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. The synthesis, X-ray structure, spectroscopic and catalytic properties of a selected series of sterically hindered salicylaldimine functionalized Copper complexes was reported. Selectively very high conversion (TON = 30,335) for chemical fixation of CO2 into cyclic carbonates was obtained while using the multinuclear catalyst ([Cu3(L2)4]Cl2·CuCl2) complex which has 5-methyl substituent on the pyridine ring.