Hydrothermal Assembly, Structures And Properties Of Organic-Inorganic Hybrid Materials Composed Of Polyoxotungstate Clusters

Polyoxometalates have unique structures and diverse properties, which endow them with applications in the fields such as catalysis, analytical chemistry and medicine. Compared with polyoxovanadates and polyoxomolybdates, the research of polyoxotungstates remains largely unexplored. In this paper, we have hydrothermal assembled seven new polyoxotungstates associated with organic amines or their transition metal coordination groups. They have been characterized by elemental analysis, single crystal X-ray diffraction, IR, thermal analysis, UV-Vis DRS, cyclic voltammetry, and catalytic activity. These seven hybrid compounds are (H₂en)(Hen)₂[W₅O₁₅(HPO₄)₂]·5.4H₂O (en = ethylenediamine) (1), [Co(2,2'-bpy)₃]₂H₂-[CoW₁₂O₄₀]·5.93H₂O (2,2'-bpy = 2,2'-bipyridine) (2), [Cu^I(en)₂H₂O]₂[Cu^I(en)₂}HPW₁₁^VIW^VO₄₀]-·2H₂O (3), [Cu(enMe)₂(H₂O)][{Cu(enMe)₂}{Cu(enMe)₂(H₂O)W₁₂O₄₀(H₂)}] (enMe = 1,2-propanediamine) (4), (H₂en)₃[P₂W₁₈O₆₂]·6.48H₂O (5), (H₂pn)₂(H₃O)₂[P₂W₁₈O₆₂]·3.65H₂O (pn = 1,3-propanediamine) (6), and [Cu(enMe)₂][{Cu(enMe)₂}{H₂P₂W₁₆^VIW₂^VO₆₁}]·2enMe-·2H₂O (7), in which compound 1 is the first Strandberg type polyoxotungstate, compounds 2-4 are Keggin type polyoxotungstates, compounds 5-7 are Dawson type polyoxotungstates.The synthesis experiments indicated that the employed hydrothermal process was relatively complicated and affected by many factors such as the systematic pH, temperature and descending rate of temperature. The organic amines in compounds 1, 5 and 6 only act as template cations; the metal-ligand subunits in compound 2 serve as isolated coordination cations; the anionic [{Cu(en)₂}HPW₁₂O₄₀]^2- in compound 3 is formed by the mixed valance {HPW₁₁^VIW^VO₄₀}^3- Keggin unit covalently linked by a {Cu(en)₂}⁺ group. The connectivities between the Keggin clusters and the metal-ligand subunits in compound 4 are unique, where the decorated Keggin clusters and {Cu(enMe)₂}²⁺ bridging groups are linked through Cu-O interaction into one dimensional chains with a sinusoidal wavilness. Each Dawson anion in compound 7 is connected through a terminal oxygen atom and a {Cu(enMe)₂}²⁺ group with its adjacent cluster to generate an infinite chain. Owing to the hydrogen bonds, compounds 1-7 all form extended three-dimensional supermolecular network.The cyclic voltammetric measurements of compound 1 showed that a W(VI) / W(V) reducing reaction was taken placed at -1.32 V. Compounds 3, 4 and 7 all have catalytic activity...