Iron(III) Complexes with Pyridine Group Coordination\nand Dissociation Reversible Equilibrium: Cooperative Activation of\nCO<sub>2</sub> and Epoxides into Cyclic Carbonates

Herein,\na series of [ONSN]-type iron(III) complexes were synthesized.\nA binary catalytic system in combination with iron complexes and tetrabutylammonium\nbromide (TBAB) exhibited high activity for the synthesis of cyclic\ncarbonates from CO<sub>2</sub> (1 atm) and terminal epoxides at room\ntemperature. Additionally, single-component iron complexes without\nusing additional TBAB as nucleophiles also showed high activity for\nthe cycloaddition of CO<sub>2</sub> and terminal epoxides under 80\n°C and 0.5 MPa of CO<sub>2</sub>. This study demonstrates that\nsingle-component iron catalysts provide a competitive alternative\nto binary catalytic systems for the synthesis of cyclic carbonates\nfrom CO<sub>2</sub> and epoxides. Mechanistic studies on a single-component\niron catalytic system suggest that the temperature serves as a role\nof responsive switch for controlling the coordination and dissociation\nof pyridine bearing iron catalysts detected using in situ infrared\nspectroscopy, and uncoordinated pyridine activates CO<sub>2</sub> to\nform carbamate. Studies of electrospray ionization high-resolution\nmass spectrometry reveal that an iron center was used as a Lewis acidic\nsite, free halogen anions from the iron center were used as a nucleophilic\nsite, and coordinated pyridine was released from iron complexes to\nactivate CO<sub>2</sub>.