Polyanion Polyoxometalate

For acid forms, polyacids = polyoxoacids, including heteropolyacids (e.g., H3PW12O40) and isopolyacids (e.g., H2Mo60, ) and for oxoanions, polyanions = polyoxoanions = polyoxometalates, including heteropolyanions (e.g., PW12O40") and isopolyanions (e.g., Mo60 9 ). 

In addition, such generic terms as metal-oxygen cluster ion, metal-oxide molecule, etc., are used for polyanions (and polyacids). Since the traditional heteropoly- and isopoly- are unsatisfactory terms to express a variety of polyacids and polyanions, the terms polyoxometalates and polyoxoacids have been used recently (3). The terminology is still changing, thus reflecting the rapid expansion of the chemistry. 

Heteropolyanions and isopolyanions are polymeric oxoanions (polyoxometalates) (2, 3, 5, 6). The structure of a heteropolyanion or polyoxoanion molecule itself is called a primary structure (5, 6, 77). There are various kinds of polyoxoanion structure (Section II.A. 1). In solution, heteropoly anions are present in the unit of the primary structure, being coordinated with solvent molecules and/ or protonated. Most heteropolyanions tend to hydrolyze readily at high pH (Section 1I.C). Protonation and hydrolysis of the primary structure may be major structural concerns in solution catalysis. Heteropoly compounds in the solid state are ionic crystals (sometimes amorphous) consisting of large polyanions, cations, water of crystallization, and other molecules. This three-dimensional arrangement is called the secondary structure. For understanding catalysis by solid heteropoly compounds, it is important to distinguish between the primary structure and the secondary structure (5, 6, 17). Recently, it has been realized that, in addition... 

Chen et al. recently reported the stable ruthenium-oxo-oxalato cluster Na7[Ru4(p3-0)4(C204)g] (2) (Fig. lc) as a particularly potent inhibitor of HIV-1 reverse transcriptase (RT) with an IC50 value of 1.9 nM [17], Apparently, the polyanionic cluster mimics poly anionic nucleic acid in its binding to HIV-1 RT through electrostatic interactions. The cluster shows promising anti-HIV-1 activity without being cytotoxic. In contrast to many polyoxometallates, this cluster has been demonstrated to be stable under physiological conditions. 

Polyoxometalates (POMs), also known as heteropolyacids (HPA),3 are a class of compounds formed from negatively charged inorganic metal-oxygen building blocks. When charge-balanced with cationic species, POMs self-assemble into unusual 3D structures with specific topological and electronic properties.214 POMs are commonly formed from polyanions of early transition metals such as W, Mo or V. These anions can be substituted with other transition metals. The diversities in POMs composition and structure make them attractive for many applications, particularly as Bronsted acid and redox catalysts. For example,... 

A templating method was presented to fabricate novel three-dimensional ordered macroporous silica materials ftinctionalized by polyoxometalates. Characterization of the products indicated that the starting XMo, remained intact regardless of the functionality of the polyanions, and their primary lacunry structures tended to get saturated. The future development of the method holds promise for the formation of advanced new materials with more novel properties, which can be applied in more wide fields. 

Fan, L., Xu, L., Gao, G et al. (2006) A novel polyoxometalate chain constructed from sandwich lanthanide-containing polyanion ICe(PWn039)2] and sodium ion linker. Inorganic Chemistry Communications, 9, 1308-1311. 

Polyoxometallates. Polyoxometallates of the group 6 transition metals (iso- and hetero-polyoxoanions) form a special class of metallate ligands for the actinide elements. These species can incorporate other atoms as either primary or secondary (peripheral) heteroatoms. Primary heteroatoms are necessary to complete the polyoxoanion structure secondary heteroatoms can be removed without disruption of the stable polyanion unit. The early actinides serve in both roles in known compounds. 

Recent years have seen the reports of increasing numbers of very large polyoxometalate anions. These may be divided into two classes (i) composite anions constructed of assemblies of simpler, usually lacunary, polyanions and (ii) mixed-valence polymolybdate anions which comprise common building blocks, e.g., Mo8, Mo6, and Mo2 clusters that have no separate, stable existence. 

Organometallic derivatives of polyoxometalates are perfectly characterized at the molecular level and in turn provide models for the grafting of an organometallic catalyst on an oxide surface, especially in the case of the supported derivatives. Beyond, they also display their own reactivity and catalytic activity.Some synergy between the organometallic and oxide parts has been reported in the literature, as well as bifunctional activity. The mobility of organometallic fragments at an oxide surface could also be reproduced on a polyanion. 

Films of oxides can be produced by anodization of metal electrodes. For example, AI2O3 forms on an aluminum anode immersed in a solution of H3PO4. The thickness of the film can be controlled by the applied potential and the time of anodization. Such a film can be used as a support for other materials, such as poly(vinylpyridine) (PVP). Oxide films of other metals, such as Ti, W, and Ta, can be produced in a similar way. Oxide films can also be produced by CVD, vacuum evaporation and sputtering, and deposition from colloidal solution. Related inorganic films are those of polyoxometallates (iso- and heteropolyacids and their salts) (20). For example, the heteropolyanion P2W17M0O62K6 shows a number of reduction waves at a glassy carbon electrode. A wide variety of metallic polyanionic species (e.g., of W, Mo, V) exist and have a rich chemistry. Films of such materials are interesting for their electrocatalytic possibilities. 

Contents Introduction. - Preparation, Structural Principles, Properties and Applications. - Isopolyanions. - Heteropolyanions. - Heteropolyanions and Ligands. - Redox Chemistry and Heteropoly Blues. Organic and Organometallic Derivatives. - Polyoxometalate Chemistry. Current Limits and Remaining Challenges. - Appendix Nomenclature of Polyanions. - References. - Subject Index... 

Polyoxometalates (POMs) are molecular nanosized polyanionic scaffolds, with multi-metal oxide structure and thus with a general motif being at the interface between molecular complexes and extended oxides.They offer well-defined models for the reactivity of metal bulk oxides, displaying, in addition, the typical tunability of the molecular species. 

Figure 15 Vesicle template-directed self-assembly of a core-shell structure with ring-shaped polyoxometalates on the surface through electrostatic interactions between the cationic amphiphiles and the polyanionic ring.

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