Synthesis, Structures And Properties Of Heteroploymolybdates And Heteroployniobates

Polyoxometalates（POMs） based on molybdenum, tungsten, vanadium, niobium,tantalum, because of their diversities in structure, size and property have attracted wide attention and applications in the field of catalysis, magnetic, photochemical, electrochemical and biological system, particularly for the heteropolyacids. Therefore, the design of novel heteropolyacids and explore their unique properties is still a hotspot in the field of POMs. In this paper, we mainly explore the synthesis, structure and properties of heteropolymolybdates and heteropolyniobates.In this paper, two heteropolymolybdate compounds and one heteropolyniobate compound were synthesized by conventional aqueous method and hydrothermal method,respectively. The crystal structures were characterized by single crystal X-ray diffractions,elemental analysis, IR spectra, thermogravimetry（TG） analysis and Powder X- ray diffraction.And we studied the thermotropic liquid crystal properties of two heteropolymolybdate compounds.1. Two heteropolymolybdate compounds with heteroatom Ge were synthesized by conventional aqueous method.（H₂en Me）₃K₂[Ge₂Mo₁6O₅8]·4H₃O·9H₂O（1）（en Me = 1,2-propanediamine） Na₁2[Ge₂Mo₁6O₅8]·30H₂O（2）Compounds 1 and 2 have the same polyanion [Ge₂Mo₁6O₅8]12-, which was a hexavacant polyanion composed of two trivacant Keggin units {β-A-Ge Mo₉O₃4} fused together by two{MoO₆} octahedra. This structural type is a new vacant POM building blocks different from that of the well-known basic structural types, which might be further used as a inorganic polydentate ligand for constructing polymolybdate clusters. Further, the DODACl was used as the surfactant to react with compounds 1 and 2 by ion-exchange, resulting in the surfactant-encapsulated complex, which exhibits typical thermotropic liquid crystal properties,formula is as follows, [DODA]₁2[Ge₂Mo₁6O₅8]·16H₂O（SEC-Mo₁6）.2. One heteropolyniobate compound with heteroatom Si was synthesized by hydrothermal method.[Cu（en）₂]₈[H₅KSi₄Nb₁4V^IV₂O₅6（V^IVO）₂]₂·24H₂O（3）Compound 3 is a {Nb₁6} cage encapsulating four {SiO₄} tetrahedra in its inner cavity,replaced by two {VO₆} octahedral and caped by two {VO₅} pyramid. This is the first example of transition metal-substituted {Nb₁6} cage. The {Si₄Nb₁4V₄} cage is connected by two K+ ion into a dimer with sandwich-type structure. Furthermore, The sandwich-type structure can been connected into a 2D framework by [Cu（en）₂]₂+ unit, this is really rare in polyoxoniobate.