Catalytic activity of new manganese(II) complexes in epoxidation of cyclooctene with H2O2

Catalytic epoxidation of olefins in the presence of transition metal complexes is an important transformation in organic synthesis since it can generate the reactive three membered epoxide rings. This process, by using H2O2 as a green oxidant, is one of the most environment-friendly processes for the transformation of olefins to important intermediates in organic chemistry. Although a variety of transition metal ions can act as a catalyst in the catalytic epoxidation of olefins, manganese complexes have attracted considerable attention during the past decades. This is mainly due to the high selectivity and activity of manganese complexes in the epoxidation of various olefins. Here, we report the synthesis, characterization and catalytic properties of two new dinuclear Mn(II) complexes. The Mn(II) complexes, [Mn(μ-L1)(NCS)(CH3OH)]2 (1) and [Mn(μ-L2)(NCS)(CH3OH)]2 (2), were synthesized from the reaction of MnCl2.4H2O, KSCN and HL1-2 in methanol where the HL1-2 are the Schiff base ligands (HL1 = (E)-4-((2-hydroxy-3-methoxybenzylidene)amino)-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one and HL2 =(E)-4-((2-hydroxybenzylidene)amino)-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one). The complexes were characterized by spectroscopic methods and single crystal X-ray diffraction analysis. The X-ray analysis indicated that the manganese atoms have a distorted octahedral coordination environment in both 1 and 2. The phenolate oxygen atom of the Schiff base ligands acts as bridging group between two Mn(II) ions. The octahedral coordination environment around the Mn(II) ions is created by coordination of nitrogen and two oxygen atoms from Schiff base ligand (L1 or L2), oxygen atom of the bridging phenolate ligand, a nitrogen atom from thiocyanate anion and one oxygen atom from coordinated methanol molecule. Since the Mn(II) complexes have high activity in the catalytic epoxidation of olefins, the catalytic activity of the obtained complex was investigated in epoxidation of cyclooctene. The catalytic reactions were carried out in aerial atmosphere and H2O2 was used as a green oxidant. The effects of various parameters including the molar ratio of oxidant, temperature and solvent were studied in order to obtain optimum condition. The progress of reaction was monitored by GC. The results of catalytic studies indicated that the obtained Mn(II) complexes have high catalytic activity and selectivity in the epoxidation of cyclooctene and acetonitrile is an ideal solvent for this catalytic system.

Keywords:

Mn(II) complex , Schiff base , Catalytic activity , Cyclooctene epoxidation , H2O2