Isopolyoxometalates

M.T. Pope, Heteropoly and isopolyoxometallates inorganic cooncepts, Vol. 8, Springer, Berlin, 1983. 

The versatility and accessibility of polyoxometalates (POMs) have led to the various applications in the fields of structural chemistry, analytical chemistry, surface science, medicine, electrochemistry, photochemistry, and catalysis. Especially, POMs have received much attention in the area of oxidation and acid catalysis [25-31]. Several categories of POMs are formed by proper selection of the starting components and by the adjustment of pH and temperature. Typical examples are shown in Figure 13.1 (i) isopolyoxometalates of the general formula, MxO> 1, produced by condensation... 

Isopolyoxometalates and heteropolyoxometalates are formulated as [MmOy]p and [X MmOy] (xc to), respectively, where M is the addenda atom and X is the heteroatom. The most common addenda atoms are the d°-early-transition-metal cations such as W6 +, Mo6+ and Vs+. The heteroatoms can be p-, d- or f-block elements such as P5 +, As5 +, Si4 +, Ge4 + and B3 +. Among a wide variety of heteropolyoxometalates, the Keggin structures are the most stable and more easily available. Keggin anions, typically represented by the formula [X + M O ] 8- , contain one central heteroatom and twelve addenda atoms (four M3013 triads). 

Isopolyoxometalates and heteropolyoxometalates such as [W6019]2-, [W10O32]4-and [PWi2O40]3- could act as efficient, stable, selective photooxidation catalysts... 

By virtue of their solubility in aprotic organic solvents and their availability as pure crystalline materials, tetrabutylammonium isopolyoxometalates have found widespread application as starting materials for the synthesis of... 

The synthesis of alkylammonium isopolyoxometalates was pioneered by Jahr, Fuchs, and Oberhauser who first prepared salts of [MoeOjg] ", [WgOig] ", and by controlled hydrolysis of molybdic and... 

The heteropoly- and isopolyoxometalates of early transition metals have been known for more than a century and have been studied extensively. Their structures are characterized by networks of MOe octahedra in which the early transition metals M (typically M = V, Nb, Mo, W) appear in their highest oxidation states in which they have a a configuration. A characteristic of many, but not all, of such structures is their reducibility to highly colored mixed oxidation state derivatives often given the trivial names of molybdenum or tungsten blues. The reducibility of early transition metal polyoxometalates requires the presence of M06 octahedra in which only one of the six oxygen atoms is a terminal oxygen atom. Such an MO ... 

Additional features of polyoxometalates impact some of the six limitations. First, polyoxometalates are oxidatively resistant as a great majority of them are composed of d° transition metal ions (most commonly W(VI), Mo(V), and/or V(V)) and oxide ions. Second, the unusually high stability of polyoxometalates coupled with the tunability of their catalytically relevant properties defines a considerable ability to avoid problems associated with intermediate oxidation states, p-oxo dimers, and product inhibition. Illustrations of two representative polyoxometalates, Wio0324 , an isopolyoxometalate (or isopolyanion) and (TM)XWii039 , a transition metal (TM) substituted heteropolyoxometalate (or heteropolyanion) in polyhedral notation are given in Figure 1. 

The synthesis of alkylammonium isopolyoxometalates was pioneered by Jahr, Fuchs, and Oberhauser who flrst prepared salts of [Mo60i9], [WjO,] , and [W, 032] " by controlled hydrolysis of molybdic and tungstic acid esters. More convenient preparations of these and other salts that utilize readily available starting materials were developed later. The preparations reported here are modifications of published procedures. They have been selected on the basis of convenience and ease of reproducibility. Ail of the anion structures have been determined by X-ray crystallography they are shown below for [Mo207] (a), [Mo60i9] and [W 0,9] "... 

M. T. Pope, Hetewpoly and Isopolyoxometalates, Springer-Verlag, New York, 1993. 

Two major subsets of polyoxometalates are the isopolyoxometalates (or isopolyanions) which contain only transition metal and oxide ions, and the heteropolyoxometalates (or heteropolyanions) which contain one or more "heteroatoms" in addition to the transition metal and oxide ions. The heteroatom in polyoxometalates can be one of many elements in the p or d blocks. In this review we have highlighted the photochemistry of two polyoxometalates, dodecatungstophosphate, a representative heteropolyanion of the common Keggin structure, and decatungstate, a representative isopolyanion, whose structures are given in Figure 1. 

Duval, P. B., Bums, C. J., Clark, D. L., et al. 2001. Synthesis and structural characterization of the first uranium cluster containing an isopolyoxometalate core. Angew. Chem. Int. Ed. Engl. 40 3357-3361. 

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