Efficient Photocatalytic CO2 Reduction under a Dilute CO2 Atmosphere by Iron Complexes with Hydrophobic Cavity in the Second Coordination Sphere

The direct utilization of CO2-diluted industrial flue gas streams for solar-driven fuel synthesis requires catalysts capable of efficient CO2 capture and conversion under low CO2 concentrations. We report that FeII complexes having hydrophobic pockets in the second coordination sphere (SCS) perform photocatalytic CO2 reduction to afford CO in high efficiency and selectivity. Using an Ir-based photosensitizer and a sacrificial reductant in CH3CN/H2O (4:1, v/v), FeII complexes bearing mesityl or anthracenyl substituents in the SCSs exhibited approximately 2-fold higher turnover numbers for photocatalytic CO2-to-CO conversion under a pure CO2 atmosphere compared to the unsubstituted complex while maintaining CO selectivity above 90%. Remarkably, the anthracenyl-substituted complex maintained high catalytic activity and selectivity even under highly diluted CO2 conditions, enabling efficient conversion of CO2 to CO at concentrations as low as 5000 ppm (0.5% CO2). Mechanistic investigations indicated that capturing a nonpolar CO2 molecule in the hydrophobic SCS of the doubly reduced Fe0 species may serve as a key step governing the catalytic activity under CO2-diluted conditions and that hydrophobic interactions facilitate this process by increasing the local concentration of CO2 near the catalytic center. These findings demonstrate the potential of the engineering of hydrophobic SCS for the development of molecular catalysts capable of efficient low-concentration CO2 utilization.