Study On The Effect Of The Ionic Substituents On The Ligand Of The Metal Complex On Its Catalytic Activity

Conventional organic substituents on the ligand of metal complex catalysts have long been widely used to tune their activities and selectivities. Due to the difficult to synthesis different conventional organic substituents on the ligand of metal complex and this type of catalysts are also not easily recovered, it needs an efficient method to solve the problems mentioned above for both academia and industry. Modifying the ligand of metal complex by ionic compound is a recent development in this field. These metal complex catalysts with ionic substituents on the ligand are easily retrieved and recycled because their solubility can be adjusted by using different cation-anion combinations, enabling phase separation from less polar organic solvents to aqueous media. Recently, ionic substituents, which usually enable the homogeneous catalyst dissolved in water and recovered, are also involved to improve the activity of catalyst. Though it has been well known that ionic substituents on the ligand can change catalyst solubility, it is still unclear how exactly it influences their catalytic ability. Hence, the research of the effect of ionic substituent on the ligand on the catalytic activity is necessary.We designed and synthesized three types of acetylacetone metal complexes catalysts. The most appealing features of the catalysts were that the ionic substituent connected with the metal center by conjugated double bond, thus the cation-anion interaction could easily deliver to the metal center. The catalysts were used in the allylic oxidation of4-methyl guaiacol, and the oxidation reaction conditions, reaction mechanism, Vanillin isolation and reaction medium recycling were researched. We also studied the influences on the catalytic activity of catalysts with ionic substituents on the ligand.In this dissertation, two types of acetylacetone metal complexes catalysts with cationic ionic substituents on the ligand Co-[Salen-X][Y]2(X=Py, Mim; Y=PF6-, ClO4-, BF4-, NO3-, Br-, BPh4-) and [Co(acac-Mim)2][Y]2(Y=Cl-, NO3-, BF4-,PF6-, C1O4-) and one types of acetylacetone metal complexes catalysts with anionic ionic substituents on the ligand [X][Co(F6-acac)3](X=NH4+, C2Py+, N44,4,4,4+, C4Mim+, P4,4,4,10+) were designed and synthesized. These metal complexes were charactered by IR, MS and element analysis to check the molecular structure. At the same time, to further prove the identity of these complexes, we have succeeded obtain the single crystal for Cu-[Salen-Mim][Br]2, Cu-[Salen-Py][NO3]2,[Salen-Py][Br2]2, Cu-[Salen-Py][BPh4]2for X-ray analysis.Later on, a cobalt Schiff base catalyst with ionic substituents on the ligand Co-[Salen-Py][PF6]2was used for the oxidation of4-methyl guaiacol to vanillin. The results showed that the catalytic activities of this modified catalysts were significantly improved compared with their unmodified counterpart. Tentative reaction mechanism research indicated that the electron-withdrawing pyridinium substituent on the ligand of Co(acacen) is responsible for the high selectivity of vanillin. Meanwhile, utilizing ethylene glycol and water as solvent, vanillin can be isolated by simple crystallization in the form of a sodium salt, and the mother liquid of the crystallization, with a large amount of NaOH (the mole ratio of NaOH/4-methyl guaiacol=2.38/1), can be successfully recycled at least three times, thereby decreasing the mole ratio of base/substrate from3.3:1to1.05:1when the mother liquid of crystallization was recycled. This strategy provides a potentially greener alternative for the synthesis of vanillin in industry.At last, the influences on the catalyst reactivity of the catalysts with ionic substituents on the ligand by changing the counter ions of the ionic substituents were explored. No matter what kinds of counter ions were changed--cationic or anionic, we still found that the counter ions were only moderate improved the activity. To further investigate the electronic effect of different counter ions of ionic substituents on the central metal of the catalyst, the NBO charge of C-3in the simply model of the real catalysts (ionic substituent-monosubtituted benzenes) was investigated by means of quantum chemical calculations utilizing Density Functional Theory (DFT). The result shown that the cationic ionic substituents and the anionic ionic substituents are electron-withdrawing substituents, the counter ions of ionic substituents also have moderate effect on electron density on the central metal. The inflence of different central metals and solvents on the catalytic activity of the acetylacetone metal complexes catalysts with imidazolium ionic substituents on the ligand were investigated. The result indicated that changed the central metal was a more effective method than change the counter ions of the ionic substituents to improve the catalytic activity. In short, the oxidation of4-methyl guaiacol to vanillin by the metal complexes with ionic substituents on the ligand was studied, and three types of acetylacetone metal complexes catalysts with ionic substituents on the ligand were involved. The cationic ionic substituents and the anionic ionic substituents are electron-withdrawing substituents, and the counter ions of the ionic substituents were only moderate improved the activity may enlighten us to use the ionic substituents to design novel and efficient catalysts. At the same time, the oxidation4-methyl guaiacol to vanillin by the acetylacetone metal complexes with ionic substituents on the ligand is a potentially greener alternative for the synthesis of the vanillin in industry.