Mechanism of the Formation of a Mn-Based CO2 Reduction Catalyst Revealed by Pulse Radiolysis with Time-Resolved Infrared Detection

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• 作者：David C. Grills ; Jaime A. Farrington ; Bobby H. Layne ; Sergei V. Lymar ; Barbara A. Mello ; Jack M. Preses ; James F. Wishart
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：April 16, 2014
• 年：2014
• 卷：136
• 期：15
• 页码：5563-5566
• 全文大小：257K
• 年卷期：v.136,no.15(April 16, 2014)
• ISSN：1520-5126

文摘

Using a new technique, which combines pulse radiolysis with nanosecond time-resolved infrared (TRIR) spectroscopy in the condensed phase, we have conducted a detailed kinetic and mechanistic investigation of the formation of a Mn-based CO₂ reduction electrocatalyst, [Mn(^tBu₂-bpy)(CO)₃]₂ (^tBu₂-bpy = 4,4鈥?^tBu₂-2,2鈥?bipyridine), in acetonitrile. The use of TRIR allowed, for the first time, direct observation of all the intermediates involved in this process. Addition of excess [ⁿBu₄N][HCO₂] to an acetonitrile solution of fac-MnBr(^tBu₂-bpy)(CO)₃ results in its quantitative conversion to the Mn鈥揻ormate complex, fac-Mn(OCHO)(^tBu₂-bpy)(CO)₃, which is a precatalyst for the electrocatalytic reduction of CO₂. Formation of the catalyst is initiated by one-electron reduction of the Mn鈥揻ormate precatalyst, which produces the bpy ligand-based radical. This radical undergoes extremely rapid (蟿 = 77 ns) formate dissociation accompanied by a free valence shift to yield the five-coordinate Mn-based radical, Mn^{鈥?/sup>(tBu₂-bpy)(CO)₃. TRIR data also provide evidence that the Mn-centered radical does not bind acetonitrile prior to its dimerization. This reaction occurs with a characteristically high radical鈥搑adical recombination rate (2k_dim = (1.3 卤 0.1) 脳 109 M鈥? s鈥?), generating the catalytically active Mn鈥揗n bound dimer.}