Electrocatalytic Reduction of CO₂by Group 6 M(CO)₆Species without “Non-Innocent” Ligands

Abstract

To understand the electrocatalytic CO2 reduction of metal carbonyl complexes without "non-innocent" ligands, the electrochemical responses of group 6 M(CO)6 (M = Cr, Mo, or W) and group 7 M2(CO)10 (M = Mn or Re) complexes were examined under Ar and CO2 at a glassy carbon electrode. All of the complexes showed changes in their cyclic voltammograms under CO2. The group 6 hexacarbonyl species show a significant increase in current under CO2 during metal-based reduction, corresponding to catalytic reduction of CO2. Bulk electrolysis experiments with Mo(CO)6 showed that CO was the primary product. The group 7 dimers showed very little change during metal-based reduction, but return oxidation responses disappeared, indicative of a chemical reaction after exposure to CO2 without catalysis. Addition of H2O, a proton source, to the solutions under CO2 decreased the catalytic current of the group 6 carbonyls and had no effect on the responses of the group 7 carbonyls. The group 6 M(CO)6 species are notable in that that they are effective catalysts without the need for an added "non-innocent" ligand such as 2,2'-bipyridine.

Progress and Perspective of Electrocatalytic CO2 Reduction for Renewable Carbonaceous Fuels and Chemicals

“…Bipyridine (bpy) complexes with earth‐abundant metal atoms were also considered as promising molecular electrocatalysts for reducing CO 2 to CO or hydrocarbons, such as HCOOH. There are many transition metals explored in this group, such as Ru, Cu, W, Mo, Mn, Re, Cr, and so on . For instance, a metal complex composed of Ru atom and 6,6′‐dimesityl‐2,2′‐bipyridine (mesbpy) ligands was applied to the generation of CO with high turnover frequency and Faradaic efficiency in the presence of Brønsted acids .…”

Photo‐Assisted Electrocatalytic Reduction of CO2: A New Strategy for Reducing Catalytic Overpotentials

“…In recent years, catalysts based on the unsustainable Earth-rare metals (Re, Ru, Rh, Ir) [2] have been relegated to benchmark status, as the community refocuses efforts on the Earth-abundant metals such as Mn, [10][11][12][13][14][15] Fe [16][17][18] and more recently the Group-6 triad (Cr, Mo, W). [19][20][21][22][23][24][25][26] For this reason, two viable catalyst precursors, fac-[Mn(CO) 3 (bipy)Br] (1) and [Mo(CO) 4 (6,6'-dmbipy)] (6,6'-dmbipy = 6,6'-dimethyl-2,2'-bipyridine) (2) have been selected for this investigation. In the very first reports, the reduction of [Mn(CO) 3 (bipy)Br] in dry organic solvents was noted to trigger no electrocatalytic CO 2 conversion.…”

“…To our knowledge, this is a unique, yet unexplored way to address the sometimes rather negative reduction potentials along the reduction path of these complexes needed to generate the electrocatalyst, which we have termed as photo‐assisted electrochemical reduction . In recent years, catalysts based on the unsustainable Earth‐rare metals (Re, Ru, Rh, Ir) have been relegated to benchmark status, as the community refocuses efforts on the Earth‐abundant metals such as Mn, Fe and more recently the Group‐6 triad (Cr, Mo, W) . For this reason, two viable catalyst precursors, fac‐ [Mn(CO) 3 (bipy)Br] ( 1 ) and [Mo(CO) 4 (6,6′‐dmbipy)] (6,6′‐dmbipy=6,6′‐dimethyl‐2,2′‐bipyridine) ( 2 ) have been selected for this investigation.…”

Group 6 Metal Complexes as Electrocatalysts of CO2 Reduction: Strong Substituent Control of the Reduction Path of [Mo(η3-allyl)(CO)2(x,x′-dimethyl-2,2′-bipyridine)(NCS)] (x = 4–6)

“…Recently, it was reported that Group-6 hexacarbonyl complexes can behave catalytically toward CO 2 . 35 This is highly unusual, as most catalysts reported to date bear a redox-active ligand acting as both an electron reservoir and a parking place for the protons required for catalysis. The hexacarbonyl is reduced by 2eto form [M(CO) 5 ] 2that can bind CO 2 and transform to [M(CO) 5 (CO 2 )] 2-; under anhydrous conditions, the major product of the reduction is CO. Interestingly, addition of water or another proton source inhibits the activity of the catalyst, most likely due to competing formation of H 2 from [M(CO) 5 H] -.…”