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.

The phosphotungstate anion, an example of a polyoxometalate

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:[1]

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.

The assembly of polyoxometalates upon acidification of solutions is an example for covalent self-assembly. This process produces homogeneous solutions of highly organized structures. Under a specific set of conditions only one polyoxometalate structure, or a small subset thereof will form. Evidence shows that this occurs via a dense-phase type mechanism whereupon small oxometalate ions first assemble non-covalently to form supramolecular structures that may then condense to form a covalently bound polyoxometalate.[2]

History
Dr. James F. Keggin, the discoverer of the Keggin Structure.

The first example of a polyoxometalate compound was ammonium phosphomolybdate, containing the PMo
12O3−
40 anion, discovered in 1826.[3] 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.[4] Following this discovery, other fundamental structures such as the Wells–Dawson ion were found, and their chemistry and applications as catalysts were determined.

Ramazzoite, the first example of a mineral with a polyoxometalate cation, was discovered in 2016 in Mt. Ramazzo Mine, Liguria, Italy. This polyoxometalate has not been reported in a synthetic compound.[5]

Structure
The cluster (NH4)(25 ± 5)[Mo154(NO)14O420(OH)28(H2O)70] · ca. 350 H2O consists of more than 700 atoms and is the size of a small protein. The anion is in the form of a tire (the cavity has a diameter of more than 20 Å) and an extremely large inner and outer surface. Here shown: Two views of the cluster without water and counter ions as well as the calculated X-ray powder diffractogram.

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 row 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[6] 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,[7][8] new potential applications based on unusual magnetic[9] and optical[10] properties of some POMs, and potential medical applications such as antitumor,[11] antibacterial[12] 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).[13]

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:

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.[14] 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.[15] In 2014, vanadate species with similar, selective metal-binding properties were reported.[16]

Polyoxotantalates, niobates, and vanadates
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).

The properties of polyniobates and polytantalates are similar, but substantially different from the polyoxovanadates. In fact, polyvanadates are more similar to the oxomolybdates and tungstates.[17]

Molecular structures of polyoxometalates.

The similarities between the polyniobates and polytantalates 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.[18] 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.[19] The decametalates with the formula M
10O6−
28 (M = Nb,[20] Ta[21]) 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.[22] Typically they lack terminal oxo ligands. Examples include the dodecatitanate Ti12O16(OPri)16 (where OPri stands for an alkoxy group),[23] the iron oxoalkoxometalates[24] and iron[25] and copper[26] 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−.[3][27] Sulfur-substituted POMs are called polyoxothiometalates. Other ligands replacing the oxide ions have also been demonstrated, such as nitrosyl and alkoxy groups.[22][28]

Applications

POMs are employed as commercial catalysts for oxidation of organic compounds.[29][30]

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,[31] a method of decontaminating water,[32] and a method to catalytically produce formic acid from biomass (OxFA process).[33] Polyoxometalates have been shown to catalyse water splitting.[34]
  • non-volatile (permanent) storage components, also known as flash memory devices.[35][36] Some POMs exhibit unusual magnetic properties[37] and are being investigated as possible nanocomputer storage devices (see qubits).[38]
  • In artificial photosynthesis, polyoxometalates containing copper have been proposed as catalysts for photochemical water splitting and production of solar fuels.[39]
  • 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.[40] 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.[41][42]

POM with a Wells-Dawson structure can efficiently inhibit amyloid β (Aβ) aggregation in a therapeutic strategy for Alzheimer's Disease.[43]

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Further reading
  • Long, D. L.; Burkholder, E.; Cronin, L. (2007). "Polyoxometalate clusters, nanostructures and materials: From self-assembly to designer materials and devices". Chem. Soc. Rev. 36 (1): 105–121. doi:10.1039/b502666k. PMID 17173149.
  • Pope, M. T. (1983). Heteropoly and Isopoly Oxometalates. New York: Springer Verlag.
  • Pope, M. T.; Müller, A. (1991). "Polyoxometalate Chemistry: An Old Field with New Dimensions in Several Disciplines". Angew. Chem. Int. Ed. Engl. 30: 34–48. doi:10.1002/anie.199100341.
  • Hill, C. L. (1998). "Special volume on polyoxometalates". Chem. Rev. 98 (1): 1–2. doi:10.1021/cr960395y. PMID 11851497.
  • Cronin, Leroy; Müller, Achim (2012). Cronin, L.; Müller, A. (eds.). "Special issue on polyoxometalates". Chem. Soc. Rev. 2012 (22): 7325–7648. doi:10.1039/C2CS90087D. PMID 23052289.
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