Selectivity in CO2 Reduction: Beyond Two Electrons

The development of catalysts for the reduction of CO2 to value-added chemicals is a major contemporary global effort. A consistent problem in the reduction of CO2 is selectivity. While two-electron reduction products such as CO and HCOOH have been extensively investigated, achieving selective multielectron, multiproton reduction to more reduced C1 products, including HCHO, CH3OH, and CH4, as well as C2+ products, remains significantly more difficult due to the involvement of multiple proton- and electron-transfer steps and competing reaction pathways. Molecular catalysts provide fertile ground for investigating the reaction mechanism of CO2 reduction and determine the chemical steps responsible for the selectivity of CO2 reduction. Iron porphyrins are attractive choices as catalysts to navigate the CO2 reduction landscape because they not only are efficient catalysts for the reduction of CO2 to CO but also provide a versatile canvas for catalyst design due to their synthetic amenability and because they are very suitable for spectroscopic investigations. In a previous Account, the factors that determine the selectivity between CO and HCOOH, both 2e-/2H+ reduction products of CO2, had been discussed [Acc. Chem. Res. 2022, 55 (2), 134-144]. In this Account, we focus on the latest development of the mechanistic principles that enable selective CO2 reduction beyond two electrons. The key to achieving further reduction of CO2 is stabilization of the reduced carbonyl species by hydrogen bonding. The further reduction of CO2 proceeds through intermediates, better described as branching points, where the site of protonation determines if partially reduced CO2 species like the 4e-/4H+ reduced HCHO or the 6e-/6H+ reduced CH3OH is released or if the CO2 is reduced completely by 8e-/8H+ to produce CH4. The site of protonation appears to be governed by the spin state of these intermediates. This reaction landscape can be navigated by ligand design, which includes second-sphere hydrogen bonding/proton transfer residues and rates of electron transfer to the catalytic site. The knowledge of the intermediates allows their alternative uses including the formation of C-C bonds in a monometallic active site using a concept of "parking space".