title: High-valent chromium-oxo complex acting as an efficient catalyst precursor for selective two-electron reduction of dioxygen by a ferrocene derivative.
abstract: Efficient catalytic two-electron reduction of dioxygen (O2) by octamethylferrocene (Me8Fc) produced hydrogen peroxide (H2O2) using a high-valent chromium(V)-oxo corrole complex, [(tpfc)Cr(V)(O)] (tpfc = tris(pentafluorophenyl)corrole) as a catalyst precursor in the presence of trifluoroacetic acid (TFA) in acetonitrile (MeCN). The facile two-electron reduction of [(tpfc)Cr(V)(O)] by 2 equiv of Me8Fc in the presence of excess TFA produced the corresponding chromium(III) corrole [(tpfc)Cr(III)(OH2)] via fast electron transfer from Me8Fc to [(tpfc)Cr(V)(O)] followed by double protonation of [(tpfc)Cr(IV)(O)](-) and facile second-electron transfer from Me8Fc. The rate-determining step in the catalytic two-electron reduction of O2 by Me8Fc in the presence of excess TFA is inner-sphere electron transfer from [(tpfc)Cr(III)(OH2)] to O2 to produce the chromium(IV) superoxo species [(tpfc)Cr(IV)(O2(•-))], followed by fast proton-coupled electron transfer reduction of [(tpfc)Cr(IV)(O2(•-))] by Me8Fc to yield H2O2, accompanied by regeneration of [(tpfc)Cr(III)(OH2)]. Thus, although the catalytic two-electron reduction of O2 by Me8Fc was started by [(tpfc)Cr(V)(O)], no regeneration of [(tpfc)Cr(V)(O)] was observed in the presence of excess TFA, regardless of the tetragonal chromium complex being to the left of the oxo wall. In the presence of a stoichiometric amount of TFA, however, disproportionation of [(tfpc)Cr(IV)(O)](-) occurred via the protonated species [(tpfc)Cr(IV)(OH)] to produce [(tpfc)Cr(III)(OH2)] and [(tpfc)Cr(V)(O)].
authors.affiliation: Department of Chemistry, Purdue University , 560 Oval Drive, West Lafayette, Indiana 47907, United States.
authors.forename: Shuo
authors.initials: S
authors.lastname: Liu
authors.forename: Kentaro
authors.initials: K
authors.lastname: Mase
authors.forename: Curt
authors.initials: C
authors.lastname: Bougher
authors.forename: Scott D
authors.initials: SD
authors.lastname: Hicks
authors.forename: Mahdi M
authors.initials: MM
authors.lastname: Abu-Omar
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
mesh_terms: D002384:Catalysis; D002857:Chromium / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005296:Ferrous Compounds / Q000737:chemistry*; D000075163:Metallocenes; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D005296:Ferrous Compounds; D000075163:Metallocenes; D002857:Chromium; D010100:Oxygen; C004998:ferrocene