Synthesis and Structural Characterization of ... - Jacobs University

Chapter 5
Published Results
Each of the {W 2 } subunits shares three corners with {W 3 } of another pentatungstate {W 5 }
unit, consequently forming the isopolytungstate unit {W 15 } with a D 3 point group symmetry.
The cavity of the {W 15 } isopolytungstate unit adapts the tellurium atom which is coordinated
to the three {W 2 } subunits resulting in a trigonal pyramidal geometry, and the sodium atom.
The Te and Na atoms have 50 % occupancy eachin their positions. Crystallographically, the
pentadecatungstate unit {W 15 } has an asymmetric unit consisting of eight WO 6 octahedra,
{W 8 }, with one of them being unique. A related pentadecatungstate {W 15 } unit capped by two
Ce 3+ ions has been reported recently by the Cao group. 16
Bond valence sum (BVS) calculations for 26 and 27 are consistent with all tungsten atoms
being in the +6 oxidation state, and all tellurium ions being in the +4 oxidation state. 17 In
addition, in 26, one monoprotonated oxygen was identified in the asymmetric half-unit, (BVS
~ 1.06), one of the three μ 2 -oxo bridges of the {W 3 } fragment, namely the μ 2 -oxo group of
W7 and W8 (pink in Figure 5.4). This results in two OH groups for 26 and a total charge of -
22 for the polyanion. The range of BVS for non-protonated bridging oxygens is (2.06 – 1.49).
The terminal W=O oxygen BVS values are low (~ 1.5), but this is typical for the distorted
W=O geometry and does not indicate additional protonation. Polyanion 27 has no apparent
protonation sites.
Thermogravimetric analyses (TGA) were performed on Na-26 to determine its degree of
hydration and thermal stability. The TGA graph showed a region of weight loss due to
dehydration, with a one-step weight loss in the range ~25-400 ˚C. We calculated 64 water
molecules per formula unit of Na-26 which agrees well with the results obtained by singlecrystal
XRD. Furthermore, the absence of any additional weight loss indicated that Na-26,
after loss of the crystal waters, was thermally stable up to 800-900 °C (see Figure 5.7). As
stated above, polyanion 27 was synthesized and co-crystallized in a mixture of products,
preventing the interpretation of the thermal stability of Na-27. However, we were able to
obtain the IR spectrum of Na-27 (see Figure 5.8).
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