Abstract

In this paper we have reinvestigated the correlations between the electrocatalytic activity of metallophthalocyanines for O2 reduction with the redox properties of the metal in the complex. We have extended these studies to the electro-oxidation of thiols. For O2 reduction, plots of log k at constant potential versus the M(III)/M(II) redox potential of the metallophthalocyanine containing Cr, Mn, Fe, and Co, give two parallel straight lines of slopes close to 2RT/F, instead of a volcano plot, as reported earlier. These lines correspond to two families with one including only Co complexes. For the oxidation of the thiols L-cysteine and 2-mercaptoethanol, single straight lines are obtained for each case when plotting log k versus the M(II)/M(I) redox potential of the metallophthalocyanine. The slope is close to 2RT/F. For O2 reduction, log k decreases with driving force, whereas for the oxidation of thiols, it increases. For O2 reduction, for the oxidation of L-cysteine and of 2-mercaptoethanol when activity (as log k) is compared along the transition series volcano-shaped curves are obtained, which illustrates the concept of 'tuning' frontier orbital energies for maximum reaction rates. Preliminary calculations of the electronic coupling between the metal in a TAP (tetraazaporphyrin) environment and the sulfur atom in a thiol show that this parameter exhibits the same trend (volcano correlation) found when log k is plotted versus the number of d-electron in the central metal. © 1998 Elsevier Science Ltd. All rights reserved.