| Based on vacant POMs as precursor, we explored the process of reaction condition, which includes pH in aqueous solution, temperature, ligand, counterion and ratio of mixture, and successfully synthesized five new lanthanide polyoxometalates (LnPOMs) at room temperature in aqueous solution. We analyzed the structures of these compounds through single X-ray diffraction, and characterized them via elemental analysis, IR, TG and UV. Magnetic properties of these new compounds were studied in the range from 2 to 300K.1. Two new lanthanide polyoxometalates based on [γ-GeW10O36]8- were successfully synthesized: Na12[Dy4(α-GeW10O38)2(H2O)6]·36H2O (1) Na12[Er4(α-GeW10O38)2(H2O)6]·29H2O (2) Compound 1 is isomorphous with 2. In these two compounds, four lanthanide ions were located in the central belt between two [α(1,4)-GeW10O38]12- units. Meanwhile, each lanthanide ion resides in the vacant site of the [α(1,4)-GeW10O38]12- fragment through bonding to five oxygen atoms, four of which are from the WO6 octahedra, while the other one is from central GeO4 tetrahedron group. Compounds 1 and 2 are the first two examples of LnPOMs dimeric polyoxoanions accommodating four Ln3+ ions between two dilacunary Keggin-type polyanion units.2. On the basis of trivacant Keggin-type polyoxoaion [α-GeW9O34]10-, a new 1:2 type structure of sandwich LnPOMs was isolated by adjusting proper pH of the reaction solution: (C4H9NO)(C4H10NO)13[(GeW11O39)2Nd]·9H2O (3)3. On the basis of monovacant Keggin-type polyoxoaion [α-BW11O39]9–, two classical structures of sandwich-type LnPOMs were synthesized in sodium acetate buffer solution: Na2(C3H5N2)12H[(BW11O39)2Pr]·7H2O (4) K2Na6[N(CH3)4]3H4[(BW11O39)2Dy]·23H2O (5) In these two compounds, the polyanions [(BW11O39)2Ln]15- (Ln = Pr, Dy) are classic sandwich-type structure, where the Ln3+ ion is sandwiched by two monolacanary polyanions [α-BW11O39]9–. Meanwhile, each monolacunary polyanion [α-BW11O39]9– acts as an effective tetradentate coordinating with the Pr atom center through four unsaturated oxygen atoms. Furthermore, adjacent sandwich-type polyanions [(BW11O39)2Pr]15- of compound 4 are linked by [Na2(C3H5N2)2]4+ units through O-Na-O bridges into a 2D layer parallel to the ab plane.
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