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

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journal contribution
posted on 18.08.2014 by Bin Sun, Zhongping Ou, Deying Meng, Yuanyuan Fang, Yang Song, Weihua Zhu, Pavlo V. Solntsev, Victor N. Nemykin, Karl M. Kadish
Cobalt porphyrins having 0–4 meso-substituted ferrocenyl groups were synthesized and examined as to their electrochemical properties in N,N′-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate as a supporting electrolyte. The examined compounds are represented as (Fc)n(CH3Ph)4–nPorCo, where Por is a dianion of the substituted porphyrin, Fc and CH3Ph represent ferrocenyl and/or p-CH3C6H4 groups linked at the four meso-positions of the macrocycle, and n 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 (n) 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 n 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 n value of 4.0. Our results in the current study indicate that appending ferrocene groups directly to the meso 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.