Polyoxometalate
In
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.[1]
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:[3]
- 6 MoO2−4 + 10 HCl → [Mo6O19]2− + 10 Cl− + 5 H2O
An abbreviated condensation sequence illustrated with vanadates is:[1][4]
- 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.[5] The isostructural phosphotungstate anion was characterized by X-ray crystallography 1934. This structure is called the Keggin structure after its discoverer.[6]
The 1970's witnessed the introduction of
Ramazzoite, the first example of a mineral with a polyoxometalate cation, was described in 2016 in Mt. Ramazzo Mine, Liguria, Italy.[8]
Structure and bonding
The typical framework building blocks are
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.[9] 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.[10] However, metal-metal bonds are not completely absent in polyoxometalates and they are often present among the highly reduced species.[11]
-
Lindqvist hexamolybdate, Mo6O2−19
-
Decavanadate, V10O6−28
-
Line drawing of disodium decavanadate, V10O6−28
-
Paratungstate B, also called dihydrogen paratungstate, H2W12O10−42
-
Mo36-polymolybdate, Mo36O112(H2O)8−16
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
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.[12] Corresponding ammonium polyoxotechnetate salt was recently isolated from trifluoromethanesulfonic acid and it has very similar structure.[13] 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.[14]
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.[15] 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.[16]
The decametalates with the formula M
10O6−
28 (M = Nb,[17] Ta[18]) 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.[19] Lanthanides can behave like Lewis acids and perform catalytic properties.[20] Lanthanide-containing polyoxometalates show chemoselectivity[21] and are also able to form inorganic–organic adducts, which can be exploited in chiral recognition.[22]
Oxoalkoxometalates
Oxoalkoxometalates are clusters that contain both oxide and alkoxide ligands. 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
Polyfluoroxometalate are yet another class of O-replaced oxometalates.[30]
Other
Numerous hybrid organic–inorganic materials that contain POM cores,[31][32][33]
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).[34]
Use and aspirational applications
Oxidation catalysts
POMs are employed as commercial catalysts for oxidation of organic compounds.[35][36]
Efforts continue to extend this theme. POM-based aerobic oxidations have been promoted as alternatives to chlorine-based wood pulp bleaching processes,[37] a method of decontaminating water,[38] and a method to catalytically produce formic acid from biomass (OxFA process).[39] Polyoxometalates have been shown to catalyse water splitting.[40]
Molecular electronics
Some POMs exhibit unusual magnetic properties,[41] which has prompted visions of many applications. One example is storage devices called qubits.[42] non-volatile (permanent) storage components, also known as flash memory devices.[43][44]
Drugs
Potential
See also
References
- ^ ISBN 978-0-7506-3365-9.
- ^ Pope, M. T. (1983). Heteropoly and Isopoly Oxometalates. New York: Springer Verlag.
- ^ ISBN 9780470132586.
- PMID 32990698.
- ^ a b Gouzerh, P.; Che, M. (2006). "From Scheele and Berzelius to Müller: polyoxometalates (POMs) revisited and the "missing link" between the bottom up and top down approaches". L'Actualité Chimique. 298: 9.
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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. 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. .
- Hill, C. L. (1998). "Special Volume on Polyoxometalates". Chem. Rev. 98 (1): 1–2. PMID 11851497.
- Cronin, L.; Müller, A. (2012). Cronin, L.; Müller, A. (eds.). "Special Issue on Polyoxometalates". Chem. Soc. Rev. 2012 (22): 7325–7648. PMID 23052289.