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

Abstract

Herein, we present the synthesis of three new neutral aryl-BIAN ZnCl₂ complexes (where aryl-BIAN = bis(aryl-imino)acenaphthene) with formulations [Zn(4-iPrC₆H₄-BIAN)Cl₂] (1), [Zn(2-iPrC₆H₄-BIAN)Cl₂] (2) and [Zn(4-NO₂C₆H₄-BIAN)Cl₂] (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-iPr₂C₆H₃-BIAN)Cl₂] (4) and [Zn(2,4,6-Me₃C₆H₂-BIAN)Cl₂] (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.