Scandium ion-enhanced oxidative dimerization and N -demethylation of N, N -dimethylanilines by a non-heme iron(IV)-oxo complex

Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [Fe^IV(O)(N4Py)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by [Fe^IV(O)(N4Py)]²⁺ is markedly enhanced by the presence of scandium triflate, Sc(OTf)₃ (OTf = CF₃SO₃^-), when TMB is further oxidized to the radical cation (TMB^•+). In contrast, we have observed the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by [Fe^IV(O) (N4Py)]²⁺ is also markedly enhanced by the presence of Sc(OTf) ₃. In the case of para-substituted DMA derivatives with electron-donating substituents, radical cations of DMA derivatives are initially formed by Sc³⁺ ion-coupled electron transfer from DMA derivatives to [Fe^IV(O)(N4Py)]²⁺, giving demethylated products. Binding of Sc³⁺ to [Fe^IV(O)(N4Py)]²⁺ enhances the Sc³⁺ ion-coupled electron transfer from DMA derivatives to [Fe ^IV(O)(N4Py)]²⁺, whereas binding of Sc³⁺ to DMA derivatives retards the electron-transfer reaction. The complicated kinetics of the Sc³⁺ ion-coupled electron transfer from DMA derivatives to [Fe^IV(O)(N4Py)]²⁺ are analyzed by competition between binding of Sc³⁺ to DMA derivatives and to [Fe^IV(O)(N4Py)] ²⁺. The binding constants of Sc³⁺ to DMA derivatives increase with the increase of the electron-donating ability of the para-substituent. The rate constants of Sc³⁺ ion-coupled electron transfer from DMA derivatives to [Fe^IV(O)(N4Py)]²⁺, which are estimated from the binding constants of Sc³⁺ to DMA derivatives, agree well with those predicted from the driving force dependence of the rate constants of Sc³⁺ ion-coupled electron transfer from one-electron reductants to [Fe^IV(O)(N4Py)]²⁺. Thus, oxidative dimerization of DMA and N-demethylation of para-substituted DMA derivatives proceed via Sc³⁺ ion-coupled electron transfer from DMA derivatives to [Fe^IV(O)(N4Py)]²⁺.