Polyoxometalates cations

Neumann and Fish have studied the novel polyoxometalate salt 25, which features 12 fluorous ammonium cations [12]. This material was insoluble in EtOAc (and toluene) at room temperature, but dissolved at 80 °C to give an effective catalyst system for the oxidation of alkenes and alcohols by 30% aqueous H2O2. CooHng precipitated the catalyst, which was reused. Additional examples of thermomorphic fluorous catalysts have been briefly described in meeting abstracts [60,61] and will Hkely soon appear in the peer-reviewed literature. 

The formation of sandwich-type POMs from solutions of metal cations and triva-cant heteropolytungstate species may alternatively be viewed as means to sequester cations from solution [116]. The question then arises if reversible dissociation of the metal cations from the multidentate ligands is possible as well. The sequestration and subsequent release of metal cations under appropriate conditions is of considerable interest in waste decontamination technology as well as in medicine [124]. Following these ideas, the demetallation process of ten multi-iron Wells-Dawson polyoxometalates is studied by cyclic... 

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... 

The present chapter reviews the structural chemistry (Section 2) and properties and applications (Section 3) of polyoxometalates that incorporate one or more rare-earth elements. In most cases these are discrete anionic entities within the crystal and in solution, but there are also extended lattices in which POM groups are linked by rare-earth cations. Solids which can best be described as mixed oxides, or which appear to be salts of common polyoxometalate architectures such as the... 

In acetonitrile solution the luminescence of [Eu(H20)4(o i-P2Wi706i)]7 increases with the addition of 1,10-phenanthroline or 2,2-bipyridine and reaches a limiting value with a 2 1 ratio of the bidentate ligand to polyoxometalate complex, consistent with the replacement of the four aqua ligands of the Eu cation (Boglio et al., 2006). 

Polyoxometalate anions, especially those of hexavalent molybdenum and tungsten interact with rare-earth cations to generate a seemingly endless variety of complexes in which the rare earths function as encrypted cations, as core heteroatoms, or as linkers of polyoxometalate fragments to yield discrete anionic assemblies currently incorporating as few as 8 and as many as 164 metal centers,... 

In one such approach we seek to exploit the potential signal transduction properties of the thermochromic Dawson [40] polyoxometalates and combine this with a fluorescent POM-hybrid to produce a device that could respond optically as a function of the local environment, see Figure 2.12. In this example, the clusters would be positioned on a gold surface using SAMs (Self-assembled monolayer) with a cationic tail and local positional control could be aimed using self-assembly, or even by means of an atomic force microscope tip. 

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,... 

An interesting example of the use of a recyclable, thermoresponsive catalyst in a micellar-type system was recently reported by Ikegami et al. [38]. A PNI-PAM-based copolymer containing pendant tetraalkylammonium cations and a polyoxometalate, PW1204o, as the counter anion was used as a catalyst for the oxidation of alcohols with hydrogen peroxide in water (Fig. 9.25). At room temperature the substrate and the aqueous hydrogen peroxide, containing the catalyst, formed distinct separate phases. When the mixture was heated to 90 °C a... 

Figure 5.10 The structure of [ Ce(H20)s 4 Na(H20)P5W3oOno ] cluster [35]. (Reprinted from Inorganica Chimica Acta, 360, no. 6, Y. Lu, Y.G. Li, et al., A new family of polyoxometalate compounds built up of Preyssler anions and trivalent lanthanide cations, 2063-2070 (Figure 1), 2007, with permission from Elsevier.)...

Figure 5.21 The one-dimensional chain in [Ce(H20)3(SiWn039)] [67b], (Reproduced with permission from M. Sadakane, et ai, Controlled assembly of polyoxometalate chains from lacunary building blocks and lanthanide-cation linkers, Angewandte Chemie International Edition, 2000, 39, no.l6, 2914-2916 (Figure 1). Wiley-VCH Verlag GmbH Co. KGaA.)...

Figure 5.24 The two-dimensional layer structure of [Ln2(H20)9(a2-P2 Vi706i)]" [36]. (Reprinted with permission fromY. Lu, etal., New polyoxometalate compounds built up of lacunary Wells-Dawson anions and trivalent lanthanide cations, Inorganic Chemistry, 45, no. 5, 2055-2060 (Figure 3), 2006. 2006 American Chemical Society.)...

Creaser, I., Heckel, M.C., Neitz, R.J. et al. (1993) Rigid nonlabile polyoxometalate cryptates [ZPsWsoOiio] that exhibit unprecedented selectivity for certain lanthanide and other multivalent cations. 

Lu, Y, Li, Y, Wang, E. et al. (2007) A new family of polyoxometalate compounds built up of Preyssler anions and trivalent lanthanide cations. Inorganica Chimica Acta, 360, 2063—2070. 

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