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 (H(2)L) were used for the analyses of the redox behaviour of the copper(ii) complexes [Cu(L(R,R/S,S))] (rac-) and [Cu(L(R,S))] (meso-). 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- was higher than that for rac-. 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- in the visible region by the change of solvent from dichloromethane to pyridine, while rac- 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.