Syntheses, Structures And Properties Of Organic-Polyoxophosphatomolybates

Polyoxometalates are found to unique structures and diverse properties, which endow them with applications in the fields such as catalysis, photo- and magno-materials. In this paper, six organic-polyoxophosphatomolybates and one organic-molybates have been prepared through hydrothermal techniques under mild temperature and with the introduction of organic templates and structure-directing agents. These seven compounds are [H2tren]2[Mo5O15(HPO4)2]·4H2O (tren = diethylenetriamine) (1), [H2tren]3[Htren][Na- Mo12O24(OH)6(H2PO4)2(HPO4)6]·nH2O (n = 13.82) (2), [Htren]3[NdMo18O48(OH)- (HPO4)4(PO4)2]·nH2O (n = 11.43) (3), [Htren]2[NdMo18O49(HPO4)4(PO4)2]·2C6H5NO2-·0.5C2H6O·nH2O (n = 13.38) (4), [H17NdMo18O73][C6H4NO2]3·2C6H5NO2·nH2O (n = 11.16) (5), [Htren]3[ZnMo18O49(HPO4)4(PO4)2]·nH2O (n = 12.66) (6) and NH4[Mo2O6(C6NH4O2)] (7). These compounds have been characterized by elemental analysis, single crystal X-ray diffraction, IR, thermal analysis, UV-Vis, cyclic voltammetry, and catalytic test measurements.It has been proven that among many synthetic factors of the hydrothermal reaction, the medium pH values and the employed organic templates should have greater influences on the products. The studies have shown that compounds 1·6 exhibit discrete structures, where compound 1 belongs to Strandberg-type polyoxomolybates, the Na[Mo6P4]2 unit in compound 2 is a sandwich-shaped cluster, and the [HxMMo18P6O73]n·polyoxomolybate in compounds 3·6 exhibits a novel shape. The new [HxMMo18P6O73]n·polyoxomolybate cluster has a Chinese dumpling shape and is made up of 18 distorted MoO6 octahedra, six PO4 tetrahedra and one NdO9 or ZnO8 polyhedron by sharing edges and corners. Such structure is the first observation in polyoxometalates science. In compound 7, the distorted MoO6 octahedra are linked to one another via edge-sharing to produce a zigzag polymeric chain, in which the two carboxylic oxygen atoms of each nicotinic group are coordinated to two neighboring molybdenum sites in a gymnastic ring fashion. Owing to the hydrogen bonds, compounds 1?7 all form the extended three-dimensional supra-molecular network.The cyclic voltammetric measurements suggested that compounds 3 and 7 possess a reversible characteristic. Through the investigation of the oxidation reaction of acetaldehyde to acetic acid, it was revealed that compounds 2, 3 and 6 have high catalytic activities in some organic oxidation reaction, where that of compound 2 is over 2 times higher than those of compound 3 and 6.