Electrochemistry and Catalytic Properties for Dioxygen Reduction Using Ferrocene-Substituted Cobalt Porphyrins

Cobalt porphyrins having 0–4 <i>meso</i>-substituted ferrocenyl groups were synthesized and examined as to their electrochemical properties in <i>N</i>,<i>N</i>′-dimethylformamide (DMF) containing 0.1 M tetra-<i>n</i>-butylammonium perchlorate as a supporting electrolyte. The examined compounds are represented as (Fc)_<i>n</i>(CH₃Ph)_4–<i>n</i>PorCo, where Por is a dianion of the substituted porphyrin, Fc and CH₃Ph represent ferrocenyl and/or <i>p-</i>CH₃C₆H₄ groups linked at the four <i>meso</i>-positions of the macrocycle, and <i>n</i> varies from 0 to 4. Each porphyrin undergoes two reversible one-electron reductions and two to six one-electron oxidations in DMF, with the exact number depending upon the number of Fc groups on the compound. The first electron addition is metal-centered to generate a Co­(I) porphyrin. The second is porphyrin ring-centered and leads to formation of a Co­(I) π-anion radical. The first oxidation of each Co­(II) porphyrin is metal-centered to generate a Co­(III) derivative under the given solution conditions. Each ferrocenyl substituent can also be oxidized by one electron, and this occurs at more positive potentials. Each compound was investigated as a catalyst for the electoreduction of dioxygen when adsorbed on a graphite electrode in 1.0 M HClO₄. The number of electrons transferred (<i>n</i>) during the catalytic reduction was 2.0 for the three ferrocenyl substituted compounds, consistent with only H₂O₂ being produced as a product of the reaction. Most monomeric cobalt porphyrins exhibit <i>n</i> values between 2.6 and 3.1 under the same solution conditions, giving a mixture of H₂O and H₂O₂ as a reduction product, although some monomeric porphyrins can give an <i>n</i> value of 4.0. Our results in the current study indicate that appending ferrocene groups directly to the <i>meso</i> positions of a porphyrin macrocycle will increase the selectivity of the oxygen reduction, resulting in formation of only H₂O₂ as a reaction product. This selectivity of the electrocatalytic oxygen reduction reaction is explained on the basis of steric hindrance by the ferrocene substituents which prevent dimerization.