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

Thecatalytic conversion of greenhouse gas CO2 intovaluable chemicals is a vital goal toward carbon balance and sustainability.In recent decades, the chemical fixation of CO2 into cycliccarbonates has gained much attention. In this work, a series of zinccomplexes bearing tetradentate aminopyridine (N4) ligands have beensynthesized and characterized. These zinc complexes were applied tothe coupling of CO2 with epoxides in excellent yields andwith a broad substrate scope under cocatalyst- and solvent-free conditions.Moreover, the zinc catalysts could be readily recovered and reusedfive times without an obvious loss in catalytic activity. Based onspectroscopic 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 presenceof a Lewis acidic zinc center and a nuclephilic halide anion, andone pyridine is released for the activation of CO2 duringthe reaction.

Automated summary

A series of zinc complexes with tetradentate aminopyridine ligands were synthesized and characterized. These zinc catalysts were able to couple CO2 with epoxides in excellent yields and broad substrate scope under solvent- and cocatalyst-free conditions. The catalysts could be easily recovered and reused without significant loss in catalytic activity. Catalyst Zn-3 was found to be multifunctional due to a Lewis acidic zinc center, a nucleophilic halide anion, and the release of a pyridine molecule for CO2 activation.