title: Clarification of the oxidation state of cobalt corroles in heterogeneous and homogeneous catalytic reduction of dioxygen.
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 (F 5PhMes 2Cor)Co, and (5,10,15-trismesitylcorrole)cobalt (Mes 3Cor)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 O 2 in benzonitrile containing 10 (-2) M HClO 4. The corroles were also investigated as to their redox properties in nonaqueous media. A reversible one-electron oxidation occurs at E 1/2 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 E 1/2 = 0.86 to 1.18 V in CH 2Cl 2, THF, or PhCN. Two reductions of each corrole are also observed in the three solvents. A linear relationship is observed between E 1/2 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 C 6F 5 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 5PhMes 2Cor)Co, which is unambiguously assigned as a Co(III) radical cation rather than the expected Co(IV) corrole with an unoxidized macrocyclic ring.
authors.affiliation: Department of Chemistry, University of Houston, Houston, Texas 77204-5003, USA. kkadish@uh.edu
authors.forename: Karl M
authors.initials: KM
authors.lastname: Kadish
authors.forename: Jing
authors.initials: J
authors.lastname: Shen
authors.forename: Laurent
authors.initials: L
authors.lastname: Frémond
authors.forename: Ping
authors.initials: P
authors.lastname: Chen
authors.forename: Maya
authors.initials: M
authors.lastname: El Ojaimi
authors.forename: Mohammed
authors.initials: M
authors.lastname: Chkounda
authors.forename: Claude P
authors.initials: CP
authors.lastname: Gros
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Barbe
authors.forename: Kei
authors.initials: K
authors.lastname: Ohkubo
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
authors.forename: Roger
authors.initials: R
authors.lastname: Guilard
mesh_terms: D002384:Catalysis; D003035:Cobalt / Q000737:chemistry*; D004563:Electrochemistry; D004566:Electrodes; D004578:Electron Spin Resonance Spectroscopy; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010472:Perchlorates / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D009942:Organometallic Compounds; D010472:Perchlorates; D011166:Porphyrins; C431863:corrole; D003035:Cobalt; D010100:Oxygen