Linear versus volcano correlations between electrocatalytic activity and redox and electronic properties of metallophthalocyanines

Abstract

In this paper we have reinvestigated the correlations between the electrocatalytic activity of metallophthalocyanines for O 2 reduction with the redox properties of the metal in the complex. We have extended these studies to the electro–oxidation of thiols. For O 2 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 2 RT/ 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 2 RT/ F. For O 2 reduction, log k decreases with driving force, whereas for the oxidation of thiols, it increases. For O 2 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.