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

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 (2k_dim = (1.3 ± 0.1) × 10⁹ M^–1 s^–1), generating the catalytically active Mn–Mn bound dimer.