Design, synthesis and photocatalytic activities of functionalized Re(I) bipyridine tricarbonyl complexes toward CO2 reduction and carboxylation.

Natural metalloenzymes efficiently catalyze chemical transformations by precisely controlling the local environment around the active site, thereby enabling selective substrate activation and reaction control. This general concept has motivated extensive efforts to improve molecular catalysts by ligand modification and functionalization. In this work, we designed and synthesized functionalized derivatives of the fac-Re(2,2'-bipyridine)(CO)3Cl complex (Re-1), which has long been recognized as a benchmark catalyst for photochemical CO2 reduction, and which we have recently demonstrated to also catalyze the carboxylation of organic substrates with CO2. Using these complexes, we investigated how substituents on the bipyridine ligand affect photocatalytic activities in both CO2 reduction and carboxylation reactions. All new complexes were fully characterized by single-crystal X-ray diffraction analysis as well as by spectroscopic and electrochemical methods. The introduction of electron-withdrawing functional groups induces clear changes in the electronic structures of the complexes, as evidenced by redshifts in their absorption spectra, CO stretching frequencies, and reduction potentials. In photocatalytic CO2 reduction, the catalytic activities of the functionalized complexes depend strongly on these electronic properties, whereas in carboxylation reactions, all complexes show comparable catalytic performance regardless of the nature of the functional groups. Moreover, the present system also exhibits additional reactivities, including [2 + 2] dimerization and hydroacylation reactions. Collectively, this work reports the design, synthesis, and characterization of a new series of functionalized Re(I) complexes, and provides a useful experimental basis for understanding how ligand modification influences their properties and reactivity, thereby offering guidance for future development of Re-based molecular systems for CO2 utilization.