Iron(III) Complexes with Pyridine Group Coordination and Dissociation Reversible Equilibrium: Cooperative Activation of CO₂ and Epoxides into Cyclic Carbonates

Herein, a series of [ONSN]-type iron(III) complexes were synthesized. A binary catalytic system in combination with iron complexes and tetrabutylammonium bromide (TBAB) exhibited high activity for the synthesis of cyclic carbonates from CO₂ (1 atm) and terminal epoxides at room temperature. Additionally, single-component iron complexes without using additional TBAB as nucleophiles also showed high activity for the cycloaddition of CO₂ and terminal epoxides under 80 °C and 0.5 MPa of CO₂. This study demonstrates that single-component iron catalysts provide a competitive alternative to binary catalytic systems for the synthesis of cyclic carbonates from CO₂ and epoxides. Mechanistic studies on a single-component iron catalytic system suggest that the temperature serves as a role of responsive switch for controlling the coordination and dissociation of pyridine bearing iron catalysts detected using in situ infrared spectroscopy, and uncoordinated pyridine activates CO₂ to form carbamate. Studies of electrospray ionization high-resolution mass spectrometry reveal that an iron center was used as a Lewis acidic site, free halogen anions from the iron center were used as a nucleophilic site, and coordinated pyridine was released from iron complexes to activate CO₂.