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Chapter 3 Published Results Figure 3.25. IR spectra of Na-14 (top) and its cerium analogue (bottom). The bonding modes of the lanthanide centers in 13 (Yb 3+ ) and 14 (La 3+ ) are different. The ytterbium ion in 1 is connected to the unique ‘belt’ tungsten via a -oxo bridge, whereas both lanthanum ions in 14 are bound to each other via a -aqua bridge, but not to the unique ‘belt’ tungsten. The W···Yb distance in 13 is ca. 4.0 Å whereas the average W···La distance in 14 is ca. 5.2 Å, which can be correlated with the size difference of Yb 3+ vs. La 3+ . The smaller ytterbium(III) ion is incorporated more deeply into the As 2 W 19 framework than the lanthanum(III) ion. We also performed thermogravimetric analyses (TGA) on the salts KNa-13 and Na-14 to determine the degree of hydration and thermal stability. The thermograms showed the expected weight loss domain between 25 and 400 ºC corresponding to dehydration. We calculated 25 and 16 water molecules per formula unit of KNa-13 and Na-14, respectively. 107
Chapter 3 Published Results 3.B.1.5. Conclusions The mono-ytterbium(III) and di-lanthanum(III) 19-tungsto-2-arsenates(III) [Yb(H 2 O) 2 K(H 2 O) 2 As 2 W 19 O 67 (H 2 O)] 10− (13) and [La 2 (H 2 O) 6 As 2 W 19 O 67 (H 2 O)] 8− (14), have been structurally characterized in the solid state by IR spectroscopy, single-crystal X-ray diffraction and TGA. Polyanions 13 and 14 were synthesized in simple, one-pot reactions of Yb 3+ and La 3+ with the dilacunary and trilacunary polyanion precursors As 2 W 19 and AsW 9 , respectively. The structure of 13 comprises an As 2 W 19 fragment with one incorporated Yb 3+ ion, whereas 14 contains two La 3+ ions in the vacant site of As 2 W 19 . Different synthetic strategies were needed to deliberately and selectively prepare mono- and di-lanthanide derivatives of As 2 W 19 . In this context it was discovered that counterions (K + vs. Na + ) play an important role. The terminal, labile water ligands in 13 and 14 are of interest for structural modifications and applications. We plan to explore aqua ligand substitution for other monoand polydentate ligands, including chiral ones, and we envision potential applications in Lewis acid catalysis. Efforts to isolate lipophilic salts of 13 and 14 in order to study their catalytic properties in organic media are also underway. 108
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