Inverted Linear Correlation Between the Catalytic Activity of Iron Phthalocyanines and the Formal Potential of the Catalyst in the Electrooxidation of L-Cysteine Cristian Gutierrez & J. Francisco Silva & Jorge Pavez & Fethi Bedioui & José H. Zagal Published online: 17 May 2012 # Springer Science+Business Media, LLC 2012 Abstract The formal potential of macrocyclic complexes is a reactivity index that can serve to predict their electrocatalytic activity. We have investigated the effect of the Fe(II)/(I) formal potential E°′ of several iron phthalocyanines preadsorbed on graphite electrodes on their catalytic activity for the electro- oxidation of L-cysteine. A linear and inverted correlation is observed between the catalytic activity (as log I at constant E) versus the Fe(II)/(I) formal potential of the catalyst, i.e., the activity decreases as the driving force of the catalyst increases. The slope of the correlation is close to -2RT/F and seems to be the falling region of an incomplete volcano plot so the decrease of the activity with increasing E°′ can be attributed to a gradual increase in the coverage of L-cysteine molecules adsorbed on the Fe sites with a subsequent decrease in free Fe (II) active sites. Keywords Iron phthalocyanines . L-Cysteine oxidation . Electrocatalysis . Catalyst formal potential . Falling region Introduction Since the publication of the seminal paper by Jasinski [1] in 1964, when the electrocatalytic activity of cobalt phthalocy- anine for the electroreduction of molecular oxygen was reported for the first time, many papers have been reported on the subject [2, 3]. In recent years, the focus has deviated to metal complexes of N 4 -ligands, such as metallophthalo- cyanines and metalloporphrins as catalysts for a great vari- ety of other electrode processes [2–5]. In particular, it has been shown by many authors that metallophthalocyanines (MPcs) confined on electrode surfaces catalyze the electro- chemical oxidation of thiols to give the corresponding disul- fides, i.e., RSH ! 1 2 RS  SR þ 1 2 H þ þ e[3–36] with Co and Fe derivatives being the most active complexes. Since most common electrode materials present passivation prob- lems that are endemic of organosulfur compounds and in spite of the rather abundant literature published on the subject, there remains great interest in assessing the electro- catalysis of the electrochemical oxidation of thiols by MPcs. Several approaches are continuously developed [3–35], in particular: (1) the synthesis of new catalysts, (2) the elabo- ration of diverse types of MPc-based molecular electrodes, (3) the design of new electrode configurations for the fabri- cation of electrochemical sensors, (4) the precise tuning of the formal potential of the MPc for a better control of the kinetic performances of the catalyst, etc. In all previous publications [2–36], it was shown that the reactivity of the electrode strongly depends on the nature of the complex confined on the electrode surface. This indicates that the reaction proceeds via an inner-sphere mechanism, involving an interaction of the thiol with the active sites provided by the central metal complex anchored on the electrode. C. Gutierrez : J. F. Silva : J. Pavez : J. H. Zagal (*) Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago 9170022, Chile e-mail: jose.zagal@usach.cl F. Bedioui Chimie ParisTech, CNRS 8151, Paris, France F. Bedioui INSERM 1022, Université Paris Descartes, Paris, France Electrocatal (2012) 3:153–159 DOI 10.1007/s12678-012-0097-y