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

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posted on 2014-04-16, 00:00 authored by David C. Grills, Jaime A. Farrington, Bobby H. Layne, Sergei V. Lymar, Barbara A. Mello, Jack M. Preses, James F. Wishart
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′-tBu2-2,2′-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–formate 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–formate precatalyst, which produces the bpy ligand-based radical. This radical undergoes extremely rapid (τ = 77 ns) formate dissociation accompanied by a free valence shift to yield the five-coordinate Mn-based radical, Mn(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–radical recombination rate (2kdim = (1.3 ± 0.1) × 109 M–1 s–1), generating the catalytically active Mn–Mn bound dimer.

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