Comparisons of bpy and phen Ligand Backbones in Cr-Mediated (Co-)Electrocatalytic CO2 Reduction

Due to the rise in atmospheric carbon dioxide (CO2) concentrations, there is a need for the development of new strategies to enhance the selectivity and activity of the electrocatalytic conversion of CO2 to value-added products. The incorporation of redox mediators (RMs) as cocatalysts to enhance the transfer of redox equivalents during catalysis has been gaining more attention in recent years across a variety of small molecule transformations. We have shown that using Cr-centered complexes with sulfone-based RMs leads to an enhancement of CO2 reduction electrocatalysis under protic conditions via an inner-sphere mechanism. In these cocatalytic systems, an oxygen atom of the reduced RM binds to the Cr center to form a key intermediate stabilized by pancake bonding between the reduced aromatic components of the catalyst ligand backbone and the RM. This interaction facilitates the transfer of an electron and accesses a more kinetically favorable reaction pathway. Here, we show that expanding the aromatic character of the ligand backbone of the catalyst as well as the RM can cause a greater enhancement of coelectrocatalytic activity. These results suggest that further activity improvements can be achieved by focusing on the kinetic and thermodynamic parameters which control association between the catalyst and RM.