Combined steric and electronic effects of positional substitution on dimethyl-bipyridine rhenium(I)tricarbonyl electrocatalysts for the reduction of CO2

Abstract

The use of CO2 as a chemical feedstock has been the focus of much research in recent years due to the promise for carbon neutral fuel storage in chemical bonds. Specifically, electrocatalysts of the type Re(bpy)(CO)3Cl (bpy=2,2′-bipyridine and analogues thereof) have been studied for their ability to reduce CO2 to CO. These catalysts are among the most active, selective, and robust homogeneous catalysts for CO2 reduction in the literature. Previous studies have involved understanding the mechanism for catalysis and the inductive effect of bipyridine functional groups on catalysis. In the study reported herein, we have assessed the CO2 reduction capabilities of a series of Re(n,n′-dimethyl-bpy)(CO)3Cl (n=3 and 5) catalysts with a bpy modified at the 3 and 5 positions with methyl substituents. While the 5,5′-positional substituents have the effect of increasing catalytic activity (icat/ip=29.6) over the unsubstituted catalyst (icat/ip=20.3); steric hindrance in the 3,3′-substituted bpy has the effect of decreasing catalytic activity (icat/ip=17.0). This decreased catalytic current response can be explained by steric hindrance in the 3,3′-substituted catalyst disfavoring optimal charge transfer in the catalytic cycle.