SUBSTITUENT EFFECTS IN THE ELECTROCHEMICAL-BEHAVIOR OF SOME MIXED-LIGAND COPPER(II) CHELATES CONTAINING BETA-KETOENOLS AND L,2-DIAMINES

The electrochemical behaviour of a series of bis(beta-dionato)copper(I I) compounds, Cu(beta dionato)(2) and mixed-ligand cationic chelates c ontaining N,N-dimethyl-N'-ethyl-1,2-diaminoethane, [Cu(beta-dionato)dm een]+, was investigated in dimethylsulphoxide solutions at carbon fibr e electrodes in the presence of 0.1 mol dm(-3) NaNO3, at 25 degrees C. Two cathodic peaks are observed and the irreversibility of the second is due to the instability and decomposition of the reduced species. T he one-electron reduction potentials and the reversibility of the hete rogeneous electron transfer depends on the substituents within the bet a-dionato moiety. Despite the strong influence of the substituents, th e linear correlation with the ionization energies of the Cu(beta-diona to)(2) chelates suggest that the electron transfer processes are assoc iated with the singly occupied molecular orbital which is essentially localized on the copper(II) ion. The interdependence of the E(1/2) val ues on the pK(a) of the beta-ketoenols and the Hammett sigma functions of the substituents indicates that inner-sphere electronic effects ar e operative even when other ligands are present.