One-Step Synthesis of Tetraalkylammonium Salts of the β Isomer of 12-Molybdophosphate

Abstract

One of the properties of heteropoly complexes with the central P, Si, As, and Ge atoms and peripheral Mo, W, and V atoms is the capability to exist in the form of geometric isomers [1]. The geometric isomerism is characteristic, in particular, of heteropoly complexes with the tetrahedral surrounding of the central atom, i.e., for Keggin and Dawson structures. The coordination sphere of such complexes consists of M 3 O 13 triads each of which is built of three distorted tetrahedra sharing common edges. In the Keggin structure of 12-molybdophosphate a-PMo 12 O 4 3 0 ! , four Mo 3 O 13 triads share common vertices with each other and with the central PO 4 tetrahedron [2].The Keggin structure may exist in the form of five geometric isomers. A turn of one, two, three, or all the four Mo 3 O 13 triads by 60o around the C 3 axis of the symmetrical a isomer, according to a theoretical analysis [3], gives rise to b, g, g`, and g``forms, respectively. In the g forms, in contrast to the a and b forms, the triads share common edges rather than vertices, which results in shorter Mo3Mo distances and stronger electrostatic repulsion. Such anions are thermodynamically unstable. Indeed, only a and b isomers were obtained in solution and isolated in the solid state. The structure of the a isomers is more favorable energetically than that of the b isomers; therefore, b isomers, as a rule, irreversibly isomerize into a isomers in aqueous solutions [4]. Reduction of heteropoly complexes, on the contrary, is accompanied by the a 6 b isomerization [5]. The published procedure [6] for preparing the tetrabutylammonium salt of the b isomer of 12-molybdophosphate, b-[(n-C 4 H 9 ) 4 N] 3 . [PMo 12 O 40 ] (I), involves reoxidation of the reduced four-electron acid b-H 7 PMo 12 O 40 . The synthesis is performed in three steps: (a) preparation of a Mo(V) solution by acidification and electrolytic reduction of a Na 2 MoO 4 solution, (b) synthesis of b-[PMo 12 O 40 ] 7! in aqueous solutions from H 3 PO 4 , Mo(V), and Na 2 MoO 4 , and (c) reoxidation of b-H 7 PMo 12 O 40 and isolation of solid compound I with tetrabutylammonium bromide. Hori and Fujinaga [7] reported on formation of b-[PMo 12 O 40 ] 3! in aqueous-organic solutions from a-[PMo 12 O 40 ] 3! under the action of acetone or other aprotic solvents. The formation of the b isomer was judged from the spectrophotometric data only. However, the increased molar extinction coefficient, which served as criterion of the a 6 b isomerization, could also be due to a higher actual content of the a isomer in the presence of aprotic organic solvents because of a lower degree of its hydrolytic dissociation to [PMo 11 O 39 ] 7! [8]. Furthermore, in the presence of the above-mentioned solvents, depending on the concentration of the complexes with respect to P(V), formation of [P 2 Mo 18 O 64 ] 6! anions (d P 32.40 ppm) is possible [8].We found by 31 P NMR that additions of lowmolecular-weight monobasic carboxylic acids (formic, acetic, propionic) suppress the dissociation of 12-molybdophosphate (33.20 p...