Clarification of the oxidation state of cobalt corroles in heterogeneous and homogeneous catalytic reduction of dioxygen

Karl M. Kadish, Jing Shen, Laurent Frémond, Ping Chen, Maya El Ojaimi, Mohammed Chkounda, Claude P. Gros, Jean Michel Barbe, Kei Ohkubo, Shunichi Fukuzumi, Roger Guilard

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Abstract

Co(III) corroles were investigated as efficient catalysts for the reduction of dioxygen in the presence of perchloric acid in both heterogeneous and homogeneous systems. The investigated compounds are (5,10,15- tris(pentafluorophenyl)corrole)cobalt (TPFCor)Co, (10-pentafluorophenyl-5,15- dimesitylcorrole)cobalt (F5PhMes2Cor)Co, and (5,10,15-trismesitylcorrole)cobalt (Mes3Cor)Co, all of which contain bulky substituents at the three meso positions of the corrole macrocycle. Cyclic voltammetry and rotating ring-disk electrode voltammetry were used to examine the catalytic activity of the compounds when adsorbed on the surface of a graphite electrode in the presence of 1.0 M perchloric acid, and this data is compared to results for the homogeneous catalytic reduction of O2 in benzonitrile containing 10-2 M HClO4. The corroles were also investigated as to their redox properties in nonaqueous media. A reversible one-electron oxidation occurs at E1/2 values between 0.42 and 0.89 V versus SCE depending upon the solvent and number of fluorine substituents on the compounds, and this is followed by a second reversible one-electron abstraction at E1/2 = 0.86 to 1.18 V in CH2Cl2, THF, or PhCN. Two reductions of each corrole are also observed in the three solvents. A linear relationship is observed between E1/2 for oxidation or reduction and the number of electron-withdrawing fluorine groups on the compounds, and the magnitude of the substituent effect is compared to what is observed in the case of tetraphenylporphyrins containing meso-substituted C6F5 substituents. The electrochemically generated forms of the corrole can exist with Co(I), Co(II), or Co(IV) central metal ions, and the site of the electron-transfer in each oxidation or reduction of the initial Co(III) complex was examined by UV-vis spectroelectrochemistry. ESR characterization was also used to characterize singly oxidized (F 5PhMes2Cor)Co, which is unambiguously assigned as a Co(III) radical cation rather than the expected Co(IV) corrole with an unoxidized macrocyclic ring.

Original languageEnglish
Pages (from-to)6726-6737
Number of pages12
JournalInorganic Chemistry
Volume47
Issue number15
DOIs
StatePublished - 4 Aug 2008

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