Polyoxometalate

The phosphotungstate anion, an example of a polyoxometalate

In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are usually group 6 (Mo, W) or less commonly group 5 (V, Nb, Ta) transition metals in their high oxidation states. They are usually colorless or orange, diamagnetic anions. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and oxide, and heteropolymetalates, composed of one metal, oxide, and a main group oxyanion (phosphate, silicate, etc.). Many exceptions to these general statements exist.

Formation

The oxides of d⁰ metals such as V₂O₅, MoO₃, WO₃ dissolve at high pH to give orthometalates, VO³⁻
₄, MoO²⁻
₄, WO²⁻
₄. For Nb₂O₅ and Ta₂O₅, the nature of the dissolved species is less clear. As the pH is lowered, these orthometalates protonate to give oxide–hydroxide compounds such as W(OH)O⁻
₃ and V(OH)O²⁻
₃. These species condense via the process called olation. Condensation proceeds via loss of water and the formation of M–O–M linkages. An abbreviated condensation sequence illustrated with vanadates is:

4 VO³⁻
₄ + 8 H⁺ → V
₄O⁴⁻
₁₂ + 4 H₂O

​2 ¹⁄₂ V
₄O⁴⁻
₁₂ + 6 H⁺ → V
₁₀O
₂₆(OH)⁴⁻
₂ + 2 H₂O

When such acidifications are conducted in the presence of phosphate or silicate, then one obtains a heteropolymetalate. For example, the phosphotungstate anion PW
₁₂O³⁻
₄₀ consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.

History

The first example of a polyoxometalate compound was ammonium phosphomolybdate, containing the PMo
₁₂O³⁻
₄₀ anion, discovered in 1826. This anion has the same structure as the phosphotungstate anion, whose structure was reported in 1934. This structure is called the Keggin structure after its discoverer. Following this discovery, other fundamental structures such as the Wells–Dawson ion were found, and their chemistry and applications as catalysts were determined.

Structure

Certain structural motifs recur. The Keggin ion for example is common to both molybdates and tungstates with different central heteroatoms. Examples of some fundamental polyoxometalate structures are shown below. The Lindqvist ion and the rest of the first raw structures in the following figure are iso-polyoxometalates (isopolyanions in this case), since only one type of transition metal atoms is involved in their composition. The other structures are of the hetero-polyoxometalate type (heteropolyanions) since they involve more than one type of metal atom. The Keggin and Dawson structures have tetrahedrally-coordinated heteroatoms, such as P or Si, and the Anderson structure has an octahedral central atom, such as aluminium.

Lindqvist hexamolybdate, Mo 6O2− 19	Decavanadate, V 10O6− 28	Paratungstate B, H 2W 12O10− 42	Mo36-polymolybdate, Mo 36O 112(H 2O)8− 16
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

Polyoxomolybdates include the wheel-shaped molybdenum blue anions and spherical keplerates. Numerous hybrid organic–inorganic materials that contain POM cores, new potential applications based on unusual magnetic and optical properties of some POMs, and potential medical applications such as antitumor and antiviral uses.

Framework

The typical framework building blocks are polyhedral units, with 4-, 5-, 6- or 7-coordinate metal centres. These units share edges and/or vertices, or, less commonly, faces (such as in the ion CeMo
₁₂O⁸⁻
₄₂, which has face-shared octahedra with Mo atoms at the vertices of an icosahedron).

The most common unit for polymolybdates is the octahedral {MoO₆} unit, often distorted by the Mo atom being off-centre to give one shorter Mo–O bond. Some polymolybdates contain pentagonal bipyramidal units; these are the key building blocks in the molybdenum blues.

Heteroatoms

H₄V₁₈O₄₂ cage containing a Cl⁻ ion.

Heteroatoms are present in many polyoxometalates. Many different elements can serve as heteroatoms, with various coordination numbers:

• 4-coordinate (tetrahedral) in the Keggin, Dawson, and Lindqvist structures (e.g., PO³⁻
  ₄, SiO⁴⁻
  ₄, AsO³⁻
  ₄)
• 6-coordinate (octahedral) in the Anderson structure (e.g., Al(OH)³⁻
  ₆, TeO⁶⁻
  ₆)
• 8-coordinate (square antiprismatic) in (CeO
  ₈)W
  ₁₀O⁸⁻
  ₂₈
• 12-coordinate (icosahedral) in (UO
  ₁₂)Mo
  ₁₂O⁸⁻
  ₃₀

The heteroatom may be located in the centre of the anion, as in the Keggin structure, or in the center of a structural fragment, such as the two phosphorus atoms in the Dawson ion, which are central to its two symmetric fragments.

Polyoxometalates bear similarities to clathrate structures. The Keggin ion, for example, could be formulated as PO³⁻
₄@M
₁₂O
₃₆, and the Dawson as (XO²⁻
₄)
₂@M
₁₈O
₅₄. The @ notation denotes the physical enclosure of the left-hand side in the right-hand side. Unlike clathrates however, the guest anions cannot be reversibly removed.

Some cage structures that contain other ions are known. For example, the vanadate cage V₁₈O₄₂ can enclose a Cl⁻ ion. This structure has 5-coordinate square-pyramidal vanadium units linked together.

Isomerism

Isomerism is observed in some POMs. For example, 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 defective structure called a lacunary structure. An example of a compound with a Dawson lacunary structure is As₂W₁₅O₅₆. In 2014, vanadate species with similar, selective metal-binding properties were reported.

Polyoxotantalates, niobates, and vanadates

The properties of polyniobates and polytantatalates are similar, but substantially different from the polyoxovanadates. In fact, polyvanadates are more similar to the oxomolybdates and tungstates.

Molecular structures of polyoxometalates.

The similarities between the polyniobates and polytantatalates arise primarily from the equivalence in the charge of their stable 5+ ion and their size (64 pm) due to the lanthanide contraction. The most common members are M
₆O⁸⁻
₁₉ (M = Nb, Ta), which adopt the Lindqvist structure. These octaanions form in strongly basic conditions from alkali melts of the extended metal oxides (M₂O₅), or in the case of Nb even from mixtures of niobic acid and alkali metal hydroxides in aqueous solution. The hexatantalate can also be prepared by condensation of peroxotantalate Ta(O
₂)³⁻
₄ in alkaline media. These polyoxometalates display an anomalous aqueous solubility trend of their alkali metal salts inasmuch as their Cs⁺ and Rb⁺ salts are more soluble than their Na⁺ and Li⁺ salts. The opposite trend is observed in group 6 POMs. The decametalates with the formula M
₁₀O⁶⁻
₂₈ (M = Nb, Ta) are isostructural with decavanadate and are formed exclusively by edge-sharing {MO₆} octahedra whereas the structure of decatungstate W
₁₀O⁴⁻
₃₂ comprises edge-sharing and corner-sharing tungstate octahedra.

Oxoalkoxometalates

Oxoalkoxometalates are clusters that contain both oxide and alkoxide ligands. Typically they lack terminal oxo ligands. Examples include the dodecatitanate Ti₁₂O₁₆(OPri)₁₆ (where OPri stands for an alkoxy group), the iron oxoalkoxometalates and iron Keggin ions.

Sulfido, imido, and other O-replaced oxometalates

The terminal oxide centers of polyoxometalate framework can in certain cases be replaced with other ligands, such as S²⁻, Br⁻, and NR²⁻. Sulfur-substituted POMs are called polyoxothiometalates. Other ligands replacing the oxide ions have also been demonstrated, such as nitrosyl and alkoxy groups.

Applications

POMs are employed as commercial catalysts for oxidation of organic compounds.

Potential and emerging applications

The range of size, structure, and elemental composition of polyoxometalates leads to a wide range of properties and a corresponding wide range of potential applications. Some of the applications include the following:

• "Green" oxidation catalysts as alternatives to chlorine-based wood pulp bleaching processes, a method of decontaminating water, and a method to catalytically produce formic acid from biomass (OxFA process). Polyoxometalates have been shown to catalyse water splitting.
• non-volatile (permanent) storage components, also known as flash memory devices. Some POMs exhibit unusual magnetic properties and are being investigated as possible nanocomputer storage devices (see qubits).
• The catalytic epoxidation of olefins by using modified silver polyoxometalate catalyst (Ag/Ag-POM) and gold catalyst supported on the barium salt of the POM (2% Au/BaPOM) is very important in the chemical industry since epoxides are versatile and important intermediates in the synthesis of many fine chemicals and pharmaceuticals.

Biomedical research

Potential antitumor and antiviral drugs. The Anderson-type polyoxomolybdates and heptamolybdates exhibit activity for suppressing the growth of some tumors. In the case of (NH₃Pr)₆[Mo₇O₂₄], activity appears related to its redox properties.

POM with a Wells-Dawson structure can efficiently inhibit amyloid β (Aβ) aggregation in a therapeutic strategy for Alzheimer's Disease.