Polyoxometalates are negatively charged aggregates of transition metals mainly Vanadium, Molybdenum and tungsten with oxygen

The chemistry of polyoxometalates has been reviewed extensively. Much useful information about the synthesis, study of structures and various applications of heteropoly acids and their salts have been reported in the literature. Thus, POMs inorganic complexes composed of transitional metals mainly Mo, W and V. The present review gives an introduction into the chemistry of these compounds and overviews of the principal studies of their catalytic effect on nitration of the phenols, acetylation of alcohols, phenols and their derivatives. Key word: Eco-friendly catalysts, Heteropoly acids, Polyoxometalates Introduction Polyoxometalates are negatively charged aggregates of transition metals mainly vanadium, molybdenum, tungsten with oxygen. M.T.Pope named these classes of compounds as heteroatomic oxometalates. These compounds are cluster type with metaloxygen and metaloxygenmetal linkages with large number of lattice water in the interstitial position. Any element can be incorporated into the frame work of polyoxometalates. If heteropoly compounds are categorized as condensation heteroatomic inorganic polymers, then the repeating unit would be M3O13 group where M=V, Nb, Ta, Mo and W. A classical heteropoly anion condenses numerous oxygen atoms, sometimes hydrogen atom and atoms of other elements in positive oxidation state. These complexes generally represent thermodynamically relatively stable arrangements, although especially in the case of polyoxotungstates. They characteristically maintain their identities in aqueous and non-aqueous solution as well as in ionic crystals. A heteropoly contains a high atomic proportion of one kind of atom in +ve oxidation state called addendum atom and much smaller proportion(s) of the other kind(s) of atom(s) in the positive oxidation state(s) known as heteroatom.V,Mo,W function as addendum atom in a great many heteropoly anions. A few additional atoms in their highest oxidation states for eg Nb,Ta,Re,I can less commonly acts as addenda atom. Over 60 other elements including non metal and transitional metals, can function as heteroatom. The atoms that can function as addenda atom are those that 1. Change their coordination with oxygen from 4 to 6 as they polymerize in the acidic solution,. 2. Have a high positive charge and are among the smaller atoms that fall within the radius for octahedral packing with oxygen. The ability to act as addenda is enhanced if the atoms are able to form double bond with unshared oxygen of their MO6 octahedra by Pπ-dπ interaction. The more the double bond shortens the addendum-oxygen distance, the greater is the polarization and the stronger is the ion in the induced dipole interaction. Heteropoly complexes are generally strong oxidizing agent. In the cases of complexes that do not contain any addenda that have just one unshared oxygen, K.C.Dey et al /Int.J. ChemTech Res.2010,2(1) 369 reduction mainly disintegrates the complex forming species containing lower oxidation states of the addenda. The reduction products are characteristically deep blue in colour and comprise a very large group of complexes known as “heteroply blues”. Further reduction of heteropoly blue complexes leads to formation of so called heteropoly brown anion. These are species that retain the gross structures of parent complexes but the addenda in some or all subunits (for M3O13 group) are reduced by two electrons. The added electrons in heteropoly brown are not delocalized. The application of POMs in the field of Chemical analysis, catalysis, material science, nuclear waste treatment and medicine are developed. As inorganic pharmaceuticals are still rare compared to more common organic compounds, it draws the attention of the researchers outside the inorganic chemistry to study the biomedical applications of POMs. However, there is still plenty of scope for further work in this field as many fundamental questions regarding the structural principles, mechanism of synthesis and reactivity of POMs remain unanswered.. Synthesis Generally two types of polyoxometalates are distinguished as based on the chemical composition – Isopolyanions [MmOy] and heteropolyanions [XxMmOy] (x≤m) where M is the addenda atom and X is the heteroatom also called central atom when located in the centre of the polyanion. Most common addenda atoms are molybdenum, tungsten, less frequently V and Nb or mixed of these elements in their highest oxidation state [d,d].Almost all elements in the periodic table can be incorporated in heteropoly anion. Mo and W are best polyoxometalates formers due to favorable combination of ionic radius, charge and accessibility of empty d orbitals for metaloxygen π bond. By 1908 approximately 750 heteropoly compounds have been reported and analysed by over 250 authors 13 .As a result of these efforts it had been widely noted that heteropoly species containing a 6:1 or 12:1 atomic ratio of addenda to heteroatom were the most common. Transition metal cations are coordinated by aqua hydroxo (OH) or Oxo (O) ligand in the aqueous solution. The acidity of a coordinated ligand parallels the metal cations. The higher the positive charge of the metal, the easier the proton on the ligand dissociate. Therefore, highly charged d cations of group 5 and 6 (V,Mo and W) form stable complex with oxo ligands in aqueous alkaline solution(VO4,MO4 and WO4).A condensation reaction takes place forming M-O-M bridges on acidification.More condensed structures could be formed with more expanding coordination number. Thus the nature of the oxometalates depends on stoichiometry, solvent used, pH, temp, concentration etc. Many different POMs may be synthesized by systematic variation of these factors. The M-O bonds in POMs can be cleaved by the addition of base and many more lacunary compounds may be obtained by controlled degradation. POMs are formed by self assembly process, typically in an acidic aqueous solution and can be isolated as solid with an appropriate counteraction e.g –H, alkali metal cations, NH4 etc. For the synthesis of polyoxometalates, the concentrations of the reactants are chosen in such a way by trial and error method so that the immediate precipitation may not occur. When a graph is plotted between the volumes of the heteroatomic solution added against the pH of the resulting solution, then a gradual fall in pH is observed initially but after some addition, the pH of the solution become constant. The constant plateau of the graph indicates the formation of some polymeric species may take place in the solution phase, the excess of H ion added is being consumed in the formation of such species. The close observation of the graph also indicates the exact volume of the solution containing reactants required for such type of condensation process to take place. Therefore from the pH studies, it can easily be predicted that the exact pH and exact concentration of the reactants to be taken for the preparation of a particularl heteropoly complex. Common structures: Among the different structures known for POMs, the following four structures are more common. 1. Keggin structure-(1933) [XM12O40]-Four trimetallic M3O13 groups are arranged around a central tetrahedron(XO4).Each MO6 octahedra is sharing two edges with other MO6 and the four M3O13 groups are attached to one another by corner sharing. The total assembly contains 40 close packed oxygen and a tetrahedral pocket in its centre for the heteroatom. Keggin discovered that the structure of the compound H3[PW12O40].5H2O contained 12WO6 octahedra linked by the edge and the corner sharing with the heteroatom PO4 occupying tetrahedral hole in the centre. 2. a.AndersonThe Anderson structures (1937) consist of six coplanar MO6 octahedra arranged in the ring sharing edges around the central heteroatom in [XM6O24]. This leaves an octahedral pocket in the centre of the ring for the heteroatom(X). b.Anderson-Evans structure-The structure [TeMo6O24] proposed by Anderson went without experimental verification until 1948 when Evans 20-21 confirmed the structures by a single crystal X-ray determination of the position of the central heteroatom. This structure is now referred as Anderson Evans structure. 3. Linqvist structure (1952) [M6O19] –It consist of an octahedral arrangement of six MO6 octahedra. Each octahedral is sharing four edges with K.C.Dey et al /Int.J. ChemTech Res.2010,2(1) 370 neighbouring octahedral. This structure can be seen as fragment of a cubic closed packed metal oxide. 4. Well Dawson structure (X2M18O62) – A.F.Wells in 1945 suggested a detailed structure of [P2W18O62] .In 1952, Tsigdinos established the Well’s proposed structures for the molybdo complex. In 1953, Dawson 25 determined the position of the heteroatom by single X-ray crystal study.This structure is referred as Well Dawson structure which is closely related as Keggin structure. Prior to the 1959, all of the structural X-ray crystallography studies of heteropoly complexes determine only the position of the heavier atom. The X-ray crystal structure of K5[CoW12O40].20H2O determine in 1959, was the first to locate directly all the oxygen atoms on the heteropoly complexes. In 1962,Simmons reported the first heteropoly complexes containing two different elements as heteroatoms in the international conference on coordination chemistry[I.C.C.C]. The structure of this red complex was reported in 1966 by Baker.et.al . It was 11Tungstosilicate where one W atom of the Keggin structure [SiW12O40] had been replaced by Co.Thereafter Weakly, Mallick Tournes 30 ,Puscaus 31 Souchay 32 and many others reported preparations of large number of 11-Tungsto,17-Tungsto,11-Molybdo, 17-Molybdo complexes with various central heteroatom and various lower valent octahedral metal atom substituted for W,Mo in a Keggin and Well Dawson structure.In 1980-90 extensive works have been done with expanding applications of polyoxometalates. By 1995, the structure of approx 180 POMS have been reported. Applications: An increasing number of potential applications for polyoxometalates continues to attract significant attention. The applications of POMs are centered primarily on their redox properties, photochemical response, ionic charge, conductivity and ionic weights. The majority of the applications of POMs are found in the area of the catalysis. About 80-85% of the literature claims POMs for their catalytic activity. The remaining 1520% of the application includes coating, membranes or thin films. These include corrosion, resistant and in conductive and non conductive polymers membranes and POMs as surface modifiers of substrate. POMs are also used as pigments, toners, wood pulp bleaching agent, reagent for chemical and biochemical analysis and for nuclear waste processing and some miscellaneous applications. 1. Heteropoly acids and polyoxometalates as catalysts A series of eco friendly solid acid and their salts have been used as catalysts in many organic reactions particulary in nitration of phenols, acetylation of alcohols and phenols with acetic anhydride.The performance of different forms of heteropoly acids and POMs have been compared from their percentage yield. Mononitration of phenol with metal nitrites The mononitration of phenols using Fe(NO3)3.9H2O and Bi(NO3)5.H2O in dichloromethane at room temperature are promoted with catalytic amount of heteropoly acids and polyoxometalates.