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 a large, closed 3-dimensional framework.
The metal atoms are usually group 5 or group 6 transition metals in their high oxidation states. In this state, their electron configuration is d0 or d1. Examples include vanadium(V), niobium(V), tantalum(V), molybdenum(VI), and tungsten(VI).
The framework of transition metal oxyanions may enclose one or more hetero atoms such as phosphorus or silicon, themselves sharing neighbouring oxygen atoms with the framework. For example, the phosphotungstate anion [PW12O40]3− consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.
The first example of a polyoxometalate compound was ammonium phosphomolybdate, containing the [PMo12O40]3− anion, discovered in 1826. This anion has the same structure as the phosphotungstate anion, whose structure was determined 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.
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 anti-tumor and anti-viral uses.
Some structural types are found in many different compounds. The first known example of this was the Keggin ion, whose structure was found to be common to both molybdates and tungstates with different central hetero atoms. Examples of some fundamental polyoxometalate structures are shown below. The Lindqvist ion is an iso-polyoxometalate, the other three are hetero-polyoxometalates. The Keggin and Dawson structures have tetrahedrally coordinated hetero-atoms, such as P or Si, and the Anderson structure has an octahedral central atom, such as aluminium.
|Lindqvist hexamolybdate, Mo6O192−||Decavanadate, V10O286−||Paratungstate B, H2M12O4210−||Mo36-polymolybdate, Mo36O112(H2O)168−|
|Strandberg structure, HP2Mo5O234−||Keggin structure, XM12O40n−||Dawson structure, X2M18O62n−|
|Anderson structure, XM6O24n−||Allman-Waugh structure, XM9O32n−||Weakley-Yamase structure, XM10O36n−||Dexter-Silverton structure, XM12O42n−|
The oxide centers of polyoxometallate framework can be replaced by other ligands, as S and Br. A sulfur-substituted POM is called a polyoxothiometalate. Other ligands replacing the oxide ions have also been attested, such as nitrosyl and alkoxy groups.
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 CeMo12O428−, 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.
Hetero atoms are present in many polyoxometalates. Many different elements can act as hetero atoms, with various coordination numbers:
- 4-coordinate (tetrahedral) in Keggin, Dawson and Lindqvist structures (e. g., PO4, SiO4, AsO4)
- 6-coordinate (octahedral) in Anderson structure (e. g., Al(OH)6, TeO6)
- 8-coordinate (square antiprism) in ((CeO8)W10O28)8−
- 12-coordinate (icosahedral) in (UO12)Mo12O30 8−
The hetero atom may be located in the centre of the anion, such 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, can be formulated as PO3−
36, and the Dawson as (XO2−
54. The @ notation denotes the physical enclosure of the left-hand side in the right-hand side.
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.
Structural isomerism is common in 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 Keggin structure.
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 have been reported.
Polyoxometalates outside Group 5 and 6 metals
Polyoxometalates with addenda atoms outside Group 5 and 6 transition metals are known. Examples include the dodecatitanates Ti12O16(OPri)16 (where OPri stands for an alkoxy group), the iron oxoalkoxometalates and iron keggin ion. These structures are also categorised as POMs, and are known as polyoxoalkoxometalates due to the presence of the alkoxy groups.
Potential and emerging applications
The wide range of size, structure and elemental composition of polyoxometalates leads to a wide range of different properties and a corresponding wide range of potential applications
Spherical nanoporous polyoxomolybdate-based capsules of different types containing more than 100 metal atoms reported have distinctive properties regarding their assembly into vesicles and the chemistry which can be conducting within the pores and cavities. A discrete polyoxometalate Lindqvist ion of the form W6O192− was imaged recently for the first time within the capillary of a carbon nanotube following steric locking of the anion with the tubule. In situ relaxation of the anion in its equatorial plane was demonstrated.
Some potential "green" applications have been reported, such as a non-chlorine based, wood pulp bleaching process, 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.
Polyoxometalates can be reduced from conduction band of wide-band-gap semiconductors, which suggests that polyoxometalates may be useful in energy storage and conversions.
Potential medicinal applications include anti-tumoral and anti-viral drugs.
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