| Polyoxometalates (POMs) have received much attention due to their fascinating variety of structure and chemical composition, and potential applications in catalysis, magnetism, materials science, environmental protection and medicine. The synthesis of new type compounds with special structures and compositions is the research hotspot and nodus in this field all the time. In this paper, the main work has been focused on the rare reported arsenomolybdates, and nine novel compounds have been synthesized. Sandwich arsenomolybdates:(NH4)12[Cr2(AsMo7O27)2]·11H2O (1)(NH4)14[Cu2(AsMo7O27)2]·13H2O (2) Double sandwich arsenomolybdates:(NH4)12Cu(H2O)4[As2Fe6Mo22O85(H2O)]·19H2O (3)(NH4)15[As2CoFe5Mo22O85(H2O)]·16H2O(4)(NH4)14Mn0.5[As2Fe5MnMo22O85(H2O)]·22H2O(5) Cagelike arsenomolybdate:(NH4)11[AgAs2Mo16O54]·9H2O (6) Organic-inorganic molybdenum arsenate complexes:(CuAs6Mo6O30){[Cu(imi)4]3[CuAs6Mo6O30]}2·6H2O (7),[Cu(enMe)2]3[As3Mo3O15]2·2H2O (8)(NH4)10{Cu(H2O)4}[AsMo6O21(O2CCH3)3]2·12H2O (9)All of the above polyoxomolybdates were characterized by IR spectroscopy, TG-DSC, elemental analysis, and single-crystal X-ray analysis to identify their structures and chemical compositions. Furthmore, some of them were characterized by UV-vis and luminescent spectroscopy, and electrochemistry analysis. X-ray structural analysis revealed that the structures of compounds 1-2 are isostructural. They all have sandwich-like structures wrapping two transition metal atoms between two {As(Ⅲ)Mo7O27} fragments. The double-sandwich molybdates 3-5 are isostructural. They are all formed by the condensation of two {As(Ⅲ)Mo7O27} fragments and a novel monocapped cinque-substituted P-Keggin bridging cluster {M(Fe5Mo8)O41(H2O)}(M = Fe3+ for 3, Co2+ for 4 and Mn2+ for 5). Arsenomolybdates 1-5 all contain the unprecedented fragment {As(Ⅲ)Mo7O27} which can be viewed as a mono-capped hexavacant B-a-Keggin subunit with a central AsO3 group. Compounds 1-2 are the first type of sandwich-like heteropolymolybdate with As(Ⅲ) as the heteroatom. Compound 3-5 are the frist type of double-sandwich C-shaped heteropolymolybdate and the cinque-substituted P-Keggin bridging cluster {M(Fe5Mo8)O41(H2O)} is unprecedented. Compound 6 exhibits cagelike structure which is composed of one [AsMo6O24]9- subunit and one [AsMo6O27]15- subunit connected by three MoO4 tetrahedra. Ag atom locates at the center of the cage and forms coordinated bonds with two As atoms. To the best of our knowledge, Ag-As bond is very rare in POMs. 7-9 are organic-inorganic complexes in which molybdearsenate clusters are used as SUBs. Compound 7 possesses a 2D framework with 436 topology structure composed of copper coordination complexes connected by [CuAs6Mo630]4- via two kinds of weak Cu-O interactions. Compound 8 represents the 2D network structure with honeycomb cavities, which constructed from infinite 1D chains which composed of copper coordination complexes and molybdenum arsenate clusters of [As3Mo3O15]3-. Compound 9 is composed of two [AsMo6O21(O2CCH3)3]6- clusters linked together by one Cu(H2O)42+ complex via two teminal oxygen atoms of two MoO6 octahedra, respectively.The investigation of this paper indicates that the formation and successful isolation of these new type compounds depend on many factors, such as the composition and the ratio of reactants, solvent, counterion, temperature and pH value and the last two are especially important.
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