Preparation,Structures and Properties of Polyoxometalate-based Complexes Constructed by Tridentate N-donor Ligands
Author:Zhang Zhuan Fang
Supervisor:ma hui zuo
Polyoxometalates（POMs）,as a kind of environmental friendly polymetallic oxygen clusters,has been attracting the attention of researchers for years because they possess the superior potential applications in catalysis and electrochemistry.However,pure POMs are easy soluble in solution and can not recycle during the process of catalytic reaction.Therefore,in order to effectively utilize property of POMs and further meet the requirements for environment pollutant treatment,developing a novel type of POMs-based catalytic materials with good stability,easing-recycling and high efficient catalytic activity has captivated the attention of many investigators.Introducing POMs into metal organic complexes by means of molecular self-assembly technology is an effective method.Compared with other simple nitrogen ligands,tridentate rigid nitrogen ligands as structural boosters are more conducive to obtain the novel POMs-based comlexes.In this paper,19 kinds of unique POMs-based complexes have been synthetised under hydrothermal methods by selecting two N-donor rigid ligands.And they were characterized by IR,single crystal X-ray diffraction,powder X-ray diffraction,elemental analysis,EDS,XPS et al.We investigated the reaction conditions,such as the p H value,the organic ligands,metal anions and POM anions,the results show that these factors could affect the structure of the products.Further,some properties of the compounds were investigated,and the relationships between structures and properties for these new compounds are studied.1.We choose 1,3,5-tris-（1-imidazolyl）-benzene（tib）and Keggin type [Ge W12O40] as raw material,by adjusting the PH value in the reaction system,six new POM-based complexes with distinct architectures were synthesized.Compounds 1,2 were obtained in Co/tib/POMs reaction systems by adjusting the p H value,when p H is 4.0,the structure of compound 1 is 1D chain,when the p H value became 4.5,the structure of compound 2 is 2D network.Compounds 3–6 were obtained by using the Mn2+ metal ions,and adjusting p H in the reaction systems.Compounds 3 and 4 were prepared at lower p H（p H ≈ 2.1 for 3 and 3.2 for 4）.Compound 3 exhibits independent body and compound 4 is a dimer.Compounds 5 and 6 were prepared at higher p H（p H ≈ 4.2 for 5 and 4.9 for 6）,and compounds 5 and 6 display a one-dimensional（1D）chain and a（4,4）-connected 2D wave-like layer,respectively.The informative structures of these six compounds contribute to systematically understanding of the protonation extent of the organic ligand can control the coordination behavior of metal ions on construction of organic-inorganic hybrids based on Keggin type POM.The catalytic properties of the compounds 4–6 have been investigated.In addition,the fluorescence of compounds 4–6 was also analyzed.2.We choose tib and 3,5-bis（pyrid-4-yl）pyridine（bbp）as ligand,[PW12O40]4-and [Ge W12O40]4-as inorganic linkers,and four new POM-based complexes with distinct architectures were synthesized which have never been reported.Compounds 7 and 9 were obtained in the Ni/tib/POMs systems,and compound 7 is a 0D dimer,in which each tib ligand coordinates to Ni ion in a μ1 coordination mode.Compound 9 is 2D network,in which each tib ligand coordinates to Ni ion in a μ3 coordination mode.Compounds 8 and 10 were obtained in the Ni/bbp/POMs systems.Compounds 8 and 10 were two fold 3D frameworks,in which bbp ligand coordinates with Ni ion in a μ3 mode.By comparing the four compounds,we found that the bbp is “V” type rigid ligand with the flexible coordination ability,which easing to form complex three-dimensional structure.The ligands tib and bbp have been introduced into Zn2++[PMo12O40]3-eaction system,NH4VO3 as mineralizer,and two different structure compounds were obtained.In compound 11,Zn2+ is coordinated with “Y” rigid ligand tib to form helical structure unit,the neighboring double helixes are further fused together by sharing tib ligands and Zn atoms to form a wavy Zn-tib metal-organic layer.In the compound 12,Zn2+ is coordinated with “V” rigid ligand bbp and the POMs to form a 1D chain,further a 1D + 1D →2D interdigitated structure was formed by intermolecular interaction.The photocatalytic properties of compounds 7,8,11 and 12 were studied,and the different structures of compounds have different ability to degration rhodamine B under visible light.In compound 12,the vanadium cap in the structure is equivalent to extra metal elements,so the doped metal element are easing to produce electron-hole pair,which enhances the catalytic activity point,and it performs the best catalytic effect among four compounds.3.Choosing bbp and [SiW12O40]4-as raw material,by changing different metal salts,such as cobalt acetate,copper sulfate,silver nitrate in the reaction system,three new POM-based complexes with distinct architectures were synthesized.The three different transition metal ions Co2+,Cu2+,Ag+ with different radius and electron configuration way.In compounds 13 and 14,the Co2+,Cu2+ are easing to coordinate with “V” type rigid bbp ligand,and the compounds 13 and 14 are 3D framework.In compound 15,Ag+ is easing to coordinate with bbp to form 1D chain,then to form honeycomb two-dimensional supramolecular structure through the inermolecules π–π forces,and the POMs embedded into the cave.[BW12O40]5-,[SiW12O40]4-,[H4W12O40]4-anions were introduced into the same Co2++tib reaction system,and three no reported compounds 16–18 were obtained.Compounds 16 and 17 are isomorphic compounds.They are 2D layered structure with POMs as the templates,and the 3D supermolecule structure is formed by hydrogen bonding.However,compound 18 is a 1D chain,further,a graphine-like structure is formed by hydrogen bonding.The photocatalytic properties of the compounds 13–15 were made a systematic study.The relationship between microstrucural and property has been studied,and the results that the photocatalytic property of compound 14 toward to degradation rhodamine B is best among three compounds,and the degradation of rhodamine B dye rate is as high as 88.2% after 80 min.It is because that 14 is a complex 3D structure,which is good for the transmission of photoelectric electrons.Compounds 13–15 carbon paste electrodes were tested by cyclic voltammograms under different scanning speed,and the test results show that the compounds 13–15 electrochemical behavior are surface control process,electron transfer rate is faster.The electrocatalytic activity of 13–15-CPEs towards nitrite anions and iodate were also investigated,and the results show that compounds 13–15 have obvious electrocatalytic reduction activity towards to nitrite and iodate.4.Based on the above experiments,choosing the bbp,cobalt acetate and [SiW12O40]4-as raw material,NH4VO3 as mineralizer.The compound 19 has been synthesized under 160 °C hydrothermal system,which is a polyoxometalates-encapsulated metal-organic 3D porous POMOF.The molecular formula of compound 19 is as follows: Co2（bbp）（bpy）（dip）]（H2O）3][Si W12O40],dip=3,4-dipyridine,bbp=3,5-bis（pyrid-4-yl）pyridine,bpy=3,3’-bis（pyrid-4-yl）dipyridine,the ligand bbp was transformed to form two new ligands dip and bpy.Compound 19 is the first case of POMOF,which is constructed by using the mixed ligand,in which the ligand in situ transformation to form new ligand.The compound provides a new idea for the design of a new POMOF material.In addition,the structure analysis indicates that the compound 19 has two kinds of approximate nanometer channel,so it has large specific surface area for convenient transfer electron.Compound 19 performs very good photocatalytic degradation of rhodamine B under visible irradiation,and the degradation of rhodamine B dye rate is as high as 92% after 120 min.