Electrochemistry and catalytic properties for dioxygen reduction using ferrocene-substituted cobalt porphyrins

Bin Sun, Zhongping Ou, Deying Meng, Yuanyuan Fang, Yang Song, Weihua Zhu, Pavlo Solntsev, Victor N. Nemykin, Karl M. Kadish

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)8600-8609
Number of pages10
JournalInorganic Chemistry
Volume53
Issue number16
DOIs
StatePublished - Aug 18 2014

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Sun, B., Ou, Z., Meng, D., Fang, Y., Song, Y., Zhu, W., Solntsev, P., Nemykin, V. N., & Kadish, K. M. (2014). Electrochemistry and catalytic properties for dioxygen reduction using ferrocene-substituted cobalt porphyrins. Inorganic Chemistry, 53(16), 8600-8609. https://doi.org/10.1021/ic501210t