Photocatalytic Reduction of CO₂ with Re-Pyridyl-NHCs

A series of Re(I) pyridyl N-heterocyclic carbene (NHC) complexes have been synthesized and examined in the photocatalytic reduction of CO₂ using a simulated solar spectrum. The catalysts were characterized through NMR, UV-vis, cyclic voltammetry under nitrogen, and cyclic voltammetry under carbon dioxide. The complexes were compared directly with a known benchmark catalyst, Re(bpy) (CO)₃Br. An electron-deficient NHC substituent (PhCF₃) was found to promote catalytic activity when compared with electron-neutral and -rich substituents. Re(PyNHC-PhCF₃) (CO)₃Br was found to exceed the CO production of the benchmark Re(bpy) (CO)₃Br catalyst (51 vs 33 TON) in the presence of electron donor BIH and photosensitizer fac-Ir(ppy)₃. Importantly, Re(PyNHC-PhCF₃) (CO)₃Br was found to function without a photosensitizer (32 TON) at substantially higher turnovers than the benchmark catalyst Re(bpy) (CO)₃Br (14 TON) under a solar simulated spectrum.