Multifunctional Zn-N4 Catalysts for the Coupling of CO2 with Epoxides into Cyclic Carbonates

The catalytic conversion of greenhouse gas CO2 into valuable chemicals is a vital goal toward carbon balance and sustainability. In recent decades, the chemical fixation of CO2 into cyclic carbonates has gained much attention. In this work, a series of zinc complexes bearing tetradentate aminopyridine (N4) ligands have been synthesized and characterized. These zinc complexes were applied to the coupling of CO2 with epoxides in excellent yields and with a broad substrate scope under cocatalyst- and solvent-free conditions. Moreover, the zinc catalysts could be readily recovered and reused five times without an obvious loss in catalytic activity. Based on spectroscopic characterizations and experimental results, catalyst Zn-3 (DAP-ZnBr2, DAP = 1,4-bis(2-pyridymethyl)-1,4-diazepane) has been found to be a multifunctional catalyst because of the presence of a Lewis acidic zinc center and a nuclephilic halide anion, and one pyridine is released for the activation of CO2 during the reaction.