Controllable Synthesis And Property Of Polyoxotungstate Clusters

Polyoxometalates (POMs),represent a fascinating class of metal-oxygen cluster compounds, have attracted extensive attention for many years due to their wide structural variety and diverse range of applications in analytical chemistry, medicinal chemistry, materials science, and especially in catalysis. The synthesis and study of novel POMs have become one of the major power sources in POMs chemistry.This dissertation has been divided into two parts:Part 1:Three kinds of new POMs have been synthesized by introducing transition metals or their complexes into trivacant Keggin-type polyoxotungstates via conventional method and hydrothermal condition through changing the reaction conditions, such as pH value, reaction temperature, ratio of reactants, organic ligand, inductive agent. The structures of new POMs have been determined by single crystal X-ray diffraction analysis, and they were characterized by IR spectra, UV-vis spectra, elemental analyses, XRD and TG analyses. The magnetic properties of some compounds have been investigated.1.Three new different dimensions (OD, 1D,2D) of Kerbs type POMs have been synthesized by changing the ratio of Mn2+ ion and trivacant Keggin-type [BiW9O33]9-and (or) in the existence of oxalic under pH=3.8-4.5 aqueous solution.Na10[Mn2(H2O)6Mn1.82(H2O)3.64W0.18O0.36(BiW9O33)2]·30H2O (1)Na10[Mn2(H2O)4(WO2)1.4Mn0.6(H20)1.2(β-BiW9O33)2]·25H2O (2)Na10[Mn2(H2O)4WO2Mn(H2O)2(BiW9O33)2]·30H2O (3)The polyanion of compound 1 consists of a dimeric assembly of two trivacant β-B-[BiW9O33]9- Keggin fragments connected by four transition metals located in the inner and outer positions of the central belt. Two inner transition metals of the central belt were statistically occupied by 1.82 Mn and 0.18 W. The polyanion presents the discrete state in 1.Two inner transition metals of the central belt of Kerbs type polyanion in 2 were statistically occupied by 1.6 Mn and 0.4 W. It is interesting that one of the all outer Mn coordinated sites in central belt of the polyanion is occupied by one terminal oxygen from the adjacent parallel polyanion, which leads to an unusual 1D chain. Two inner transition metals of the central belt of Kerbs type polyanion in 3 were statistically occupied by 1 Mn and 1 W. The all outer Mn coordinated sites in central belt of the polyanion is also occupied by one terminal oxygen from the adjacent polyanion, in which one is connected by parallel polyanion and the other connected by orthogonal one, which leads to an unusual 2D layer. Magnetic study reveals that the antiferromagnetic coupling interactions exist in compounds 1 and 2.The successful synthesis of different dimensions of Kerbs type sandwich POMs compounds 1-3 indicated that the Mn/W ratio of two inner transition metals of the central belt of Kerbs type polyanion can change by adjusting the ratio of Mn2+ ion and trivacant Keggin-type [BiW9O33]9- to achieve the adjustion of the composition of Kerbs type sandwich POMs at the atomic level. In addition, the results also indicated that the different dimensions of Kerbs type POMs could be isolated in the existence of proper inductive agents.2. Two new Herve type sandwich POMs 4 and 5 modified with aminopyrazine ligand to substitute coordinated water molecules attached to transition metal ions of the central belt, have been synthesized via conventional aqueous solution method under pH=7.0-7.5.Na4[Ni2(apy)(H20)10]1/2[(Ni(Hapy))3(Na(H2O)2)3(SbW9O33)2]-23H2O (4)Na8H4(C4H5N3)2[(MnC4H5N3)2Mn(H2O)(SbW9O33)2]-26H2O (5)Two new POMs are all based on trivacant Keggin-type polyoxoanion building block [SbW9O33]9- and transition ion. Compound 4 can be described as two trivacant Keggin-type [a-SbW9O33]9- subunits encapsulating a hexagonal{Ni3Na3} core through exposed 12 bridging O atoms from lacunae of two [a-SbW9O33]9- subunits. In the hexagonal{Ni3Na3} core, three Na+ ions alternately linked three Ni2+ ions coordinated by aminopyrazine to form a six-membered ring. Compound 5 constructed from two trivacant Keggin-type [a-SbWgO33]9- subunits encapsulated a hexagonal{Mn3Na3} core through exposed 12 bridging O atoms from lacunae of two [a-SbW9O33]9- subunits. In the hexagonal{Mn3Na3} core, three Na+ ions alternately linked three Mn2+ ions to form a six-membered ring. Different from 4, only two Mn2+ ions are modified with aminopyrazine ligands. The magnetic investigation for 4 suggests the existence of antiferromagnetic coupling in trinuclear Ni(Ⅱ) clusters. The magnetic investigation for 5 suggests the existence of ferromagnetic coupling in trinuclear Mn(II) clusters.The successful synthesis of pyrazine substituting water molecules which coordinated to the transition metals of central belt of Herve type sandwich POMs indicated that the existence of a strong coordination competition between water and nonaqueous ligands in aqueous medium. It is beneficial for the formation of sandwich-type POMs modified with N-donor ligand under higher pH value and appropriate competitive organic ligands.3. Three new extended organic-inorganic compounds have been synthesized by reacting the trivacant Keggin-type polyoxotungstates, transition metal ions and N-donor ligand in the hydrothermal condition under pH=5.0 aqueous solution.[Cu(CioH8N2)(CioHioN3)2][GeW1204o]·2H2O (6)[Cu(CioH8N2)(C6H6N4)2[GeW12O40] (7)H2C10N3H8[Cu(C10N2H8)2]2[AS2W18O62]·7H2O (8)Compounds 6 and 7 are all constructed from saturated Keggin-type polyoxoanion [GeW12O40]4- and transition metal complex. Compound 8 is made of Dawson-type polyoxoanion unit [As2W18O62]6- and transition metal complex. The saturated Keggin-type polyoxoanion building block [GeW12O40]4- of compound 6 acts as bidentate ligand and coordinates to two [Cu(C10H8N2)(C10HioN3)2]4+ cations through the terminal oxygen atoms to form a 1D chains. The saturated Keggin-type polyoxoanion unit [GeW12O40]4- acts as tetradentate ligand and coordinates to four [Cu(C10H8N2)(C6H6N4)]2+ cations through the terminal oxygen atoms to form a 2D layers in compound 7. In compound 8, the adjacent Dawson-type polyoxoanion units [AS2W18O62]6+ are interconnected head to head through double [Cu(CioH8N2)(C6H6N4)]2+bridging groups to form a copper doubly bridged alternate 1D chains.The successful synthesis of three organic-inorganic compounds based on Keggin/Dawson polyanion indicated that trivacant Keggin polyanion was inclined to form saturated structure and the valence of heteroatom arsenic was changed from +3 to+5 under acidic medium.Part 2:Two mesoporous molecular sieve SBA-15 supported heteropolyacid catalysts have been synthesized by two isostructural sandwich POMs Na12[(Cu(H2O))3(XW9O33)2]·nH2O(X=Sb, n= 46; X= Bi, n= 29) immobilized on amine-functionalized mesoporous SBA-15 molecular sieves. The catalytic activities of the supported heteropolyacid are tested in the selective oxidation of benzyl alcohol to benzaldehyde and their recyclability was investigated.1. These mesoporous supported heteropolyacid catalysts have been characterized by IR, XRD, N2 sorption measurements, SEM and TEM. The results indicated that heteropolyacid was uniformly decentralized on the surface of pore canal and the parent structure of heteropolyacid was still kept, and the catalysts possessed good mesoporous character of hexagonally arranged pore array. But the pore size and specific surface area of mesoporous molecular sieve SBA-15 decreases after immobilization.2. The results indicated that the catalytic activities of sandwich POMs have been improved after immobilized on amine-functionalized mesoporous SBA-15, and these may caused by the cooperation of sandwich POMs and mesoporous SBA-15.3. The effect of the reaction conditions on the catalytic activities of catalysts were investigated by adjusting the heteropolyacid loading, reaction temperature, reaction time, H2O2 dosage and catalysts dosage. The optimized conditions for the oxidation of benzyl alcohol to benzaldehyde are as follows:0.05 g catalyst,5 mLtoluene as solvent, H2O2/benzyl alcohol=3:1, and 80℃ for 8 h. Catalyst 1 exhibits 92% conversion. Catalyst 2 exhibits 90% conversion.4. We investigated the recyclability of catalysts and the results indicated that the catalytic activities of catalysts still remained after four cycles. Fourth recycled Catalyst 1 has 91% conversion compared with 92% conversion of fresh catalyst 1. For the corresponding catalyst 2, fourth recycled catalyst exhibits 87% conversion. These results indicate that catalysts 1 and 2 are efficient, stable and easy recycled.