title: Mechanism of the formation of a Mn-based CO2 reduction catalyst revealed by pulse radiolysis with time-resolved infrared detection.
abstract: 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((t)Bu2-bpy)(CO)3]2 ((t)Bu2-bpy = 4,4'-(t)Bu2-2,2'-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 [(n)Bu4N][HCO2] to an acetonitrile solution of fac-MnBr((t)Bu2-bpy)(CO)3 results in its quantitative conversion to the Mn-formate complex, fac-Mn(OCHO)((t)Bu2-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(•)((t)Bu2-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) × 10(9) M(-1) s(-1)), generating the catalytically active Mn-Mn bound dimer.
authors.affiliation: Chemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York 11973-5000, United States.
authors.forename: David C
authors.initials: DC
authors.lastname: Grills
authors.forename: Jaime A
authors.initials: JA
authors.lastname: Farrington
authors.forename: Bobby H
authors.initials: BH
authors.lastname: Layne
authors.forename: Sergei V
authors.initials: SV
authors.lastname: Lymar
authors.forename: Barbara A
authors.initials: BA
authors.lastname: Mello
authors.forename: Jack M
authors.initials: JM
authors.lastname: Preses
authors.forename: James F
authors.initials: JF
authors.lastname: Wishart