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

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, H-1 NMR, C-13 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(2)O and (MnSO4H2O)-H-.. 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-1) exhibited relatively poor activity under identical experimental conditions. (c) 2006 Elsevier B.V. All rights reserved.