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

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• 作者：Seungwoo Hong ; Yong-Min Lee ; Muniyandi Sankaralingam ; Anil Kumar Vardhaman ; Young Jun Park ; Kyung-Bin Cho ; Takashi Ogura ; Ritimukta Sarangi ; Shunichi Fukuzumi ; Wonwoo Nam
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：July 13, 2016
• 年：2016
• 卷：138
• 期：27
• 页码：8523-8532
• 全文大小：568K
• 年卷期：0
• ISSN：1520-5126

文摘

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.