Polyoxometalate

Polyoxometalate

The term polyoxometalate (abbreviated POM) is applied to an extremely large group of generally anionic clusters with frameworks built from transition metal oxo anions linked by shared oxide ions. The term is usually applied to clusters of 3 or more transition metal atoms from group 5 and group 6 in their high oxidation states, (d0 and d1 configuration), e.g. (V(V), Nb(V), Ta(V)), (Mo(VI) and W(VI)).
Historically, the first example, the ammonium phosphomolybdate containing the [PMo12O40]3− ion, was discovered in 1826. The structure of the related phosphotungstate anion was determined in 1934, and is generally called the Keggin structure for its discoverer. In the period following this other fundamental structures, e.g. the Wells-Dawson ion, were discovered, and their chemistry and applications as catalysts were determined. Whilst this work still continues new areas of interest have emerged, for example:

  • the discovery of large, highly symmetric polyoxomolybdates such as the wheel-shaped molybdenum blue anions and spherical keplerates.
  • numerous hybrid organic/inorganic materials that contain POM cores (see as an example )
  • new potential applications based on unusual magnetic and optical properties of some POM's
  • potential medicinal applications, in particular anti-tumoral and anti-viral.

Fundamental polyoxometalate structures

Some structural types are found in many different compounds. The first known example of this was the Keggin ion whose structure was found to be common to both molybdates and tungstates with different central hetero atoms. Examples of some fundamental polyoxometalate structures are shown below. The Lindqvist ion is an iso-polyoxometalate, the other three are hetero-polyoxometalates. The Keggin and Dawson structures have tetrahedrally coordinated hetero-atoms e.g. P or Si, Anderson structure has an octahedral central atom e.g. Al.

Lindqvist structure M6O19n− Keggin structure, XM12O40n− Dawson structure, X2M18O62n− Anderson structure, XM6O24n−

Build up of polyoxometalate structures

The metal atoms that make up the framework (termed "addenda atoms") are typically Mo, W, and V. When more than one element is present the cluster is called a "mixed addenda" cluster.
The ligands coordinated to metal atoms that together form the bridged framework are usually oxide ions, but other elements, such as S and Br have been substituted for some of the oxide ions. (Note that a sulfur substituted POM is often termed a polyoxothiometalates.) Another development is the use of other ligands e.g. nitrosyl and alkoxy to replace oxide ions.
The typical framework building blocks are polyhedral units, with 4, 5, 6 or 7 coordinate metal centres. These units usually share edges and/or vertices, (face sharing is uncommon, but not unknown, for example the ion CeMo12O428− has face shared octahedra with Mo atoms at the vertices of an icosahedron). The most common unit for polymolybdates is the octahedral MoO6 unit, which is a distorted octahedron where the Mo atom moves off centre to give one short Mo-O bond. In some polymolybdates there are pentagonal bipyramidal units, and these are key building blocks in the molybdenum blues.
Hetero atoms are present in many polyoxometalates. Many different elements can act as hetero-atoms. Examples of various coordination numbers around the hetero-atom are known:

  • 4 coordinate (tetrahedral) in Keggin, Dawson and Lindqvist structures (e.g. PO4, SiO4, AsO4)
  • 6 coordinate (octahedral) in Anderson structure (e.g. Al(OH)6, TeO6
  • 8 coordinate (square antiprism) in ((CeO8)W10O28)8−
  • 12 coordinate (icosahedral) in (UO12)Mo12O30 8−

Often the hetero-atom is centrally located in the anion (e.g. Keggin structure) or in a structure fragment e.g. the 2 phosphorus atoms in the Dawson ion are central to the two symmetric fragments.
There are similarities to clathrate structures. The Keggin ion can be formulated as PO4 2−@ M12O36 and the Dawson as (XO42-)2@M18O54.

Structural isomerism is common. For example the Keggin structure has 5 isomers which can be considered to contain one or more of the four M3O13 units being rotated through 60°.

Many compounds share the same framework architectures or frameworks derived from a larger framework with one or more addenda atoms and attendant oxide ions removed, to give defect structure, usually called a "lacunary" structure. An example of a compound with a Dawson lacunary structure is As2W15O56.

Some cage structures containing ions are known, e.g.. An example is the vanadate cage, V18O 42 containing a Cl−.ion . This has 5 coordinate, square pyramidal vanadium units linked together. H4V18O42 cage containing Cl

Polyoxometalates outside Group 5 and 6

Polyoxoalkoxometalates of titanium and iron are known, e.g. Til2Ol6(OPri)16,the dodecatitanates and iron oxoalkoxometalate . It is a moot point as to whether these should be categorised as POM's .

Properties and Applications

The huge range of size, structure and elemental composition of known polyoxometalates leads to a wide range of different properties.The Keggin ions are well known to be thermally stable, to be reversibly reduced (by accepting electrons) and are used as catalysts for a range of organic reactions. Some potential "green" applications have been reported e.g. a non-chlorine based, wood pulp bleaching process and a method of decontaminating water. Some structures containing transition metal atoms with unpaired electrons have unusual magnetic properties and are being investigated as nano computer storage devices (see qubits).. Some compounds exhibit luminescence . There are many reported potential medicinal applications e.g. anti tumoral and anti-viral There have been reports on the role of weak or non bonding interactions on the crystal engineering of hybrid polyoxometalates. .
Spherical nanoporous polyoxomolybdate based capsules of different types containing more than 100 metal atoms reported by Achim Müller and his group have versatile unique properties regarding their assembly to vesicles and the chemistry which can be done inside the pores and cavities. A discrete polyoxometalate Lindqvist ion of the form W6O192− was successfully imaged recently for the first time within the capillary of a carbon nanotube following steric locking of the anion with the tubule. In situ relaxation of the anion in its equatorial plain was demonstrated. .

Footnotes

For a general good overview read

  • D. L. Long, E. Burkholder, and L. Cronin, 'Polyoxometalate clusters, nanostructures and materials: From self assembly to designer materials and devices', Chem. Soc. Rev., 2007, 36, 105-121.DOI: 10.1039/b502666k
  • M.T. Pope "Heteropoly and Isopoly Oxometalates", Springer Verlag, New York, (1983).
  • M.T. Pope, A. Müller, Polyoxometalate Chemistry: An Old Field with New Dimensions in Several Disciplines, Angew. Chem. Int. Ed. Engl. 1991, 30, 34.
  • Special volume on "Polyoxometalates", Chem.Rev.,1998, 98, 1

References

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