Zinc-Azatrane Complexes as Efficient Catalysts for the Conversion of Carbon Dioxide into Cyclic Carbonates

Three Zn complexes based on the N-4-tris(2-aminoethyl)amine (tren) chelating ligand and presenting a C-3-symmetrical axis were synthesized and successfully applied in the coupling of CO2 with terminal and internal epoxides. These complexes proved to be efficient catalysts if associated with tetrabutylammonium iodide, even at a low catalyst loading (0.005 mol%) or at room temperature, and allowed the production of cyclic carbonates in good to high yields. Variation of the substitution pattern on the tren ligand was shown to impact the catalyst performance greatly, and the highest turnover number (TON) (up to 11 200) was achieved with the less sterically hindered methyl-substituted Zn-II-azatrane complex. These binary Zn-azatrane/NBu4I catalytic systems could be applied to a wide range of epoxide substrates, including the more challenging internal epoxides. Moreover, although soluble in the reaction medium, Zn-azatrane catalysts could be easily recovered and reused up to three times without any substantial loss in activity, proving their robustness under the reaction conditions.