Highly efficient photocatalytic reduction of CO2 to CO under visible light using rhenium benzo[d]oxazole complexes

A series of six isomeric rhenium(i) tricarbonyl complexes featuring asymmetric diimine ligands 5,7-di-tert-butyl-2-(pyridin-2-yl)benzo[d]oxazole, 2-(pyridin-2-yl)naphtho[1,2-d]oxazole, 2-(pyridin-2-yl)naphtho[2,3-d]oxazole, 2-(quinolin-2-yl)benzo[d]oxazole, 2-(quinolin-2-yl)naphtho[1,2-d]oxazole, and 2-(isoquinolin-1-yl)benzo[d]oxazole, was synthesized and comprehensively characterized through analytical techniques, spectroscopy, and single-crystal X-ray diffraction. These complexes were assessed as catalysts for visible-light-driven CO2 reduction, both with and without an external photosensitizer (PS), and also exhibited catalytic activity in electrochemical CO2 reduction, effectively functioning in both regimes. In the presence of a proton donor, trifluoroethanol (TFE), the complexes produced methane and carbon monoxide (CO) electrochemically. Under photochemical conditions, high selectivity for CO production was achieved in acetonitrile with triethanolamine (TEOA) as the proton source. This work highlights the tunability of metal complex photocatalysts for solar-to-fuel conversion through ligand design. Among the complexes studied, the rhenium complex featuring the 2-(pyridin-2-yl)naphtho[1,2-d]oxazole ligand demonstrated the highest catalytic efficiency, achieving a turnover number of 660 for CO2 reduction to CO after 5 hours. Mechanistic studies employing NMR, UV-vis spectroscopy, and time-dependent density functional theory (TD-DFT) calculations provided insights into the catalytic process. These rhenium(i) complexes demonstrate promising potential for photocatalytic CO2 reduction to CO, offering valuable insights into the design and development of efficient catalysts for artificial photosynthesis and solar-to-fuel conversion.