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.