Steric, geometrical and solvent effects on redox potentials in salen-type copper(II) complexes.

Two diastereomers of the Schiff base ligand [N,N'-bis(2'-hydroxyphenyl)phenylmethylidene]-1,2-diamino-1,2-diphenylethane (H2L) were used for the analyses of the redox behaviour of the copper(II) complexes [Cu(L-R,L-R/S,L-S)] (rac-1) and [Cu(L-R,L-S)] (meso-1). Both complexes were structurally characterised by X-ray crystallographic studies and showed square planar geometries. The reduction potential of Cu-II to Cu-I for meso-1 was higher than that for rac-1. This is due to the steric effect of the phenyl substituents and the geometrical change in the copper(I) state, which is supported by DFT calculations. A red shift of the absorption spectrum was observed for meso-1 in the visible region by the change of solvent from dichloromethane to pyridine, while rac-1 did not show a significant change. The effect of solvent on the reduction potential was found to be relatively small. The geometrical effect is more important for understanding the electrochemical behaviour in this system.