Oxygen reduction on adsorbed iron tetrapyridinoporphyrazine

The electrocatalytic properties of iron tetrapyridino porphyrazine FeTPyPz for the reduction on O₂ in alkaline media have been examined with cyclic voltammetry and rotating ring disk techniques. Four distinct redox peaks are observed in the absence of O₂ in solution for this material adsorbed at monolayer coverages on ordinary pyrolytic graphite surfaces. The onset for O₂ reduction appears to coincide with the voltammetric peak associated with the metal centered Fe(II)TPyPz/Fe(III)TPyPz transition. No hydrogen peroxide is detected at the ring in this potential range indicating that the reaction takes place by a four electron pathway. This is in contrast with the behavior observed at more negative potentials for which sizable currents for H₂O₂ oxidation are observed. This change in the mechanism has been attributed to the further reduction of the iron center in the macrocycle rendering an Fe(I)TPyPz species. If the electrode is polarized at potentials more negative than -0.8 V vs SCE, however, the reduction proceeds once again without generation of H₂O₂ in the solution phase. Quantum mechanical arguments involving orbital symmetry and overlap indicate that the activation could involve a simultaneous bonding of O₂ to the iron center and a bridge nitrogen in the macrocycle ring. Such a potential dependent mechanism is similar to that reported earlier for iron tetrasulfonated phthalocyanine for which theoretically predicted electronic properties are essentially the same as those of FeTPyPz.