An Unexpected Iron (II)-Based Homogeneous Catalytic System for Highly Efficient CO₂-to-CO Conversion under...

We present two as-synthesized Fe(II)-based molecular catalysts with 1,10-phenanthroline (phen) ligands; Fe(phen)₃Cl₂ (1) and [Fe(phen)₂(CH₃CH₂OH)Cl]Cl (2), and their robust catalytic properties for the conversion of CO₂ to CO in DMF/TEOA (DMF = N,N'-dimethylformamide; TEOA = triethanolamine) solution containing Ru(bpy)₃²⁺ and BIH (1,3-dimethyl-2-phenyl-2,3- dihydro-1H-benzo-[d]-imidazole). High turnover numbers (TONs) of 19,376 were achieved with turnover frequencies (TOFs) of 3.07 s^-1 for complex 1 (1.5 × 10^-7 M). A quantum efficiency of 0.38% was observed after 5 h irradiated by 450 nm monochromatic light. The generation rate of CO₂ and H₂ were tuned by optimizing the experimental conditions, resulting in a high CO selectivity of 90%. The remarkable contribution of the photosensitizer to the total TON_CO was found being 19.2% (as shown by tests under similar conditions without catalysts) when BIH was employed as a sacrificial electron donor. The product selectivity in complex 2 reached 95%, and the corresponding TON_CO and TOF_CO were 33,167 and 4.61 s^-1 in the same concentration with complex 1 used as catalyst; respectively. This work provides guidance for future designs of simple, highly efficient and selective molecular catalytic systems that facilitate carbon-neutral solar-to-fuel conversion processes.