Light-driven CO₂ reduction with substituted imidazole-pyridine Re catalysts favoring formic acid production

Removing carbon dioxide from the atmosphere is an attractive way to mitigate the greenhouse gas effect that contributes to climate change. A series of donor-pi (D-π), acceptor-pi (A-π), and π Re(i) pyridyl imidazole complexes have been synthesized and examined under photocatalytic conditions for the photocatalytic reduction of CO₂. The catalytic activity of the complexes was further supported by cyclic voltammetry through the presence of a catalytic current under CO₂ atmosphere. The D-π, π, and A-π complexes were studied to elucidate the effects of incorporating conjugated electron donating vs. withdrawing groups on the catalytic rates and product selectivity. The synthesized complexes were compared with Re(bpy)(CO)₃Br (where bpy is 2,2'-bipyridine), the benchmark catalyst for this transformation. Remarkably, the complex with A-π pendant (RC4) outperformed the π (RC2-3) and D-π (RC5) complexes for the production of formic acid (HCO₂H) in the presence of photosensitizer [Ru(bpy)₃]²⁺ and sacrificial electron donor BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]-imidazoline). Among the investigated catalysts, RC4 with the A-π pendant showed the highest turnover number (TON) value of 844 for HCO₂H production with 86% carbon selectivity. In stark contrast to the imidazole-pyridine based catalysts reported here that favor formic acid as a product, Re(bpy)(CO)₃Br generated no formic acid under the same conditions. The imidazole-pyridine complexes also function as catalysts for CO₂ reduction without an added photosensitizer, however, the TON values under self-sensitized conditions are poor.