[M(Me6Tren)X]X complex as efficacious bifunctional catalyst for CO2 cycloaddition: The synergism of the metal and halogen ions

Efficient catalysis for fixation and conversion of CO2 is of significance to carbon neutralization. Here, a series of C3 symmetric bifunctional complex catalysts with different metals (Ni, Co, Cu) have been strategically synthesized and evaluated in the CO2 cycloaddition with epoxides. Among these catalysts, [Ni(Me6Tren)I]I 3c exhibits the best performance affording up to 99.1% yield of chloropropylene carbonate under ambient (60 °C, CO2 balloon), cocatalyst- and solvent-free conditions. TPD and XPS showed 3c possessed excellent acid center. Single crystal X-ray diffraction indicated the facile dissociation of Ni-I based on the bond length information. The synergistic effect of the Ni(II) acidic center, the axial and outer iodine ions was discussed. Based on the structure-activity relationship, plausible reaction pathways were proposed for the bifunctional catalysts. Moreover, a large-scale synthesis of chloropropylene carbonate (350 mmol) and recyclability of 3c catalyst highlight its potential for industrial applications.