Reactivity of electrodes modified with substituted metallophthalocyanines. Correlations with redox potentials, Hammett parameters and donor–acceptor intermolecular hardness

We have carried out a systematic study of the electrocatalytic activity of metallophthalocyanines (M(II)-Pcs) adsorbed on graphite electrodes for the electro-oxidation of thiols (R-SH) and for the electroreduction of molecular oxygen. We have found for both reactions that when comparing the activity of substituted phthalocyanines of the same metal, plots of log k at constant potential versus the redox potential of the metallophthalocyanine, the rate of the reaction decreases with increasing driving force of the catalyst. In this work we present evidence that this behavior can be rationalized in terms of the donor–acceptor intermolecular chemical hardness ( η DA). The energies of frontier orbitals involved in the estimation of η DA were calculated using the PM3 semi-empirical theoretical method. For the donor–acceptor pairs the lower is the hardness, the higher is the reactivity.