Modifying the steric and electronic character within Re(I)-phenanthroline complexes for electrocatalytic CO 2 reduction

Abstract

We have synthesized and characterized a series of fac-[Re(R2phen)(CO)3Cl] complexes (R2phen = 2,9-disubstituted-1,10-phenanthroline) that function as electrocatalysts for the reduction of CO2 to CO. The 2,9-disubstituted phenanthroline ligands contain proton (phen), methyl (2,9-Me2phen), trimethylphenyl (Mes2phen), or trimethoxyphenyl ((2,4,6-tmp)2phen and (3,4,5-tmp)2phen) groups as steric and electronic modifiers to provide insight into factors impacting catalytic activity. Cyclic voltammograms (CVs) recorded in CO2-saturated CH3CN or DMF solutions reveal that following two-electron reduction and chloride dissociation to form the active [Re(R2phen)(CO)3]− intermediate, current enhancement indicative of CO2 reduction to CO was observed and confirmed by controlled potential electrolysis (CPE). Using current enhancement values (icat/ip, where icat and ip are the current response under CO2 and N2, respectively) to estimate catalytic activity, it was observed that catalysts with more cathodic ReI/0 potentials displayed greater activity, in accord with an electronic effect driving catalysis. Interestingly, the trimethoxyphenyl-substituted complexes Re((2,4,6-tmp)2phen) and Re((3,4,5-tmp)2phen) showed different degrees of activity in DMF (icat/ip = 14.8 and 6.0, respectively) and CH3CN (icat/ip = 8.5 and 7.7, respectively). These results suggest that while electronics dominate the observed catalytic activity within this fac-[Re(R2phen)(CO)3Cl] architecture, factors such as sterics and solvent play an important role as well.