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 L1–L3 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 (L3) 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)2·4H2O and MnSO4·H2O. However, another two salen–Mn complexes 1 and 2 derived from ligands N,N′-bis(salicylidene)cyclohexanediamine (L1) and N,N′-bis(2-pyridine carboxalidene)cyclohexanediamine (L2) exhibited relatively poor activity under identical experimental conditions.

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