Metal-centered redox chemistry of substituted cobalt phthalocyanines adsorbed on graphite and correlations with MO calculations and Hammett parameters.: Electrocatalytic reduction of a disulfide

We have studied the metal-centered redox chemistry of unsubstituted cobalt-phthalocyanine (Co-Pc) and the cobalt complexes of the substituted ligands, cobalt-octaethylhexyloxyphthalocyanine (CoOEHPc), cobalt-tetramethoxyphthalocyanine (CoMeOPc), cobalt-tetraneopentoxyphthalocyanine (CoTNPPc), cobalt-tetraaminophthalocyanine (CoTAPc), cobalt-tetrasulfonatophthalocyanine (CoTSPc) and cobalt hexadecafluorophthalocyanine (CoPcF16) adsorbed on ordinary pyrolytic graphite (OPG). These complexes exhibit two distinct metal-centered reversible redox processes attributed to the Co(II)/Co(I) and Co(III)/Co(II) couples. The redox potentials of these couples are shifted according to the electron withdrawing or donating character of the groups located at the periphery of the phthalocyanine ligand, as seen by comparison to molecular orbital theoretical calculations. Correlations between the redox potential and the Hammett parameters of the substituents provide good linear correlations. The slopes of the linear free energy correlations give values of 0.41 and 0.49 V sigma (-1) for the Co(III)/Co(II) and Co(II)/Co(I) redox couples, respectively. We have also tested the electrocatalytic activity of these adsorbed phthalocyanines for the reduction of 2-hydroxyethyldisulfide, A plot of log i (current density at constant potential) versus the Co(II)/Co(I) redox potential gives a linear correlation where the rate of the reaction increases with the driving force of the catalyst, suggesting that that the reaction takes place via an outer-sphere mechanism. (C) 2000 Elsevier Science B.V. All rights reserved.