Tuning the Reactivity of Chromium(III)-Superoxo Species by Coordinating Axial Ligands

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• 作者：Yi Re Goo ; Annada C. Maity ; Kyung-Bin Cho ; Yong-Min Lee ; Mi Sook Seo ; Young Jun Park ; Jaeheung Cho ; Wonwoo Nam
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：November 2, 2015
• 年：2015
• 卷：54
• 期：21
• 页码：10513-10520
• 全文大小：494K
• ISSN：1520-510X

文摘

Metal-superoxo species have attracted much attention recently as key intermediates in enzymatic and biomimetic oxidation reactions. The effect(s) of axial ligands on the chemical properties of metal-superoxo complexes has never been explored previously. In this study, we synthesized and characterized chromium(III)-superoxo complexes bearing TMC derivatives with pendant pyridine and imidazole donors, such as [Cr^III(O₂)(TMC-Py)]²⁺ (1, TMC-Py = 4,8,11-trimethyl-1-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) and [Cr^III(O₂)(TMC-Im)]²⁺ (2, TMC-Im = 4,8,11-trimethyl-1-(2-methylimidazolmethyl)-1,4,8,11-tetraazacyclotetradecane). The reactivity of chromium(III)-superoxo complexes binding different axial ligands, such as 1, 2, and [Cr^III(O₂)(TMC)(Cl)]⁺ (3, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was then investigated in C鈥揌 bond activation and oxygen atom transfer reactions. Kinetic studies revealed that the reactivity of the Cr(III)-superoxo complexes depends on the axial ligands, showing the reactivity order of 1 > 2 > 3 in those electrophilic oxidation reactions. It was also shown that there is a good correlation between the reactivity of the chromium(III)-superoxo complexes and their redox potentials, in which the redox potentials of the chromium(III)-superoxo complexes are in the order 1 > 2 > 3. DFT calculations reproduced the reactivity order between 1 and 3 in both C鈥揌 bond activation and oxygen atom transfer reactions, and the latter reaction is described using orbital interactions. The calculations are also in agreement with the experimentally obtained redox potentials. The present results provide the first example showing that the reactivity of metal-superoxo species can be tuned by the electron-donating ability of axial ligands bound trans to the metal-superoxo moiety.