Polyoxometalates Keggin structure

Submitted by JOSE ALVES DIAS, SILVIA CLAUDIA LOUREIRO DIAS, and 

The following syntheses involve some of the most common heteropoly compounds with the Keggin structure. These compounds can help students develop a good understanding of how subunits can be connected together to form condensed structures. In addition, the preparations expose the students to a variety of useful laboratory techniques such as vacuum filtration, solvent 

Laborat6rio de Catalise, Instituto de Qufmica, Universidade de Brasilia, Brasilia, DF 70919-970, Brazil. 

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Fig. 15. Crystal structure of the Keggin-type polyoxometallate ion [PMo427W7.73O40]6. Adapted from (608).

High-Nuclearity Mixed-Valence Clusters Localization vs. Delocalization of the Electronic Pair in the Double Reduced Polyoxometallates with Keggin Structure... 

The Keggin-structure anions (a-XM O ), particularly the tungsto- and molybdophosphates and -silicates, continue to be the most frequently investigated polyoxometalates, almost always for potential and realized catalytic applications. 

Some new structural polyoxometalates were obtained, which are listed in the Table 1. In Table 1, the compound [Bu4N]6[H30]2[Moi304o]2 synthesized by Professor K.L. Tang was a new Keggin-type isopolyoxomolybdate, in which, the [BuaN] cations, [H3O] cations, [Moi304o] anions have different symmetry. Both anions have Keggin-type structures. The structures are based on a central Mo(A)04 slightly distorted tetrahedron surrounded by 12 Mo(B)Oe octahedra. 

It has always been intriguing that the Keggin structure, so well known for d configurations, had not been reported for open-shell configurations. Work from Bino et al. has corrected this, with the structure of [Fei304F24(OMe)i2] which has the full symmetry required by a Keggin ion. The involvement of fluoride appears vital, and possibly this may be the trick to produce further polyoxometallate analogs terminal F may function for middle 3d metals the way terminal 0 functions for early metals. 

It is important to note that the synthesis of many heterocycles is often carried out under acid-catalyzed reactions, so much effort has been put into the search for solid acid catalysts (Rosati et al., 2007 Dhakshinamoorthy et al., 2011 Sreekumar and Padmakumar, 1998 Kandarpa et al., 2011 Krishnakumar and Swaminathan, 2011 Huang et al., 2008). From this point of view, catalysis by heteropoly acids (HPAs) and related compounds is a field of increasing importance worldwide. To avoid the use of conventional acid catalysts (sulfuric, phosphoric, and hydrofluoric acids and boron trifluoride) and the related environmental pollution and corrosion problems (Vdzquez et al., 2002), insoluble solid acid catalysts such as HPAs can be used. HPAs are mixed oxides composed of a central ion or heteroatom, generally P, As, Si or Ge, bonded to an appropriate number of oxygen atoms and surrounded by a shell of octahedral MOg units. HPAs with Keggin structure and related polyoxometalates are quite common and are represented by the formula Hg [XM,204o], where X is the... 

Rocchiccioli-Deltcheff, C. Fournier, M. Franck, R. Thouvenot, R. (1983). Vibrational Investigations of Polyoxometalates. 2. Evidence for Anion-Anion Interactions in Molybdenum(VI) and Timgsten(VI) Compounds Related to the Keggin Structure. Inorg. Chem., Vol.22, pp. 207-216... 

Figure 9.1 Polyoxometalates with the Keggin structure (a) [XIVIi204o] and (b) [PMi2V204o]. ...

Almost all reported ionic liquids consist of organic cations and inorganic anions. Only a few publications deal with ionic liquids completely formed by inorganic compounds and most of them treat eutectic mixtures. The work of Shan and co-workers is an exception in this field, as it describes ionic liquids formed by just inorganic cations and anions [63]. In their work, they describe a class of anions formed by polyoxometallates with the Keggin structures bound... 

Systems based on Keggin-type polyoxometalates (POMs) (phosphomolybdic acid derivatives) have been the most effective and widely used catalysts in gas-phase isobutane selective oxidation research. This Keggin POM catalyst application was hrst claimed in 1981 by Rohm and Haas, and after several catalyst mod-ihcations has been patented by other companies such as Asahi Kasei, Sumitomo, Mitshubishi, etc. However, in addition to a Keggin phosphomolybdate, vanadium must also be present in the anion structure in order to obtain a catalyst effective in the selective oxidation of isobutane. Moreover, the addition of - -3 metal cations to the phosphomolybdate anion increases the thermal stability of the catalyst, significantly minimizing its deactivation. " ... 

Two major subsets of polyoxometalates are the isopolyoxometalates (or isopolyanions) which contain only transition metal and oxide ions, and the heteropolyoxometalates (or heteropolyanions) which contain one or more "heteroatoms" in addition to the transition metal and oxide ions. The heteroatom in polyoxometalates can be one of many elements in the p or d blocks. In this review we have highlighted the photochemistry of two polyoxometalates, dodecatungstophosphate, a representative heteropolyanion of the common Keggin structure, and decatungstate, a representative isopolyanion, whose structures are given in Figure 1. 

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