Surface-Modified Electrodes Based on Nickel(II) and Copper(II) Bis(salicylaldimine) Complexes

Surface-modified electrodes have been prepared by the oxidative polymerization of nickel(II) N,N'-ethylenebis(salicylaldimine) {[Ni(H-sal(2)en)]}, copper(II) N-N'-ethylenebis(salicylaldimine) {[Cu(H-sal(2)en)]}, and substituted nickel(II) and copper(II) bis(salicylaldimine) complexes. The modified electrodes were prepared in weak-donor solvents by repetitively cycling the potential in the oxidative region or by constant-potential electrolysis. For [Ni(H-sal(2)en)], the surface coverage increased linearly for the first several voltammetric scans, and the initial rate of film growth increased linearly with the concentration of [Ni(H-sal(2)en)]. For [Cu(H-sal(2)en)], the rate of film growth decreased steadily from the initial scan. The resulting modified electrodes exhibited single reversible redox waves of modest stability in fresh electrolyte solution. Scan rate dependence of the current and peak potential separations are consistent with slow electron transfer for surface-immobilized redox couples. Electrodes prepared in various weak-donor solvents (acetonitrile, acetone, methylene chloride) showed essentially identical electrochemical behavior in fresh acetonitrile electrolyte, although scanning electron microscopy revealed a difference in surface morphology. The properties of this new family of surface-modified electrodes are discussed in terms of the proposed mechanism for polymer film formation and electron transfer.