Polyoxometalate Keggin structure

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. 

D. G. Musaev, K. Morokuma, Y. V. Geletii, C. L. Hill, Computational modeling of di-transition-metal-substituted /-Keggin polyoxometalate anions. Structural refinement of the protonated divacant lacunary silicodecatimgstate, Inorg. Chem. 43 (2004) 7702. 

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... 

Musaev, D., Morokuma, K., Geletii, Y., etal. (2004). Computational Modeling of Di-Transition-Metal-Substituted g-Keggin Polyoxometalate Anions. Structural Refinement of the Protonated Divacant Lacunary Silicodecatungstate, Inorg. Chem., 43, pp. 7702-7708. 

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. 

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

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