title: Autocatalytic formation of an iron(IV)-oxo complex via scandium ion-promoted radical chain autoxidation of an iron(II) complex with dioxygen and tetraphenylborate.
abstract: A non-heme iron(IV)-oxo complex, [(TMC)Fe(IV)(O)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was formed by oxidation of an iron(II) complex ([(TMC)Fe(II)](2+)) with dioxygen (O2) and tetraphenylborate (BPh4(-)) in the presence of scandium triflate (Sc(OTf)3) in acetonitrile at 298 K via autocatalytic radical chain reactions rather than by a direct O2 activation pathway. The autocatalytic radical chain reaction is initiated by scandium ion-promoted electron transfer from BPh4(-) to [(TMC)Fe(IV)(O)](2+) to produce phenyl radical (Ph(•)). The chain propagation step is composed of the addition of O2 to Ph(•) and the reduction of the resulting phenylperoxyl radical (PhOO(•)) by scandium ion-promoted electron transfer from BPh4(-) to PhOO(•) to produce phenyl hydroperoxide (PhOOH), accompanied by regeneration of phenyl radical. PhOOH reacts with [(TMC)Fe(II)](2+) to yield phenol (PhOH) and [(TMC)Fe(IV)(O)](2+). Biphenyl (Ph-Ph) was formed via the radical chain autoxidation of BPh3 by O2. The induction period of the autocatalytic radical chain reactions was shortened by addition of a catalytic amount of [(TMC)Fe(IV)(O)](2+), whereas addition of a catalytic amount of ferrocene that can reduce [(TMC)Fe(IV)(O)](2+) resulted in elongation of the induction period. Radical chain autoxidation of BPh4(-) by O2 also occurred in the presence of Sc(OTf)3 without [(TMC)Fe(IV)(O)](2+), initiating the autocatalytic oxidation of [(TMC)Fe(II)](2+) with O2 and BPh4(-) to yield [(TMC)Fe(IV)(O)](2+). Thus, the general view for formation of non-heme iron(IV)-oxo complexes via O2-binding iron species (e.g., Fe(III)(O2(•-))) without contribution of autocatalytic radical chain reactions should be viewed with caution.
authors.affiliation: Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology (JST) , Suita, Osaka 565-0871, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Nishida
authors.forename: Yong-Min
authors.initials: YM
authors.lastname: Lee
authors.forename: Wonwoo
authors.initials: W
authors.lastname: Nam
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
mesh_terms: D002384:Catalysis; D005296:Ferrous Compounds / Q000737:chemistry; D005609:Free Radicals; D007202:Indicators and Reagents; D007501:Iron / Q000737:chemistry*; D000075163:Metallocenes; D019159:Nonheme Iron Proteins / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D012538:Scandium / Q000737:chemistry*; D013775:Tetraphenylborate / Q000737:chemistry*
chemical_list: D005296:Ferrous Compounds; D005609:Free Radicals; D007202:Indicators and Reagents; D000075163:Metallocenes; D019159:Nonheme Iron Proteins; D013775:Tetraphenylborate; D007501:Iron; D010100:Oxygen; C004998:ferrocene; D012538:Scandium