Substituent effects in the electrochemical behaviour of some mixed-ligand copper(II) chelates containing β-ketoenols and 1,2-diamines

Abstract

The electrochemical behaviour of a series of bis(β-dianato)copper(II) compounds, Cu(β-dionato) 2 and mixed-ligand cationic chelates containing N,N-dimethyl-N′-ethyl-1,2-diaminoethane, [Cu(β-dianato)dmeen] +, was investigated in dimethylsulphoxide solutions at carbon fibre electrodes in the presence of 0.1 mol dm −3 NaNO 3 at 25°C. Two cathodic peaks are observed and the irreversibility of the second is due to the instability and decomposition of the reduced species. The one-electron reduction potentials and the reversibility of the heterogeneous electron transfer depends on the substituents within the β-dionato moiety. Despite the strong influence of the substituents, the linear correlation with the ionization energies of the Cu(β-dianato) 2 chelates suggest that the electron transfer processes are associated with the singly occupied molecular orbital which is essentially localized on the copper(II) ion. The interdependence of the E 1 2 values on the p K a of the β-ketoenols and the Hammett σ functions of the substituents indicates that inner-sphere electronic effects are operative even when other ligands are present.