A Manganese(V)-Oxo Complex: Synthesis by Dioxygen Activation and Enhancement of Its Oxidizing Power by Binding Scandium Ion

A mononuclear non-heme manganese(V)-oxo complex, [Mn^V(O)(TAML)]^- (1), was synthesized by activating dioxygen in the presence of olefins with weak allylic C-H bonds and characterized structurally and spectroscopically. In mechanistic studies, the formation rate of 1 was found to depend on the allylic C-H bond dissociation energies (BDEs) of olefins, and a kinetic isotope effect (KIE) value of 16 was obtained in the reactions of cyclohexene and cyclohexene-d₁₀. These results suggest that a hydrogen atom abstraction from the allylic C-H bonds of olefins by a putative Mn^IV-superoxo species, which is formed by binding O₂ by a high-spin (S = 2) [Mn^III(TAML)]^- complex, is the rate-determining step. A Mn(V)-oxo complex binding Sc³⁺ ion, [Mn^V(O)(TAML)]^--(Sc³⁺) (2), was also synthesized in the reaction of 1 with Sc³⁺ ion and then characterized using various spectroscopic techniques. The binding site of the Sc³⁺ ion was proposed to be the TAML ligand, not the Mn-O moiety, probably due to the low basicity of the oxo group compared to the basicity of the amide carbonyl group in the TAML ligand. Reactivity studies of the Mn(V)-oxo intermediates, 1 and 2, in oxygen atom transfer and electron-transfer reactions revealed that the binding of Sc³⁺ ion at the TAML ligand of Mn(V)-oxo enhanced its oxidizing power with a positively shifted one-electron reduction potential (ΔE_red = 0.70 V). This study reports the first example of tuning the second coordination sphere of high-valent metal-oxo species by binding a redox-inactive metal ion at the supporting ligand site, thereby modulating their electron-transfer properties as well as their reactivities in oxidation reactions.