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Chapter 1 Introduction potassium salt by hydrolysis of P 2 W 18 at pH 6-7 using KHCO 3 . The trivacant anion [P 2 W 15 O 56 ] 12− (P 2 W 15 ) (Figure 1.9) is however formed by hydrolysis of P 2 W 18 using Na 2 CO 3 without the presence of potassium at pH 9-10, and is isolated as a sodium salt. Two isomers of the monolacunary α 1 -[P 2 W 17 O 61 ] 10− and α 2 -[P 2 W 17 O 61 ] 10− species exist, where the vacant site in the belt or in the cap, respectively. However, the α 1 -P 2 W 17 anion is unstable and readily undergoes isomerization to the α 2 -P 2 W 17 , which is depicted in Figure 1.9. 1f 1.4.2 The hexa‐lacunary species The hexavacant anion [H 2 P 2 W 12 O 48 ] 12− (P 2 W 12 ) (Figure 1.9) is formed by interaction of P 2 W 18 with the base (tris-hydroxymethylaminomethane) and is isolated as a potassium salt. 1f Although P 2 W 12 , being hexavacant, is regarded as a multi-dentate ligand for interaction with electrophiles, it rearranges in acidic aqueous medium to the monolacunary α 1 -P 2 W 17 which in turn is unstable and rearranges to the more stable α 2 -P 2 W 17 , due the metastability of P 2 W 12 in solution. 1a,1f Contant and Tézé were able to condense four P 2 W 12 units in lithium acetate buffer, resulting in the cyclic polyanion [H 7 P 8 W 48 O 184 ] 33− (P 8 W 48 ) which can be isolated as a mixed potassium/lithium salt. 21 The wheel-shaped phosphotungstate P 8 W 48 polyanion is stable at an unusually large pH range (1-8) and has D 4h point symmetry, as depicted in Figure 1.10. The multi-vacant P 8 W 48 , having 16 donor oxygens and a large central cavity (diameter of around 10 Å), can be considered as ‘super-lacunary’, but its stability and inertness were noteworthy. 17
Chapter 1 Introduction Figure 1.10 Polyhedral representation of P 8 W 48 polyanion. Red octahedra: WO 6 , green tetrahedra: PO 4 . 1.5 Typical properties of POMS POMs are discrete and high symmetrical species with a wide versatility in terms of their size, charge, structure and molecular weight. They are thermally robust and stable against oxidants. Isopoly- and heteropolyanions can be prepared and isolated from both aqueous and nonaqueous solutions. By far, the most common preparative method involves acidification of aqueous solutions of simple oxoanions [MO n ] m− that will reorganize to a packed molecular array of MO n units. In a formal sense, POMs are formed via Brønsted acid-base condensationaddition processes, e.g. 1 7WO 4 2− + 8H + [W 7 O 24 ] 6− + 4H 2 O 12MO 4 2− + H 3 PO 4 + 21H + [PM 12 O 40 ] 3− + 12H 2 O 18
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