Iron(II) bis(pyrazolyl)phenanthroline complexes as robust and efficient homogeneous catalysts for CO2-to-CO conversion under visible light

We report the design, synthesis, and comprehensive characterization of a new series of iron(II) complexes bearing bis(pyrazolyl)phenantroline ligands, developed as homogeneous catalysts for the visible-light-driven reduction of CO 2 to CO. All complexes adopt high-spin quintet ground states and exhibit distorted octahedral geometries, with redox processes primarily localized on the ligand framework. Spectroscopic data and theoretical modeling revealed that the optical absorption in the UV region arises from ligand-centered π–π* transitions, while the weaker red-shifted bands are attributed to forbidden transitions. Electrochemical analyses confirmed ligand-centered reductions across the series, modulated by the electronic nature of the pyrazolyl substituents. All iron complexes were active for photocatalytic reduction of CO 2 to CO. Among them, Fe2 showed the highest overall activity, reaching a TON CO of 1318 and CO selectivity of 84 %. Notably, Fe4 achieved a high TON CO of 1265 and CO selectivity of 91 % in the series. Systematic variation of reaction parameters using Fe2 allowed the identification of optimal conditions for maximizing catalytic efficiency and product selectivity. At lower catalyst loadings, Fe2 achieved the highest TON CO (up to 23,138) and CO selectivity (up to 91 %). Additionally, the presence of 7.5–10 % water proved essential for effective proton-coupled electron transfer and stabilization of intermediates. Mechanistic studies, including control and Hg poisoning experiments, confirmed the homogeneous nature and robustness of the catalytic system, with deactivation arising mainly from photosensitizer deterioration. Fe2 reached a TON CO of 9754 after 4 h of irradiation (λ = 462 nm), corresponding to a TOF of 2438 h −1 and a total quantum yield of 8.24 %.