The oxides ofd0 metals such asV2O5,MoO3,WO3 dissolve at highpH to give orthometalates,VO3−4,MoO2−4,WO2−4. ForNb2O5 andTa2O5, 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 asWO3(OH)− andVO3(OH)2−. These species condense via the process calledolation. 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 theMO3(OH)n− species entails loss of water and the formation ofM−O−M linkages. The stoichiometry forhexamolybdate is shown:[2]
6 MoO2−4 + 10 HCl → [Mo6O19]2− + 10 Cl− + 5 H2O
An abbreviated condensation sequence illustrated with vanadates is:[1][3][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 ofphosphate orsilicate, heteropolymetalate can result. For example, thephosphotungstate anion[PW12O40]3− consists of a framework of twelveoctahedral tungsten oxyanions surrounding a centralphosphate group.
The 1970s witnessed the introduction ofquaternary ammonium salts of POMs.[2] This innovation enabled systematic study without the complications of hydrolysis and acid/base reactions. The introduction of17ONMR spectroscopy allowed the structural characterization of POMs in solution.[7]
Ramazzoite, the first example of a mineral with a polyoxometalate cation, was described in 2016 in Mt. Ramazzo Mine,Liguria, Italy.[8]
The typical framework building blocks arepolyhedral 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 bridgingoxo ligands. Interior oxides are typically triply bridging or even octahedral.[1] POMs are sometimes viewed as soluble fragments ofmetal oxides.[7]
Recurring structural motifs allow POMs to be classified.Iso-polyoxometalates (isopolyanions) feature octahedral metal centers. Theheteropolymetalates 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 exhibitcoordinate 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 ofcluster materials.[10] However, metal-metal bonds are not completely absent in polyoxometalates and they are often present among the highly reduced species.[11]
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 containpentagonal bipyramidal units. These building blocks are found in themolybdenum blues, which aremixed valence compounds.[1]
Polyoxotechnetates form only in strongly acidic conditions, such as inHTcO4 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 thehydronium salt[H7O3]4[Tc20O68]·4H2O by concentrating anHTcO4 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]
Mixed polyoxo(technetate-rhenate) [Tc4O4(H2O)2(ReO4)14]2- polyanion crystals that contain Tc(V) and Re(VII)were also isolated[15] and structurally characterized.
The polyniobates, polytantalates, and vanadates are derived, formally at least, from highly charged [MO4]3- precursors. For Nb and Ta, most common members areM 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 peroxotantalateTa(O 2)3− 4 in alkaline media.[16] 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 ingroup 6 POMs.[17]
The decametalates with the formulaM 10O6− 28 (M = Nb,[18] Ta[19]) are isostructural with decavanadate. They are formed exclusively by edge-sharing {MO6} octahedra (the structure of decatungstateW 10O4− 32 comprises edge-sharing and corner-sharing tungstate octahedra).
Heteroatoms aside from the transition metal are a defining feature ofheteropolymetalates. Many different elements can serve as heteroatoms but most common arePO3− 4,SiO4− 4, andAsO3− 4.
Two views of a [Mo154(NO)14On]z- cluster, omitting water and counter ions. Also shown is theX-ray powder pattern for the salt.
Polyoxomolybdates include the wheel-shapedmolybdenum blue anions and spherical "keplerates". The cluster[Mo154O420(NO)14(OH)28(H2O)70]20− 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 a large inner and outer surface.
Oxoalkoxometalates are clusters that contain both oxide and alkoxide ligands.[20] Typically they lack terminal oxo ligands. Examples include the dodecatitanate Ti12O16(OPri)16,[21] the iron oxoalkoxometalates[22] and iron[23] and copper[24] Keggin ions.
The terminal oxide centers of polyoxometalate framework can in certain cases be replaced with other ligands, such asS2−,Br−, andNR2−.[5][25] Sulfur-substituted POMs are calledpolyoxothiometalates. Other ligands replacing the oxide ions have also been demonstrated, such asnitrosyl andalkoxy groups.[20][26]
Polyfluoroxometalate are yet another class of O-replaced oxometalates.[27]
Numerous hybrid organic–inorganic materials that contain POM cores,[28][29][30]
Illustrative of the diverse structures of POM is the ionCeMo 12O8− 42, which has face-shared octahedra with Mo atoms at the vertices of an icosahedron.[31]
POM-based aerobic oxidations have been promoted as alternatives tochlorine-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 catalysewater splitting.[40]
Potentialantitumor[45] andantiviral drugs.[46][sentence fragment] 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.[47][48] The Wells-Dawson structure can efficiently inhibit amyloid β (Aβ) aggregation in a therapeutic strategy for Alzheimer's disease.[49][50] antibacterial[51] and antiviral uses.
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