A new mononuclear hydrazone-based aluminum complex as a catalyst for solvent-free CO2 cycloaddition reaction

Abstract

Recently, due to increasing concerns over carbon dioxide emissions and their impact on the environment, the study and conversion of it into valuable products have been of great interest. In general, aluminum-based complexes can be a good choice for CO₂ cycloaddition reactions due to their strong Lewis acidity. Hence, a mononuclear aluminum complex [Al(HL′)(H₂O)₃](H₂O)₂Cl containing hydrazonic based ligand (sodium 4-hydroxy-3-{[2-(pyridine-4‑carbonyl)hydrazinylidene]methyl}benzene-1-sulfonate) was synthesized and identified by FT-IR, ¹H NMR, XRD and single-crystal X-ray crystallography. The ligand coordinated to aluminum in enolate-phenolate form through oxygen atoms of phenolate and enolate and nitrogen atoms of imine. Water molecules complete the distorted octahedral geometry around the metal center. The XRD pattern of the complex exhibited that it was stable up to around 120 °C. Therefore, the prepared compound was examined as a catalyst in the presence of quaternary ammonium (TBAB) as a co-catalyst for CO₂ cycloaddition reaction under solvent-free conditions to synthesize cyclic carbonates via CO₂ and epoxide. In this work, the reaction was performed under mild conditions. The conversion of around 80 % of epichlorohydrin was obtained using the catalyst and co-catalyst at 60 °C, 24 h, and 1 bar of CO₂. A reusability study demonstrated that a catalyst can be used in at least three cycles without any decrease in catalytic activity under mild conditions.