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.

We describe a new strategy for enhancing the efficiency of electrocatalytic CO2 reduction with a homogeneous catalyst, using a room-temperature ionic liquid as both the solvent and electrolyte. The electrochemical behavior of fac-ReCl(2,2'-bipyridine)(CO)3 in neat 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][TCB]) was compared with that in acetonitrile containing 0.1 M [Bu4N][PF6]. Two separate one-electron reductions occur in acetonitrile (-1.74 and -2.11 V vs Fc(+/0)), with a modest catalytic current appearing at the second reduction wave under CO2. However, in [emim][TCB], a two-electron reduction wave appears at -1.66 V, resulting in a ∼0.45 V lower overpotential for catalytic reduction of CO2 to CO. Furthermore, the apparent CO2 reduction rate constant, kapp, in [emim][TCB] exceeds that in acetonitrile by over one order of magnitude (kapp = 4000 vs 100 M(-1) s(-1)) at 25 ± 3 °C. Supported by time-resolved infrared measurements, a mechanism is proposed in which an interaction between [emim](+) and the two-electron reduced catalyst results in rapid dissociation of chloride and a decrease in the activation energy for CO2 reduction.

Synthesis and characterization of the dimeric [fac-Re(R-OQN)(CO) 3 ] 2 and monomeric fac-Re(R-OQN)-(CO) 3 (CH 3 CN) complexes are reported where R = unsubstituted, 2-methyl, 5,7-dimethyl, or 5-fluoro and OQN = 8oxyquinolate. Facile solvolysis of the dimeric systems is observed in coordinating media quantitatively yielding the monomer complexes in situ. Due to poor synthetic yields of the dimeric precursors, a direct synthetic strategy for isolation of the acetonitrile monomer complexes with an improved yield was developed. The fac-Re(CH 3 CN) 2 (CO) 3 Cl complex was easily generated in situ as a convenient intermediate to give the desired products in quantitative yield via reaction with the appropriately substituted 8-hydroxyquinoline and tetramethylammonium hydroxide base. Key to the success of this reaction is the precipitation of the product with triflic acid, whose conjugate triflate base is here noncoordinating. Furthermore, isolation of the solvated single crystal [fac-Re(FOQN)(CO) 3 ](μ-Cl)[fac-Re(FHOQN)(CO) 3 ]•CH 3 C 6 H 5 has allowed a unique opportunity to access a possible reaction intermediate, giving insight into the formation of the [fac-Re(R-OQN)(CO) 3 ] 2 dimeric systems. Spectroscopic features (UV−vis, FTIR, and 1 H NMR) of both monomeric and dimeric structures are discussed in terms of the π-electron-donating ability of the oxyquinolate ligand. Interpretation of these electronic effects and the associated steric properties is aided by single-crystal X-ray diffraction, electrochemical, and DFT/TD-DFT computational studies.