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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 171
Chapter 5 Published Results [HTe 2 W 10 O 40 ] 11− half units related by an inversion center. Furthermore, there are three sodium ions coordinated to the central cavity of 26 and one of them is present at the inversion center of the polyanion (see Figure 5.2). Each Te 4+ ion is linked to one of the {W 10 } units through three -oxo bridges and another such bridge links to the other half-unit, resulting in coordination number four and a see-saw coordination geometry for the Te atom (see Figure 5.3). Figure 5.2. Polyhedral (left) and ball-and-stick (right) representations of [H 2 Te 4 W 20 O 80 ] 22− (26). The color code is as follows: WO 6 octahedra (red); Te (blue), W (black); O (red), Na (yellow). 172
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