Synthesis and Structural Characterization of ... - Jacobs University

Chapter 5
Published Results
5.A.1.4. Results and Discussion
We perfomed a systematic study on the interaction of tellurium(IV) ions with sodium
tungstate in neutral, aqueous medium, and at a ratio of 1 : 5 we were able to isolate crystals of
Na 22 [H 2 Te 4 W 20 O 80 ]·64H 2 O (Na-26). Four reaction parameters are crucial for the successful
formation and isolation of [H 2 Te 4 W 20 O 80 ] 22− (26) in a pure form: reaction temperature,
concentration of reagents, molar ratio of reagents, and pH of the solution. The polyanion 26 is
formed at room temperature, but if the solution is heated then a mixture of products is present:
26, the well-known paradodecatungstate ion [H 2 W 12 O 42 ] 10− , and another novel
tungstotellurate(IV), [NaTeW 15 O 54 ] 13− (27). Polyanions 26 and 27 were crystallized as sodium
salts and characterized by IR and single-crystal XRD.
The yield of 26 is affected by the concentration of the starting material. A tungstate
concentration as high as 1.5 M and a TeO 2 and Na 2 WO 4 ratio of 1: 5 are needed to isolate
pure crystalline Na-26. Finally, the pH of the reaction solution affects the formation of 26 and
yield of Na-26. A pH window of 7.4-7.6 resulted in a successful synthetic procedure, with pH
7.5 being optimal. A pH lower than 7.4, or higher than 7.6, resulted in paratungstate as the
main product. Experimentally, the filtration of the reaction solution after one hour of stirring
is important to decrease the amount of paratungstate formed. Obviously, the formation of
paratungstate accounts for the low yield of 26. Interestingly, we discovered 26 in the first
place when trying to reproduce and crystallize [TeW 9 O 33 ] 8– following the synthetic procedure
of Gaunt et al. 11
The polyanion 26 crystallized as a hydrated sodium salt in the triclinic space group P1̅.
The single-crystal X-ray analysis showed that 26 consists of four tellurium atoms sandwiched
between two novel decatungstate {W 10 } units. The structure of 26 comprises two
[HTe 2 W 10 O 40 ] 11− fragments which are connected through Te−O−W -oxo bridges. Hence
polyanion 26 can be described as a dimeric molecular entity composed of two
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