Keggin structure

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Keggin structure
James F. Keggin, the discoverer of the Keggin Structure.
Keggin structure

The Keggin structure is the best known structural form for

catalysts
.

History

The first α-Keggin anion,

coordination compounds ideas of Copaux, attempted to explain the structure of silicotungstic acid. He assumed a central group, [SiO4]4− ion, enclosed by four [RW2O6]+, where R is a unipositive ion. The [RW2O6]+ are linked to the central group by primary valences. Two more R2W2O7 groups were linked to the central group by secondary valences. This proposal accounted for the characteristics of most poly-acids, but not all.[citation needed
]

In 1928,

dehydrated acids.[citation needed
]

X-ray diffraction experimentally determined the structure of α-Keggin anions in 1934. The Keggin structure accounts for both the hydrated and dehydrated α-Keggin anions without the need for significant structural change. The Keggin structure is the widely accepted structure for the α-Keggin anions.[2]

Structure and physical properties

Keggin structure of phosphotungstate anion = Cage of the Keggin structure +Central tetrahedron

The structure has full tetrahedral symmetry and is composed of one heteroatom surrounded by four oxygen atoms to form a tetrahedron. The heteroatom is located centrally and caged by 12 octahedral MO6 units linked to one another by the neighboring oxygen atoms. There are a total of 24 bridging oxygen atoms that link the 12 addenda atoms. The metal centres in the 12 octahedra are arranged on a sphere almost equidistant from each other, in four M3O13 units, giving the complete structure an overall tetrahedral symmetry. The bond length between atoms varies depending on the heteroatom (X) and the addenda atoms (M). For the 12–phosphotungstic acid, Keggin determined the bond length between the heteroatom and each the four central oxygen atoms to be 1.5 Å. The bond length form the central oxygen to the addenda atoms is 2.43 Å. The bond length between the addenda atoms and each of the bridging oxygen is 1.9 Å. The remaining 12 oxygen atoms that are each double bonded to an addenda atom have a bond length of 1.70 Å. The octahedra are therefore distorted.[3][4] This structure allows the molecule to hydrate and dehydrate without significant structural changes and the molecule is thermally stable in the solid state for use in vapor phase reactions at high temperatures (400−500 °C).[5]

Isomerism

Including the original Keggin structure there are 5 isomers, designated by the prefixes α-, β-, γ-, δ- and ε-. The original Keggin structure is designated α. These isomers are sometimes termed Baker, Baker–Figgis or rotational isomers,[6] These involve different rotational orientations of the Mo3O13 units, which lowers the symmetry of the overall structure.

Lacunary Keggin structures

The term lacunary is applied to ions which have a fragment missing, sometimes called defect structures. Examples are the [XM11O39]n− and [XM9O34]n− formed by the removal from the Keggin structure of sufficient Mo and O atoms to eliminate 1 or 3 adjacent MO6 octahedra. The Dawson structure is made up of two Keggin lacunary fragments with 3 missing octahedra.

Group 13 cations with the Keggin structure

The cluster cation [Al13O4(OH)24(H2O)12]7+ has the Keggin structure with a tetrahedral Al atom in the centre of the cluster coordinated to 4 oxygen atoms. The formula can be expressed as [AlO4Al12(OH)24(H2O)12]7+.[7] This ion is generally called the Al13 ion. A Ga13 analogue is known[8] an unusual ionic compound with an Al13 cation and a Keggin polyoxoanion has been characterised.[9]

The iron Keggin ion

Due to the similar aqueous chemistries of aluminium and iron, it has been long thought that an analogous iron polycation should be isolatable from water. Moreover, in 2007, the structure of ferrihydrite was determined and shown to be built of iron Keggin ions.[10] This further captured scientists' imagination and drive to isolate the iron Keggin ion. In 2015, the iron Keggin ion was isolated from water, but as a polyanion with a −17 charge; and protecting chemistry was required.[11] Iron-bound water is very acidic; so it is difficult to capture the intermediate Keggin ion form without bulky and nonprotic ligands instead of the water that is found in the aluminum Keggin ion. However, more important in this synthesis was the bismuth (Bi3+) counterions that provided high positive charge to stabilize the high negative charge of the heptadecavalent polyanion.[citation needed]

Chemical properties

The stability of the Keggin structure allows the metals in the anion to be readily

acidity of the solution and the charge on the α-Keggin anion, it can be reversibly reduced in one- or multiple-electron steps.[12] For example, the silicotungstate anion can be reduced a −20 state.[13] Some anions such as silicotungstic acid are strong enough as an acid as sulfuric acid and can be used in its place as an acid catalyst.[citation needed
]

Preparation

In general α-Keggin anions are synthesized in acidic solutions. For example, 12-phosphotungstic acid is formed by condensing phosphate ion with tungstate ions. The heteropolyacid that is formed has the Keggin structure.[5]

PO3−4 + 12 WO2−4 + 27 H+ → H3PW12O40 + 12 H2O

Uses

α-Keggin anions have been used as catalyst in the following reactions:

THF.[14][15]

Suppliers

12-

Aldrich Chemicals, Fisher Chemicals, Alfa Aesar, VWR Chemical, American Elements, and others.[citation needed
]

References

  1. ISBN 978-0-13-039913-7
    .
  2. ^ Bailar, J. C. Jr. (1956). The Chemistry of the Coordination Compounds. Reinhold Publishing. pp. 472–482.
  3. doi:10.1098/rspa.1934.0035
    .
  4. ISSN 0567-7408
    .
  5. ^
    ISBN 978-0-12-007841-7
    .
  6. doi:10.1021/ja00715a047
    .
  7. ISBN 978-0-08-037941-8
    .
  8. doi:10.1039/DT9900000413
    .
  9. PMID 12817975
    .
  10. S2CID 36998522
    .
  11. S2CID 206634621
    .
  12. ^ Okuhara, T.; Mizuno, N.; Misono, M. (1996). Catalytic Chemistry of Heteropoly Compounds. Advances in Cayalysis. Vol. 41. Academic Press. pp. 191–193.[ISBN missing]
  13. ^ Pope, M. T. (1983). Heteropoly and Isopolyoxometalates. Inorganic Chemistry Concepts. Vol. 8. Heidelberg: Springer-Verlag. pp. 101–107.[ISBN missing]
  14. ^ Pope, M. T.; Müller, A. (1994). Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity. Netherlands: Kluwer Academic Publications. pp. 262–265.[ISBN missing]
  15. ISSN 0897-4756
    .
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