Polyoxometalate complexes, application

Iu search for efficieut aud greeuer processes over the past few years various heterogeneous catalysts such as titanium incorporated mesoporous molecular sieves [45,46], Schiff-base complexes supported on zeolite [47] and Zn(II)-Al(III) layered double hydroxide (LDH) [48], oxomolybdenum(VI) complexes supported on MCM-41 and MCM-48 [49], polyoxometallate supported materials [50], Co and Mn-AlPO s [51] etc. have been developed and studied for the catalytic epoxidatiou of a-pinene. Many of these processes suffer from drawbacks and limited applicability due to the poor conversion and also the selectivities. Sacrificial aldehydes are often used as an oxygen acceptor in such processes to achieve reasonable yield and selectivities but, this procedure leads to an increase in the E-factors and decrease in the atom economy [51]. 

Applications of spectroscopic methods in studies of polyoxometalates and their complexes with lanthanide (III) ions Lis (2000)... 

Polyoxometalates are important reagents in analytical chemistry and they also find applications in catalysis, molecular biology, materials sciences, and medicine. A recent study of nine Ndm polyoxometalates (POM) showed their aqueous 5 mM solutions to be weakly luminescent, whereas no luminescence at all is seen for the aquo ion. In particular, the bis(POM) complexes better protect the Ndm ion from nonradiative deactivations, for instance t(4F3/2) = 411 6nsfor[Nd(PWn039)2]11- and67 2 ns for [Nd(As4W4oOi4o)]25 (But et al., 2005). 

The explosion of activity and subsequent literature in the polyoxometalate field during the past two decades may reflect the increasing access to reliable methods of characterization in solution (multinuclear NMR) and the solid state (X-ray diffraction), but is certainly stimulated by the realization that there are few fields of science to which these complexes do not have important applications. One result of this expansion has been the necessity to devote two chapters to the chemistry of polyoxometalates one focusing on structure and one on reactivity instead of the single chapter in the original Comprehensive Coordination Chemistry (CCC, 1987). 

To a large extent the discussion will be limited to those complexes that have a discernible solution chemistry (e.g., by spectroscopy). The boundary between metal-oxide lattices and solid-state materials incorporating known and new polyoxometalate structures is becoming less and less well defined. Although this is an interesting and important area, with many potential applications, it is beyond the scope of the present endeavor. 

In the preparation of materials, aqueous lanthanide and polyoxometalate chemistry is fraught with problems owing to multi-equilibria. Occluded water is often hard to remove and may preclude some applications. Additionally, in aqueous syntheses, different counterions complicate the issue as shown from our work and that of others. Therefore, it is very difficult to predict complexes and structures that will be formed in reactions carried out in water. We intend to branch into s)mtheses using organic soluble lacunary polyoxoanions and organic soluble or organometallic lanthanide precursors. We hope to establish reactions that can be depended on to produce complexes that can be reliably used in synthesis of extended materials. 

Menon D, Thomas RT, Narayanan S, Maya S, Jayakumar R, Hussain F, Lakshmanan VK, Nair SV. A novel chitosan/polyoxometalate nano-complex for anti-cancer applications. Carbohydr Polym. 2011 84(3) 887-93. 

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. 

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