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 molecula r oxygen. We have found for both reactions that when comparing the activity of substituted phthalocyanines of the same metal, plots of log k at consta nt potential versus the redox potential of the metallophthalocyanine, the r ate of the reaction decreases with increasing driving force of the catalyst . In this work we present evidence that this behavior can be rationalized i n terms of the donor-acceptor intermolecular chemical hardness (eta (DA)). The energies of frontier orbitals involved in the estimation of eta (DA) we re calculated using the PM3 semi-empirical theoretical method. For the dono r-acceptor pairs the lower is the hardness, the higher is the reactivity. ( C) 2001 Elsevier Science Ltd. All rights reserved.