Synthesis, Crystal Structures And Properties Study On New Polyoxomolybdates

Polyoxometalates reveal extensive potential applications in the fields, such as catalysis, magnetism, adsorption-separation, medicine and material science, not only because of the fascinating variety of structures but also the attractive properties. The design and synthesis, structure characterization as well as property research on polyoxometalates can give support to the application. Moreover, it will infuse a strong power into the chemistry and the subject in point. In this thesis, thirteen new polyoxomolybdates have been obtained by adjusting the reagents and conditions under hydrothermal and solvothermal synthesis, and determined by single-crystal X-ray diffraction analysis, ICP, IR, TG, XRD, EPR. The catalytic properties of the compounds have also been tested in the styrene oxidation with H₂O₂.1. Three new polyoxomolybdates built on the capped Keggin units have been obtained via hydrothermal synthesis: (NH₄)[Mo^VI₆Mo^V₆O₃₅(AsVO₄)(MoVO)₂] (1) {[Ni^II(en)]₂[Ni^II(en)(H₂O)][Ni^II(H₂O)₃][Ni^IIMo^V₆W^VI₆O₄₀H₄]}·5H₂O (2) {[Cu^II(2,2'-bipy)₂][Cu^II(2,2'-bipy)]₂[SiMo₁₂O₄₀]}·2H₂O (3)Single crystal X-ray diffraction shows that compound (1) represents a 1-D infinite chain, in which the bi-capped Keggin anions are connected by sharing one terminal oxygen atom from the caps. It represents a new mode of connection between two adjacent units. Compound (2) exhibits an unusual Mo/W mixed Ni^II centered Keggin structure, which is tetra-capped by two [Ni(en)]²⁺, one [Ni(en)(H₂O)]²⁺, one [Ni(H₂O)₃]²⁺ units. The adjacent neutral clusters are linked and represent a novel inorganic-organic zigzag chain built on the Ni^II centered Keggin species. Compound (3) represents a discrete tri-capped Keggin neutral cluster. Two [Cu(2,2'- bipy)]²⁺ units cap the [SiMo₁₂O₄₀]^6- through fourμ3-O atoms respectively, while the [Cu(2,2'-bipy)₂]²⁺ unit is coordinated to the bi-capped cluster via oneμ3-O atom. The hydrogen bonding andÏ€-Ï€packing interaction connect the neutral clusters to form the 3-D supermolecular structure. All the three compounds are effective in the oxidation of styrene.2. Two new inorganic-organic hybrid polyoxomolybdates based on the Keggin unit and one built on the [Mo₈O₂₆]^4- have been obtained via hydrothermal and solvothermal synthesis: {[Cu(4,4'-bipy)]₃(4,4'-bipy)}[PMo₁₂O₄₀]·2H₂O (4) (C₆H₁₄N₂)₂[MoV₂ Mo^VI₁₀O₃₆(SO₄)]·H₂O(C₆H₁₂N₂= 1,4-Diazabicyclo[2.2.2]octane) (5) (C₈H₂₀N)₃Na[Mo₈O₂₆] (6)Single crystal X-ray diffraction illuminates that compound (4) exhibits a three-dimen- sional structure, in which Keggin anions [PMo₁₂O₄₀]^3- are linked by infinite [Cu(4,4'-bipy)]⁺n chains through Cu-O, and come into being a 3-D inorganic-organic hybrid open framework. The guest 4,4'-bipy molecules and water molecules are filled in the channels formed by Keggin anions [PMo₁₂O₄₀]^3- and [Cu(4,4'-bipy)]⁺n chains. The polyanion [MoV₂Mo^VI₁₀O₃₆ (SO₄)]^4- and the protonated 1,4-Diazabicyclo[2.2.2] octane generate a 3-D supermolecular structure via hydrogen bonding in compound (5), and the channels are filled with the guest water molecule. In compound (6), the adjacent polyanions [Mo₈O₂₆]^4- are connected by Na atoms and generate a straight chain along a-axis, while the tetraethyl ammonium cations locate beside the anion chains and are involved in the hydrogen bonding interaction .3. Four new inorganic-organic hybrid polyoxometalates based on the {Mo₆P₄} unit have been hydrothermally synthesized in different pH conditions: (H₄TETA)₂[(Co^IITREN)2Co^IIMo^V₁₂P₈O₅₂(OH)₁₀]·4H₂O (7) (H₄TETA)₂(C₆H₁₈N₃)₂[Co^IIMo^V₁₂P₈O₅₄(OH)₈]·5H₂O (8) [H₄TREN]₄[Co^IIMo^V₁₂P₈O₅₆(OH)₆]·4H₂O (9) [H₄TETA]₂[Co^IIMo^V₁₂P₈O₄₈ (OH)₁₄]·4H₂O (10)Single crystal X-ray diffraction shows that compound (7) which is obtained at pH=8.9, represents a {Mo₆P₄} unit supported transition metal complex, in which the Co^II atom is five-coordinated by four N atoms from TREN and one O atom from PO₄ tetrahedron, while the other Co^II atom joins two {Mo₆P₄} units together to form a dimer by sharing sixμ3-O atoms. Compound (8)-(10) obtained from lower pH conditions (6.4, 4.5 and 3.1) represent the same anion [Co^IIMo^V₁₂P₈O₆₂]^n- but different cations. Both TETA and N-(2-Aminoethyl) piperazine which was obtained via the isomerization of TETA and the deamination of TREN are observed in compound (8). The TREN in compound (9) take on a clawlike look, with different extents of proton-accepting process. Compound (10) includes the disordered protonated TETA with the occupancy factors of 0.5. The discrete anions and the organic amine construct 3-D structures via hydrogen bonding interaction. The guest water molecules are filled in the channels, and also involved in the hydrogen bonding with the inorganic- organic hybrid framework, which makes the structures more stable. The catalytic properties of the four compounds are also tested in the styrene oxidation, all of which are less effective than the capped-Keggin species.4. A novel compound (NH₄)₆[Mo₉O₃₀]·H₂O (11) was hydrothermally synthesized. Structural analysis indicates that four MoO₆ octahedra connect each other to form a {MO₄} ring by sharing the edges and corners, and two {MO₄} rings are further linked by the central MoO₄ tetrahedron to generate the dumbbell-shaped polyanion. The NH₄⁺ and water molecules surround the [Mo₉O₃₀]^6- and also are involved in the hydrogen bonding interaction.5. Two new molybdate compounds with 2-D and 3-D extend structures have been synthesized by hydrothermal and solvothermal method: (en)_0.5[Ce(MoO₄)₂] (12) NH₄[Fe^IIFe^III(MoO₄)₃]·H₂O (13)Structural analysis of compound (12) indicates that MoO₄ tetrahedra and CeO₈ polyhedra are linked by bridging O atoms to form the 2-D layers. Ethylenediamine molecules are inserted between the Mo-Ce layers and further connect the layers to generate the 3-D supermolecular structure. In compound (13), the MoO₄ tetrahedra and FeO₆ octahedra share the corners to generate the 3-D open framework. The NH₄⁺ and water molecules which act as the structure-directing agent locate in the center of cages constructed by Mo₆Fe₄ and Mo₆Fe₆, and are involved in the hydrogen bonding interaction with the O atoms from the 3-D open frameworks. The 3-D structure of compound (13) shows a chiral. Both L- and R-helix chains can be observed along c-axis. Compound (13) also shows a week antiferromagnetic exchange coupling within the molecular unit.