Visible-light-driven photocatalytic CO₂ reduction using ruthenium complexes having a CNC-type pincer ligand: comparison of substituent effect on a CNC ligand

A series of ruthenium complexes bearing a CNC-type pincer ligand with different substituents (CF₃, H, and OMe) were synthesized and characterized for visible-light-driven photocatalytic CO₂ reduction. The photocatalytic reactions were conducted using [Ru(dmbpy)₃]²⁺ as a photosensitizer and BI(OH)H as a sacrificial electron donor in a DMA-TEOA mixed solvent under LED light irradiation. The main product was formic acid, along with small amounts of CO and H₂. Contrary to previous findings that Papish <i>et al.</i> reported, the catalyst with an electron-withdrawing CF₃ substituent on the CNC ligand exhibited higher turnover number for formic acid production (TON_HCOOH = 4742) compared to the electron-donating OMe substituent (TON_HCOOH = 2152) after 24 hours of irradiation. Addition experiments revealed that the cessation of CO₂ reduction was primarily due to the degradation of the photosensitizer rather than deactivation of the catalyst. ESI-MS analysis detected the formation of a carbonate complex prior to the photocatalytic reaction. This study provides insights into the effect of substituents on CNC pincer ligands in ruthenium complexes for photocatalytic CO₂ reduction.