In this work we discuss different approaches for achieving electrodes modified with N(4) macrocyclic complexes for the catalysis of the electrochemical oxidation of thiols. These approaches involve adsorption, electropolymerization and molecular anchoring using self assembled monolayers. We also discuss the parameters that determine the reactivity of these complexes. Catalytic activity is associated with the nature of the central metal, redox potentials and Hammett parameters of substituents on the ligand. Correlations between catalytic activity (log i at constant E) and the redox potential of catalysts for complexes of Cr, Mn, Fe, Co, Ni and Cu are linear with an increase of activity for more positive redox potentials. For a great variety complexes bearing the same metal center (Co) correlations between log i and E(o') of the Co(II)/Co(I) couple have the shape of an unsymmetric volcano. This indicates that the potential of the Co(II)/Co(I) couple can be tuned using the appropriate ligand to achieve maximum catalytic activity. Maximum activity probably corresponds to a DeltaG of adsorption of the thiol on the Co center equal to zero, and to a coverage of active sites by the thiol equal to 0.5.

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The electrocatalytic activity of a series of Co macrocyclic complexes confined on graphite electrodes for the oxidation of thiols : 2mercaptoethanol, 2-mercaptoacetate, 2-aminoethanethiol and l-cysteine give volcano correlations between catalytic activity of the metallo complexes and the formal potential of the complex. The formal potential that gives the maximum activity correlates linearly with the pKa of the thiol. We have extended previous studies for the oxidation of hydrazine using Co and Fe macrocyclics and obtain volcano correlations for this reaction as well. The slope of the rising region of the volcano (where the catalytic activity increases with the driving force of the catalyst) gives values of ca. +0.120 V/decade which is the theoretical value when using a Langmuir isotherm for the adsorbed thiol or hydrazine molecule on the reacting site. These results indicate that is possible to "tune" the redox potential of the complexes to attain maximum activity.