Heteropolymetalate

In chemistry, a heteropolymetalate is a subset of the polyoxometalates. They consist of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional molecular framework. In contrast to isopolymetalates, the heteropolymetallates contain main group oxy anions. The metal atoms are usually group 6 (Mo, W) or less commonly group 5 (V, Nb, Ta) transition metals in their highest oxidation states. They are usually colorless to orange, diamagnetic anions. For most heteropolymetalates the W, Mo, or V, is complemented by main group oxyanions phosphate and silicate. Many exceptions to these general statements exist, and the class of compounds includes hundreds of examples. [1][2]

The phosphotungstate anion, an example of a heteropolymetalate.

Structure

Certain structural motifs recur. The Keggin ion for example is common to both molybdates and tungstates with diverse central heteroatoms. The Keggin and Dawson structures have tetrahedrally-coordinated heteroatoms, such as P or Si, and the Anderson structure[3] has an octahedral central atom, such as aluminium.

Strandberg structure, HP
2Mo
5O4−
23		 Keggin structure, XM
12On−
40		 Dawson structure, X
2M
18On−
62
Anderson structure, XM
6On−
24		 Allman–Waugh structure, XM
9On−
32		 Weakley–Yamase structure, XM
10On−
36		 Dexter–Silverton structure, XM
12On−
42

Heteropolyacids

Generally, the heteropolymetalates are more thermally robust than homopolymetalates. This trend reflects the stabilizing influence of the tetrahedral oxyanion that "glues" together the transition metal oxo framework. One reflection of their ruggedness, heteropolymetalates can be isolated in their acid form, whereas homopolymetalates typically cannot. Examples include:[4][5]

Isomerism

The Keggin structure has 5 isomers, which are obtained by (conceptually) rotating one or more of the four {M3O13} units through 60°.

The five isomers of the Keggin structure
α-XM
12On−
40		 β-XM
12On−
40		 γ-XM
12On−
40		 δ-XM
12On−
40		 ε-XM
12On−
40

Lacunary structures

The structure of some POMs are derived from a larger POM's structure by removing one or more addenda atoms and their attendant oxide ions, giving a defect structure called a lacunary structure. An example of a compound with a Dawson lacunary structure is As2W15O56.[6] In 2014, vanadate species with similar, selective metal-binding properties were reported.[7]

Uses
A polyoxovanadate (POV) of the species V14Sb8O42 in the alpha form. The Isomers differ by the position of the half-rings towards each other. This POVs can be linked e.g. by octahedrally coordinated nickel(II).

This type of acid is a common re-usable acid catalyst in chemical reactions.[8]

Dawson ion Dawson structure, X2M18O62n−

The heteropolyacids are widely used as homogeneous and heterogeneous catalysts,[9] particularly those based on the Keggin structure as they can possess qualities such as good thermal stability, high acidity and high oxidising ability. Some examples of catalysis are:[10]

Heteropolyacids have long been used in analysis and histology and are a component of many reagents e.g. the Folin-Ciocalteu reagent, folins phenol reagent used in the Lowry protein assay and EPTA, ethanolic phosphotungstic acid.

See also

Citations
  1. Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 978-0-7506-3365-9.
  2. Pope, M. T. (1983). Heteropoly and Isopoly Oxometalates. New York: Springer Verlag.
  3. Blazevic, Amir; Rompel, Annette (January 2016). "The Anderson–Evans polyoxometalate: From inorganic building blocks via hybrid organic–inorganic structures to tomorrows "Bio-POM"". Coordination Chemistry Reviews. 307: 42–64. doi:10.1016/j.ccr.2015.07.001.
  4. Dias, J. A.; Dias, S. C. L.; Caliman, E. (2014). "Keggin Structure Polyoxoometalates". Inorganic Syntheses. 36: 210-217. doi:10.1002/9781118744994.ch39.
  5. Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY.
  6. Mbombekalle, I. M.; Keita, B.; Nadjo, L.; Berthet, P.; Neiwert, W. A.; Hill, C. L.; Ritorto, M. D.; Anderson, T. M. (2003). "Manganous heteropolytungstates. Synthesis and heteroatom effects in Wells–Dawson-derived sandwich complexes". Dalton Trans. 2003 (13): 2646–2650. doi:10.1039/b304255c.
  7. Kastner, K.; Margraf, J. T.; Clark, T.; Streb, C. (2014). "A Molecular Placeholder Strategy To Access a Family of Transition-Metal-Functionalized Vanadium Oxide Clusters". Chem. Eur. J. 20 (38): 12269–12273. doi:10.1002/chem.201403592. PMID 25082170.
  8. Mizuno, Noritaka; Misono, Makoto (1998). "Heterogeneous Catalysis". Chemical Reviews. 98: 199–217. doi:10.1021/cr960401q.
  9. Kozhevnikov, I. V. (1998). "Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions". Chemical Reviews. 98 (1): 171–198. doi:10.1021/cr960400y. PMID 11851502.
  10. "Oxide catalysts in solid state chemistry". T Okuhara, M Misono. Encyclopedia of Inorganic Chemistry. Editor R Bruce King (1994). John Wiley and Sons. ISBN 0-471-93620-0

References
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