Design of Cr-Based Molecular Electrocatalyst Systems for the CO₂ Reduction Reaction

ConspectusHuman influence on the climate system was recently summarized by the sixth Intergovernmental Panel on Climate Change (IPCC) Assessment Report, which noted that global surface temperatures have increased more rapidly in the last 50 years than in any other 50-year period in the last 2000 years. Elevated global surface temperatures have had detrimental impacts, including more frequent and intense extreme weather patterns like flooding, wildfires, and droughts. In order to limit greenhouse gas emissions, various climate change policies, like emissions trading schemes and carbon taxes, have been implemented in many countries. The most prevalent anthropogenic greenhouse gas emitted is carbon dioxide (CO₂), which accounted for 80% of all U.S. greenhouse gas emissions in 2022. The reduction of CO₂ through the use of homogeneous electrocatalysts generally follows a two-electron/two-proton pathway to produce either carbon monoxide (CO) with water (H₂O) as a coproduct or formic acid (HCOOH). These reduced carbon species are relevant to industrial applications: the Fischer-Tropsch process uses CO and H₂ to produce fuels and commodity chemicals, while HCOOH is an energy dense carrier for fuel cells and useful synthetic reagent. Electrochemically reducing CO₂ to value-added products is a potential way to address its steadily increasing atmospheric concentrations while supplanting the use of nonrenewable petrochemical reserves through the generation of new carbon-based resources. The selective electrochemical reduction of CO₂ (CO₂RR) by homogeneous catalyst systems was initially achieved with late (and sometimes costly) transition metal active sites, leading the field to conclude that transition metal complexes based on metals earlier in the periodic table, like chromium (Cr), were nonprivileged for the CO₂RR. However, metals early in the table have sufficient reducing power to mediate the CO₂RR and therefore could be selective in the correct coordination environment. This Account describes our efforts to develop and optimize novel Cr-based CO₂RR catalyst systems through redox-active ligand modification strategies and the use of redox mediators (RMs). RMs are redox-active molecules which can participate cocatalytically during an electrochemical reaction, transferring electrons─often accompanied by protons─to a catalytic active site. Through mechanistic and computational work, we have found that ligand-based redox activity is key to controlling the intrinsic selectivity of these Cr compounds for CO₂ activation. Ligand-based redox activity is also essential for developing cocatalytic systems, since it enables through-space interactions with reduced RMs containing redox-active planar aromatic groups, allowing charge transfer to occur within the catalyst assembly. Following a summary of our work, we offer a perspective on the possibilities for future development of catalytic and cocatalytic systems with early transition metals for small molecule activation.