Light-driven CO 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 CO. The catalytic activity of the complexes was further supported by cyclic voltammetry through the presence of a catalytic current under CO atmosphere. The D-π, π, and A-π complexes were studied to elucidate the effects of incorporating conjugated electron donating withdrawing groups on the catalytic rates and product selectivity. The synthesized complexes were compared with Re(bpy)(CO)Br (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 (HCOH) in the presence of photosensitizer [Ru(bpy)] and sacrificial electron donor BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1-benzo[]-imidazoline). Among the investigated catalysts, RC4 with the A-π pendant showed the highest turnover number (TON) value of 844 for HCOH 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)Br generated no formic acid under the same conditions. The imidazole-pyridine complexes also function as catalysts for CO reduction without an added photosensitizer, however, the TON values under self-sensitized conditions are poor.