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 (L<inf>1</inf>H = N-[allylamine]-3,5-di- tert-butyl salicylaldimine, L<inf>2</inf>H=N-[2-amino-5-methyl pyridine]-3,5-di-tert-butyl salicylaldimine and L<inf>3</inf>H=N-[2-amino-6- methyl pyridine]-3,5-di-tert-butyl salicylaldimine), and their mononuclear Cu(II) complex for L<inf>1</inf>H with multinuclear Cu(II) complexes for L <inf>2</inf>H and L<inf>3</inf>H, 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 CuCl<inf>2</inf>•2H <inf>2</inf>O. 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 L<inf>2</inf>H and L<inf>3</inf>H ligands in a 1:1 mol ratio with CuCl <inf>2</inf>•2H<inf>2</inf>O afforded ionic copper metal(II) complexes in the presence of NEt<inf>3</inf>. 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. [Cu<inf>3</inf>(L <inf>2</inf>)<inf>4</inf>]Cl<inf>2</inf>•CuCl<inf>2</inf> 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. Graphical Abstract: 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 CO<inf>2</inf> into cyclic carbonates was obtained while using the multinuclear catalyst ([Cu<inf>3</inf>(L<inf>2</inf>) <inf>4</inf>]Cl<inf>2</inf>•CuCl<inf>2</inf>) complex which has 5-methyl substituent on the pyridine ring. © 2010 Springer Science+Business Media, LLC.