Autocatalytic formation of an iron(IV)-oxo complex via scandium ion-promoted radical chain autoxidation of an iron(II) complex with dioxygen and tetraphenylborate

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.