Hexamethylenetetramine Complexes as Easy-to-Handle and Efficient Lewis Acid–Base Heterogeneous Catalysts for Coupling of CO₂ under Mild Conditions

In terms of the multisynergetic effects among Lewis acid, halogen anions, and Lewis base, a series of Lewis acid–base hexamethylenetetramine complexes with tertiary amines, different metal centers, and halogen anions were conveniently synthesized at room temperature. Hexamethylenetetramine is cheap, nontoxic, rich in tertiary amine, and with a cage structure. This is the first time that the hexamethylenetetramine complexes were used as efficient and stable heterogeneous catalysts for the generation of cyclic carbonates by the coupling reaction of CO₂ under relatively mild conditions (60 °C, 1.5 MPa, 12 h) with outstanding performance (yield = 98.9%) without use of solvent and cocatalyst. Hatm complexes that are different in nucleophilicity and Lewis acidity are designed and evaluated, which suggest that the stronger Lewis acidity of metal together with stronger nucleophilicity of anions means higher catalytic performance. A plausible multisynergetic mechanism over ZnBr₂–Hatm is proposed for the CO₂ coupling. In addition, the ZnBr₂–Hatm can be conveniently recovered and reused in a test of five times without remarkable decline of activity.