Cyclic voltammetric characterization of rate constants for conformational change in an electron-transfer square scheme involving a copper(II)/(I) macrocyclic tetrathiaether complex

Abstract

With a glassy-carbon ultramicroelectrcde and appropriate circuitry, cyclic voltammograms have been generated for solutions containing the Cu/'( [ 141aneS4) system at scan rates up to 60 kV s-l in 80% methanol (w/w) at 25 OC. These rapid-scan voltammograms provide direct evidence for the existence of metastable intermediates of both the Cu'L and the CuL species in conformance with our previously proposed square scheme mechanism. On the basis of the experimental data, estimates have been made for all the specific homogeneous and heterogeneous rate constants associated with this square scheme. The values of these constants have then been further refined using digital simulation. The resulting simulated voltammograms are shown to correlate reasonably well with the experimental observations at even the most rapid scan rates achieved. The availability of the specific rate constant values representing CulL and CuL conformational interconversions makes it possible to predict the conditions under which conformational change of the CulL species should become rate limiting, resulting in the appearance of gated electron-transfer behavior in homogeneous cross reactions.