Polyoxometalate
Encyclopedia
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.
, 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.
Recent new developments include the discovery of large, highly symmetric polyoxomolybdates such as 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 POM's, and potential medical applications such anti-tumor and anti-viral uses.
coordinated hetero-atoms, such as P
or Si
, and the Anderson structure has an octahedral
central atom, such as aluminium
.
, W
, and V
. When more than one element is present the cluster is called a mixed addenda cluster.
The ligands coordinated to metal atoms that together form the bridged framework are usually oxide ions, but other elements, such as S
and Br
may be substituted for some of the oxide ions. 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 blue
s.
s:
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 , and the Dawson as . 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.
The five isomers of Keggin structure.
The Keggin ions
are well-known to be thermally stable and reversibly reduced
by accepting electrons. This makes them useful as catalysts for a range of organic reactions.
Some POM's exhibit luminescence.
There have been reports on the role of weak- or non-bonding interactions on the crystal engineering of hybrid polyoxometalates.
Spherical nanoporous polyoxomolybdate-based capsules of different types containing more than 100 metal atoms reported by Achim Müller
and his group have versatile unique properties regarding their assembly into vesicles and the chemistry which can be done inside the pores and cavities. A discrete polyoxometalate Lindqvist ion of the form W6O192− was successfully 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 structures containing transition metal atoms with unpaired electrons have unusual magnetic properties and are being investigated as possible nanocomputer
storage devices (see qubit
s).
Some potential "green" applications have been reported, such as a non-chlorine
based, wood pulp bleaching process and a method of decontaminating water.
Many potential medicinal applications have been reported, such as anti-tumoral and anti-viral applications.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, a polyoxometalate (abbreviated POM) is a polyatomic ion
Polyatomic ion
A polyatomic ion, also known as a molecular ion, is a charged species composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix "poly-" means "many," in...
, usually an anion, that consists of three or more transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
oxyanions linked together by shared oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atoms to form a large, closed 3-dimensional framework.
The metal atoms are usually group 5
Group 5 element
A Group 5 element is a chemical element in the fifth group in the periodic table. In the modern IUPAC nomenclature, Group 5 of the periodic table contains vanadium , niobium , tantalum and dubnium . This group lies in the d-block of the periodic table...
or group 6
Group 6 element
A Group 6 element is one in the series of elements in group 6 in the periodic table, which consists of the transition metals chromium , molybdenum , tungsten , and seaborgium ....
transition metals in their high oxidation states. In this state, their electron configuration
D electron count
The d electron count is a chemistry formalism used to describe the electron configuration of the valence electrons of a transition metal center in a coordination complex. The d electron count is an effective way to understand the geometry and reactivity of transition metal complexes...
is d0 or d1. Examples include vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
(V), niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
(V), tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
(V), molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
(VI), and tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
(VI).
The framework of transition metal oxyanions may enclose one or more hetero atoms such as phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
or silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, themselves sharing neighbouring oxygen atoms with the framework. For example, the phosphotungstate anion
Phosphotungstic acid
Phosphotungstic acid , tungstophosphoric acid , is a heteropoly acid with the chemical formula 31240. It normally present as a hydrate. EPTA is the name of ethanolic phosphotungstic acid, its alcohol solution used in biology. It has the appearance of small, colorless-grayish or slightly...
[PW12O40]3− consists of a framework of twelve octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
tungsten oxyanions surrounding a central phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
group.
History
The first example of a polyoxometalate compound was ammonium phosphomolybdateAmmonium phosphomolybdate
Ammonium phosphomolybdate is the inorganic salt of phosphomolybdic acid with the chemical formula 3PMo12O40. It contains the remarkable phosphomolybdate ion complex.-Synthesis:...
, containing the [PMo12O40]3
Keggin structure
Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α–Keggin anions, which have a general formula of [XM12O40]n−, where X is the heteroatom , M is the addenda atom , and O represents oxygen...
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.
Recent new developments include the discovery of large, highly symmetric polyoxomolybdates such as the wheel-shaped molybdenum blue
Molybdenum blue
Molybdenum blue is a term applied to:*reduced heteropolymolybdate complexes, polyoxometalates containing Mo, Mo, and a hetero atom such as phosphorus or silicon...
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 POM's, and potential medical applications such anti-tumor and anti-viral uses.
Structure
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 tetrahedrallyTetrahedral molecular geometry
In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...
coordinated hetero-atoms, such as P
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
or Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, and the Anderson structure has an octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
central atom, such as aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
.
| Lindqvist hexamolybdate, Mo6O192- | Decavanadate, V10O286− | Paratungstate B, H2M12O4210− | Mo36-polymolybdate, Mo36O112(H2O)168− |
| Strandberg structure, HP2Mo5O234- | Keggin structure Keggin structure Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α–Keggin anions, which have a general formula of [XM12O40]n−, where X is the heteroatom , M is the addenda atom , and O represents oxygen... , XM12O40n- |
Dawson structure, X2M18O62n− | |
| Anderson structure, XM6O24n− | Allman-Waugh structure, XM9O32n− | Weakley-Yamase structure, XM10O36n− | Dexter structure, XM12O42n− |
Framework
The metal atoms that make up the framework, called addenda atoms, are typically MoMolybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, W
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
, and V
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
. When more than one element is present the cluster is called a mixed addenda cluster.
The ligands coordinated to metal atoms that together form the bridged framework are usually oxide ions, but other elements, such as S
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
and Br
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
may be substituted for some of the oxide ions. A sulfur-substituted POM is called a polyoxothiometalate. Other ligands replacing the oxide ions have also been attested, such as nitrosyl and alkoxy
Alkoxy group
In chemistry, the alkoxy group is an alkyl group singular bonded to oxygen thus: R—O. The range of alkoxy groups is great, the simplest being methoxy . An ethoxy group is found in the organic compound phenetol, C6H5OCH2CH3 which is also known as ethoxy benzene...
groups.
The typical framework building blocks are polyhedral
Polyhedron
In elementary geometry a polyhedron is a geometric solid in three dimensions with flat faces and straight edges...
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
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 blue
Molybdenum blue
Molybdenum blue is a term applied to:*reduced heteropolymolybdate complexes, polyoxometalates containing Mo, Mo, and a hetero atom such as phosphorus or silicon...
s.
Hetero atoms
Hetero atoms are present in many polyoxometalates. Many different elements can act as hetero atoms, with various coordination numberCoordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
s:
- 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 , and the Dawson as . 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
Vanadate
In chemistry, a vanadate is a compound containing an oxoanion of vanadium generally in its highest oxidation state of +5. The simplest vanadate ion is the tetrahedral, orthovanadate, VO43− anion, which is present in e.g. sodium orthovanadate and in solutions of V2O5 in strong base...
cage V18O42 can enclose a Cl
Isomerism
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°.| α-XM12O40n- | β-XM12O40n- | γ-XM12O40n- | δ-XM12O40n- | ε-XM12O40n- |
|---|---|---|---|---|
The five isomers of Keggin structure.
Lacunary structures
The structure of some POM's 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.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) and the iron oxoalkoxometalates. These structures are also categorised as POM's, and are known as polyoxoalkoxometalates due to the presence of the alkoxy groups.Properties and applications
The huge range of size, structure and elemental composition of known polyoxometalates leads to a wide range of different properties.The Keggin ions
Keggin structure
Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α–Keggin anions, which have a general formula of [XM12O40]n−, where X is the heteroatom , M is the addenda atom , and O represents oxygen...
are well-known to be thermally stable and reversibly reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
by accepting electrons. This makes them useful as catalysts for a range of organic reactions.
Some POM's exhibit luminescence.
There have been reports on the role of weak- or non-bonding interactions on the crystal engineering of hybrid polyoxometalates.
Spherical nanoporous polyoxomolybdate-based capsules of different types containing more than 100 metal atoms reported by Achim Müller
Achim Müller
Achim Müller is a German scientist . He is working now with his research group at the Faculty of Chemistry, University of Bielefeld.- Academic career :...
and his group have versatile unique properties regarding their assembly into vesicles and the chemistry which can be done inside the pores and cavities. A discrete polyoxometalate Lindqvist ion of the form W6O192
Some structures containing transition metal atoms with unpaired electrons have unusual magnetic properties and are being investigated as possible nanocomputer
Nanocomputer
Nanocomputer is the logical name for a computer smaller than the microcomputer, which is smaller than the minicomputer. More technically, it is a computer whose fundamental parts are no bigger than a few nanometers...
storage devices (see qubit
Qubit
In quantum computing, a qubit or quantum bit is a unit of quantum information—the quantum analogue of the classical bit—with additional dimensions associated to the quantum properties of a physical atom....
s).
Some potential "green" applications have been reported, such as a non-chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
based, wood pulp bleaching process and a method of decontaminating water.
Many potential medicinal applications have been reported, such as anti-tumoral and anti-viral applications.
Further reading
- D. L. Long, E. Burkholder, and L. Cronin, 'Polyoxometalate clusters, nanostructures and materials: From self assembly to designer materials and devices', Chem. Soc. Rev., 2007, 36, 105-121.DOI: 10.1039/b502666k http://www.rsc.org/ej/CS/2007/b502666k.pdf
- M.T. Pope "Heteropoly and Isopoly Oxometalates", Springer Verlag, New York, (1983).
- M.T. Pope, A. Müller, Polyoxometalate Chemistry: An Old Field with New Dimensions in Several Disciplines, Angew. Chem. Int. Ed. Engl. 1991, 30, 34.
- Special volume on "Polyoxometalates", Chem.Rev.,1998, 98, 1