Volcano correlations for the reactivity of surface-confined cobalt N4-macrocyclics for the electrocatalytic oxidation of 2-mercaptoacetate

We have investigated the electrocatalytic activity of several substituted and unsubstituted cobalt-phthalocyanines of substituted tetraphenyl porphyrins and of vitamin B-12, for the electro-oxidation of 2-mercaptoacetate, with the complexes pre-adsorbed on a pyrolytic graphite electrode. Several N4-macrocyclic were used to have a wide variety of Co(II)/(I) formal potentials. The electrocatalytic activity, measured as current at constant potential, increases with the Co(II)/(I) redox potential for porphyrins as Co-pentafluorotetraphenylporphyrin < Co-tetrasulfonatotetraphenylporphyrin < Co-2,2',2 '',2'''tetra-aminotetraphenylporphyrin and decreases for cobalt phthalocyanines as Co-3,4-octaethylhexyloxyphthalocyanine > Co-octamethoxyphthalocyanine > Co-tetranitrophthalocyanine Co-tetraaminophthalocyanine > Co-unsubstituted phthalocyanine > Co-tetrasulfonatophthalocyanine > Co-perfluorinated phthalocyanine. Vitamin B-12 exhibits the maximum activity. A correlation of log I (at constant potential) versus the Co(II)/(I) formal potential of the catalysts gives a volcano curve. This clearly shows that the search for better catalysts for this reaction point to those N-4-macrocyclic complexes with Co(II)/(I) formal potentials close to -0.84 V versus SCE, which correspond to an optimum situation for the interaction of the thiol with the active site.