Novel Pyridine Schiff-base Mn(Ⅱ) Complexes:Synthesis,Immobilization And Catalytic Reactivity Of Epoxidation Of Cyclohexene

Epoxidation of cyclohexene is among the most important reactions in organic synthesis, because epoxide compound is valuable precursor for the synthesis of drugs, agrochemicals and food additives. In traditional production process, there were many problems concerning low yield, catalyst lost, harsh condition and serious pollution. Consequently, the development of oxidation reaction using molecular oxygen as a cheap, environmentally clean and convenient oxidant has attracted significant attention. Unfortunately, it is observed that the oxidation efficiency of molecular oxygen is poorer than other oxidants. Therefore, it is the key challenge to find effective catalysts for activation of molecular oxygen in developing new procedures using O2as a green oxidant. Herein, a series of novel Schiff-base Mn(Ⅱ) complexes and their montmorillonite-based catalysts were designed and prepared, then their activity and selectivity as catalysts for the epoxidation of cyclohexene were studied. The effects of various reaction conditions on the catalytic reaction were optimized.A series of ligands and their Mn(Ⅱ) complexes were prepared by pyridine-2,6-dicarboxylic acid as starting material, based on pyridine ring as main framework as well as C=N as bridging group, ligands containing nitrogen heterocycle were introduced into hydroxyl group and other electron donating group. All the ligands and intermediates were characterized by IR and1H NMR. The complexes were characterized with elemental analysis, TG-DSC, IR and EI-MS. The complexes were proved to promote the epoxidation of cyclohexene efficiently. At last The Proposed Radical-chain Sequence Mechanism was studied.We immobilized Mn-LR into Montmorillonite to reach the recyclable property. All the montmorillonite-based catalysts were characterized by FT-IR、XRD、UV-dis、SEM and ICP. The heterogeneous catalysts were proved to promote the epoxidation of cyclohexene efficiently. Mn-L2-mont displayed the highest catalytic activity and selectivity due to providing electronic of methyl and100%conversion and92.0% selectivity were obtained under the optimized conditions (10mmol cyclohexene;0.01mol%(20.0mg) catalyst;20mL solvent,10mmol isobutyraldehyde, temperature,40℃; molecular oxygen rate,10mL/min,5h). Repeated runs indicated that the catalysts were stable for3cycles without noticeable loss of their catalytic activity. It was demonstrated that these pyridine Schiff-base catalysts were highly selective catalysts than other Schiff-base catalysts.