Integrating four-, five- and six-coordinated Zn catalytic centers in an ionic-type zinc catalyst for the coupling of CO2 with epoxides

Effectively converting greenhouse gas CO2 into value-added chemicals, without the need for cocatalysts or solvents, is a vital objective in order to attain carbon neutrality and promote sustainable development. However, achieving this goal continues to pose significant challenges. In this study, the in-situ formed octadentate N5O3 tripodal ligand (H3L) reacts with ZnBr2 and ZnI2 to produce two ionic-type zinc catalysts, Zn2(L)2ZnBr4·4CH3OH (IZ-1) and Zn2(L)I·CH3OH (IZ-2), respectively. In the Zn2(L) subunit, the two Zn ions exhibit non-symmetry and are coordinated with the N5O3 ligand in a five- and six-coordinate mode, respectively. Due to the coexistence of Lewis acidic Zn sites and nucleophilic Br- ions, IZ-1 shows remarkable catalytic performance in the cycloaddition of CO2 with epoxides. Under cocatalyst- and solvent-free conditions, it achieves an excellent yield of cyclic carbonates, reaching up to 99 %. The ionic-type zinc catalyst can be readily recovered and reused at least for five times without a noticeable loss in catalytic activity. Mechanistic investigation demonstrates that the improved activity of IZ-1 is attributed to the synergistic effect of Lewis acidic Zn centers and nucleophilic Br-. Moreover, the broad substrate scope of IZ-1 further highlights its versatility and applicability in the cycloaddition reaction.