The Organic-inorganic Hybrid Supramolecuiar Self-assembties Based On N-heterocyclic Ligands And Polyoxometaiates: Synthesisormance, Structure And Catalytic Epoxidation Performence

Polyoxometalates (POMs) have always attracted many attentions due to theirdefinite structures, controllable sizes and unique charge transfer properties, and haveimportant theoretical research and practical application values in many fields. As animportant epoxidation catalyst system of olefin, polyoxometalates showed excellentcatalytic performance for many types of olefin epoxidation. However, these catalystsoften dissolved during the process of reactions, and then were involved in reaction ashomogeneous catalysts. So people always are looking forward to prepare apolyoxometalate-based heterogeneous catalyst system which has the advantages ofexcellent performance, easy separation and recycling.Recently, with the rapid developments of the coordination chemistry and structuralchemistry and other related disciplines, a class of inorganic-organic hybrid materialswhich were builded by polyoxometalates and organic components (ligands) has beendeveloping rapidly. It was found that the structures and properties ofpolyoxometalates could be of effective regulation and control by using the advantagesof organic molecules. Thus these polyoxometalate-based functional materials wereoften showed the advantages both of inorganic POMs and organic components, andhave extensive research and development space in the fields of molecular recognition,bio-conductive materials, magnetic materials, materials of gas adsorption andcatalysis. Recently, there have been some reports about polyoxometalate-basedinorganic-organic hybrid materials as heterogeneous olefin epoxidation catalyst.Based on the above, in this paper the polyoxometalates and N-heterocyclic ligandswere used to be as the main structural units to synthesize a series of organic-inorganichybrid-polyoxometalate catalytic materials under hydrothermal conditions, and thento study the catalytic properties of such materials on the different types of olefinepoxidation reaction, especially the dysoxidizable election-deficient terminal olefin. Inaddition, the compositions, structures and bonding (coordination) modes of these materials were characterized through related characterization techniques, and combined thestudy of theoretical calculation of chemiatry on the relationship between structure andcatalytic properties of these materials to explore the catalytic properties of active sitesand catalytic mechanism. The main research contents and results were summarized asfollows:1. Structures and catalytic epoxidation properties of hybrid supramolecularself-assemblies based on imidazole and β-octamolybdate.Two hybrid supramolecular self-assemblies with muti-dimensional spatialconformations were synthesized under the hydrothermal conditions, which include(Himi)₄[(Mo₈O₂₆)(imi)2]·H₂O (catalyst1)[imi=imidazole] and (Himi)4[(Mo₈O₂₆)](catalyst2). Their structures were directed by the interactions, such as covalent bondsand hydrogen bonds and Ï€-Ï€ stacking, between imidazole and β-octamolybdate. Inaddition, the catalyst3was prepared through calcining catalyst2at340°C to removethe imidazole ligands. The catalytic performance evaluations were given to thesecatalysts by the epoxidation reactions of cyclooctene and1-octene with tert-butylhydrogen peroxide as oxidant. The results indicated that the catalytic activities andstabilities of catalyst1and2were much better than catalyst3. Further results ofexperiments showed that to the dysoxidizable terminal olefin, such as1-octene, catalyst1alsoshowed high reactivity and stability, and can be recycled many times. Compared thedifferences of three catalysts' structures and catalytic performances, it can bespeculated that the key factor for the high activities and stabilities of catalyst1and2was the multi-dimensional supramolecular structure they owned, which were formedby a variety of weak interactions, including hydrogen bonding and Ï€-Ï€ stacking, etc.Moreover, synthesized catalyst4,(Himi)4[(nic-O)2(Mo₈O₂₆)](nic=nicotinic acid),according to the literature, and compared the structures and catalytic performanceswith catalyst1. The results showed that these two materials were very similar eitheron the connection positions of the ligands and polyoxometalates or on the bondingmodes, as well as supramolecular spatial structure. The main difference between thesetwo catalysts was the types of ligands which linked with polyoxometalates covalently.Catalytic reaction results show that the reactivity of the catalyst1was evidentlyhigher than catalyst4. It indicated that when the spatial structures (supramolecularstructures) of catalysts were similar, the different types of organic ligands coordinatedwith polyoxometalates may noticeably influence the catalytic properties of catalysts. the results of theoretical chemistry calculation it can be thought that different kinds ofligands may take different electronic effects on the polyoxometalates, and then affectthe the performance of the catalysts. The specific situation to catalysts1and4is: theimidazole ring connected to a Mo atom of [Mo₈O₂₆]4-to form a conjugative Ï€ bondwith5centers of6electrons, and this electronic withdrawing effect of conjugationwas stronger than that of carboxyl group.Therefore, catalyst1exhibited higher activities.2. Structures and catalytic epoxidation properties of hybrid supramolecularself-assemblies based on1,1'-(1,4-butanediyl)bis(imidazolium) andβ-octamolybdate.Two hybrid supramolecular self-assemblies with muti-dimensional spatialconformations were synthesized under the hydrothermal conditions, which include(H₂bbi)₂[Mo₈O₂₆](catalyst5)(H₂bbi=1,1'-(1,4-butanediyl)bis(imidazolium)) and(H₂bbi)[(Hbbi)2(Mo₈O₂₆)]· H₂O (catalyst6). In the structure of catalyst6,1,1'-(1,4-butanediyl) bis(imidazolium) ligands through one nitrogen atom of aimidazole ring covalently connected with one molybdenum atom of β-octamolybdate. This is a novel connection mode between1,1'-(1,4-butanediyl)bis(imidazolium) ligands and polyoxometalates. In a symmetric structural unit, twoligand molecules covalently attached to octamolybdate at two positions of centersymmetry, and then many this kind of units connected into supramolecular structuresthrough hydrogen bonds between the bridge O atoms of polyoxometalates and N-Hbond of imidazole ring in the ligand.The catalytic performance evaluations were given to these catalysts by theepoxidation reactions of cyclooctene with tert-butyl hydrogen peroxide as oxidant.The results indicated that both catalyst5and6have good activity and selectivity, buthave the phenomenons of active specie loss. In comparison the stability of catalyst5was a little better than catalyst6, but the activity was a little lower. This could be dueto the active species loss of catalyst6is relatively more, and the homogeneouscatalysis made the activity of catalyst6higher. Combined with the results of XPS andother characterization, as well as the structural characteristics of catalyst1, it can beconsidered that in the structures of catalysts5and6, the electron donating effect oforganic ligands covalently bonding with octamolybdates would reduced theelectropositivity of molybdenum atoms of octamolybdates, which can lead to make against the molybdenum atoms combined with nucleophilic group. It is may be themain reasons which affected the catalytic properties of catalysts5and6.3. Structures and catalytic epoxidation properties of hybrid supramolecularself-assemblies based on organosilver and phosphomolybdateA hybrid supramolecular self-assembly with muti-dimensional spatialconformations were synthesized under the hydrothermal conditions using1,2,4-triazole organic derivative of silver and phosphomolybdate as the building blocks,which is [Ag12(C2N3)9](PMo12O40)·4H₂O (catalyst7). Catalyst7has a verynovel topological space configuration: metal silver ions and small-moleculeN-heterocyclic-ligand1,2,4-triazole use covalent bond together to form themacromolecules pseudorotaxane configuration, which have interlockmulti-dimensional supramolecular structure. The structural units of thismulti-dimensional organicsilver framework is octahedral cages consisted of silverions and1,2,4-triazole ligands, and these units connected together trough angle andangle interlock. In these octahedral cages, silver ions are unsaturated coordinations(the coordination number is2). Polyoxometalate anions present in the gap of thesecages by electrostatic force, and a certain extent play a role to stabilize structure. Thisstructure in this type of material is very new and rare.The catalytic performance evaluations were given to catalyst7by the epoxidationreactions of cyclooctene with tert-butyl hydrogen peroxide and molecular oxygen asoxidant. The results indicated that catalyst7have high activity and stability. Theability of catalyst7to direct activation of oxygen molecules is very meaningful, andworthy to further study and discussion. Combined with the structural characteristics ofcatalyst7, it can be considered the phosphomolybdate anion should be the main activecenter. The silver ions showed electropositivity because of their unsaturatedcoordinations, so there are electrostatic force between silver ions andphosphomolybdate anion, and then this interaction acted stabilization on activecenters of catalyst. In addition, during the reaction, there maybe exist synergisticeffect between inorganic polyoxometalates and organic organic metal framework, sothat the catalyst showed high catalytic performance.