The eventual deployment of large-scale systems for the electrochemical reduction of carbon dioxide (CO2) to fuels and commodity chemicals depends on the development of stable, highly active, and selective catalysts. The fac-Re(bpy-R)(CO)3X system, originally reported three decades ago, is very efficient for CO2 reduction to carbon monoxide (CO) even in the presence of proton sources. Recent studies from our group and others that have improved the catalyst activity and significantly expanded the understanding of these catalysts are highlighted in this report. The 4,4′-tert-butyl-substituted complexes fac-Re(bpy-tBu)(CO)3X have been found to be more active than the parent 2,2′-bipyridine complexes. The presence of Brønsted acids increases the activity of these catalysts, with stronger acids leading to more rapid catalysis. The catalytically relevant [Re(bpy-R)(CO)3]− 1 anions have been isolated and studied in order to elucidate their structures and reactivities. X-ray crystallography, quantum chemical calculations, and synchrotron radiation experiments have shown that the electronic structures of the anions are best described as Re0(bpy-R)− 1 states, with electron density delocalized over both the metal and the ligand. This delocalized ground state is thought to enable better overlap with CO2 compared to protons, which explains the selectivity for CO2 reduction with these types of catalysts even in the presence of acids. Recent reports have also shown that earth-abundant manganese can be substituted for rhenium to yield fac-Mn(bpy-R)(CO)3X catalysts that approach the CO2 reduction activity of the analogous rhenium compounds. Indeed, the anionic [Mn(bpy-tBu)(CO)3]− 1 species has recently been crystallized and studied, and it possesses a similar structure to the [Re(bpy-tBu)(CO)3]− 1 anion. Future directions for the study of fac-M(bpy-R)(CO)3X catalysts are also discussed.
Available at: http://works.bepress.com/kyle_grice/3/