Synthesis and Structural Characterization of ... - Jacobs University

Chapter 3
Unpublished Results
The two structures of polyanions 15–20 are very similar as both are based on the Sb 2 W 20
unit. The main difference lies in the composition and coordination to this Sb 2 W 20 unit in 15–
17 is condensed by a WO 6 octahedral and coordinated to one lanthanide(III) ion while in 18–
20 is coordinated to two lanthanide(III) ions. This can be rationalized by the fact that pH is
the main parameter affects the syntheses of 15–17 and 18–20. It is well known that the pH is a
crucial parameter in POM synthesis in general.
Interestingly, when we followed the Krebs procedure, 5 to synthesize Ln 2 Sb 2 W 20 instead
of M 2 Sb 2 W 20 using the Sb 2 W 22 precursor and the respective lanthanide salt instead of the d-
transition salt, we did not get any crystalline product. Nevertheless, we have developed a
synthetic approach which has led to obtain the desired crystalline products of Na-15 – Na-17
using the Sb 2 W 22 precursor with higher yields than using the SbW 9 precursor. Several
variations of the synthetic procedure with Sb 2 W 22 (pH, solvent, temperature, ratio) till
polyanions 18–20 were obtained as well. However a high concentration of NaCl (1.0 M) and a
pH value ~5 were needed in order to obtain the desired crystalline products of Na-15 – Na-20
using the Sb 2 W 22 precursor with ratios (1:1 and 1:2) to the lanthanide salts, respectively,
based on FTIR and single-crystal XRD. This can be rationalized by the fact that ratio of the
reagents is the main parameter affects the syntheses of 15–20 using the Sb 2 W 22 precursor.
Thermogravimetric analyses (TGA) were performed on the salts of 15–20 to determine
the respective degree of hydration and the thermal stability. The thermograms of all six
compounds showed the expected weight loss domain between 25 and 400 ºC corresponding to
dehydration (lattice water molecules), see Figures 3.33 and 3.34. The degree of hydration was
calculated for all compounds and gave a range of 40 – 42 water molecules per formula unit
for Na-15 – Na-17 while 37– 38 for Na-18 – Na-20.
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