Scandium Ion-Enhanced Oxidative Dimerization andN-Demethylation ofN,N-Dimethylanilines by a Non-Heme Iron(IV)-Oxo Complex

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Abstract
Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [FeIV(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 [FeIV(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 [FeIV(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 [FeIV(O)(N4Py)]2+, giving demethylated products. Binding of Sc3+ to [FeIV(O)(N4Py)]2+ enhances the Sc3+ ion-coupled electron transfer from DMA derivatives to [FeIV(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 [FeIV(O)(N4Py)]2+ are analyzed by competition between binding of Sc3+ to DMA derivatives and to [FeIV(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 [FeIV(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 [FeIV(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 [FeIV(O)(N4Py)]2+.

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