Multinuclear Cu(II) Schiff Base Complex as Efficient Catalyst for the Chemical Coupling of CO2 and Epoxides: Synthesis, X-ray Structural Characterization and Catalytic Activity

Abstract  

The synthesis, X-ray structure, spectroscopic and catalytic properties of sterically hindered Schiff-base ligands (L₁H = N-allylamine]-3,5-di-tert-butyl salicylaldimine, L₂H=N-2-amino-5-methyl pyridine]-3,5-di-tert-butyl salicylaldimine and L₃H=N-2-amino-6-methyl pyridine]-3,5-di-tert-butyl salicylaldimine), and their mononuclear Cu(II) complex for L₁H with multinuclear Cu(II) complexes for L₂H and L₃H, were described. The copper(II) complexes of these ligands were synthesized by treating an methanolic solution of the appropriate ligand with an appropriate amount of CuCl₂·2H₂O. The ligands and their copper(II) complexes were characterized by FT-IR, UV–Vis, ¹H-NMR, elemental analysis, measurement of room temperature magnetic moment, and X-ray structural determination. The reaction of the L₂H and L₃H ligands in a 1:1 mol ratio with CuCl₂·2H₂O afforded ionic copper metal(II) complexes in the presence of NEt₃. The Cu(II) metal complexes tested as catalysts for the formation of cyclic organic carbonates from carbon dioxide and liquid epoxides which served as both reactant and solvent. Cu₃(L₂)₄]Cl₂·CuCl₂ complex which has 5-methyl substituent on the pyridine ring showed high catalytic activity for chemical coupling carbon dioxide with epoxides (propylene oxide (PO), epichlorohydrine (EC) and 1,2-epoxy butane (EB)) selectively.