Enhanced Electrochemical CO<sub>2</sub> Reduction by a Series of Molecular Rhenium Catalysts Decorated with Second-Sphere Hydrogen-Bond Donors
Kallol Talukdar, Sayontani Sinha Roy, Eva Amatya, Elizabeth A. Sleeper, Pierre Le Maguerès, Jonah W. Jurss
Abstract
A series of rhenium(I) fac -tricarbonyl complexes containing pendent arylamine functionality in the second coordination sphere have been developed and studied as electrocatalysts for carbon dioxide (CO 2 ) reduction. Aniline moieties were appended at the 6 position of a 2,2′-bipyridine (bpy) donor in which the primary amine was positioned at the ortho - ( 1-Re ), meta - ( 2-Re ), and para - ( 3-Re ) sites of the aniline substituent to generate a family of isomers. The relationship between the catalyst structure and activity was explored across the series, and the catalytic performance was compared to that of the benchmark catalyst Re(bpy)(CO) 3 Cl ( ReBpy ). Catalysts 1-Re, 2-Re, and 3-Re outperform the benchmark catalyst both in anhydrous acetonitrile and with added trifluoroethanol (TFE) as an external proton source. In the presence of TFE, the aniline-substituted catalysts convert CO 2 to carbon monoxide (CO) with high Faradaic efficiencies (≥89%) and have superior turnover frequencies (TOFs) relative to ReBpy (72.9 s –1 ), with 2-Re having the highest TOF of the series at 239 s –1, a value that is twice that of the next most active catalyst. TOFs of 123 and 109 s –1 were observed for the ortho - and para -substituted aniline complexes ( 1-Re and 3-Re ), respectively. Indeed, catalytic activities vary widely across the series, showing a high sensitivity to the position of the amine functionality relative to the rhenium active site. IR and UV–vis spectroelectrochemical experiments were conducted on the aniline-substituted systems, revealing important differences between the catalysts and mechanistic insight.