The Hydrothermal Synthesis,Structure And Properties Of Novel Keggin Polyoxometalates

Keggin polyoxometalates exhibit excellent coordination ability due to their rich protons,electrons,oxygen atoms,and can coordinate easily with the transition metals,rare earth elements,alkali/alkaline earth metals and organic ligands through non-covalent interactions（electrostatic interactions,hydrogen bonding,van der Waals interactions,π-πstacking）,ionic bonding or covalent interactions.Thus,a large number of new supramolecular compounds with unique structures,higher dimensions,superior performance have been synthesized and this is a research hotspot.In this paper,six novel supramolecular compounds have been synthesized under solvethermal conditions by utilizing the 1:12 Keggin polyoxometalates H₃PMo₁₂O₄₀,H₃PW₁₂O₄₀,H4SiMo₁₂O₄₀ as precursors,Bi³⁺,Zr4+,Ni²⁺,Fe³⁺as metal center,2,2’-bipy,4,4’-bipy,H₂C2O4 as organic coligands and characterized by X-ray diffraction,infrared（IR）spectroscopy,thermal gravimetric analysis etc.The photodegradation and selective absorption toward organic dyes of compound 1 have also been studied.The main research as follows:（1）A new purely inorganic 2D framework {PMo₁₂O₄₀Bi₂（H₂O）2}·4H₂O（1）were synthesized under solvothermal condition by using H₃PMo₁₂O₄₀,Bi（NO₃）₃,ethanol,HNO3 and characterized by single-crystal X-ray diffraction,infrared（IR）spectroscopy,UV-vis spectroscopy,thermal gravimetric analysis.Structural analysis reveals that compound 1consists of the well-known Keggin unit {PMo₁₂O₄₀} with two bismuth atoms capping two opposite pits,leading to the bi-bismuth-capping Keggin polyoxometalate.Compound 1 not only displays excellent photodegradation activity for methyl orange（MO）,rhodamine B（RhB）and methylene blue（MB）under acidic conditions,but also has high absorption capacity for cationic dyes（MB and RhB）at room temperature.Moreover,compound 1 is extremely stableand easily separated from the reaction system for reuse,and it is a kind of excellent photocatalytic/adsorption material.（2）Two new inorganic-organic hybrid materials were synthesized under solvothermal condition by using H4SiMo₁₂O₄₀ as precursor,4,4’-bipy、2,2’-bipy as organic coligands,Bi³⁺,Ni²⁺ as metal center,namely: {(H₃SiMo₁₂O₄₀)（4,4’-bipy）3}·3H₂O（2）,{SiMo₁₂O₄₀}{Ni（2,2’-bipy）3}2（3）.Then,they were characterized by single-crystal X-ray diffraction and infrared（IR）spectroscopy.In compound 2,the Keggin unit [SiMo₁₂O₄₀]4-and4,4’-bipy are connected through hydrogen bonding and intermolecular interactions to form a3 D supramolecular structure;Similarly,[SiMo₁₂O₄₀]4-and [Ni（2,2’-bipy）3]²⁺ are connected through hydrogen bonding and intermolecular interactions to form a 3D supramolecular structure in compound 3.（3）Three novel supramolecular compounds were synthesized under high temperature and pressure conditions by using H₃PW₁₂O₄₀,H₃PMo₁₂O₄₀ as precursors,2,2’-bipy,H₂C2O4 as organic coligands,Zr4+,Bi³⁺,Fe³⁺ as metal center,methanol,ethanol,DMF,acetonitrile as solvents,namely: {H₃PW₁₂O₄₀}₂·5H₂O（4）,[Mo₄O₁₅]·2H₂O（5）,H[Fe₄（C₂O₄）₆][Fe（2,2’-bipy）3]2（6）.Then,they were characterized by single-crystal X-ray diffraction and infrared（IR）spectroscopy.In compound 4,The two polyoxometalate units{H₃PW₁₂O₄₀} are connected via the W-O bonds to form a dimer,then,a 2D layer structure constructed from the dimer is formed through intermolecular hydrogen bonding,and finally expand to form a 3D supramolecular structure.In compound 5,{H₃PMo₁₂O₄₀} is decomposed into the isopolyanions {Mo₄O₁₅},then,the unit {Mo₄O₁₅} is extended into a 1D chain structure by Mo-O bonds,finally,the 3D supramolecular structure is formed through intermolecular hydrogen bonding.In compound 6,[Fe₄（C₂O₄）₆]4-and the organic fragments[Fe（2,2’-bipy）3]²⁺ are connected by intermolecular hydrogen bonding and π-π stacking to form a 3D supramolecular structure.