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"From Scheele and Berzelius to Müller: polyoxometalates (POMs) revisited and the "missing link" between the bottom up and top down approaches" P. Gouzerh, M. Che; L’Actualité Chimique, 2006, 298, 9] . The structure of the related phosphotungstate anion was determined in 1934, and is generally called the Keggin structure for its discoverer ["The Structure and Formula of 12-Phosphotungstic Acid" J.F. Keggin. Proc. Roy. Soc., A, 144, 851, 75-100 (1934) doi|10.1098/rspa.1934.0035] . 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 ['Y.-F. Song, D.-L. Long, and L. Cronin, 'Non covalently connected frameworks with nanoscale channels assembled from a tethered polyoxometalate- pyrene hybrid', Angew. Chem. Int. Ed., 2007, 46, 3900-3904doi|10.1002/anie.200604734.and 'A novel 3D organic–inorganic hybrid based on sandwich-type cadmium hetereopolymolybdate: [Cd₄(H₂O)₂(2,2′-bpy)₂] Cd [Mo₆O₁₂(OH)₃(PO₄)₂(HPO4)₂] ₂ [Mo₂O₄(2,2′-bpy)₂] ₂·3H₂O" Hong-Xu Guo and Shi-Xiong Liu Inorganic Chemistry Communications 7, 11, (2004), 1217 doi:10.1016/j.inoche.2004.09.010 ] )
*new potential applications based on unusual magnetic ["Classical and Quantum Magnetism in Giant Keplerate Magnetic Molecules" Achim Müller, Marshall Luban, Robert Modler, Paul Kögerler, Maria Axenovich, Jürgen Schnack, Paul Canfield, Sergey Bud'ko, Neil Harrison. ChemPhysChem 2001, 2, 517.] and optical ["Field-dependent magnetic parameters in Ni₄Mo₁₂: Magnetostriction at the molecular level?" Jürgen Schnack, Mirko Brüger, Marshall Luban, Paul Kögerler, Emilia Morosan, Ronald Fuchs, Robert Modler, Hiroyuki Nojiri, Ram C. Rai, Jinbo Cao, Janice L. Musfeldt, Xing Wei. Phys. Rev. B 2006, 73, 094401.] 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.

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 ["Direct Bromination of Keggin Fragments To Give [PW₉O₂₈Br₆] ³⁻: A Polyoxotungstate with a Hexabrominated Face" R. John Errington,Richard L. Wingad, William Clegg,Mark R. J. Elsegood Angewandte Chemie 39, 21 ,3884 – 3886 doi|10.1002/1521-3773(20001103)39:21<3884::AID-ANIE3884>3.0.CO;2-M] 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 ["Functionalization of polyoxomolybdates: the example of nitrosyl derivatives" P. Gouzerh, Y. Jeannin, A. Proust, F. Robert and S. G. Roh Molecular Engineering 3, (1993) 79 doi|10.1007/BF00999625] and alkoxy"Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity" By Michael Thor Pope, Achim Müller Springer (1994) ISBN 0792324218 ] 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 ["A New Structural Type for Heteropoly Anions.The Crystal Structure of (NH₄)₂H₆(CeMo₁₂042)12H₂0" DD Dexter, JV Silverton - Journal of the American Chemical Society, 1968, 3589 ] ).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 (XO₄^2-)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 ["Manganous heteropolytungstates. Synthesis and heteroatom effects in Wells–Dawson-derived sandwich complexes " I.M. Mbomekalle, B. Keita, L. Nadjo, P. Berthet, W. A. Neiwert, C.L. Hill, M.D. Ritorto and T. M. Anderson, Dalton Trans., 2003, 2646 - 2650, doi|10.1039/b304255c] .

Some cage structures containing ions are known, e.g.. An example is the vanadate cage, V₁₈O ₄₂ containing a Cl^−.ion ["Supramolecular Inorganic Chemistry: Small Guests in Small and Large Hosts" A. Müller, H. Reuter, S. Dillinger. Angew. Chem. Int. Ed. Engl. 1995, 34, 2328.] . 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. Ti_l2O_l6(OPri)₁₆,the dodecatitanates ["Dodecatitanates: a new family of stable polyoxotitanates" V. W. Day, T. A. Eberspacher, W. G. Klemperer, and C. W. Park J. Am. Chem. Soc.; 1993; 115(18) pp 8469 - 8470; doi|10.1021/ja00071a075] and iron oxoalkoxometalate ["Synthesis and Structure of [Fe₁₃O₄F₂₄(OMe)₁₂] ⁵⁻: The First Open-Shell Keggin Ion" Avi Bino, Michael Ardon, Dongwhan Lee, Bernhard Spingler, and Stephen J. Lippard J. Am. Chem. Soc., 124 (17), 4578 -4579, 2002. doi|10.1021/ja025590a] . 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 ["Alternatives for lignocellulosic pulp delignification using polyoxometalates and oxygen: a review" A. R. Gaspar, J. A. F. Gamelas, D. V. Evtuguin and C P Neto Green Chem., 2007, 9, 717 - 730, doi|10.1039/b607824a] and a method of decontaminating water ["Polyoxometallate photocatalysis for decontaminating the aquatic environment from organic and inorganic pollutants" A. Hiskia, A.Troupis, S. Antonaraki, E. Gkika, P. Kormali, E. Papaconstantinou, International Journal of Environmental Analytical Chemistry, 86, Issue 3 & 4 (2006), 233, doi|10.1080/03067310500247520] . Some structures containing transition metal atoms with unpaired electrons have unusual magnetic properties ["Polyoxovanadates: High-Nuclearity Spin Clusters with Interesting Host-Guest Systems and Different Electron Populations. Synthesis, Spin Organization, Magnetochemistry, and Spectroscopic Studies" A Müller, R Sessoli,E Krickemeyer, H Bögge, J Meyer, D Gatteschi, L Pardi, J Westphal, K Hovemeier, R Rohlfing, J Döring, F Hellweg, C Beugholt and M Schmidtmann Inorg. Chem., 36 (23), 5239 -5250, 1997. ] and are being investigated as nano computer storage devices (see qubits). ["Spin qubits with electrically gated polyoxometalate molecule" J. Lehmann, A. Gaita-Ariño, E. Coronado, D. Loss Nature Nanotechnology 2, 312 - 317 (2007) doi|10.1038/nnano.2007.110] . Some compounds exhibit luminescence ["Regular Two-Dimensional Molecular Array of Photoluminescent Anderson-type Polyoxometalate Constructed by Langmuir-Blodgett Technique" Takeru Ito, Hisashi Yashiro, Toshihiro Yamase, Langmuir, 22 (6), 2806 -2810, (2006) doi|10.1021/la052972w S0743-7463(05)02972-0] . There are many reported potential medicinal applications e.g. anti tumoral and anti-viral ["Polyoxometalates in Medicine" Jeffrey T. Rhule, Craig L. Hill, and Deborah A. Judd Chem. Rev., 98 (1), 327 -358, 1998.] There have been reports on the role of weak or non bonding interactions on the crystal engineering of hybrid polyoxometalates. ["Role of H-Bonded Interactions in the Crystal Packing of Phenylenediammonium Phosphomolybdates Shailesh Upreti and Arunachalam Ramanan, Crystal Growth & Design, 2006, 6(9), 2066-2071 doi|10.1021/cg0601610] " ["Structure-Directing Role of Hydrogen-Bonded Dimers of Phenylenediammonium Cations: Supramolecular Assemblies of Octamolybdate-Based Organic-Inorganic Hybrids, Shailesh Upreti and Arunachalam Ramanan, Crystal Growth & Design, 2005, 5(5), 1837 – 1843 doi|10.1021/cg050100m] ".
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"From Scheele and Berzelius to Müller: polyoxometalates (POMs) revisited and the "missing link" between the bottom up and top down approaches" P. Gouzerh, M. Che; L’Actualité Chimique, 2006, 298, 9] . 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. [ "Direct Imaging of the Structure, Relaxation, and Sterically Constrained Motion of Encapsulated Tungsten Polyoxometalate Lindqvist Ions within Carbon Nanotubes", Jeremy Sloan, Gemma Matthewman, Clare Dyer-Smith, A-Young Sung, Zheng Liu, Kazu Suenaga, Angus I. Kirkland, and Emmanuel Flahaut, ACS Nano, 2(5), 966–976, 2008. doi|10.1021/nn7002508] .

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 [http://www.rsc.org/ej/CS/2007/b502666k.pdf]
*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