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Trilacunary Keggin-Type POMs as Versatile Building Blocks for Lanthanoid Silicotungstates


  • Dedicated to Professor Michael T. Pope on the occasion of his 80th birthday


Three new lanthanoid-substituted polyoxotungstates based on open Wells–Dawson fragments {[Ln2(H2O)7SiW18O66]10– [Ln = EuIII (Eu-1 and Eu-2), TbIII (Tb-1)]} and a new sandwich-type polyoxometalate constituted of Keggin-type monolacunary anions {[Eu(α-SiW11O39)2]13– (Eu-3)} were obtained from the trilacunary Keggin POM Na10[SiW9O34]·23 H2O and lanthanoid salts as precursors. All compounds were comprehensively characterized by inductively coupled plasma (ICP) and thermogravimetric analyses and by FT-IR and UV/Vis spectroscopy, as well as by powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. Eu-1 and Tb-1 exhibit 3D architectures of dimeric [Ln2(H2O)7SiW18O66]10– units connected through LnIII linkers, whereas Eu-2 displays a 2D sheet-like framework. Eu-3 exists in two enantiomeric forms, due to the C2 symmetry of the central square-antiprismatic EuO8 polyhedron. The structure-directing influence of the Ln cations on the emerging POM architectures is investigated in detail. Photoluminescence of Eu-1, Eu-2, Eu-3 and Tb-1 can be ascribed to the characteristic emissions of the EuIII and TbIII cations, respectively. The electrochemical properties of all compounds were investigated by cyclic voltammetry in sodium acetate buffer solution as supporting electrolyte. The magnetic susceptibility of Tb-1 was studied in the temperature range between 5 and 300 K and indicated paramagnetic behavior.