Synthesis, characterization and catalytic property of tetradentate Schiff-base complexes for the epoxidation of styrene

Abstract

A series of Schiff-base complexes has been synthesized by the condensation of 1,2-diaminocyclohexane with salicylaldehyde, 2-pyridinecarboxaldehyde, and 2-hydroxy-1-naphthaldehyde, followed by the metallation with manganese ( 1, 2, 3a), cobalt ( 3b), copper ( 3c) and iron ( 3d) salts. These Schiff-base ligands L 1 – L 3 and complexes 1, 2, 3a– d were then characterized by IR, 1H NMR, 13C NMR, UV–vis spectra, and DSC measurement. Schiff-base Mn complex ( 3a) resulting from N, N′-bis(2-hydroxy-1-naphthalidene)cyclohexanediamine ( L 3 ) ligand was considerably active for the catalytic epoxidation of styrene under mild conditions, in which the highest yield of styrene oxide reached 91.2 mol%, notably higher than those achieved from simple salt catalysts Mn(Ac) 2·4H 2O and MnSO 4·H 2O. However, another two salen–Mn complexes 1 and 2 derived from ligands N, N′-bis(salicylidene)cyclohexanediamine ( L 1 ) and N, N′-bis(2-pyridine carboxalidene)cyclohexanediamine ( L 2 ) exhibited relatively poor activity under identical experimental conditions.