Polyoxometalate-supported transition metal

Shivaiah and Dah [69] reported the characterization of Al " and Cr " polyoxometalate-supported transition metal complexes (Figure 9). 

Detailed discussions on supramolecular structures of giant polyoxometalate clusters have been reported by Muller and co-workers." " The number of ionic lattices formed from anionic POM clusters and organic cations-coordination complex cations resulting in supramolecular structures is substantial and literally more than hundreds of new compounds in this class appear annuallyIn this overview, the supramolecular features of POM-based systems will be discussed mainly under three headings (a) supramolecular features of polyoxometalate supported transition metal complexes, (b) polyoxometalate-crown ether complexes with supramolecular cations, and (c) supramolecular water clusters associated with polyoxometalates. 

II. SUPRAMOLECULAR FEATURES OF POLYOXOMETALATE-SUPPORTED TRANSITION-METAL COMPLEXES... 

In this volume we have collected 10 review chapters from distinguished scientists who have contributed extensively to the study and development of supramolecular assemblies that contain metals and metal-like elements with unusual structures and morphologies and possess potentially useful (and applicable) physical and biological properties. The first chapter by K. Ariga et al. is a general discussion of supramolecular structures that contain inorganic building blocks for hybrid lipid thin films, layer-by-layer assemblies, structure transcription, and functional mesoporous hybrids. This is followed by two chapters, the first by M. L. Kistler et al., who describe the self-assembly of hydrophilic polyoxometalate (POM) macro-anions and examine the structure and behavior of POM macro-ions in solution. This is followed by a chapter by S. K. Das, who provides an overview of the supramolecular features of POM-supported transition metal complexes, POM-crown ether complexes with supramolecular cations, and supramolecular water clusters associated with POMs. 

Polyoxometalates (POMs) are transition metal oxygen clusters with well-defined atomic coordination structures. POMs are used as functional nano-colloidal materials and also as supports for catalysts via ion-pair interactions due to their acidic properties. Combinations of chiral diamines and POM 225 effectively catalyze enamine-based aldol reactions. Less than 1 mol% of chiral amine loading is suf-ficientto catalyze the reaction (Table 28.10, entries 1 and 2) [114]. Highly diastereo-and enantioselective cross-aldol reactions of aldehydes are accomplished using chiral diamine-POM 226 under emulsion conditions (entries 3 and 4) [115]. Sul-fonated polystyrene or fluoropolymer Nafion NR50 are also good supports for the immobilization of primary-tertiary diamines. The catalyst 227 can be recovered by filtration and reused for at least four cycles with no loss of stereoselectivity (entries 5 and 6) [116]. 

In the previous section, transition-metal-substituted polyoxometalates (TMSPs) supported on Si02 [61, 62] have been mentioned. TMSPs have been also tested when immobilized on Ti02- For instance, the H5PW10V2O40 supported on Ti02 was active in the photocatalytic decomposition of dimethyl sulphide (DMS) in gas-... 

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