II) complexes of tetradentate hydroquinone-containing Schiff bases†

Abstract

Reaction of 2,5-dihydroxybenzaldehyde with 0.5 equivalent of 1,2-diaminoethane, trans-1,2-diaminocyclohexane or 1,2-diaminobenzene in the absence or presence of hydrated Cu(O₂CMe)₂·H₂O in refluxing MeOH respectively afforded the ligands H₂L and complexes [Cu(L)] [H₂L = N,N′-bis(2,5-dihydroxybenzylidene)-1,2-diaminoethane (H₂L¹), N,N′-bis(2,5-dihydroxybenzylidene)-trans-1,2-diaminocyclohexane (H₂L²), N,N′-bis(2,5-dihydroxybenzylidene)-1,2-diaminobenzene (H₂L³)] in yields of 66–86%. Using X- and Q-band EPR and ¹H and ¹⁴N X-band ENDOR data, the following fractional unpaired spin densities were calculated: ρ{Cu} = 0.75, ρ{N} = 0.07, ρ{O} ≤ 0.04, ρ{H} = 0.01. Density functional (DF), intermediate neglect of differential overlap with spectroscopic parameterisation (INDO/S) and extended Hückel calculations of [Cu(MeOsalen)] broadly reproduced these results, the DF calculations demonstrating that the phenoxide oxygen lone pair is misdirected away from the Cu–O bond. The cyclic voltammograms of the ligands and complexes in dmf–0.1 M NBuⁿ₄PF₆ at 293 K showed a single oxidation of the two ligand hydroquinone groups, and two principal daughter processes: an irreversible reduction of the initial oxidised quinone, probably in a monoprotonated or metallated form; and a more cathodic reduction and associated reoxidation indicative of a proton- and metal ion-induced electrochemical step–chemical step–electrochemical step reaction.