Polyoxometalate
Template:Short description Polyoxometalates (POMs) are a group of inorganic anionic molecular metal oxides.
Formation
The oxides of d0 metals such as Template:Chem2, Template:Chem2, Template:Chem2 dissolve at high pH to give orthometalates, Template:Chem2, Template:Chem2, Template:Chem2. For Template:Chem2 and Template:Chem2, the nature of the dissolved species at high pH is less defined, but these oxides also form polyoxometalates.
As the pH is lowered, solutions of orthometalates give oxide–hydroxide compounds such as Template:Chem2 and Template:Chem2. 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.<ref name=G&E/>
Condensation of the Template:Chem2 species entails loss of water and the formation of Template:Chem2 linkages. The stoichiometry for hexamolybdate is shown:<ref name=wgk/>
An abbreviated condensation sequence illustrated with vanadates is:<ref name=G&E>Template:Cite book</ref><ref>Template:Cite journal</ref><ref name=Pope>Template:Cite book</ref>
When such acidifications are conducted in the presence of phosphate or silicate, heteropolymetalate can result. For example, the phosphotungstate anion Template:Chem2 consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.
History
Ammonium phosphomolybdate, Template:Chem2 anion, was reported in 1826.<ref name = gouzerh>Template:Cite journal</ref> The isostructural phosphotungstate anion was characterized by X-ray crystallography 1934. This structure is called the Keggin structure after its discoverer.<ref>Template:Cite journal</ref>
The 1970s witnessed the introduction of quaternary ammonium salts of POMs.<ref name=wgk>Template:Cite book</ref> 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.<ref name=VWD>Template:Cite journal</ref>
Ramazzoite, the first example of a mineral with a polyoxometalate cation, was described in 2016 in Mt. Ramazzo Mine, Liguria, Italy.<ref>Template:Cite journal</ref>
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.<ref name=G&E/> POMs are sometimes viewed as soluble fragments of metal oxides.<ref name=VWD/>
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 Template:Chem2, each addenda center connects to single terminal oxo ligand, four bridging μ2-O ligands and one bridging μ3-O deriving from the central heterogroup.<ref name=Mingos_Book>Template:Cite book</ref> 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.<ref name=Cotton>Template:Cite journal</ref> However, metal-metal bonds are not completely absent in polyoxometalates and they are often present among the highly reduced species.<ref>Template:Cite journal</ref>
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Lindqvist hexamolybdate, Template:Chem2 -
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Line drawing of disodium decavanadate, Template:Chem2 -
Paratungstate B, also called dihydrogen paratungstate, Template:Chem2 -
Mo36-polymolybdate, Template:Chem2
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.<ref name=G&E/>
Polyoxotechnetates and rhenates
Polyoxotechnetates form only in strongly acidic conditions, such as in [[pertechnetic acid|Template:Chem2]] or trifluoromethanesulfonic acid solutions. The first empirically isolated polyoxotechnetate was the red Template:Chem2. It contains both Tc(V) and Tc(VII) in ratio 4: 16 and is obtained as the hydronium salt Template:Chem2 by concentrating an Template:Chem2 solution.<ref>Template:Cite journal</ref> Corresponding ammonium polyoxotechnetate salt was recently isolated from trifluoromethanesulfonic acid and it has very similar structure.<ref>Template:Cite journal</ref> The only polyoxorhenate formed in acidic conditions in presence of pyrazolium cation. The first empirically isolated polyoxorhenate was the white Template:Chem2. It contains Re(VII) in both octahedral and tetrahedral coordination.<ref>Template:Cite journal</ref>
Mixed polyoxo(technetate-rhenate) [Tc4O4(H2O)2(ReO4)14]2- polyanion crystals that contain Tc(V) and Re(VII)were also isolated <ref>Template:Cite journal</ref> and structurally characterized.
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 Template:Chem (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 Template:Chem in alkaline media.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
The decametalates with the formula Template:Chem (M = Nb,<ref>Template:Cite journal</ref> Ta<ref>Template:Cite journal</ref>) are isostructural with decavanadate. They are formed exclusively by edge-sharing {MO6} octahedra (the structure of decatungstate Template:Chem comprises edge-sharing and corner-sharing tungstate octahedra).
Heteroatoms
Template:Main Heteroatoms aside from the transition metal are a defining feature of heteropolymetalates. Many different elements can serve as heteroatoms but most common are [[Phosphate|Template:Chem]], [[Silicate|Template:Chem]], and [[Arsenate|Template:Chem]].
Giant structures
Polyoxomolybdates include the wheel-shaped molybdenum blue anions and spherical "keplerates". The cluster Template:Chem2 consists of more than 700 atoms. The anion is in the form of a tire (the cavity has a diameter of more than 20 Å) and an large inner and outer surface.
Oxoalkoxometalates
Oxoalkoxometalates are clusters that contain both oxide and alkoxide ligands.<ref name = Pope&Muller /> Typically they lack terminal oxo ligands. Examples include the dodecatitanate Ti12O16(OPri)16 (where OPri stands for an alkoxy group),<ref>Template:Cite journal</ref> the iron oxoalkoxometalates<ref>Template:Cite journal</ref> and iron<ref>Template:Cite journal</ref> and copper<ref>Template:Cite journal</ref> 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−.<ref name = gouzerh /><ref>Template:Cite journal</ref> Sulfur-substituted POMs are called polyoxothiometalates. Other ligands replacing the oxide ions have also been demonstrated, such as nitrosyl and alkoxy groups.<ref name=Pope&Muller>Template:Cite book</ref><ref>Template:Cite journal</ref>
Polyfluoroxometalate are yet another class of O-replaced oxometalates.<ref>Template:Cite journal</ref>
Other
Numerous hybrid organic–inorganic materials that contain POM cores,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Illustrative of the diverse structures of POM is the ion Template:Chem, which has face-shared octahedra with Mo atoms at the vertices of an icosahedron.<ref>Template:Cite journal</ref>
Uses
POMs are employed as commercial catalysts for oxidation of organic compounds.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Research
Lanthanides can behave like Lewis acids and perform catalytic properties.<ref>Template:Cite journal</ref> Lanthanide-containing polyoxometalates show chemoselectivity<ref>Template:Cite journal</ref> and are also able to form inorganic–organic adducts, which can be exploited in chiral recognition.<ref>Template:Cite journal</ref>
POM-based aerobic oxidations have been promoted as alternatives to chlorine-based wood pulp bleaching processes,<ref>Template:Cite journal</ref> a method of decontaminating water,<ref>Template:Cite journal</ref> and a method to catalytically produce formic acid from biomass (OxFA process).<ref>Template:Cite journal</ref> Polyoxometalates have been shown to catalyse water splitting.<ref>Template:Cite journal</ref>
Molecular electronics
Some POMs exhibit unusual magnetic properties,<ref>Template:Cite journal</ref> which has prompted visions of many applications. One example is storage devices called qubits.<ref>Template:Cite journal</ref> non-volatile (permanent) storage components, also known as flash memory devices.<ref>"Flash memory breaches nanoscales", The Hindu.</ref><ref>Template:Cite journal</ref>
Drugs
Potential antitumor<ref>Template:Cite journal</ref> and antiviral drugs.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> The Wells-Dawson structure can efficiently inhibit amyloid β (Aβ) aggregation in a therapeutic strategy for Alzheimer's disease.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> antibacterial<ref>Template:Cite journal</ref> and antiviral uses.