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

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 H-1 and N-14 X-band ENDOR data, the following fractional unpaired spin densities were calculated: rho{Cu}=0.75, rho{N}=0.07, rho{O}less than or equal to 0.04, rho{H}=0.01. Density functional (DF), intermediate neglect of differential overlap with spectroscopic parameterisation (INDO/S) and extended Huckel 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 (NBu4PF6)-P-n 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.