Unified View of Oxidative C-H Bond Cleavage and Sulfoxidation by a Nonheme Iron(IV)-Oxo Complex via Lewis Acid-Promoted Electron Transfer

Oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by a nonheme iron(IV)-oxo complex, [(N4Py)Fe-IV(O)](2+) (N4Py = N,N-bis (2-pyridylmethyl)-N-bis (2-pyridyl)methylamine), were remarkably enhanced by the presence of triflic acid (HOTf) and Sc(OTf)(3) in acetonitrile at 298 K. All the logarithms of the observed second-order rate constants of both the oxidative C-H bond cleavage and sulfoxidation reactions exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes between PCET and MCET were taken into account, respectively. Thus, the mechanisms of both the oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by [(N4Py)Fe-IV(O)](2+) in the presence of HOTf and Sc(OTf)(3) have been unified as the rate-determining electron transfer, which is coupled with binding of [(N4Py)FeIV(O)](2+) by proton (PCET) and Sc(OTf)(3) (MCET). There was no deuterium kinetic isotope effect (KIE) on the oxidative C-H bond cleavage of toluene via the PCET pathway, whereas a large KIE value was observed with Sc(OTf)(3), which exhibited no acceleration of the oxidative C-H bond cleavage of toluene. When HOTf was replaced by DOTf, an inverse KIE (0.4) was observed for PCET from both toluene and [Ru-II(bpy)(3)](2+) (bpy =2,2'-bipyridine) to [(N4Py)Fe-IV(O)](2+). The PCET and MCET reactivities of [(N4Py)FeIV(O)](2+) with Bronsted acids and various metal triflates have also been unified as a single correlation with a quantitative measure of the Lewis acidity.