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) and group 7 (Tc , Re) transition metals in their high oxidation states. Polyoxometalates are often colorless, orange or red diamagnetic anions. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and oxide, and heteropolymetalates, composed of one or more metals, oxide, and eventually 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 at high pH is less clear, but these oxides also form polyoxometalates. As the pH is lowered, orthometalates protonate to give oxide–hydroxide compounds such as WO₃(OH)⁻ and VO₃(OH)²⁻. These species condense via the process called olation. The replacement of terminal M=O bonds, which in fact have triple bond character, is compensated by the increase in coordination number. The nonobservation of polyoxochromate cages is rationalized by the small radius of Cr(VI), which may not accommodate octahedral coordination geometry.

Condensation of the MO₃(OH)ⁿ⁻ species entails loss of water and the formation of M−O−M linkages. The stoichiometry for hexamolybdate is shown:

6 MoO2−4 + 10 HCl → [Mo₆O₁₉]²⁻ + 10 Cl⁻ + 5 H₂O

An abbreviated condensation sequence illustrated with vanadates is:

4 VO3−4 + 8 H⁺ → V₄O4−12 + 4 H₂O

5 V₄O4−12 + 12 H⁺ → 2 V₁₀O₂₆(OH)4−2 + 4 H₂O

When such acidifications are conducted in the presence of phosphate or silicate, heteropolymetalate result. For example, the phosphotungstate anion [PW₁₂O₄₀]³⁻ consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.

History

Ammonium phosphomolybdate, [PMo₁₂O₄₀]³⁻ anion, was reported in 1826. The isostructural phosphotungstate anion was characterized by X-ray crystallography 1934. This structure is called the Keggin structure after its discoverer.

The 1970's witnessed the introduction of quaternary ammonium salts of POMs. This innovation enabled systematic study without the complications of hydrolysis and acid/base reactions. The introduction of ¹⁷O NMR spectroscopy allowed the structural characterization of POMs in solution.

Ramazzoite, the first example of a mineral with a polyoxometalate cation, was described in 2016 in Mt. Ramazzo Mine, Liguria, Italy.

Structure and bonding

The typical framework building blocks are polyhedral units, with 6-coordinate metal centres. Usually, these units share edges and/or vertices. The coordination number of the oxide ligands varies according to their location in the cage. Surface oxides tend to be terminal or doubly bridging oxo ligands. Interior oxides are typically triply bridging or even octahedral. POMs are sometimes viewed as soluble fragments of metal oxides.

Recurring structural motifs allow POMs to be classified. Iso-polyoxometalates (isopolyanions) feature octahedral metal centers. The heteropolymetalates form distinct structures because the main group center is usually tetrahedral. The Lindqvist and Keggin structures are common motifs for iso- and heteropolyanions, respectively.

Polyoxometalates typically exhibit coordinate metal-oxo bonds of different multiplicity and strength. In a typical POM such as the Keggin structure [PW₁₂O₄₀]³⁻, each addenda center connects to single terminal oxo ligand, four bridging μ₂-O ligands and one bridging μ₃-O deriving from the central heterogroup. Metal–metal bonds in polyoxometalates are normally absent and owing to this property, F. Albert Cotton opposed to consider polyoxometalates as form of cluster materials. However, metal-metal bonds are not completely absent in polyoxometalates and they are often present among the highly reduced species.

Polymolybdates and tungstates

The polymolybdates and polytungstates are derived, formally at least, from the dianionic [MO₄]^2- precursors. The most common units for polymolybdates and polyoxotungstates are the octahedral {MO₆} centers, sometimes slightly distorted. Some polymolybdates contain pentagonal bipyramidal units. These building blocks are found in the molybdenum blues, which are mixed valence compounds.

Polyoxotechnetates and rhenates

Polyoxotechnetates form only in strongly acidic conditions, such as in HTcO₄ or trifluoromethanesulfonic acid solutions. The first empirically isolated polyoxotechnetate was the red [Tc₂₀O₆₈]⁴⁻. It contains both Tc(V) and Tc(VII) in ratio 4: 16 and is obtained as the hydronium salt [H₇O₃]₄[Tc₂₀O₆₈]·4H₂O by concentrating an HTcO₄ solution. Corresponding ammonium polyoxotechnetate salt was recently isolated from trifluoromethanesulfonic acid and it has very similar structure. The only polyoxorhenate formed in acidic conditions in presence of pyrazolium cation. The first empirically isolated polyoxorhenate was the white [Re₄O₁₅]²⁻. It contains Re(VII) in both octahedral and tetrahedral coordination.

Polyoxotantalates, niobates, and vanadates

The polyniobates, polytantalates, and vanadates are derived, formally at least, from highly charged [MO₄]^3- precursors. For Nb and Ta, 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. They are formed exclusively by edge-sharing {MO₆} octahedra (the structure of decatungstate W
₁₀O⁴⁻
₃₂ comprises edge-sharing and corner-sharing tungstate octahedra).

Heteroatoms

Heteroatoms aside from the transition metal are a defining feature of heteropolymetalates. Many different elements can serve as heteroatoms but most common are PO³⁻
₄, SiO⁴⁻
₄, and AsO³⁻
₄.

Giant structures

Polyoxomolybdates include the wheel-shaped molybdenum blue anions and spherical keplerates. The cluster [Mo₁₅₄O₄₂₀(NO)₁₄(OH)₂₈(H₂O)₇₀]²⁰⁻ 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. The incorporation of lanthanide ions in molybdenum blues is particularly intriguing. Lanthanides can behave like Lewis acids and perform catalytic properties. Lanthanide-containing polyoxometalates show chemoselectivity and are also able to form inorganic–organic adducts, which can be exploited in chiral recognition.

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 and copper 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.

Polyfluoroxometalate are yet another class of O-replaced oxometalates.

Other

Numerous hybrid organic–inorganic materials that contain POM cores,

Illustrative of the diverse structures of POM is the ion CeMo
₁₂O⁸⁻
₄₂, which has face-shared octahedra with Mo atoms at the vertices of an icosahedron).

Use and aspirational applications

Oxidation catalysts

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

Efforts continue to extend this theme. POM-based aerobic oxidations have been promoted 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.

Molecular electronics

Some POMs exhibit unusual magnetic properties, which has prompted visions of many applications. One example is storage devices called qubits. non-volatile (permanent) storage components, also known as flash memory devices.

Drugs

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. The Wells-Dawson structure can efficiently inhibit amyloid β (Aβ) aggregation in a therapeutic strategy for Alzheimer's disease. antibacterial and antiviral uses.

See also

• Superatom
• Perovskite
• Metal aromaticity

References

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.; 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, L.; Müller, A. (2012). Cronin, L.; Müller, A. (eds.). "Special Issue on Polyoxometalates". Chem. Soc. Rev. 2012 (22): 7325–7648. doi:10.1039/C2CS90087D. PMID 23052289.