Heteropolymetalate

The heteropolymetalates are a subset of the polyoxometalates, which consist of three or more transition metal oxyanions linked together by shared oxygen atoms to form a closed 3-dimensional molecular framework. In contrast to isopolymetalates, which contain only one kind of metal atom, the heteropolymetalates contain differing main group oxyanions. 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]

Heteropolymetalates:
K₅[IMo₆O₂₄]·nH₂O
Ag₇[PV₁₂O₃₆]·nH₂O
(NH₄)₄[NiMo₆O₂₄H₆]·5H₂O
K₃[CrMo₆O₂₄H₆]·nH₂O
(NH₄)₈[CeMo₁₂O₄₂]·8H₂O

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, [HP2Mo5O23]4−	Keggin structure, [XM12O40]n−	Dawson structure, [X2M18O62]n−
Anderson structure, [XM6O24]n−	Allman–Waugh structure, [XM9O32]n−	Weakley–Yamase structure, [XM10O36]n−	Dexter–Silverton structure, [XM12O42]n−

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]

• Silicotungstic acid, H₄SiW₁₂O₄₀·nH₂O
• Phosphomolybdic acid, H₃Mo₁₂PO₄₀·nH₂O
• Phosphotungstic acid, H₃W₁₂PO₄₀·nH₂O

Isomerism

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

The five isomers of the Keggin structure

α-[XM12O40]n−	β-[XM12O40]n−	γ-[XM12O40]n−	δ-[XM12O40]n−	ε-[XM12O40]n−

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 toward 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, [X₂M₁₈O₆₂]ⁿ⁻

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]

• Homogeneous acid catalysis
  • hydrolysis of propene to give propan-2-ol by H₃PMo₁₂O₄₀ and H₃PW₁₂O₄₀
  • Prins reaction by H₃PW₁₂O₄₀
  • polymerisation of tetrahydrofuran by H₃PW₁₂O₄₀
• Heterogeneous acid catalysis
  • dehydration of propan-2-ol to propene and methanol to hydrocarbons by H₃PW₁₂O₄₀
  • reformation of hexane to 2-methylpentane (isohexane) by H₃PW₁₂O₄₀ on SiO₂
• Homogeneous oxidation
  • cyclohexene + H₂O₂ to adipic acid by the mixed addenda H₃PMo₆V₆O₄₀
  • ketone by O₂ to acid and aldehyde by mixed addenda H₅PMo₁₀V₂O₄₀

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

• Phosphomolybdic acid
• Phosphotungstic acid
• Silicotungstic acid