Synthesis And Conductivity Of Triol-Ligand-Modified Anderson/Lindqvist Polyoxometalates

Proton conducting materials are electrolyte materials with hydrogen ions as charge carriers and have certain conductivity under specific conditions.Due to the small size and low mass of hydrogen ions,proton conducting materials are more likely to obtain high mobility,which are widely used in dye batteries,hydrogen sensors,electroluminescence and other fields.Crystalline materials usually have high structural stability and can be combined with multi-functional building units to adjust the proton conductivity,which has great potential for development and has attracted wide attention.Therefore,how to develop more conductive crystal materials and fully understand the functional properties of each component in proton conductive materials is an important research topic in this field.Polyoxometalates are a class of stable inorganic clusters with many electrons,and their surfaces are mostly covered by oxygen atoms,which can be used as hydrogen bond acceptors to construct hydrogen bond networks,so they have been developed rapidly in the field of proton conduction.The organic component and the polyoxometalate cluster are combined by a covalent modification method,the number and the position of a hydrogen bond acceptor on the surface of the polyoxometalate cluster are adjusted,the proton conductivity is changed,and the method is an important means for developing a material with high proton conductivity.At the same time,materials with different conductivity can be obtained by controlling the modified organic components and the original inorganic components,and then the relationship between structure and materials can be understood,which provides a basis for designing materials with higher performance.Based on the above discussion,the research work of this paper is mainly carried out from the following three aspects:（1）A triol ligand with a pyrazole group at the end is covalently modified on the inorganic cluster by taking the Anderson-type polyoxometalate cluster with an Al center as an inorganic core.Affected by the central metal ions and the synthesis conditions,the organic components are connected to the polyoxometalates by unilateral modification,and the molecular structures of the polyoxometalates are verified by elemental analysis,IR and ¹H NMR spectra.In the introduced organic components,there are both hydrogen bond donors and hydrogen bond acceptors,thus changing the hydrogen bond network structure around the polyoxometalates.It was found that the proton conductivity of the polyoxometalate hybrid was 3.3×10^-3 S/cm at 60℃and 88%relative humidity when the counterion of the polyoxometalate hybrid was sodium ion.In contrast,when the counterion is tetrabutylammonium cation,its strong hydrophobicity is not conducive to proton transfer,so that its conductivity is greatly reduced,indicating that the counterion has an important impact on the proton conduction process.（2）By replacing the central heteroatom of the Anderson-type polyoxometalate cluster from Al to Mn,the bilateral-modified products with pyrazole groups at the end were obtained through similar reactions.Compared with the structure of Al center,the double-side modified Mn center hybrid has higher symmetry and better crystallinity,so its structure is accurately characterized by single crystal X-ray diffraction experiment.The results show that the one-dimensional pore structure is formed by the hydrogen bonding between the pyrazole group and the adjacent polyoxometalates,and the hydrogen bonding network of proton transfer is formed by the crystal water filling in the pore.Impedance experiments show that the proton conductivity reaches 1.0×10-3 S/cm at 60℃and 88%relative humidity.Although the bilaterally modified structure did not improve the conductivity,its better crystallinity proved the existence of hydrogen bond network in the structure,and then explained the proton transmission channel.（3）Replacing the Anderson-type structure of the inorganic cluster with the Lindqvist type structure,and successfully stabilizing the single vacant Lindqvist type{V₅}cluster by using the coordination effect of triol ligands and combining with the{Mo₂}cluster to obtain the organic-inorganic hybrid anion{V₅Mo₂O₁₉[RC（CH₂O）₃]}^4-,R=CH₃ or C₂H₅.In the sodium salt of hybrid anions,sodium ions are combined with coordinated water molecules to form a two-dimensional layer structure,which is beneficial to proton transmission,while methyl or hydroxyl at the end of triol ligands can not participate in the formation of hydrogen bonds,which reduces the proton transmission efficiency to a certain extent.Impedance measurements show that the proton conductivity of the methyl-terminated hybrid is 1.2×10^-4S/cm at 30℃and 98%relative humidity,which is lower than that of the pyrazole-terminated hybrid,indicating the influence of the structure of organic components on the conductivity.Andreson and Lindqvist type polyoxometalates were modified by organic components with different functional groups,and a series of organic-inorganic hybrid compounds were prepared.Their structures were determined by 1H NMR,IR,elemental analysis and X-ray single crystal diffraction.The proton conductivity of the hybrids obtained under different temperature and humidity conditions was studied,the relationship between proton conductivity and structure was explored,and the influence of organic components with hydrogen bonding to acceptor groups on the construction of hydrogen bonding network was revealed.