Zinc complexes bearing BIAN ligands as efficient catalysts for the formation of cyclic carbonates from CO2 and epoxides

Herein, we present the synthesis of three new neutral aryl-BIAN ZnCl2 complexes (where aryl-BIAN = bis(aryl-imino)acenaphthene) with formulations [Zn(4-iPrC(6)H(4)-BIAN)Cl-2] (1), [Zn(2-iPrC(6)H(4)-BIAN)Cl-2] (2) and [Zn(4-NO2C6H4-BIAN)Cl-2] (3) obtained through a green synthesis methodology. Compounds 1 and 2 were fully characterized by elemental analysis, ESI-LS mass spectrometry, FT-IR ATR mode, and multinuclear NMR spectroscopic techniques. Compound 3 was characterized by elemental analysis, ESI-LS mass spectrometry and FT-IR, ATR mode. The solid-state molecular structures of compounds 1 and 2 were determined by single crystal X-ray diffraction. The new complexes 1, 2 and 3, and the known complexes of formulations [Zn(2,6-iPr(2)C(6)H(3)-BIAN)Cl-2] (4) and [Zn(2,4,6-Me3C6H2-BIAN)Cl-2] (5) were tested as catalysts in the cycloaddition reaction of CO2 and epoxides using the tetrabutylammonium bromide salt as a co-catalyst. Primary studies revealed that complex 4 presented the highest catalytic activity among all synthetized complexes. Good reaction TOFs were obtained for the conversion of propylene oxide (112 h(-1)) after 3 hours and for the conversion of styrene oxide (23 h(-1)) after 8 hours, at 4 MPa and 333 K. For more sterically hindered cyclohexene oxide, no cyclic carbonate formation was observed, under the same operational conditions. Furthermore, the effect of reaction time (1-40 h), pressure (0.5-40 MPa) and nucleophile to metal ratio (0.5-100) on final cyclic carbonate formation was investigated.

Automated summary

In this study, three new neutral aryl-BIAN ZnCl2 complexes were synthesized using a green synthesis methodology. The complexes were fully characterized and tested as catalysts in the cycloaddition reaction of CO2 and epoxides. The results showed that the complexes exhibited good catalytic activity in the reaction.