Mechanistic Insights into CO₂-to-CO Photoreduction by Proton-Responsive Imidazole-Pyridine Re(I) Complexes

Two Re(I) tricarbonyl complexes with imidazole-pyridine ligands, fac-[Re(CO)₃(pbiH)Cl], (Re-pbiH), and fac-[Re(CO)₃(bbzp)Cl] (Re-bbzp), where pbiH= 2-(2-pyridy)benzimidazole, bbzp = 2,6-bis(2-benzimidazolyl)pyridine, were synthesized, and their photophysical, electrochemical, and photochemical properties were investigated for application as photocatalysts for CO₂-to-CO reduction. Upon light irradiation (λ > 370 nm) in CO₂-saturated CH₃CN using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial donor, Re-bbzp promotes CO formation with a turnover number (TON_CO) of 45 ± 2, whereas Re-pbiH displayed a significantly lower activity (TON_CO = 8 ± 1). The role of the bbzp ligand in the photocatalytic behavior was examined in detail using IR spectroelectrochemistry (IR-SEC) together with in situ FTIR and UV-vis spectroscopy under photocatalytic conditions, revealing the formation of key intermediates involved in CO₂ activation. The superior activity of Re-bbzp was rationalized by its ability to function as a proton relay and two-electron acceptor. The role as a proton relay was further supported by the observation of a kinetic isotope effect (KIE = 1.5) when the deuterated electron donor BID was employed, in agreement with the unusual decrease in catalytic activity observed upon addition of Brønsted bases. Overall, these results provide new insights into the role of ligand-based proton-responsive sites in Re(I) tricarbonyl complexes and their impact on CO₂ reduction photocatalysis.