Synthesis, Structures And Properties Of Polyoxovanadium Borate Compounds

Polyoxometalate clusters have attracted considerable interest for their unmatched structural characteristic and wide applications in many fields, such as catalysis chemistry, electrochemistry and as optical and magnetic materials.Seven novel polyoxovanadium borates and one vanadate have been synthesized under hydrothermal condition and determinated by single crystal X-ray diffraction analyses. Spectral properties study and electrochemical performance research of the obtained compounds supply theoretical basis for its potential application. The main work is as follows:1. Seven novel polyoxovanadium borates contain several different cluster units: V6B20 in compound 1、2、3、4, V12B18 in 5, and V12B16 in 7. It is noteworthy that the V6B20 cluster unit in compound 1 and the V6B20 cluster units in compound 2,3,4 are conformational isomers, and the the eclipsed conformation in compound 1 is unprecedented. There is no cluster unit isomer reported in polyoxovanadium borates, and the conformational isomerism in metal-oxygen cluster chemistry is unreported. In addition, we introduced the alkali cations Li+ and Cs+ into the polyoxovanadium borates and the polyoxovanadium borates contain Cs+ is unreported, at the same time explore the influence of alkali cations with different radius to the two comformations of V6B20 cluster unit. Compound 1 and 4 connected by Li+ forming 3D and 2D structures respectively. Compound 2 connected via Cs+ forming 2D layer structure, and compound 3,5 connected via Cs+ forming 3D framework structures.2. XRD, IR spectra, Two-demensional Infrared Correlation Spectra (2D-IR COS), TGA, ultraviolet-visible absorption spectrum (UV-Vis DRIS) and electrochemistry experiments were used to characterize the above-mentioned compounds. IR spectra research the region of characteristic peaks of V=O and V-0 bonds in different polyoxovanadium borate cluster units,2D-IR COS indicates the stretching vibrations of B-O bond in BO3 and V=Ot bond are sensitive under thermal perturbation, the groups linked Ⅴ(Ⅳ) are sensitive with magnetic perturbation. TGA analysis indicates the polyoxovanadium borate cluster exsists stably below 450℃, experimental results accord with crystal structures determinated by X-ray single-crystal diffractometer.3. In order to research the two different conformations (eclipsed and staggered) of the V6B20 cluster unit further, theoretical calculations were performed with the Gaussian 03 program package. Theoretical calculations indicate the two different conformations (eclipsed and staggered) have similar energy with slightly difference of about 0.14897 eV, which demonstrates the eclipsed is stable as same as the staggered. Analysis of natural bond orbital charge population of the V6B20 cluster unit indicates t-O atoms in BO3 triangle can be the active centers of the cluster units, this accord with that the stretching vibrations of B-O bonds in BO3 triangle of Ihe V6B20 cluster unit are sensitive to thermal perturbation.4. In order to explore the electrochemical performance of polyoxovanadium borates as anode materials of lithium battery, the charging and discharging curve, cyclic voltammetry curve and electrochemical impedence spectroscopy of 1 were tested. Electrochemical experiment indicates the potential application of polyoxovanadium borates in electrochemistry.