Linear versus volcano correlations between electrocatalytic activity and redox and electronic properties of metallophthalocyanines Jose H. Zagal*, Miguel Gulppi, Mauricio Isaacs, Gloria CaÂrdenas-Jiro n, Maria JesuÂs Aguirre Departamento de QuõÂmica Aplicada, Facultad de QuõÂmica y BiologõÂa, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile Received 12 June 1998 Abstract In this paper we have reinvestigated the correlations between the electrocatalytic activity of metallophthalocyanines for O 2 reduction with the redox properties of the metal in the complex. We have extended these studies to the electro±oxidation of thiols. For O 2 reduction, plots of log k at constant potential versus the M(III)/M(II) redox potential of the metallophthalocyanine containing Cr, Mn, Fe, and Co, give two parallel straight lines of slopes close to 2RT/F, instead of a volcano plot, as reported earlier. These lines correspond to two families with one including only Co complexes. For the oxidation of the thiols L-cysteine and 2-mercaptoethanol, single straight lines are obtained for each case when plotting log k versus the M(II)/M(I) redox potential of the metallophthalocyanine. The slope is close to 2RT/F. For O 2 reduction, log k decreases with driving force, whereas for the oxidation of thiols, it increases. For O 2 reduction, for the oxidation of L-cysteine and of 2-mercaptoethanol when activity (as log k) is compared along the transition series volcano-shaped curves are obtained, which illustrates the concept of `tuning' frontier orbital energies for maximum reaction rates. Preliminary calculations of the electronic coupling between the metal in a TAP (tetraazaporphyrin) environment and the sulfur atom in a thiol show that this parameter exhibits the same trend (volcano correlation) found when log k is plotted versus the number of d-electron in the central metal. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Metallophthalocyanines; O 2 reduction; Redox catalysis; Thiol oxidation; Electronic coupling 1. Introduction Metallophthalocyanines, metalloporphyrins and re- lated macrocyclic complexes are of great importance as models in many biological processes and are well known as electrocatalysts for the reduction of oxygen [1±3]. The reduction of oxygen to water via a total transfer of four electrons is very exoergonic but the kinetic stability of dioxygen makes life processes possible. The stability of O 2 is attributed to its triplet ground state, so, reactions with molecules having sing- let ground states (which are the majority) that involve the reduction of dioxygen are spin forbidden. Some metal complexes can overcome the spin barrier and provide a low-energy route for dioxygen to react. When the dioxygen molecule binds to the metal in the N-4 moiety, its 2p electrons interact with the partially- ®lled d-orbitals of the metal. These processes are ac- companied by intramolecular electron transfer from the metal center to the dioxygen molecule. This requires the previous reduction of the metal from M(III) to M(II) in order to interact with the O 2 mol- ecule. Thus, the redox potential of the metal in the phthalocyanine is an important parameter when com- Electrochimica Acta 44 (1998) 1349±1357 0013-4686/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S0013-4686(98)00257-6 PERGAMON * Corresponding author. Fax: +56-268-19036; e-mail: jza- gal@lauca.usach.cl