Polyoxometalate

File:AlfaPMo12 bs.jpg anion, an example of a 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.{{cite book |last=Pope |first=M. T. |title=Heteropoly and Isopoly Oxometalates |publisher=Springer Verlag |location=New York |date=1983}}

Formation

The oxides of d⁰ metals such as {{chem2|V2O5}}, {{chem2|MoO3}}, {{chem2|WO3}} dissolve at high pH to give orthometalates, {{chem2|VO4(3−)}}, {{chem2|MoO4(2−)}}, {{chem2|WO4(2−)}}. For {{chem2|Nb2O5}} and {{chem2|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 {{chem2|WO3(OH)−}} and {{chem2|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 {{chem2|MO3(OH)^{n-} }} species entails loss of water and the formation of {{chem2|M\sO\sM}} linkages. The stoichiometry for hexamolybdate is shown:

:{{chem2|6 MoO4(2-) + 10 HCl -> [Mo6O19](2-) + 10 Cl- + 5 H2O}}

An abbreviated condensation sequence illustrated with vanadates is:{{cite book |last1=Greenwood |first1=N. N. |last2=Earnshaw |first2=A. |date=1997 |title=Chemistry of the Elements |edition=2nd |location=Oxford |publisher=Butterworth-Heinemann |isbn=978-0-7506-3365-9}}{{Cite journal|last1=Gumerova|first1=Nadiia I.|last2=Rompel|first2=Annette|date=2020|title=Polyoxometalates in solution: speciation under spotlight|url=http://xlink.rsc.org/?DOI=D0CS00392A|journal=Chemical Society Reviews|language=en|volume=49|issue=21|pages=7568–7601|doi=10.1039/D0CS00392A|pmid=32990698|issn=0306-0012|doi-access=free}}

:{{chem2|4 VO4(3-) + 8 H+ -> V4O12(4-) + 4 H2O}}

:{{chem2|5 V4O12(4-) + 12 H+ -> 2 V10O26(OH)2(4-) + 4 H2O}}

When such acidifications are conducted in the presence of phosphate or silicate, heteropolymetalate result. For example, the phosphotungstate anion {{chem2|[PW12O40](3−)}} consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.

History

File:Dr. James F. Keggin, the discoverer of the Keggin Structure - b.jpg

Ammonium phosphomolybdate, {{chem2|[PMo12O40](3−)}} anion, was reported in 1826.{{cite journal |last1=Gouzerh |first1=P. |last2=Che |first2=M. |date=2006 |title=From Scheele and Berzelius to Müller: polyoxometalates (POMs) revisited and the "missing link" between the bottom up and top down approaches |journal=L'Actualité Chimique |volume=298 |pages=9 }} The isostructural phosphotungstate anion was characterized by X-ray crystallography 1934. This structure is called the Keggin structure after its discoverer.{{cite journal |last=Keggin |first=J. F. |date=1934 |title=The Structure and Formula of 12-Phosphotungstic Acid |journal=Proc. R. Soc. A |volume=144 |issue=851 |pages=75–100 |doi=10.1098/rspa.1934.0035 |bibcode=1934RSPSA.144...75K|doi-access=}}

The 1970s witnessed the introduction of quaternary ammonium salts of POMs.{{cite book|author1-link=Walter G. Klemperer|first1=W. G.|last1=Klemperer|title=Inorganic Syntheses|chapter=Tetrabutylammonium Isopolyoxometalates|year=1990|volume=27|pages=74–85 |doi=10.1002/9780470132586.ch15|isbn=9780470132586}} 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.{{cite journal |doi=10.1126/science.228.4699.533|first1=V. W.|last1=Day|last2= Klemperer|first2=W. G.|title=Metal Oxide Chemistry in Solution: The Early Transition Metal Polyoxoanions|journal=Science|year=1985|volume=228|issue=4699|pages=533–541|pmid=17736064|bibcode=1985Sci...228..533D|s2cid=32953306}}

Ramazzoite, the first example of a mineral with a polyoxometalate cation, was described in 2016 in Mt. Ramazzo Mine, Liguria, Italy.{{cite journal|title=Ramazzoite, [Mg8Cu12(PO4)(CO3)4(OH)24(H2O)20][(H0.33SO4)3(H2O)36], the first mineral with a polyoxometalate cation|journal=European Journal of Mineralogy|date=April 4, 2018|volume=30|issue=4|pages=182–186|doi=10.1127/ejm/2018/0030-2748|url=https://pubs.geoscienceworld.org/eurjmin/article/530707/?searchresult=1|access-date=21 May 2018|last1=Kampf|first1=Anthony R.|last2=Rossman|first2=George R.|last3=Ma|first3=Chi|last4=Belmonte|first4=Donato|last5=Biagioni|first5=Cristian|last6=Castellaro|first6=Fabrizio|last7=Chiappino|first7=Luigi|bibcode=2018EJMin..30..827K|s2cid=134883484}}

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 {{chem2|[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.{{cite book |title=Bonding and Charge Distribution in Polyoxometalates: A Bond Valence Approach |first1=D.M.P. |last1=Mingos |publisher=Springer |date=1999 |isbn=978-3-662-15621-6}} 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.{{cite journal |doi=10.1098/rsbm.2008.0003|first1=M. H.|last1=Chisholm|last2= of Newnham|first2= Lord Lewis of |title=Frank Albert Cotton. 9 April 1930—20 February 2007|journal=Biogr. Mem. Fellows R. Soc.|year=2008|volume=54|pages=95–115|s2cid=71372188|doi-access=}} However, metal-metal bonds are not completely absent in polyoxometalates and they are often present among the highly reduced species.{{cite journal|title=Metal-MetalBonds, Metal–metal bonds in polyoxometalate chemistry|journal=Nanoscale|year=2021|volume=13|issue=32|pages=13574–13592|doi=10.1039/D1NR02357H|last1=Kondinski|first1=Aleksandar|pmid=34477632|s2cid=237398818|doi-access=free}}

Lindquist M6.jpg|Lindqvist hexamolybdate, {{chem2|Mo6O19(2-)}}

decavanadate.jpg|Decavanadate, {{chem2|V10O28(6-)}}

sodium decavanadate.png|Line drawing of disodium decavanadate, {{chem2|V10O28(6-)}}

H2W12O42.jpg|Paratungstate B, also called dihydrogen paratungstate, {{chem2|H2W12O42(10-)}}

Mo36.jpg|Mo₃₆-polymolybdate, {{chem2|Mo36O112(H2O)16(8-)}}

=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=

File:Tc20O68_polyoxotechnetate.png

File:Re4O15 polyoxorhenate in grey and blue.png

Polyoxotechnetates form only in strongly acidic conditions, such as in pertechnetic acid or trifluoromethanesulfonic acid solutions. The first empirically isolated polyoxotechnetate was the red {{chem2|[Tc20O68](4-)}}. It contains both Tc(V) and Tc(VII) in ratio 4: 16 and is obtained as the hydronium salt {{chem2|[H7O3]4[Tc20O68]*4H2O}} by concentrating an {{chem2|HTcO4}} solution.{{cite journal |last1=German |first1=Konstantin E. |last2=Fedoseev |first2=Alexander M. |last3=Grigoriev |first3=Mikhail S. |last4=Kirakosyan |first4=Gayane A. |last5=Dumas |first5=Thomas |last6=Den Auwer |first6=Christophe |last7=Moisy |first7=Philippe |last8=Lawler |first8=Keith V. |last9=Forster |first9=Paul M. |last10=Poineau |first10=Frederic |title=A 70-Year-Old Mystery in Technetium Chemistry Explained by the New Technetium Polyoxometalate [H₇O₃]₄[Tc₂₀O₆₈]⋅4H₂O |journal=Chemistry – A European Journal |date=24 September 2021 |volume=27 |issue=54 |pages=13624–13631 |doi=10.1002/chem.202102035|pmid=34245056 |s2cid=235787236 }} Corresponding ammonium polyoxotechnetate salt was recently isolated from trifluoromethanesulfonic acid and it has very similar structure.{{Cite journal |last1=Zegke |first1=Markus |last2=Grödler |first2=Dennis |last3=Roca Jungfer |first3=Maximilian |last4=Haseloer |first4=Alexander |last5=Kreuter |first5=Meike |last6=Neudörfl |first6=Jörg M. |last7=Sittel |first7=Thomas |last8=James |first8=Christopher M. |last9=Rothe |first9=Jörg |last10=Altmaier |first10=Marcus |last11=Klein |first11=Axel |date=2022-01-17 |title=Ammonium Pertechnetate in Mixtures of Trifluoromethanesulfonic Acid and Trifluoromethanesulfonic Anhydride |journal=Angewandte Chemie International Edition |language=en |volume=61 |issue=3 |pages=e202113777 |doi=10.1002/anie.202113777 |pmid=34752692 |pmc=9299680 |issn=1433-7851}}

The only polyoxorhenate formed in acidic conditions in presence of pyrazolium cation. The first empirically isolated polyoxorhenate was the white {{chem2|[Re4O15](2-)}}. It contains Re(VII) in both octahedral and tetrahedral coordination.{{cite journal |last1=Volkov |first1=Mikhail A. |last2=Novikov |first2=Anton P. |last3=Borisova |first3=Nataliya E. |last4=Grigoriev |first4=Mikhail S. |last5=German |first5=Konstantin E. |title=Intramolecular Re···O Nonvalent Interactions as a Stabilizer of the Polyoxorhenate(VII) |journal=Inorganic Chemistry |date=10 August 2023 |volume=62 |issue=33 |pages=13485–13494 |doi=10.1021/acs.inorgchem.3c01863|pmid=37599582 }}

Mixed polyoxo(technetate-rhenate) [Tc₄O₄(H₂O)₂(ReO₄)₁₄]^2- polyanion crystals that contain Tc(V) and Re(VII)were also isolated {{Cite journal |last=Bustos |first=Jenna |last2=Shohel |first2=Mohammad |last3=Buzanich |first3=Ana Guilherme |last4=Zakharov |first4=Lev |last5=Buils |first5=Jordi |last6=Segado-Centellas |first6=Mireia |last7=Bo |first7=Carles |last8=Nyman |first8=May |title=Technetium and Rhenium Auto-reduction, Polymerization and Lability towards Group VII Polyoxometalate Chemistry |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202404144 |journal=Chemistry – A European Journal |language=en |volume=n/a |issue=n/a |pages=e202404144 |doi=10.1002/chem.202404144 |issn=1521-3765}} 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 {{chem|M|6|O|19|8−}} (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 {{chem|Ta(O|2|)|4|3−}} in alkaline media.{{cite journal |last1=Fullmer |first1=L. B. |last2=Molina |first2=P. I. |last3=Antonio |first3=M. R. |last4=Nyman |first4=M. |date=2014 |title=Contrasting ion-association behaviour of Ta and Nb polyoxometalates |journal=Dalton Trans. |volume=2014 |issue= 41|pages= 15295–15299|doi=10.1039/C4DT02394C|pmid=25189708 }} 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.{{cite journal |last1=Anderson |first1=T. M. |last2=Thoma |first2=S. G. |last3=Bonhomme |first3=F. |last4=Rodriguez |first4=M. A. |last5=Park |first5=H. |last6=Parise |first6=J. B. |last7=Alan |first7=T. M. |last8=Larentzos |first8=J. P. |last9=Nyman |first9=M. |date=2007 |title=Lithium Polyniobates. A Lindqvist-Supported Lithium−Water Adamantane Cluster and Conversion of Hexaniobate to a Discrete Keggin Complex |journal=Crystal Growth & Design |volume= 7|issue= 4|pages= 719–723|doi=10.1021/cg0606904}}

The decametalates with the formula {{chem|M|10|O|28|6−}} (M = Nb,{{cite journal |last1=Graeber |first1=E. J. |last2=Morosin |first2=B. |date=1977 |title=The molecular configuration of the decaniobate ion (Nb₁₇O₂₈⁶⁻) |journal=Acta Crystallographica B |volume= 33|issue= 7|pages= 2137–2143|doi=10.1107/S0567740877007900 |doi-access= }} Ta{{cite journal |last1=Matsumoto |first1=M. |last2=Ozawa |first2=Y. |last3=Yagasaki |first3=A. |last4=Zhe |first4=Y. |date=2013 |title=Decatantalate—The Last Member of the Group 5 Decametalate Family |journal=Inorg. Chem. |volume= 52|issue= 14|pages= 7825–7827|doi=10.1021/ic400864e |pmid=23795610 }}) are isostructural with decavanadate. They are formed exclusively by edge-sharing {MO₆} octahedra (the structure of decatungstate {{chem|W|10|O|32|4−}} 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 Phosphate, Silicate, and Arsenate.

=Giant structures=

File:POM-Wheel+PD.png for the salt.]]

Polyoxomolybdates include the wheel-shaped molybdenum blue anions and spherical keplerates. The cluster {{chem2|1=[Mo154O420(NO)14(OH)28(H2O)70]^{20-}|2=}} 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.{{Cite journal |last1=Al-Sayed |first1=Emir |last2=Rompel |first2=Annette |date=2022-03-02 |title=Lanthanides Singing the Blues: Their Fascinating Role in the Assembly of Gigantic Molybdenum Blue Wheels |journal=ACS Nanoscience Au |volume=2 |issue=3 |language=en |pages=179–197 |doi=10.1021/acsnanoscienceau.1c00036 |pmid=35726275 |pmc=9204829 |issn=2694-2496}} Lanthanides can behave like Lewis acids and perform catalytic properties.{{Cite journal |last1=Barrett |first1=Anthony G. M. |last2=Christopher Braddock |first2=D. |date=1997 |title=Scandium(III) or lanthanide(III) triflates as recyclable catalysts for the direct acetylation of alcohols with acetic acid |url=http://xlink.rsc.org/?DOI=a606484a |journal=Chemical Communications |issue=4 |pages=351–352 |doi=10.1039/a606484a}} Lanthanide-containing polyoxometalates show chemoselectivity{{Cite journal |last1=Boglio |first1=Cécile |last2=Lemière |first2=Gilles |last3=Hasenknopf |first3=Bernold |last4=Thorimbert |first4=Serge |last5=Lacôte |first5=Emmanuel |last6=Malacria |first6=Max |date=2006-05-12 |title=Lanthanide Complexes of the Monovacant Dawson Polyoxotungstate [α1-P2W17O61]10− as Selective and Recoverable Lewis Acid Catalysts |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200600364 |journal=Angewandte Chemie International Edition |language=en |volume=45 |issue=20 |pages=3324–3327 |doi=10.1002/anie.200600364 |pmid=16619320 |issn=1433-7851}} and are also able to form inorganic–organic adducts, which can be exploited in chiral recognition.{{Cite journal |last1=Sadakane |first1=Masahiro |last2=Dickman |first2=Michael H. |last3=Pope |first3=Michael T. |date=2001-06-01 |title=Chiral Polyoxotungstates. 1. Stereoselective Interaction of Amino Acids with Enantiomers of [Ce III (α 1 -P 2 W 17 O 61 )(H 2 O) x ] 7- . The Structure of dl -[Ce 2 (H 2 O) 8 (P 2 W 17 O 61 ) 2 ] 14- |url=https://pubs.acs.org/doi/10.1021/ic0014383 |journal=Inorganic Chemistry |language=en |volume=40 |issue=12 |pages=2715–2719 |doi=10.1021/ic0014383 |pmid=11375685 |issn=0020-1669}}

=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),{{cite journal |last1=Day |first1=V. W. |last2=Eberspacher |first2=T. A. |last3=Klemperer |first3=W. G. |last4=Park |first4=C. W. |date=1993 |title=Dodecatitanates: a new family of stable polyoxotitanates |journal=J. Am. Chem. Soc. |volume=115 |issue= 18|pages=8469–8470 |doi=10.1021/ja00071a075 }} the iron oxoalkoxometalates{{cite journal |last1=Bino |first1=Avi |last2=Ardon |first2=Michael |last3=Lee |first3=Dongwhan |last4=Spingler |first4=Bernhard |last5=Lippard |first5=Stephen J. |date=2002 |title=Synthesis and Structure of [Fe₁₃O₄F₂₄(OMe)₁₂]⁵⁻: The First Open-Shell Keggin Ion |journal=J. Am. Chem. Soc. |volume=124 |issue= 17|pages=4578–4579 |doi=10.1021/ja025590a |pmid=11971702 }} and iron{{cite journal |last1=Sadeghi |first1=Omid |last2=Zakharov |first2=Lev N. |last3=Nyman |first3=May |date=2015 |title=Aqueous formation and manipulation of the iron-oxo Keggin ion |journal=Science |volume=347 |issue= 6228|pages=1359–1362 |doi=10.1126/science.aaa4620 |pmid=25721507|bibcode=2015Sci...347.1359S |s2cid=206634621 }} and copper{{cite journal|author1=Kondinski, A.|author2=Monakhov, K.|year=2017|title=Breaking the Gordian Knot in the Structural Chemistry of Polyoxometalates: Copper(II)–Oxo/Hydroxo Clusters|doi=10.1002/chem.201605876|pmid=28083988|journal=Chemistry: A European Journal|volume=23|issue=33|pages=7841–7852|doi-access=free}} 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⁻.{{cite journal |last1=Errington |first1=R. John |last2=Wingad |first2=Richard L. |last3=Clegg |first3=William |last4=Elsegood |first4=Mark R. J. |title=Direct Bromination of Keggin Fragments To Give [PW₉O₂₈Br₆]³⁻: A Polyoxotungstate with a Hexabrominated Face |journal=Angew. Chem. |volume=39 |issue=21 |pages=3884–3886 |doi=10.1002/1521-3773(20001103)39:21<3884::AID-ANIE3884>3.0.CO;2-M |year=2000|pmid=29711675 }} Sulfur-substituted POMs are called polyoxothiometalates. Other ligands replacing the oxide ions have also been demonstrated, such as nitrosyl and alkoxy groups.{{cite book |title=Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity |first1=Michael Thor |last1=Pope |first2=Achim |last2=Müller |publisher=Springer |date=1994 |isbn=978-0-7923-2421-8}}{{cite journal |last1=Gouzerh |first1=P. |last2=Jeannin |first2=Y. |last3=Proust |first3=A. |last4=Robert |first4=F. |last5=Roh |first5=S.-G. |date=1993 |title=Functionalization of polyoxomolybdates: the example of nitrosyl derivatives |journal=Mol. Eng. |volume=3 |issue= 1–3|pages=79–91 |doi=10.1007/BF00999625|s2cid=195235379 }}

Polyfluoroxometalate are yet another class of O-replaced oxometalates.{{cite journal |last1=Schreiber |first1=Roy E. |last2=Avram |first2=Liat |last3=Neumann |first3=Ronny |title=Self-Assembly through Noncovalent Preorganization of Reactants: Explaining the Formation of a Polyfluoroxometalate |journal=Chemistry - A European Journal |date=2018 |volume=24 |issue=2 |pages=369–379 |doi=10.1002/chem.201704287|pmid=29064591}}

=Other=

Numerous hybrid organic–inorganic materials that contain POM cores,{{cite journal |last1=Song |first1=Y.-F. |last2=Long |first2=D.-L. |last3=Cronin |first3=L. |date=2007 |title=Non covalently connected frameworks with nanoscale channels assembled from a tethered polyoxometalate–pyrene hybrid |journal=Angew. Chem. Int. Ed. |volume=46 |issue= 21|pages=3900–3904 |doi=10.1002/anie.200604734 |pmid=17429852}}{{cite journal |last1=Guo |first1=Hong-Xu |last2=Liu |first2=Shi-Xiong |date=2004 |title=A novel 3D organic–inorganic hybrid based on sandwich-type cadmium heteropolymolybdate: [Cd₄(H₂O)₂(2,2′-bpy)₂] Cd[Mo₆O₁₂(OH)₃(PO₄)₂(HPO₄)₂]₂ [Mo₂O₄(2,2′-bpy)₂]₂·3H₂O |journal=Inorganic Chemistry Communications |volume=7 |issue=11 |pages=1217 |doi=10.1016/j.inoche.2004.09.010}}{{Cite journal|last1=Blazevic|first1=Amir|last2=Rompel|first2=Annette|date=January 2016|title=The Anderson–Evans polyoxometalate: From inorganic building blocks via hybrid organic–inorganic structures to tomorrows "Bio-POM"|journal=Coordination Chemistry Reviews|language=en|volume=307|pages=42–64|doi=10.1016/j.ccr.2015.07.001}}

Illustrative of the diverse structures of POM is the ion {{chem|CeMo|12|O|42|8−}}, which has face-shared octahedra with Mo atoms at the vertices of an icosahedron.{{cite journal |last1=Dexter |first1=D. D. |last2=Silverton |first2=J. V. |date=1968 |title=A New Structural Type for Heteropoly Anions. The Crystal Structure of (NH₄)₂H₆(CeMo₁₂O₄₂)·12H₂O |journal=J. Am. Chem. Soc. |volume=1968 |issue= 13|pages=3589–3590 |doi= 10.1021/ja01015a067 }}

Use and aspirational applications

=Oxidation catalysts=

POMs are employed as commercial catalysts for oxidation of organic compounds.{{cite journal |last=Misono |first=Makoto |date=1993 |title=Catalytic chemistry of solid polyoxometalates and their industrial applications |journal=Mol. Eng. |volume=3 |issue= 1–3|pages=193–203 |doi=10.1007/BF00999633|s2cid=195235697 }}{{cite journal |last=Kozhevnikov |first=Ivan V. |date=1998 |title=Catalysis by Heteropoly Acids and Multicomponent Polyoxometalates in Liquid-Phase Reactions |journal=Chem. Rev. |volume=98 |issue= 1|pages=171–198 |doi=10.1021/cr960400y |pmid=11851502}}

Efforts continue to extend this theme. POM-based aerobic oxidations have been promoted as alternatives to chlorine-based wood pulp bleaching processes,{{cite journal |last1=Gaspar |first1=A. R. |last2=Gamelas |first2=J. A. F. |last3=Evtuguin |first3=D. V. |last4=Neto |first4=C. P. |date=2007 |title=Alternatives for lignocellulosic pulp delignification using polyoxometalates and oxygen: a review |journal=Green Chem. |volume=9 |issue= 7|pages=717–730 |doi=10.1039/b607824a }} a method of decontaminating water,{{cite journal |last1=Hiskia |first1=A. |last2=Troupis |first2=A. |last3=Antonaraki |first3=S. |last4=Gkika |first4=E. |last5=Kormali |first5=P. |last6=Papaconstantinou |first6=E. |date=2006 |title=Polyoxometallate photocatalysis for decontaminating the aquatic environment from organic and inorganic pollutants |journal=Int. J. Env. Anal. Chem. |volume=86 |issue=3–4 |pages=233 |doi=10.1080/03067310500247520|bibcode=2006IJEAC..86..233H |s2cid=93535976 }} and a method to catalytically produce formic acid from biomass (OxFA process).{{cite journal | first1 = R. |last1= Wölfel |first2= N.|last2= Taccardi|first3= A.|last3= Bösmann|first4= P.|last4= Wasserscheid | title = Selective catalytic conversion of biobased carbohydrates to formic acid using molecular oxygen | journal = Green Chem. | volume = 13|issue= 10 | page = 2759 | date = 2011 | doi = 10.1039/C1GC15434F }} Polyoxometalates have been shown to catalyse water splitting.{{cite journal |last1=Rausch |first1=B. |last2=Symes |first2=M. D. |last3=Chisholm |first3=G. |last4=Cronin |first4=L. |date=2014 |title=Decoupled catalytic hydrogen evolution from a molecular metal oxide redox mediator in water splitting |journal=Science |volume=345 |issue= 6202|pages=1326–1330 |doi= 10.1126/science.1257443 |pmid=25214625|bibcode=2014Sci...345.1326R|s2cid=20572410 }}

=Molecular electronics=

Some POMs exhibit unusual magnetic properties,{{cite journal |last1=Müller |first1=A. |last2=Sessoli |first2=R. |last3=Krickemeyer |first3=E. |last4=Bögge |first4=H |last5=Meyer |first5=J. |last6=Gatteschi |first6=D. |last7=Pardi |first7=L. |last8=Westphal |first8=J. |last9=Hovemeier |first9=K. |last10=Rohlfing |first10=R. |last11=Döring |first11=J |last12=Hellweg |first12=F. |last13=Beugholt |first13=C. |last14=Schmidtmann |first14=M. |date=1997 |title=Polyoxovanadates: High-Nuclearity Spin Clusters with Interesting Host–Guest Systems and Different Electron Populations. Synthesis, Spin Organization, Magnetochemistry, and Spectroscopic Studies |journal=Inorg. Chem. |volume=36 |issue=23 |pages=5239–5240 |doi= 10.1021/ic9703641}} which has prompted visions of many applications. One example is storage devices called qubits.{{cite journal |last1=Lehmann |first1=J. |last2=Gaita-Ariño |first2=A. |last3=Coronado |first3=E. |last4=Loss |first4=D. |date=2007 |title=Spin qubits with electrically gated polyoxometalate molecules |journal=Nanotechnology |volume=2 |issue= 5|pages=312–317 |doi=10.1038/nnano.2007.110 |pmid=18654290 |arxiv=cond-mat/0703501 |bibcode=2007NatNa...2..312L|s2cid=1011997 }} non-volatile (permanent) storage components, also known as flash memory devices.[http://www.thehindu.com/sci-tech/technology/flash-memory-breaches-nanoscales/article6615239.ece "Flash memory breaches nanoscales"], The Hindu.{{cite journal |last1=Busche |first1=C. |last2=Vila-Nadal |first2=L. |last3=Yan |first3=J. |last4=Miras |first4=H. N. |last5=Long |first5=D.-L. |last6=Georgiev |first6=V. P. |last7=Asenov |first7=A. |last8=Pedersen |first8=R. H. |last9=Gadegaard |first9=N. |last10=Mirza |first10=M. M. |last11=Paul |first11=D. J. |last12=Poblet |first12=J. M. |last13=Cronin |first13=L. |date=2014 |title=Design and fabrication of memory devices based on nanoscale polyoxometalate clusters |journal=Nature |volume=515 |issue= 7528|pages=545–549 |doi= 10.1038/nature13951 |pmid=25409147|bibcode=2014Natur.515..545B |s2cid=4455788 }}

=Drugs=

Potential antitumor{{Cite journal |last=Bijelic |first=Aleksandar |last2=Aureliano |first2=Manuel |last3=Rompel |first3=Annette |date=2019 |title=Polyoxometalates as Potential Next-Generation Metallodrugs in the Combat Against Cancer |url=https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201803868 |journal=Angewandte Chemie International Edition |language=en |volume=58 |issue=10 |pages=2980–2999 |doi=10.1002/anie.201803868 |issn=1521-3773 |pmc=6391951 |pmid=29893459}} and antiviral drugs.{{cite journal |last1=Rhule |first1=Jeffrey T. |last2=Hill |first2=Craig L. |last3=Judd |first3=Deborah A. |date=1998 |title=Polyoxometalates in Medicine |journal=Chem. Rev. |volume=98 |issue= 1|pages=327–358 |doi=10.1021/cr960396q|pmid=11851509 }} 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.{{Cite journal|last=Hasenknopf|first=Bernold|title=Polyoxometalates: introduction to a class of inorganic compounds and their biomedical applications|journal=Frontiers in Bioscience|volume=10|issue=1–3|doi=10.2741/1527|pages=275–87|year=2005|pmid=15574368|doi-access=free}}{{Cite book|title=Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity - Springer|volume = 10|last1=Pope|first1=Michael|last2=Müller|first2=Achim|pages=337–342|doi=10.1007/978-94-011-0920-8|series = Topics in Molecular Organization and Engineering|year = 1994|isbn = 978-94-010-4397-7}} The Wells-Dawson structure can efficiently inhibit amyloid β (Aβ) aggregation in a therapeutic strategy for Alzheimer's disease.{{Cite journal|last1=Gao|first1=Nan|last2=Sun|first2=Hanjun|last3=Dong|first3=Kai|last4=Ren|first4=Jinsong|last5=Duan|first5=Taicheng|last6=Xu|first6=Can|last7=Qu|first7=Xiaogang|date=2014-03-04|title=Transition-metal-substituted polyoxometalate derivatives as functional anti-amyloid agents for Alzheimer's disease|journal=Nature Communications|language=en|volume=5|pages=3422|doi=10.1038/ncomms4422|pmid=24595206|bibcode=2014NatCo...5.3422G|doi-access=free}}{{Cite journal|last1=Bijelic|first1=Aleksandar|last2=Aureliano|first2=Manuel|last3=Rompel|first3=Annette|date=2019-03-04|title=Polyoxometalates as Potential Next-Generation Metallodrugs in the Combat Against Cancer|journal=Angewandte Chemie International Edition|language=en|volume=58|issue=10|pages=2980–2999|doi=10.1002/anie.201803868|issn=1433-7851|pmc=6391951|pmid=29893459}} antibacterial{{Cite journal|last1=Bijelic|first1=Aleksandar|last2=Aureliano|first2=Manuel|last3=Rompel|first3=Annette|date=2018|title=The antibacterial activity of polyoxometalates: structures, antibiotic effects and future perspectives|journal=Chemical Communications|language=en|volume=54|issue=10|pages=1153–1169|doi=10.1039/C7CC07549A|issn=1359-7345|pmc=5804480|pmid=29355262}} and antiviral uses.