Chemical fixation of CO2 into cyclic carbonates by azo-containing Schiff base metal complexes

Abstract

Two new ligands containing a –NN– group, 4-((E)-(4-bromophenyl)diazenyl)-2-((E)-(phenylimino)methyl)phenol (L¹H, 2), 4-((E)-(4-bromophenyl)diazenyl)-2-((E)-((4-ethylphenyl)imino)methyl)phenol (L²H, 3) and their metal complexes were synthesized. The synthesized metal complexes were used as catalysts for the chemical fixation of CO₂ into cyclic carbonates using epoxides which were used as both substrate and solvent. According to analytical, UV-visible and IR data, the metal complexes are formed by coordination of the N and O atoms of the ligands and the metal : ligand ratio was found to be 1 : 2 for all the complexes. The TG and DTA results showed that these complexes had good thermal stability. The molecular structures of ligand 2 (L¹H) and its Zn^II complex 4 (Zn(L¹)₂) were determined by single crystal X-ray diffraction studies. After choosing the most active catalyst (Zn(L¹)₂, 4), optimization studies were performed by changing various parameters. Ionic liquids have been found to have a positive effect and showed the most active performance with (bmim)PF₆ + Zn(L¹)₂ (4) as a binary catalytic system.