The cyclic 48-tungsto-8-phosphate [H₇P₈W₄₈O₁₈₄]³³⁻ Contant–Tézé polyanion and its derivatives [H₆P₄W₂₄O₉₄]¹⁸⁻ and [H₂P₂W₁₂O₄₈]¹²⁻: structural aspects and reactivity

Abstract

Polyoxometalates (POMs) are discrete, anionic metal-oxo clusters of early transition metals in high oxidation states (e.g., W^VI, Mo^VI, V^V) usually comprised of edge- and corner-shared MO₆ octahedra. Lacunary POMs are defect heteropolyanions mainly of the Keggin or Dawson type, and they can be formed by the loss of one or more MO₆ octahedra by controlled base hydrolysis. The largest subclass of POMs are tungstophosphates, and several lacunary derivatives are known, such as the Keggin-based [PW₁₁O₃₉]⁷⁻ and [PW₉O₃₄]⁹⁻, and the Dawson-based [P₂W₁₇O₆₁]¹⁰⁻ and [P₂W₁₅O₅₆]¹²⁻. This review is based on the cyclic 48-tungsto-8-phosphate [H₇P₈W₄₈O₁₈₄]³³⁻ (P₈W₄₈) as well as its smaller derivatives [H₆P₄W₂₄O₉₄]¹⁸⁻ (P₄W₂₄), and [H₂P₂W₁₂O₄₈]¹²⁻ (P₂W₁₂), with a focus on structural aspects, solution stability and reactivity. All three polyanions can be considered as inorganic multidentate O-donor ligands that coordinate with d, f or p-block metal ions. Here we provide a comprehensive overview of guest metal-containing derivatives of the P₈W₄₈ wheel, the P₄W₂₄ half-wheel and the P₂W₁₂ quarter wheel. The structures containing P₂W₁₂ as a building unit are presented in a sequence of increasing number of POM units in the resulting assembly. Transition metal-containing POMs have been of interest for decades due to their remarkable capability of forming novel and unexpected structures associated with interesting and relevant physicochemical properties (e.g., catalysis, magnetism, biomedicine, electrochemistry), and this also applies for derivatives containing P₈W₄₈, P₄W₂₄ and P₂W₁₂.

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Sib Sankar Mal

Dr Sib Sankar Mal received his Ph.D. degree in Chemistry from Jacobs University, Bremen, in 2008, under the supervision of Prof. Ulrich Kortz. Then he did postdoctoral work at the University of Ottawa, Canada, in 2011, and then moved to Hamburg University, Germany, as an Alexander von Humboldt postdoctoral fellow. After completing his postdoctoral work, he joined as an Assistant professor at the National Institute of Technology Karnataka, India, in 2013, where he is currently serving as an Associate professor in the Department of Chemistry. His primary research areas are energy storage, energy conversion, renewable energy, electrochemistry, catalysis, and polyoxometalates.

Abhishek Banerjee

Dr Abhishek Banerjee received his Ph.D. degree in Chemistry from Jacobs University, Bremen, Germany in 2012, under the supervision of Prof. Ulrich Kortz. After postdoctoral research work at Northwestern University, Evanston, USA, with Prof. Mercouri G. Kanatzidis, he joined Visvesvaraya National Institute of Technology, Nagpur, India in 2016, as an Assistant Professor. His primary research areas are in energy conversion, energy storage and oxidative catalysis using organic oxo-thio-metalates. He is the recipient of several awards, including special Ph.D. distinction from Jacobs University, Bremen, Germany as well as Early Career Research Award from Science and Engineering Research Board (SERB), India.

Ulrich Kortz

Prof. Ulrich Kortz performed Ph.D. studies at Georgetown University in Washington, DC under the supervision of Michael T. Pope (1990–1995). Then he did postdoctoral studies with Dante Gatteschi in Florence, Italy (1995–1996) as well as André Tézé and Gilbert Hervé in Versailles, France (1996–1997). In 1997 he started his independent academic career at the American University of Beirut (AUB) in Lebanon. In 2002 he joined Constructor University (formerly International University Bremen and Jacobs University Bremen) as Associate Professor and in 2007 became Full Professor. His research interests include synthetic inorganic and organometallic chemistry, structural chemistry, polyoxometalates, MOFs, catalysis, magnetism, and electrochemistry.

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1. Introduction

The field of polyoxometalate (POM) chemistry has seen increased interest in recent decades due to the extensive range of chemical and physical properties of such compounds. Berzelius reported the formation of a molydophosphate polyoxoanion in 1826, and more than a decade later, Keggin identified the crystal structure of H₃[PW₁₂O₄₀] using powder X-ray diffraction.¹ As such, POMs were elucidated as discrete anionic metal-oxo clusters formed by condensation of simple oxoanions (e.g., WO₄²⁻) in aqueous media upon acidification. POMs consist of oxo-bridged early transition metal atoms of groups V and VI in high oxidation states, such as V^V, Nb^V, Ta^V, Mo^VI, or W^VI.² For the formation of stable polyanions, the metal addendum atom should possess vacant d orbitals, which allows for d_π–p_π back bonding with terminal oxygen atoms. This phenomenon helps to terminate the condensation process at the level of discrete polyanions and disfavors the formation of extended metal oxide lattices.

Based on their chemical composition, POMs can be broadly subdivided into two main subclasses: (i) isopolyanions, which are composed exclusively of metal addenda M and oxo ligands, represented as [M_mO_y]ⁿ⁻, and (ii) heteropolyanions, which contain one or more hetero atoms inside the polyanion, represented as [X_xM_mO_y]^q− (x ≤ m), where X is usually a main group element such as P, Si, Ge, or As. Due to their high thermal and redox stability as well as their radiation-resistant nature, isopolyanions have attracted increasing attention for applications involving the separation and sequestration of radioactive species.³ It is important to note that the formation of polyoxotungstates is accompanied by very slow equilibration of the reaction system, compared to molybdates and vanadates. Prime examples of heteropolyanions are the Keggin ion (e.g. [SiW₁₂O₄₀]⁴⁻) and the Wells–Dawson ion (e.g. [P₂Mo₁₈O₆₂]⁶⁻), incorporating one or two tetrahedral XO₄ hetero groups, respectively. Pope and Müller reviewed the synthesis and characterization of POMs in 1991. Subsequently, Hill, as guest editor, highlighted popular research themes within the field of POM chemistry in a thematic issue of Chemical Reviews in 1998.^4–9

POMs have generated considerable interest across all areas of chemistry due to their versatile nature with regard to shape, size, composition, redox activity, solubility, photochemistry, and charge distribution.⁷ Contemporary areas that have emerged in the multidisciplinary field of POM chemistry are mainly associated with various applications of POMs corresponding to developing new materials.¹⁰ These have shown promise in the fields of nanotechnology,¹¹ biology,^12–15 surfaces,^14–17 catalysis,^18,19 supramolecular materials,^20,21 colloid science,²² and electronic materials,²³ sensors,^24,25 molecular materials^26,27 and magnetism.²⁸ Crucial to the fast development of structural POM chemistry are advances in single-crystal X-ray diffraction (XRD), which allows for faster measurements on smaller crystals, allowing the characterization of large (with several hundred addenda atoms) polyanions.^29,30 Polyoxotungstates, in particular, have been observed to be effective homogeneous photocatalysts for the mineralization of organic pollutants.^31–35 As such, this review will focus on recent developments of the large, cyclic 48-tungsto-8-phosphate [H₇P₈W₄₈O₁₈₄]⁴⁰⁻ (abbreviated as P₈W₄₈) and its smaller fragments [H₂P₂W₁₂O₄₈]¹²⁻ (abbreviated as P₂W₁₂) and [H₆P₄W₂₄O₉₄]¹⁸⁻ (abbreviated as P₄W₂₄). Emphasis will be placed on the synthesis and structure of these polyanions.

In 1945, A. F. Wells suggested a detailed structure for the dimeric (18 : 2) tungstophosphate ion,³⁶ based on Pauling's principles and the structure Keggin had shown for the 12-tungstophosphate. In 1952, the formula [P₂M₁₈O₆₂]⁶⁻ (M = Mo, W) proposed by Wells was experimentally established for the molybdo analogue by Tsigdinos. In 1953, Dawson investigated this polyanion using single-crystal X-ray diffraction,³⁷ and demonstrated that the positions of the W atoms in [P₂W₁₈O₆₂]⁶⁻ coincided with what was postulated by Wells. In 1975 Strandberg,³⁸ and in 1976 D'Amour,³⁹ reported complete and accurate X-ray crystal structures of [α-P₂Mo₁₈O₆₂]⁶⁻ and [α-P₂W₁₈O₆₂]⁶⁻, respectively.

2. The cyclic [H₇P₈W₄₈O₁₈₄]³³⁻ (P₈W₄₈) and its derivatives [H₆P₄W₂₄W₉₄]¹⁸⁻ (P₄W₂₄) and [α-H₂P₂W₁₂O₄₈]¹²⁻ (P₂W₁₂)

The single-crystal X-ray structure for the [P₂W₁₈O₆₂]⁶⁻ polyanion indicated that the eighteen metal centers are not equivalent. Thus, the distinction was made between the α₂ positions for the cap tungstens and the α₁ positions for the belt tungstens. The two caps in this Wells–Dawson structure are composed of three edge-shared MO₆ (M = W, Mo) octahedra, whereas the two equatorial belts are formed via alternating corner- and edge-shared MO₆ units. In 1979 Acerete showed by ¹⁸³W solution NMR that the β geometrical isomer of [P₂W₁₈O₆₂]⁶⁻ differs from the α isomer by a 60° rotation of one W₃O₁₃ cap.^40,41 The Wells–Dawson derivative can be seen as a derivative of the Keggin structure; the removal of a corner-shared W₃O₁₃ triad from [α-PW₁₂O₄₀]³⁻ produces a lacunary structure formulated as [A-α-PW₉O₃₄]⁹⁻, which combines with another equivalent moiety to form the [α-P₂W₁₈O₆₂]⁶⁻ assembly.

Careful solution studies by Contant and Tézé on the chemistry of the complete (plenary) [α-P₂W₁₈O₆₂]⁶⁻ polyanion showed that upon basification a hydrolytic cleavage of W–O(W) bonds occurs, resulting in a mixture of the monovacant (lacunary) species [α₁-P₂W₁₇O₆₁]¹⁰⁻ and [α₂-P₂W₁₇O₆₁]¹⁰⁻, respectively. Further, if the final pH is kept between 4 and 6, the [α₁-P₂W₁₇O₆₁]¹⁰⁻ anion is observed to transform to the [α₂-P₂W₁₇O₆₁]¹⁰⁻ anion readily.⁴² At about pH 10, the trilacunary polyanion [P₂W₁₅O₅₆]¹²⁻ is formed.⁴² Alternatively, in the presence of tris(hydroxymethyl)aminomethane (tris base), the plenary [α-P₂W₁₈O₆₂]⁶⁻ transforms to the hexavacant polyanion [α-H₂P₂W₁₂O₄₈]¹²⁻ (P₂W₁₂).⁴² This P₂W₁₂ is labile and transforms quickly in aqueous, acidic medium to either the unstable, monolacunary polyanion [α₁-P₂W₁₇O₆₁]¹⁰⁻, which successively rearranges to either the more stable [α₂-P₂W₁₇O₆₁]¹⁰⁻,⁴² or the large cyclic polyanion [H₇P₈W₄₈O₁₈₄]⁴⁰⁻ (P₈W₄₈).⁴³ Additionally, Contant and Tézé have revealed that two P₂W₁₂ can be connected end-on to form the dimeric [H₆P₄W₂₄O₉₄]¹⁸⁻ (P₄W₂₄) species in aqueous solution.⁴³ However, the exact linkage of the two P₂W₁₂ units is still unknown.⁴⁴

Lacunary derivatives of the plenary Wells–Dawson ion [α-P₂W₁₈O₆₂]⁶⁻ and their related terminology (α, α₁, α₂) have been thoroughly investigated by Contant.⁴² More recently, Poblet and Cronin have studied the different rotational isomerisms of non-classical Wells–Dawson anions with different heteroatoms based on theoretical and mass spectrometry techniques.⁴⁵

2.1 The metastable, hexalacunary [α-H₂P₂W₁₂O₄₈]¹²⁻ (P₂W₁₂)

The hexalacunary 12-tungsto-2-phosphate P₂W₁₂ is generated by the treatment of [α-P₂W₁₈O₆₂]⁶⁻ by tris(hydroxymethyl)aminomethane (tris base). The formation of P₂W₁₂ can be visualized by removing six tungsten-oxo groups (W-O_t), one from each of the two caps and two from each of the two belts. The P₂W₁₂ was first identified by Contant in 1985, who proved its existence by ³¹P NMR (−8.6 ppm) in lithium chloride solution. The polyanion structure can be inferred from that of the cyclic tetramer P₈W₄₈. This is due to the fact that P₂W₁₂ is labile and readily undergoes rearrangement in aqueous, acidic medium to the more stable monolacunary [α₂-P₂W₁₇O₆₁]¹⁰⁻,⁴² or cyclic P₈W₄₈,⁴³ as stated earlier. As such, the ¹⁸³W NMR spectrum of this polyanion is not clean due to the presence of one or more transformation products in solution simultaneously, as Baker reported.⁴⁶ Very recently, the polyanion P₂W₁₂ was characterized by single-crystal X-ray diffraction studies, as well as a W-bridged dimer and trimer.^47,48 Due to the presence of six lacunary sites in P₂W₁₂, the reactivity of this polyanion with guest metal ions has been explored in some detail, as will be discussed subsequently (vide infra, section 3).

2.2 The dimeric, multilacunary [H₆P₄W₂₄O₉₄]¹⁸⁻ (P₄W₂₄)

The P₄W₂₄ polyanion has been known since 1985,⁴³ and very likely it comprises of two P₂W₁₂ units, which are linked in a C- or S-shaped fashion, resulting in structures with C_2v or C_2h symmetry in solution (Fig. 1).⁴³ The ³¹P and ¹⁸³W NMR spectra have proven the coexistence of both such forms in solution.^40,41,46,49 Only a few polyanions have been reported based on the dimeric P₄W₂₄ unit till date. In 1998, Roussel et al. investigated the structure of P₄W₂₄ by single-crystal X-ray diffraction studies and electron microscopy, which suggested P₄W₂₄ to be a phosphate-containing derivative of a polyoxotungstate with the chemical formula (PO₄)₄(WO₃)_2m (m = 12).⁵⁰

Fig. 1 Polyhedral representation of the C-shaped isomer of P₄W₂₄. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink).

2.3 The crown-shaped, superlacunary [H₇P₈W₄₈O₁₈₄]³³⁻ (P₈W₄₈)

The crown-shaped polyanion [H₇P₈W₄₈O₁₈₄]³³⁻ (P₈W₄₈) comprises of four P₂W₁₂ subunits, which are linked via the caps in a cyclic fashion, resulting in a structure with idealized D_4h symmetry (Fig. 2). The structural elucidation of the polyanion by single-crystal XRD showed the presence of eight potassium ions within the central cavity. This suggests a templating effect of the alkali ions, which may play an important role in the formation process of this cyclic polyanion. The P₈W₄₈ is stable over a broad pH range (ca. 1–8), which represents a remarkable stability in POM chemistry. The presence of potassium ions inside the polyanion crown decreases the solubility of the salt and increases the tendency towards aggregation in solution, causing precipitation. Cronin reported two K-free salts of P₈W₄₈.⁵¹

Fig. 2 Polyhedral representation of P₈W₄₈. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink).⁵¹

3. Metal complexes of [α-H₂P₂W₁₂O₄₈]¹²⁻

3.1 Monomeric structures

To date, only a limited number of transition metal-containing derivatives of P₂W₁₂ have been reported.⁵² Hill reported the peroxo-niobium-containing derivative [P₂W₁₂(NbO₂)₆O₅₆]¹²⁻ (P₂W₁₂(NbO₂)₆), see Fig. 3.⁵³ This peroxo-polyanion was reported to be unstable both in solution and in the solid-state in the absence of hydrogen peroxide. However, the formation of P₂W₁₂(NbO₂)₆ in the presence of H₂O₂ has been demonstrated using several spectroscopic methods, such as infrared spectroscopy (IR), fast atom bombardment-mass spectroscopy (FAB-MS), and ³¹P and ¹⁸³W NMR spectroscopy. The P₂W₁₂(NbO₂)₆ polyanion was prepared by the reaction of P₂W₁₂ with the Lindqvist-type isopolyanion [Nb₆O₁₉]⁸⁻ in the presence of aqueous H₂O₂ as an oxidant. The six Nb atoms in P₂W₁₂(NbO₂)₆ are connected by η²-O ligands, and each has a terminal, side-on peroxo ligand.

Fig. 3 Combined polyhedral/ball-and-stick representation of the peroxo-Nb polyanion [P₂W₁₂(NbO₂)₆O₅₆]¹²⁻ (P₂W₁₂(NbO₂)₆). Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), O (red), Nb (brown).⁵³

In 2002, Nadjo reported the synthesis and characterization of five mixed 3d (Fe, Cu)-4d (Mo) metal-containing P₂W₁₂-based polyanions: α₂-[P₂W₁₂Fe(OH₂)Mo₅O₆₁]⁷⁻ (P₂W₁₂FeMo₅) and α₂-[P₂W₁₃Fe(OH₂)Mo₄O₆₁]⁷⁻ (P₂W₁₃FeMo₄), α₁- and α₂-[P₂W₁₂Cu(OH₂)Mo₅O₆₁]⁸⁻ (P₂W₁₂CuMo₅) and α₂-[P₂W₁₃Cu(OH₂)Mo₄O₆₁]⁸⁻ (P₂W₁₃CuMo₄).⁵⁴ These compounds were characterized by IR, UV-vis, and ³¹P NMR spectroscopy. No XRD structures could be obtained, but in particular, IR and ³¹P NMR spectroscopy were useful tools for purifying the Fe³⁺ and Cu²⁺-substituted derivatives. The authors have also studied the electrocatalytic properties of these polyanions, and the Cu and Fe-substituted derivatives showed good activity for the electrocatalytic reduction of nitrate.^55–57

Gouzerh reported dimeric 3d transition metal (Fe³⁺, Co²⁺, Mn²⁺, Ni²⁺) containing derivatives of P₂W₁₂. The iron(III)-containing polyanion [H₄P₂W₁₂Fe₉O₅₆(CH₃COO)₇]⁶⁻ (Fe₉P₂W₁₂), see Fig. 4a, was prepared by a simple one-pot reaction of P₂W₁₂ with an excess of iron(III) chloride in aqueous solution containing lithium chloride and lithium acetate, at room temperature.⁵⁸ The monomeric Fe₉P₂W₁₂ structure is derived from P₂W₁₂ by filling the six vacant sites with iron atoms, with the now-formed plenary Wells–Dawson type species {P₂W₁₂Fe₆} having three additional iron(III) atoms grafted onto the hexa-iron-oxo face of the polyanion, resulting in Fe₉P₂W₁₂, which has idealized C₂ symmetry. Magnetic susceptibility measurements on Fe₉P₂W₁₂ showed intramolecular antiferromagnetic coupling between the two high-spin Fe³⁺ centers.

Fig. 4 Combined polyhedral/ball-and-stick representations of (a) [H₄P₂W₁₂Fe₉O₅₆(CH₃COO)₇]₆⁻ (Fe₉P₂W₁₂), and (b) [{M(H₂O)₄}₂{H₁₂P₄W₂₈Fe₈O₁₂₀}]¹²⁻ (M = Co²⁺, Mn²⁺, Ni²⁺). Color code: WO₆ octahedra (green and brown), PO₄ tetrahedra (pink), Fe (green), M (turquoise), O (red), C (grey). H atoms on carbon have been omitted for clarity.^58,59

3.2 Dimeric structures

Upon further investigation of the Fe³⁺ and P₂W₁₂ system by Gouzerh, it was observed that lowering the Fe³⁺ : P₂W₁₂ ratio and the solution pH, as well as prolonged heating, led to the formation of dimeric polyanions with the general formula [H_yP₄W_28+xFe_8−xO₁₂₀]^{(28−y−3x)−}.⁵⁹ These species have extra tungsten atoms in the new polyanions (with slight variations in the value of x), which have been isolated as different salts. The polyanion [H₁₂P₄W₂₈Fe₈O₁₂₀]¹⁶⁻ (Fe₈P₄W₂₈) comprises two {P₂W₁₄Fe₄} units linked via Fe–O–Fe′ bonds, resulting in a cubic Fe₈ topology. The Fe₈P₄W₂₈ is formed by reaction of P₂W₁₂ with hydrous iron(III) oxide or iron(III) acetate [Fe₃O(OAc)₆(H₂O)₃]Cl·5H₂O.⁵⁸

An extra tungsten atom arising from the decomposition of P₂W₁₂ in situ or upon deliberate addition of tungstate to the reaction mixture competes with the added metal ions, as has been observed in the reactions with lanthanide ions.^60,61 Frequently, adding of extra tungstate to the reaction mixture is not essential for the formation of such structures.⁶¹ The cyclic voltammogram (CV) of the polyanion Fe₈P₄W₂₈ displays four reduction waves. The first one at −0.32 V vs. SCE is attributed to the reduction of Fe³⁺ to Fe²⁺.

A polyanion with the composition [H_yP₄W_28+xFe_8−xO₁₂₀]^{(28−y−3x)−} (x ≈ 2) was characterized among the products obtained from a 1 : 4 : 2 aqueous mixture of P₂W₁₂, sodium tungstate, and iron(III) chloride at pH 4.2.⁵⁹ When P₂W₁₂ is reacted with the mixed-metal complexes [Fe^III₂M^IIO(OAc)₆(H₂O)₃] (M = Co²⁺, Mn²⁺, Ni²⁺), the polyanions [{M(H₂O)₄}₂{H₁₂P₄W₂₈Fe₈O₁₂₀}]¹²⁻ (M₂Fe₈P₄W₂₈) (Fig. 4b) (M = Co²⁺, Mn²⁺, Ni²⁺) are obtained, together with Fe₈P₄W₂₈.⁵⁹ These compounds were isolated as potassium salts. Magnetic susceptibility measurements on Co₂Fe₈P₄W₂₈ are consistent with two non-interacting Co²⁺ centers, suggesting that the ground state of the Fe₈P₄W₂₈ unit is diamagnetic, along with the intramolecular antiferromagnetic coupling of the eight Fe³⁺ centers. In contrast, the parent polyanion Fe₈P₄W₂₈ showed weak magnetization, arising either from paramagnetic impurities or a partial substitution of Fe for W.⁵⁹ The cyclic voltammogram indicated that the Co²⁺ centers are not electrochemically active in the potential range investigated.

Wang and coworkers investigated the reactivity of P₂W₁₂ with Co²⁺ ions in an aqueous acidic medium. A new heart-shaped dimeric Co²⁺-containing polyanion [{W₂Co₂O₈(H₂O)₂}(P₂W₁₂O₄₆)₂]²⁰⁻ (W₂Co₂(P₂W₁₂)₂) was synthesized and structurally characterized by IR spectroscopy, thermogravimetry, electrochemistry, and single-crystal X-ray diffraction.⁶² The polyanion W₂Co₂(P₂W₁₂)₂ comprises two subunits of P₂W₁₂, which are fused via four W–O–W bonds and grafting of two W and two Co atoms in the vacant positions. This type of W–O–W connectivity was observed for the first time by Kortz and coworkers in the dimethyletin-containing tungstophosphate [{Sn(CH₃)₂}₄(H₂P₄W₂₄O₉₂)₂]²⁸⁻ (vide infra, section 4). Notably, all the addenda positions in W₂Co₂(P₂W₁₂)₂ are disordered in the cluster.

Very recently, a dimeric mixed-addenda Nb/W polyanion, [H₆P₂W₁₂Nb₄O₅₉(NbO₂)₂]₂⁸⁻ (P₂W₁₂Nb₄(NbO₂)₂)₂, has been isolated under acidic condition by Yue, He, and coworkers.⁶³ This Nb-peroxo-containing polyanion was synthesized by the reaction of K₇H[Nb₆O₁₉]·13H₂O and P₂W₁₂ in sodium formate buffer in the presence of H₂O₂. The polyanion was characterized by single-crystal X-ray diffraction and elemental analysis. The structure revealed that six Nb atoms occupy the vacant sites of the hexalacunary P₂W₁₂ precursor to form a {P₂W₁₂Nb₆(O₂)₂} mixed-addenda subunit. Two such {P₂W₁₂Nb₆(O₂)₂} subunits are joined by two Nb–O–Nb bridges at the belt positions to form a dimeric unit. This polyanion, (P₂W₁₂Nb₄(NbO₂)₂)₂, is structurally different from the Nb-peroxo polyanions reported by Hill and coworkers, for example, P₂W₁₂(NbO₂)₆, as discussed in section 3.1 (vide supra).⁵³ In P₂W₁₂(NbO₂)₆ structure, all niobium atoms have terminal peroxo ligands and exist as a six-membered Nb-peroxo monomer; whereas in (P₂W₁₂Nb₄(NbO₂)₂)₂, only two niobium ions have peroxo ligands, and the polyanion is a dimer. A common feature for both polyanions was that the polyanions aggregated to larger species when the peroxo groups in either P₂W₁₂Nb₄(NbO₂)₂ or P₂W₁₂(NbO₂)₆ were removed by heating or chemical reduction.^64–66

In 2015, Niu, Wang, and coworkers reported two new multi-Nb-containing POM structures, [H₁₃{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]⁷⁻ (P₂W₁₂Nb₆(O₂)₄)₂ (Fig. 5a) and [H₁₄{P₂W₁₂Nb₇O₆₃(H₂O)₂}₄{Nb₄O₄(OH)₆}]¹⁶⁻ (P₂W₁₂Nb₇)₄Nb₄ (Fig. 5b).⁶⁷ Interestingly, (P₂W₁₂Nb₇)₄Nb₄ has the highest nuclearity of all niobium-containing heteropolyanions to date. The polyanion {P₂W₁₂Nb₆(O₂)₄}₂ is a di-Nb–O–Nb-linked dimer of two P₂W₁₂ units with the six vacant sites of each P₂W₁₂ unit occupied by four niobium-peroxo groups (NbO₂) and two niobium-oxo groups (Nb–O). Further, (P₂W₁₂Nb₇)₄Nb₄ comprises dimeric [P₄W₂₄Nb₁₄O₁₂₆(H₂O)₄]¹⁸⁻ subunits and an adamantane-like Nb₄O₆ core. The [P₄W₂₄Nb₁₄O₁₂₆(H₂O)₄]¹⁸⁻ subunit in (P₂W₁₂Nb₇)₄Nb₄ comprises two [Nb₆P₂W₁₂O₆₁]¹⁰⁻ units without any niobium-peroxo groups. The [Nb₆P₂W₁₂O₆₁]¹⁰⁻ units are connected through Nb–O–Nb bridges. The adamantane-like {Nb₄O₆} core is composed of four niobium metal atoms connected by oxo ligands. The authors have demonstrated that careful synthetic control can stabilize the (Nb₆P₂W₁₂) fragment in both polyanions as a dimer or a tetramer. Moreover, ³¹P and ¹⁸³W NMR spectra indicated that (P₂W₁₂Nb₇)₄Nb₄ is stable in solution. The photocatalytic activity for H₂ evolution has been studied for both polyanions, and (P₂W₁₂Nb₇)₄Nb₄ exhibited good activity.

Fig. 5 Combined polyhedral, ball-and-stick representations of (a) [H₁₃{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]⁷⁻ and (b) [H₁₄{P₂W₁₂Nb₇O₆₃(H₂O)₂}₄{Nb₄O₄(OH)₆}]¹⁶⁻, respectively. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), Nb (brown), O (red).⁶⁷

In 2018, Kögerler and coworkers reported an open dimeric structure based on the P₄W₂₄ unit with {P₄W₂₄O₂₉} being capped by two phenyl phosphonate or -arsonate ligands, [(PhXO)₂P₄W₂₄O₉₂]¹⁶⁻ (X = P, As) (PhXOP₄W₂₄).⁶⁸ In 2006, Kortz and coworkers have also reported P₄W₂₄ units in the dimethyltin-grafted polyanion [{Sn(CH₃)₂}₄(H₂P₄W₂₄O₉₂)₂]²⁸⁻ (Sn(CH₃)₂P₄W₂₄) (vide infra).⁴⁴ For PhXOP₄W₂₄, the ³¹P NMR spectra confirm a stabilizing effect on the P₄W₂₄ unit due to the presence of the two phoshonate/arsonate ligands. Further studies on the incorporation of Co²⁺ ions into the central cavity of PhXOP₄W₂₄ resulted in the formation of [{(H₂O)₄Co}(PhXO)₂P₄W₂₄O₉₂]¹⁴⁻ units linked into an infinite 1D chain in the solid state via additional “outer” Co²⁺ centers.

Very recently, the same group reported an unprecedented discrete dimeric polyanion [(o-H₂N-C₆H₄-AsO₃)₄P₄W₂₄O₈₅]¹⁴⁻ ((o-NH₂-C₆H₆-AsO)₄P₄W₂₄O₉₂), by the condensation reaction of o-aminophenylarsonic and P₂W₁₂ in acidic media. The polyanion (o-NH₂-C₆H₆-AsO)₄P₄W₂₄O₉₂ was further reacted with divalent transition metal ions Mn²⁺, Co²⁺, Ni²⁺. The introduction of the divalent transition metal ions in the reaction mixture resulted in V-shaped one-dimensional (1D) coordination polymeric polyanions [{M(H₂O)₄}P₄W₂₄O₉₂(C₆H₆AsNO)₂]¹⁴⁻ (M = Mn²⁺, Co²⁺, Ni²⁺) (M(o-NH₂-C₆H₆-AsO)₂P₄W₂₄),^69,70 where a similar type of structure was first observed by Kortz group (vide infra). Each monomeric unit of (o-NH₂-C₆H₆-AsO)₄P₄W₂₄O₉₂ consists of P₂W₁₂, comprising two PW₄O₂₁ belts capped with two W₂O₁₀ units linked through oxygen atoms. The transition metal complex M(o-NH₂-C₆H₆-AsO)₂P₄W₂₄ exhibits dimeric anionic assembly and coordinates one oxygen atom from the upper PW₄O₂₁ belt of each P₂W₁₂L subunits.

3.3 Trimeric structures

In 2008, Wang and coworkers were able to isolate three trimeric polyoxotungstates upon the reaction of 3d metal ions with P₂W₁₂ at different pH, [K₃⊂{Mn(H₂O)₄}₂{WO₂(H₂O)₂}₂{WO(H₂O)}₃(P₂W₁₂O₄₈)₃]¹⁹⁻ (Mn₂(P₂W₁₂)₃W₅) (Fig. 6), K₃Na₇Li_5.5Ni_0.25[Na₃⊂{Ni_3.5(H₂O)₁₃}{WO₂(H₂O)₂}₂{WO(H₂O)}₃(P₂W₁₂O₄₈)₃]·64H₂O (Ni_3.5(P₂W₁₂)₃W₅), and [Na₃⊂{Cu₃(H₂O)₉}{WO₂(H₂O)₂}₂{WO(H₂O)}₃(P₂W₁₂O₄₈)₃]¹⁷⁻ (Cu₃(P₂W₁₂)₃W₅).⁷¹ These polyanions were prepared by reaction of the corresponding transition metal salts (Mn²⁺, Ni²⁺, and Cu²⁺) with P₂W₁₂ and Na₂WO₄ in aqueous acidic solutions. The polyanion Mn₂(P₂W₁₂)₃W₅ was obtained in a pH 4.0 solution, whereas the other two polyanions Ni_3.5(P₂W₁₂)₃W₅ and Cu₃(P₂W₁₂)₃W₅ were obtained in the pH range 1.5–2.5.⁷¹ As based on the formula, the Ni-containing polyanion appears to be a mixture of at least two species.

Fig. 6 Combined polyhedral/ball-and-stick representation of [K₃⊂{Mn(H₂O)₄}₂{WO₂(H₂O)₂}₂{WO(H₂O)}₃(P₂W₁₂O₄₈)₃]¹⁹⁻. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), Mn (teal), O (red).⁷¹

Yao et al. suggested that the pH plays an important role in forming these P₂W₁₂-based heteropolytungstates.⁷² Upon further decreasing the pH to 1.0, they were able to isolate two new compounds, [Na₃⊂{Co(H₂O)₄}₆{WO(H₂O)}₃(P₂W₁₂O₄₈)₃]¹⁵⁻ (Co₆(P₂W₁₂)₃W₃), and [Na₃⊂{Ni(H₂O)₄}₆{WO(H₂O)}₃(P₂W₁₂O₄₈)₃]¹⁵⁻ (Ni₆(P₂W₁₂)₃W₃).⁷² It is worth mentioning that all the vacant sites in Co₆(P₂W₁₂)₃W₃ and Ni₆(P₂W₁₂)₃W₃ are occupied by the 3d metal ions. Therefore, it is observed that a lower pH facilitates the combination of more transition metal atoms encapsulated within the {(P₂W₁₂)₃W₃} shell. All these polyanions have crown-type structures, comprising three P₂W₁₂ subunits linked by three {WO(H₂O)} fragments in a corner-sharing arrangement, resulting in the trimeric cluster [P₆W₃₉O₁₄₇(H₂O)₃]³⁰⁻ (abbreviated as P₆W₃₉). In such crown-shaped polyanions, the three W atoms in the hinges are all in a hexa-coordinated environment, with all transition metal atoms also fully coordinated. The six vacant sites on the polyanion Mn₂(P₂W₁₂)₃{WO₂(H₂O)₂}{WO(H₂O)}₃ are occupied by two W^VI atoms, two Mn²⁺ ions, and two potassium counter cations, respectively. For the polyanion Ni_3.5(P₂W₁₂)₃{WO₂(H₂O)₂}₂{WO(H₂O)}₃, two W^VI and four Ni²⁺ atoms fill up the six vacant sites of the P₆W₃₉ shell, with all the Ni²⁺ centers fully occupied, except the Ni5 position which exhibits a crystallographic site-occupancy disorder with a Na atom, resulting in a total of two W^VI and 3.5 Ni²⁺ ions being incorporated in the structure. In a similar sense, for the polyanion Cu₃(P₂W₁₂)₃{WO₂(H₂O)₂}₂{WO(H₂O)}₃, two W^VI and four Cu²⁺ guest atoms occupy the six vacant sites of the P₆W₃₉ unit. However, all these metal sites, except W40, also have site-occupancy disorder, resulting in two W^VI and 2.75 Cu²⁺ atoms as guests in the host structure.

More recently, Wang and coworkers have reported mixed 3d/4f metal ion-based P₆W₃₉ structures, [K₃⊂{GdMn(H₂O)₁₀}{HMnGd₂(tart)O₂(H₂O)₁₅}{P₆W₄₂O₁₅₁(H₂O)₇}]¹¹⁻ ({MnGd)(HMnGd₂P₆W₄₂}_∞) and [K₃⊂{GdCo(H₂O)₁₁}₂{P₆W₄₁O₁₄₈(H₂O)₇}]¹³⁻ (CoGdP₆W₄₁)_∞, with organic guests (tartrate and (CH₃)₂NH·HCl respectively).⁷³ According to the authors, the introduction of such secondary organic ligands stabilizes the lanthanide atoms and/or reduces the reactivity of the lanthanide atoms with the polyanions. The crown-shaped P₆W₃₉ is formed by the encapsulation of transition metal and lanthanide atoms inside the cavity of the polyanion. The polyanion (MnGd)(HMnGd₂P₆W₄₂)_∞ forms two-dimensional porous frameworks through the Gd and Mn linkers, whereas the polyanion (CoGdP₆W₄₁)_∞ forms a one-dimensional chain linked through Gd ions, which exhibit a square-antiprismatic geometrical environment. These are the first POM structures comprising mixed lanthanide and transition metal atoms. Wang, Niu, and coworkers have isolated a trimeric assembly of P₂W₁₂ units joined by Cr-atoms, [H₂₃(Cr(H₂O)₂)₃(H₂P₂W₁₂O₄₈)₃]⁴⁻ (Cr₃(P₂W₁₂)₃).⁷⁴ The structure bears similarity with a trimeric {P₆W₃₉} unit, except that Cr has replaced the W-atoms.

3.4 Tetrameric structures

Following Gouzerh's report of the metastable polyanion Fe₉(OAc)₇P₂W₁₂ (section 3.1, vide supra), which was isolated as a lithium–potassium salt,⁵⁸ subsequent heating of this polyanion in aqueous sodium acetate solution transformed it into different compounds, which were successively isolated as sodium–potassium salts, such as Na₁₆K₁₂[H₅₆P₈W₄₈Fe₂₈O₂₄₈] (Fe₂₈P₈W₄₈) and Na₁₆K₁₀[H₅₅P₈W₄₉Fe₂₇O₂₄₈] (Fe₂₇P₈W₄₉).⁵⁸ The polyanion Fe₂₈P₈W₄₈ was characterized by electrochemistry and magnetic measurements and by determination of the unit cell parameters. Subsequently, the formula of the polyanion Fe₂₇P₈W₄₉ was proposed on the basis of an X-ray diffraction study. Godin et al. hinted at other novel compounds’ formation by lowering either the pH or the Fe³⁺ : P₂W₁₂ ratio. However, IR spectroscopy and chemical analysis were insufficient to determine the composition and purity of these species. The polyanion Fe₂₇P₈W₄₉ comprises four {P₂W₁₂Fe₆} units, each bridged through Fe–O–Fe linkages to an {Fe₄O₆} cluster core. The linkage is through pairs of three Fe–O–Fe bridges involving the three outer iron atoms. According to crystallographic refinement, the polyanion Fe₂₇P₈W₄₉ seems to have 25% W and 75% Fe occupancy, suggesting a mixture of polyanions. The polyanion exhibits antiferromagnetic coupling between the Fe³⁺ centers (Fig. 7).

Fig. 7 Combined polyhedral/ball-and-stick representation of [H₅₅P₈W₄₉Fe₂₇O₂₄₈]²⁷⁻. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), O (red), Fe (green-yellow), Fe/W (brown).⁵⁸

Wang and coworkers have reported the 3d transition metal (Co²⁺, Ni²⁺) modified {P₈W₄₉} polyanions {[Co(H₂O)₂Cl][Co(H₂O)₃]₂[Co(H₂O)₅]_1.5[Co(H₂O)₃H₄P₈W₄₉O₁₈₇(H₂O)]}²⁶⁻ ([Co(H₂O)₂][Co(H₂O)₃]₂[Co(H₂O)₅]_1.5P₈W₄₉) and {[Ni(H₂O)₃]₂[{Ni(H₂O)₃}_1.5H₃P₈W₄₉O₁₈₇(H₂O)]}³⁰⁻ ([Ni(H₂O)₃]₂[Ni(H₂O)₃]_1.5P₈W₄₉), respectively, by the reaction of P₂W₁₂ with the respective transition metal salt.⁷⁵ The transformation from P₂W₁₂ to P₈W₄₉ happens during the reaction performed in an aqueous acidic medium. The structural arrangements of Co²⁺ and Ni²⁺ in [Co(H₂O)₂][Co(H₂O)₃]₂[Co(H₂O)₅]_1.5P₈W₄₉ and [Ni(H₂O)₃]₂[Ni(H₂O)₃]_1.5P₈W₄₉ are different from that of several other Mn²⁺ complexes reported by Cronin's and Proust's groups (see section 5).^21,76,77

In 2014, Niu and coworkers reported the gigantic Nb₂₈ cluster encapsulated in a hexa-lacunary {P₂W₁₂} precursor, [{Nb₄O₆(OH)₄}{Nb₆P₂W₁₂O₆₁}₄]³⁶⁻ ((Nb₄O₆)(Nb₆P₂W₁₂)₄).⁷⁸ This polyanion was formed directly by controlling the reaction parameters (e.g., pH, concentration, temperature) and isolated as a sodium salt. The salt was characterized by single-crystal X-ray diffraction, IR spectroscopy, and elemental analysis. The structure of (Nb₄O₆)(Nb₆P₂W₁₂)₄ reveals that the polyanion comprises two [P₄W₂₄Nb₁₂O₁₂₂]²⁰⁻ dimeric units, rotated 180° with respect to each other and connected by four Nb–O–Nb bridges, resulting in an adamantane-type {Nb₄O₆} core.

Very recently, Zhang and coworkers have reported the formation of Na₂₄[Mn₈(H₂O)₃₂P₈W₄₈O₁₈₄]·58H₂O, K₄Na₁₆H₄[Co₈(H₂O)₃₂P₈W₄₈O₁₈₄]·76H₂O, and Na₂₀H₄[Ni₈(H₂O)₃₂P₈W₄₈O₁₈₄]·72H₂O (M₈P₈W₄₈, M = Mn²⁺, Co²⁺, Ni²⁺) by reaction of the respective divalent metal ion with the hexavalent P₂W₁₂ at room temperature in aqueous solution.⁷⁹ The direct reaction of the metal ions with P₈W₄₈ was not successful.

3.5 Hexameric structures

In 2015, Niu, Wang, and coworkers reported a hexameric {Nb₆P₂W₁₂}-based Mn₁₅-oxo cluster, [H₁₂₃Nb₃₆P₁₂W₇₂Mn^III₁₂Mn^II₃NaO₄₂₄]¹⁰⁻ (Mn₁₅(Nb₆P₂W₁₂)₆), with single-molecule magnet (SMM) properties.⁸⁰ The polyanion structure consists of three main parts: six peroxo-free {Nb₆P₂W₁₂} units, four {Mn^III₃} trinuclear cores, and four {Mn^II} hinges. The {Mn^III₃} unit is made of three mutually corner-bridged {Mn^IIIO₆} octahedra (Fig. 8). Two of the Mn⋯Mn distances in the trinuclear {Mn^III₃} unit are observed to be identical and slightly longer or shorter than the third Mn⋯Mn distance, having an overall shape of an equivalent isosceles triangle. Each {Mn^III₃} unit is surrounded by three Wells–Dawson {Nb₆P₂W₁₂} units and six Mn–O–Nb bridges. Further, each {Mn^II} hinge shows crystallographic positional disorder of Mn²⁺ and Na⁺ (0.75 : 0.25), being coordinated by three μ₂-oxo groups and four-terminal oxygen atoms (water molecules), thus leading to a distorted octahedral geometry. Therefore, the authors suggest that the disordered metal centre should be formulated as [Mn^II_0.75Na_0.25(H₂O)₄]^1.75+.

Fig. 8 Combined polyhedral/ball-and-stick representation of [H₁₂₃Nb₃₆P₁₂W₇₂Mn^III₁₂Mn^II₃NaO₄₂₄]¹⁰⁻. Color code: WO₆ octahedra (green), W (black), Nb (yellow), MnO₆ octahedra (light pink), PO₄ tetrahedra (pink), O (red), Na (turquoise).⁸⁰

In 2016, Mizuno and coworkers isolated a giant hexameric ring-shaped manganese-substituted polyanion, [{γ-P₂W₁₂O₄₈Mn₄(acac)₂(OAc)}₆]⁴²⁻ (acac = acetylacetonate, OAc = acetate) {P₂W₁₂O₄₈Mn₄}₆ by reaction of the hexavacant lacunary P₂W₁₂ precursor with Mn(acac)₃ in organic medium.⁸¹ The polyanion {P₂W₁₂O₄₈Mn₄}₆ is composed of six manganese-substituted monomeric {P₂W₁₂O₄₈Mn₄} units, overall resulting in a cyclic, hexameric structure. Each {P₂W₁₂Mn₄} monomeric unit consists of two types of Mn-coordinated ligands acetyl acetonate (acac) and acetate, respectively, so the polyanion has two different active sites. Interestingly, the hexameric polyanion transforms to the tetrameric polyanion, [{γ-P₂W₁₂O₄₈Mn₄(H₂O)₆}₄(H₂O)₄]²⁴⁻ {P₂W₁₂O₄₈Mn₄}₄ in the absence of the organic ligands capping the manganese ions.

Following the report of Mizuno's 2016 work, Yamaguchi and coworkers in 2019 have reported the synthesis of the tetra-n-butylammonium (TBA) salts of a series of new isostructural divalent transition metal substituted polyanion family, (TBA₅[γ-P₂W₁₂O₄₄M₂(OAc)(CH₃CONH)₂]·nH₂O·mCH₃CN; M = Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺; OAc = acetate) (γ-P₂W₁₂O₄₄M₂(OAc)) including unique edge-shared bis (square-pyramidal) {O₂M(μ₃-O)₂(μ-OAc)MO₂} core. The metal ions occupy the vacant belt positions of the γ-P₂W₁₂ precursor, whereas the two acetamide (CH₃CONH₂) groups stabilize the γ-P₂W₁₂ unit.⁸²

3.6 Rearrangement of [H₂P₂W₁₂O₄₈]¹²⁻ in acidic medium

Lanthanide-containing POMs are interesting due to potentially attractive photoluminescence, Lewis acid catalysis, electrochemistry, and magnetic properties. Due to the larger size and resulting higher coordination number of lanthanide ions as compared to d-block metal ions, they cannot be fully incorporated into the vacant sites of lacunary POMs and hence tend to link two or more polyanions, as observed in one of the largest polyanions, [As^III₁₂Ce^III₁₆(H₂O)₃₆W₁₄₈O₅₂₄]⁷⁶⁻ (W₁₄₈), reported by Pope and coworkers.⁸³

In 2000, Pope's group reported the polyanion [Ce₄(OH₂)₉(OH)₂(α₁,α₁-P₂W₁₆O₅₉)₂]¹⁴⁻ (Ce₄(α₁α₁-P₂W₁₆)₂) in which the P₂W₁₂ precursor picks up extra tungsten atoms and then incorporates four cerium(III) ions.⁶⁰ The polyanion Ce₄(α₁α₁-P₂W₁₆)₂ exhibits a dimeric structure with C_2v symmetry, comprising two {α₁,α₁-P₂W₁₆O₅₉} (abbreviated as P₂W₁₆) units connected via a central core of four cerium atoms. There are two structural types of cerium in this polyanion, the first one has 10-coordination and the other has 9-coordination. The ³¹P and ¹⁸³W NMR studies in aqueous solution suggested that the polyanion is stable in solution.

In 2003, Kortz reported the lanthanum-substituted polyanion [{La(CH₃COO)(H₂O)₂(α₂-P₂W₁₇O₆₁)}₂]¹⁶⁻ (La(OAc)-(α₂-P₂W₁₇)₂), which was synthesized by reaction of La³⁺ ions with the hexalacunary polyanion P₂W₁₂.⁸⁴ It was observed that P₂W₁₂ rearranges quickly in an aqueous, acidic medium to the monolacunary [α₁-P₂W₁₇O₆₁]¹⁰⁻ which in turn rearranges to [α₂-P₂W₁₇O₆₁]¹⁰⁻. The polyanion La(OAc)-(α₂-P₂W₁₇)₂ is composed of two [α₂-P₂W₁₇O₆₁]¹⁰⁻ fragments connected by two lanthanum acetato dimers (La₂(CH₃COO)₂(H₂O)₄)⁴⁺, resulting in a head-on, transoid dimer with C_2h symmetry. Each La³⁺ atom exhibits a nine-coordinatied geometry.⁶¹ The monolacunary Wells–Dawson ion [α₂-P₂W₁₇O₆₁]¹⁰⁻ is known to react with lanthanide ions to form Pope's “head-on dimers” or Francesconi's “side-on dimers”.^61,85

The Enbo Wang group has shown considerable interest in synthesizing metal–organic frameworks containing the Wells–Dawson ion. In 2008, they reported three transition metal-containing Wells–Dawson-based assemblies, using the P₂W₁₂ anion as a reagent, K₄Na₁₀[α₁-CuP₂W₁₇O₆₀(OH)]₂·58H₂O (Cu₂P₄W₃₄), Na₂[H₂en][H₂hn]_0.5[Cu(en)₂]_4.5[α₁-CuP₂W₁₇O₆₀(OH)]₂·43H₂O {Cu(en)₂α₁-CuP₂W₁₇}_∞, and Na₃[H₂hn]_2.5[P₂W₁₇O₆₀Cu(OH)₂]·14H₂O ((H₂hn(CuP₂W₁₇)₂)_∞, where en = 1,2-ethylenediamine; hn = 1,6-hexamethylene diamine).⁸⁶ The Cu₂P₄W₃₄ polyanion consists of two α₁-P₂W₁₇ units with the vacant α₁ (belt) position being occupied by a Cu²⁺ atom and the two Wells–Dawson units are connected by two W-OH-Cu groups resulting in a dimeric [α₁-CuP₂W₁₇O₆₀(OH)]₂¹⁴⁻ (Cu₂P₄W₃₄) assembly.

The (Cu(en)₂α₁-CuP₂W₁₇)_∞ constitutes the first 2-D organic–inorganic hybrid network based on such double-Dawson-type polyanion (DDTP) building blocks and (Cu(en)₂)²⁺ bridging units, representing a large polyoxotungstate building block for the construction of extended organic–inorganic hybrid materials. Subsequently, {(H₂hn)(CuP₂W₁₇)₂}_∞ possesses a 3-D hybrid supramolecular framework with 1-D channels that are constructed from the half-unit of the ‘DDTP’ and ‘hn’ cations. A magnetic study of (Cu(en)₂α₁-CuP₂W₁₇)_∞ indicated weak antiferromagnetic interactions and that the two types of Cu²⁺ centers are well separated.

All the above examples consist of mono-Cu²⁺ substituted Wells–Dawson-type polyanions. Further exploration by the Wang group focused on feasible synthetic routes by adjusting the composition of Wells–Dawson polyanions to capture more transition metal ions. Subsequently, they reported a 1-D inorganic polymer formed by the reaction of P₂W₁₂ with Mn²⁺ ions, Na₈H₂L(H₂enMe)₄[Mn(H₂O)₂ (W₄Mn₄O₁₂)(P₂W₁₄O₅₄)₂]·17H₂O, {W₄Mn₄(MnP₂W₁₄)₂}_∞ (L = pyromellitic dianhydride PMDA).⁸⁷ The structure of this compound consists of two tetravacant Dawson moieties [P₂W₁₄O₅₄]¹⁴⁻ (P₂W₁₄) sandwiching an eight-metal cluster with W–O–W_(Mn) and P–O–W_(Mn) connecting modes. The metal centers in this eight-metal cluster form an almost regular cubane-like (W₄Mn₄) cluster. The solid-state network is completed by multi-Mn²⁺-substituted DDTP and Mn²⁺ linkers with idealized C₂ symmetry, which is further connected into a 3-D supramolecular network via extensive hydrogen-bonding interactions.

Fang, Kögerler, and coworkers have isolated potassium and lithium salts of a 40-manganese(III)-containing polyanion, [(P₈W₄₈O₁₈₄){(P₂W₁₄Mn₄O₆₀)(P₂W₁₅Mn₃O₅₈)₂}₄]¹⁴⁴⁻ ((P₈W₄₈)(Mn₄P₂W₁₄)₄(Mn₃P₂W₁₅)₈), using hexavacant P₂W₁₂ as a reagent.⁸⁸ Single-crystal XRD revealed that the polyanion ((P₈W₄₈)(Mn₄P₂W₁₄)₄(Mn₃P₂W₁₅)₈) consists of P₂W₁₄, P₂W₁₅, and P₈W₄₈ units. The polyanion was synthesized by reacting P₂W₁₂ with [Mn₁₂O₁₂(OAc)₁₆(H₂O)₄]·4H₂O·2HOAc in lithium acetate/acetic acid medium. Interestingly, although the cyclic P₈W₄₈ is present in the pocket of the polyanion, no Mn³⁺ ions are found inside the cavity of the P₈W₄₈ unit. It appears that P₈W₄₈ acts as a template, reducing the steric repulsion between the tri-Dawson ({P₂W₁₄Mn₄}{P₂W₁₅Mn₃}₂) units. Magnetic measurements revealed intramolecular antiferromagnetic coupling between the Mn³⁺ ions.

In 2013, Yang and coworkers reported three lanthanide derivatives of P₈W₄₈, {[Ln₂(μ-OH)₄(H₂O)x]₂(H₂₄P₈W₄₈O₁₈₄)}¹²⁻ (Ln = Nd, Sm, Tb) (Ln₄P₈W₄₈), and a manganese derivative, [K(H₂O)₂]₄[K₄(μ-H₂O)₈]₂[K(H₂O)]₈{[Mn₈(H₂O₎₁₆](H₄P₈W₄₈O₁₈₄)} (K₈Mn₈P₈W₄₈), starting from P₂W₁₂ as precursor and working under hydrothermal conditions.⁸⁹ The cavities of P₈W₄₈ in Ln₄P₈W₄₈ are occupied by lanthanide ions bridged by hydroxyl groups, and the 8 lanthanide ions have a 50% occupancy each. The K₈Mn₈P₈W₄₈ polyanion is formed by incorporating eight Mn²⁺ atoms inside the eight vacant sites within the cavity of the cyclic P₈W₄₈. These Mn²⁺ atoms are observed to be disordered over eight positions with K⁺ atoms. The compounds were characterized by single-crystal XRD, FTIR, elemental analysis, thermogravimetry, and powder XRD.

In 2020, Abramov and coworkers reported the dimeric tri-niobium-substituted P₂W₁₅ polyanion, [cis-(P₂W₁₅Nb₃O₆₁)₂]¹⁴⁻ (P₂W₁₅Nb₃), and two phases of the disordered derivative [trans-(P₂W_14.7Nb_3.3O₆₁)₂]^14.6− using P₂W₁₂ as a precursor.⁹⁰ The main structural unit consists of the dimeric [(P₂W₁₆Nb₄O₆₀)₂(μ-O)₂]ⁿ⁻ archetype anion based on two Wells–Dawson-type subunits, connected by two Nb–O–Nb bridges. Interestingly, out of the 12 niobium positions, only four are fully occupied. The addition of Me₂NH₂Cl to the reaction media yields a mixture of triclinic and orthorhombic crystalline phases. The dimeric units are comprised of two Dawson anions connected via Nb–O–Nb bridges. In 2020, Kortz and coworkers reported the gigantic, macrocyclic 48-Fe^III-96-tungsto-16-phosphate, [Fe₄₈(OH)₇₆(H₂O)₁₆(HP₂W₁₂O₄₈)₈]³⁶⁻ (Fe₄₈(P₂W₁₂)₈), which was prepared by reaction of P₂W₁₂ and the 22-iron(III)-containing coordination complex [Fe₂₂O₁₄(OH)₃(O₂CMe)₂₁(mda)₆]·(ClO₄)₂ (mdaH₂ = N-methyldiethanolamine) and isolated as a potassium salt, K₃₆[Fe₄₈(OH)₇₆(H₂O)₁₆(HP₂W₁₂O₄₈)₈].⁹¹ The crystal structure of Fe₄₈(P₂W₁₂)₈ revealed that there are eight equivalent {Fe₆P₂W₁₂} Dawson-type subunits linked to each other via Fe–O–Fe/W bonds, resulting in a cyclic assembly with idealized D₂ point group symmetry and a cavity of ca. 24 Å × 13 Å. Magnetic studies indicated that the 48 Fe³⁺ centers in Fe₄₈(P₂W₁₂)₈ share several exchange pathways. The averaged exchange coupling constant was estimated to be J_av = −7.07 K. The electrochemical study of Fe₄₈(P₂W₁₂)₈ exhibited redox transitions, suggesting the electroactivity of the Fe³⁺ and W^VI ionic states.

4. Metal complexes of [H₆P₄W₂₄O₉₄]¹⁸⁻

Kortz and co-workers first showed that the super-lacunary Preyssler–Jeannin–Pope ion [H₂P₄W₂₄O₉₄]²²⁻ (P₄W₂₄) can react with electrophiles. The dimethyltin-containing hybrid organic–inorganic polyanion [{Sn(CH₃)₂}₄(H₂P₄W₂₄O₉₂)₂]²⁸⁻ ({Sn(CH₃)₂}₄(P₄W₂₄)₂) (Fig. 9) was prepared by reaction of (CH₃)₂SnCl₂ with P₄W₂₄ in an aqueous, acidic medium at ambient temperature and isolated as a potassium salt.⁴⁴ The polyanion comprises two P₄W₂₄ units linked through four dimethyltin groups, leading to a structure with D_2d point group symmetry. The two P₄W₂₄ units of the polyanion {Sn(CH₃)₂}₄(P₄W₂₄)₂ are oriented orthogonally to each other and held together by four dimethyltin groups. Room temperature ¹¹⁹Sn [δ (ppm) = −243.2 ppm], ³¹P [δ (ppm) = −7.1, −8.7 ppm], ¹³C [δ (ppm) = 8.6 ppm], and ¹H [δ (ppm) = 0.7 ppm] NMR studies in aqueous medium proved the integrity of the solid-state structure in solution.

Fig. 9 Combined polyhedral/ball-and-stick representation of [{Sn(CH₃)₂}₄(H₂P₄W₂₄O₉₂)₂]²⁸⁻. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), C (grey), Sn (red). No hydrogen atoms are shown for clarity.⁴⁴

The continued study of Kortz and coworkers of the lacunary precursor P₄W₂₄ has produced interesting new compounds with interesting physical and chemical properties. Kortz and coworkers studied the interaction of phosphotungstates with actinide ions, especially uranyl, as these complexes can potentially have rich structural, magnetic, and electrochemical properties. In 2008, the uranyl-peroxo-containing 36-tungsto-8-phosphate [Li(H₂O)K₄(H₂O)₃{(UO₂)₄(O₂)₄(H₂O)₂}₂(PO₃OH)₂P₆W₃₆O₁₃₆]²⁵⁻ (Li(UO₂)₄(O₂)₄(PO₃OH)₂P₆W₃₆) (Fig. 10) was synthesized and structurally characterized using P₄W₂₄ as precursor.⁹² The structure comprises three P₂W₁₂ units encapsulating two independent, neutral [(UO₂)(O₂)]₄ units in the central cavity, resulting in a U-shaped (P₂W₁₂)₃ assembly. Notably, the resulting polyanion could not be isolated using P₂W₁₂ as a reagent instead of P₄W₂₄. Notably, from the single-crystal X-ray diffraction data, a lithium atom embedded in the structure was observed, which is rare in polyoxotungstate chemistry, given that the high electron density of the W atoms generally tends to obscure such a low electron-dense atom. The coordination of the central uranium-peroxo unit {(UO₂)₄(O₂)₄(H₂O)₂}₂ comprises eight uranyl-peroxo units subdivided in two [(UO₂)(O₂)]₄ squares, the η²-peroxo ions being bound side-on to each pair of uranium atoms. The room temperature ³¹P NMR spectrum of the polyanion in water was fully consistent with its solid-state structure.

Fig. 10 Combined polyhedral/ball-and-stick representation of [Li(H₂O)K₄(H₂O)₃{(UO₂)₄(O₂)₄(H₂O)₂}₂(PO₃OH)₂P₆W₃₆O₁₃₆]²⁵⁻. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), U (turquoise), P (pink), O (red).⁹²

A mixed-valent vanadium derivative [Rb₃⊂{V^VV^IV₃O₇(H₂O)₆}₂{H₆P₆W₃₉O₁₄₇(H₂O)₃}]¹⁵⁻ ((V^VV^IV₃O₇)₂(P₆W₃₉)) (Fig. 11) was also reported by Kortz and coworkers.⁹³ This polyanion was synthesized by the reaction of vanadium(IV) and vanadium(V) with P₄W₂₄ in an acidic aqueous medium (pH 3.2–3.7). A mixed rubidium/potassium salt was isolated and characterized by single-crystal XRD, elemental analysis, TGA, IR, and ³¹P NMR spectroscopy. The polyanion is composed of three P₂W₁₂ subunits, which form a macrocyclic template of P₆W₃₉, capped by two mixed-valent {(V^V=O)(V^IV=O)₃(μ₂-O)₃(H₂O)₆}³⁺ ({V^VV^IV₃}) groups. Each {V^VV^IV₃} cap comprises three octahedrally-coordinated V^IV atoms and one tetrahedrally-coordinated V^V atom. Each of the P₂W₁₂ units is linked through {WO(H₂O)} groups to form a cyclic P₆W₃₉ assembly in this polyanion. The connectivity mode of the P₂W₁₂ units in this polyanion resembles what Wang and coworkers had seen previously.⁵⁸ In the polyanion (V^VV^IV₃O₇)₂P₆W₃₉, the tetrahedral V^v=O oxo group of each {V^VV^IV₃} cap is bridged by three V^IVO₆ octahedra, which occupy the hexavacant positions in the P₆W₃₉ unit. Notably, the terminal oxo-ligands of both the V^IV and V^V metal centers are directed towards the interior of the polyanion. The solid-state structure of the polyanion (V^VV^IV₃O₇)₂P₆W₃₉ was maintained in solution, as confirmed by ³¹P NMR. The same type of connectivity and geometry of vanadium(IV/V) has been observed before for Müller's/Pope's mixed-valent vanadium-containing polyanion [K₈⊂{V^V₄V^IV₂O₁₂(H₂O)₂}₂{P₈W₄₈O₁₈₄}]²⁴⁻ ((V^V₄V^IV₂O₁₂)₂P₈W₄₈) (vide infra, section 6).⁹⁴

Fig. 11 Combined polyhedral/ball-and-stick representation of [Rb₃⊂{V^VV^IV₃O₇(H₂O)₆}₂{H₆P₆W₃₉O₁₄₇(H₂O)₃}]¹⁵⁻. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), O (red), V (yellow).⁹³

5. Metal complexes of [H₇P₈W₄₈O₁₈₄]³³⁻

Since the pioneering discovery of the Cu²⁺-containing polyanion [Cu₂₀Cl(OH)₂₄(H₂O)₁₂(P₈W₄₈O₁₈₄)]²⁵⁻ (Cu₂₀ClP₈W₄₈) (Fig. 12) in 2005, the chemistry of P₈W₄₈ has continued to inspire chemists to study this unique cyclic, multilacunary polyanion. Since then, numerous compounds have been reported using the wheel-shaped P₈W₄₈ polyanion as a precursor, which includes distinct anionic species as well as zeolitic frameworks.⁹⁵

Fig. 12 Combined polyhedral/ball-and-stick representation of [Cu₂₀X(OH)₂₄(H₂O)₁₂(P₈W₄₈O₁₈₄)]²⁵⁻ (X = Cl, Br, I). Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), O (red), Cu (blue), X (yellow).⁹⁶

The first transition metal-containing P₈W₄₈ polyanion, Cu₂₀ClP₈W₄₈, was synthesized by Kortz and coworkers by reacting Cu²⁺ ions with P₈W₄₈ in an aqueous medium, and the product was fully characterized by IR, ³¹P NMR, single-crystal X-ray diffraction and magnetic studies.⁹⁶ The 20-copper-oxo cluster in Cu₂₀ClP₈W₄₈ comprises three structurally unique types of copper(II) atoms with respect to their coordination geometry, namely octahedral, square-pyramidal, and square-planar, with a central chloride ion acting as a template. The copper ions are connected by μ₃-hydroxo-ligands, resulting in a highly symmetrical, cage-like copper-hydroxo cluster assembly, {Cu₂₀(OH)₂₄}¹⁶⁺. In order to study the variation of the magnetic properties of the cluster in the presence of different halide ions, Mal et al. prepared derivatives of Cu₂₀ClP₈W₄₈, with the central chloride guest being replaced by a bromide and an iodide ion, [Cu₂₀Br(OH)₂₄(H₂O)₁₂(P₈W₄₈O₁₈₄)]²⁵⁻ (Cu₂₀BrP₈W₄₈) and [Cu₂₀I(OH)₂₄(H₂O)₁₂(P₈W₄₈O₁₈₄)]²⁵⁻ (Cu₂₀IP₈W₄₈).⁹⁷

DFT calculations were performed on the Cu₂₀ClP₈W₄₈ polyanion in order to obtain additional information on the properties of the anionic guest inside the cavity created by the 20-copper-hydroxo cage, related to its electronic structure and energies of encapsulation.⁹⁸ The DFT calculations indicated the central halide ion to be extremely stable inside the polyanion cavity and cannot be released even at higher temperatures, without the destruction of the POM framework. Magnetic measurements showed that the Cu²⁺ atoms in all halide-derivatives are antiferromagnetically coupled, leading to an overall diamagnetic ground state.⁹⁷

The solution stability of the polyanion Cu₂₀ClP₈W₄₈ was investigated by electrochemical studies at varying pH.⁹⁸ It was observed that the resolution of the reduction wave for Cu²⁺ to Cu⁰ through Cu⁺ was much better at pH 5.0, than compared with measurements performed at lower or higher pH values. It was also observed that all 20 Cu²⁺ centers within the polyanionic complex remain electroactive, as observed using controlled potential coulometry measurements and strong electrocatalytic reduction behavior towards NO_x.^98,99

Scanning tunneling microscopic (STM) and scanning tunneling spectroscopic (STS) studies were performed on a highly oriented pyrolytic graphite (HOPG) surface at room temperature to visualize the 20-copper cluster.¹⁰⁰ STS measurements were done especially to better understand the STM results of Cu₂₀ClP₈W₄₈, such as the Cu⋯Cu distances and the orientation of the polyanion after deposition. The STM and STS images show a regular assembly of the polyanions and the regularly separated single copper atoms in the organic matrix. Several types of polyanion arrangements were observed by STM measurements on the HOPG surface, ostensibly due to the different concentration levels of Cu₂₀ClP₈W₄₈.

Most POMs exhibit hydrophilicity accompanied by high solubility of the corresponding salt in polar solvents, mainly due to a substantial negative charge and oxo/hydroxo/aqua ligands on the surface. Based on such a notion, Kortz and coworkers investigated several solution properties of the Cu₂₀ClP₈W₄₈ polyanion, for example, the investigation of supramolecular interaction resulting in “blackberry-type” structures, where the highly soluble homogeneous electrolytes tend to self-assemble into single layer, spherical, vesical-like structures in dilute solution.^101,102 Earlier reports of such studies were limited to polyoxomolybdate structures,^103–106 until Liu, Kortz, and coworkers showed that polyoxotungstates could also form “blackberry-type” arrangements. Such phenomena were mainly studied by dynamic light scattering (DLS), static light scattering (SLS), and zeta potential measurements.^103–106 The DLS studies showed that a slow supramolecular assembly formation in an aqueous Cu₂₀ClP₈W₄₈ solution starts spontaneously on the 11^th day of the experiment, accompanied by continuous growth until 40 days before finally becoming stable.^101,102 Further studies using SLS indicated that the “blackberry-type” structure solution is relatively stable over prolonged periods, as evidenced by no change in scattering intensity for a month-old solution. It should be noted that the “blackberry-type” structure for Cu₂₀ClP₈W₄₈ only forms at 50 °C. DLS and SLS also provide important information regarding the mechanism of formation of such “blackberry-type” structures. The individual Cu₂₀ClP₈W₄₈ polyanions first overcome the high kinetic energy barrier,^107–110 and then slowly nucleate together, quickly forming a “blackberry-type” structure with an average radius of 38 nm. It was also observed that the counter cations play a major role in forming the blackberry solution.¹¹¹

The Cu₂₀ClP₈W₄₈ polyanion was also investigated for the fabrication of organized thin films by the Langmuir–Blodgett (LB) technique. The polyanions can be introduced into organic–inorganic hybrid films using different LB techniques, resulting in well-defined layered structures. Dimethyldioctadecylammonium bromide (DODA) was observed to react with Cu₂₀ClP₈W₄₈ to form a surfactant-encapsulated Cu₂₀ClP₈W₄₈, which was characterized by different analytical techniques, such as NMR, FT-IR, TGA, powder X-ray diffraction (XRD), and elemental analysis. XRD studies indicated that two different types of DODA-Cu₂₀ClP₈W₄₈ structures are present (DODA/Cu₂₀ClP₈W₄₈ and DODA-Cu₂₀ClP₈W₄₈) based on the diameter and thickness of the layer spacing. The two types of LB films were successfully fabricated onto the substrate by using different deposition methods. It was also observed that Cu₂₀ClP₈W₄₈ exhibits different packing modes in the two LB films, depending on the deposition strategy used.^112,113 Further studies on Cu₂₀ClP₈W₄₈ include electrocatalytic reduction of NO_x.⁹⁸ The polyanion Cu₂₀ClP₈W₄₈ has also been shown to be a very efficient heterogeneous catalyst for the solvent-free aerobic oxidation of n-hexadecane.¹¹⁴ In such a study, Cu₂₀ClP₈W₄₈ was first supported on 3-aminopropyltriethoxysilane (apts)-modified SBA-15 and then subsequently used for the aerobic oxidation of n-hexadecane, showing an exceptionally high turnover frequency (TOF) of 20 000 h⁻¹ and resistance to CS₂ poisoning_. The efficiency of the polyanion catalyst was observed to increase dramatically upon immobilization on mesoporous support due to a large increase in the surface area, which enhanced the oxidation of n-hexadecane into ketones and alcohols. Moreover, it was found that the supported catalyst can be reused at least five times, retaining almost the same catalytic activity as the fresh catalyst.

Mialane and coworkers have been exploring POMs containing azido ligands since 2003.¹¹⁵ The azido ligands can act as connectors between the 3d metal centers embedded in a POM unit and also act as intermolecular linkers between different POM subunits, leading to high-nuclearity POM complexes. Pichon et al. have shown that azido groups can also function as ligands in P₈W₄₈ by preparing the large azido-POM [P₈W₄₈O₁₈₄Cu₂₀(N₃)₆(OH)₁₈]²⁴⁻ (Cu₂₀(N₃)₆P₈W₄₈).¹¹⁶ The polyanion Cu₂₀(N₃)₆P₈W₄₈ has two {Cu₅(OH)₄}⁶⁺ and two {Cu₅(OH)₂(μ_1,1,3,3-N₃)}⁷⁺ subunits encapsulated in the crown-shaped P₈W₄₈.

In each of the four subunits {Cu₅(OH)₄}⁶⁺ and {Cu₅(OH)₂(μ_1,1,3,3-N₃)}⁷⁺, the five Cu²⁺ atoms form a square pyramid with two μ₃-hydroxo ligands connecting the apical Cu²⁺ center to the four basal copper atoms. Interestingly, in each {Cu₅(OH)₄}⁶⁺ fragment, the apical copper atom has an axially distorted coordination geometry, and the four remaining Cu centers exhibit a distorted trigonal–bipyramidal geometry. Moreover, the {Cu₅(OH)₄}⁶⁺ fragments in Cu₂₀(N₃)₆P₈W₄₈ are crystallographically disordered between the hydroxo and azido ligands connecting the Cu²⁺ atoms. Hence, the polyanion is a mixture of species containing two different {Cu₅(OH)₄}⁶⁺ subunits.

Kortz, Müller, and coworkers have synthesized the 16-Fe³⁺ containing polyanion [P₈W₄₈O₁₈₄Fe₁₆(OH)₂₈(H₂O)₄]²⁰⁻ (Fe₁₆P₈W₄₈) (Fig. 13a), which was prepared independently.¹¹⁷ The polyanion contains a cationic 16-iron(III)-hydroxo nanocluster {Fe₁₆(OH)₂₈(H₂O)₄}²⁰⁺ in the cavity of the crown-shaped P₈W₄₈. The {Fe₁₆(OH)₂₈(H₂O)₄}²⁰⁺ cluster comprises eight pairs of structurally equivalent, edge-shared {Fe₂O₁₂} octahedra, which are connected to each other via corners. Notably, the binding mode of the 16 Fe³⁺ centers in Fe₁₆P₈W₄₈ differs from the 20 Cu²⁺ centers in Cu₂₀ClP₈W₄₈. In Fe₁₆P₈W₄₈, each of the 16 equivalent Fe³⁺ centers is connected to P₈W₄₈ by Fe–O(W) and a Fe–O(P) bonds, resulting in a tight anchoring of the 16-iron-hydroxo core, {Fe₁₆(OH)₂₈(H₂O)₄}²⁰⁺, to the wheel-shaped POM host. In Cu₂₀ClP₈W₄₈, only eight of the 20 Cu²⁺ ions form two Cu–O(W) bonds each and hence are bound to the P₈W₄₈ host. On the other hand, the eight phosphate hetero groups of P₈W₄₈ are not directly bonded to the cationic {Cu₂₀(OH)₂₄}¹⁶⁺ cluster. Nevertheless, Cu₂₀ClP₈W₄₈ is quite stable in solution. In fact, Fe₁₆P₈W₄₈ is structurally more closely related to Mialane's Cu₂₀-azide derivative, Cu₂₀(N₃)₆P₈W₄₈.¹¹⁶ In Cu₂₀(N₃)₆P₈W₄₈, 16 of the 20 Cu²⁺ ions are connected to the inner cavity of P₈W₄₈ in the same fashion as the Fe³⁺ centers in Fe₁₆P₈W₄₈. The sites of the remaining four unique Jahn–Teller distorted Cu²⁺ atoms in Cu₂₀(N₃)₆P₈W₄₈ are observed to remain empty in Fe₁₆P₈W₄₈. Kortz and coworkers further investigated the Fe₁₆P₈W₄₈ polyanion toward incorporating lanthanide ions in the remaining vacancies. They successfully prepared an unprecedented, horseshoe-shaped 16-iron(III)-containing polyanion [Fe₁₆O₂(OH)₂₃(H₂O)₉P₈W₄₉O₁₈₉Ln₄(H₂O)₁₉]¹¹⁻ (Ln = Eu, Gd) (Fe₁₆Ln₄P₈W₄₉) (Fig. 13b) with a central [Fe₁₆(OH)₂₈(H₂O)₄]²⁰⁺ guest.¹¹⁸ These Fe₁₆Ln₄P₈W₄₉ polyanions can be called open derivatives of Fe₁₆P₈W₄₈, where the P₈W₄₈ template wheel does not remain intact and is cut open. The {Fe₁₆} ring in Fe₁₆Ln₄P₈W₄₉ is cleaved between four Fe atoms (Fe1/Fe2 on one side and Fe15/Fe16 on the other). Interestingly, an extra tungsten atom is incorporated into the P₈W₄₈ framework, resulting in an open {P₈W₄₉} unit. The extra W atom occupies the cap of the P₂W₁₂ Wells–Dawson fragment and is connected to the novel open P₈W₄₈ fragment through one μ₄-oxo and two μ₂-oxo bridges.

Fig. 13 Combined polyhedral/ball-and-stick representations of (a) [P₈W₄₈O₁₈₄Fe₁₆(OH)₂₈(H₂O)₄]²⁰⁻ and (b) [Fe₁₆O₂(OH)₂₃(H₂O)₉P₈W₄₉O₁₈₉Ln₄(H₂O)₁₉]¹¹⁻ (Ln = Eu, Gd). Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), O (red), Fe (green), Ln (turquoise), W (black).^118,119

Kortz and coworkers have also reported Co²⁺, Mn²⁺, Ni²⁺, and V^V-containing derivatives based on the P₈W₄₈ wheel. For the Mn and Ni derivatives, the tungsten-oxo wheel has accumulated two extra tungsten atoms, resulting in the unprecedented P₈W₅₀ unit.¹²⁰ All four compounds, K₁₂Li₁₆Co₂[Co₄(H₂O)₁₆P₈W₄₈O₁₈₄] (Co₄P₈W₄₈),¹²¹ (Fig. 14a) K₁₂Li₁₀Mn₃[Mn₄(H₂O)₁₆(P₈W₄₈O₁₈₄)(WO₂(H₂O)₂)₂] (Mn₄P₈W₅₀),¹²¹ K₁₄Li₈Ni₃[Ni₄(H₂O)₁₆(P₈W₄₈O₁₈₄)(WO₂(H₂O)₂)₂] (Ni₄P₈W₅₀) (Fig. 14b),¹²¹ and K₂₀Li₁₆[(VO₂)₄(P₈W₄₈O₁₈₄)] ((VO₂)₄P₈W₄₈),¹²¹ were synthesized and characterized by single-crystal XRD, FTIR, elemental analysis, electrochemistry, magnetic susceptibility, and EPR techniques.

Fig. 14 (a) Combined polyhedral/ball-and-stick representations of (a) [Co₄(H₂O)₁₆P₈W₄₈O₁₈₄]³²⁻ and (b) [Ni₄(H₂O)₁₆(P₈W₄₈O₁₈₄)(WO₂(H₂O)₂)₂]²⁸⁻. Color code: WO₆ octahedra (green and brown), PO₄ tetrahedra (pink), O (red), Co (deep blue), Ni (green). The brown WO₆ octahedra have an occupancy of 50% each.¹²¹

The Co₄P₈W₄₈ and (VO₂)₄P₈W₄₈ were prepared by reacting Co²⁺ and VO²⁺ with P₈W₄₈ in aqueous solution, respectively. Bassil et al. have isolated the manganese(II) derivative Mn₄P₈W₅₀ and its nickel(II) analogue Ni₄P₈W₅₀, using similar synthetic procedures but in the presence of small amounts of H₂O₂.¹²¹

The solid-state structure of Co₄P₈W₄₈ consists of four Co²⁺ atoms coordinated to the hinge-oxygens at the inner rim of P₈W₄₈. As perceived from the structures of similar transition metal derivatives of P₈W₄₈,^71,72 the terminal W–O oxygen atoms are observed to point towards the center of the polyanion. Interestingly, in Co₄P₈W₄₈ only half of the eight equivalent hinge sites are occupied by the four Co²⁺ atoms that are coordinated in a cis fashion to two oxo(W) ligands from two adjacent P₂W₁₂ subunits. Aqua ligands occupy the remaining four terminal coordination sites. In addition to the four inner Co²⁺ ions, two outer Co²⁺ ions were found in Co₄P₈W₄₈, linking adjacent polyanions and forming a one-dimensional chain in the solid state. Magnetic studies indicated that the two types of Co²⁺ centers are non-interacting with each other.

Cronin and coworkers have reported a cobalt(II) salt of a cobalt(II)-containing P₈W₄₈ assembly {Co₄[Co₆(P₈W₄₈O₁₈₄)]}_∞ (Co₄(Co₆P₈W₄₈)_∞) with six internal and four external Co²⁺ ions bridging neighboring polyanions, resulting in a 1D chain or 3D network, respectively.¹²² Kortz's group reported the tetra-cobalt(II)-containing polyanion Co₄P₈W₄₈ (which crystallized with two extra cobalt(II) counter cations). The coordination of two pairs of Co²⁺ ions in diagonally related positions of Co₄P₈W₄₈ led to a subtle distortion of the P₈W₄₈ assembly, as reflected by a difference of ca. 1.6 Å between the polyanion diameter of opposite W centers where the Co²⁺ ions are bound and the diameter of opposite W centers perpendicular to the previous one. Wang, Su, and coworkers have synthesized three Co²⁺ linked derivatives of P₈W₄₈, Na₈Li₈Co₅[Co_5.5(H₂O)₁₉P₈W₄₈O₁₈₄]·60H₂O (Co₅(Co_5.5P₈W₄₈)_∞), K₂Na₄Li₁₁Co₅[Co₇(H₂O)₂₈P₈W₄₈O₁₈₄]Cl·59H₂O (Co₅(Co₇P₈W₄₈)_∞), and K₂Na₄LiCo₁₁[Co₈(H₂O)₃₂P₈W₄₈O₁₈₄](CH₃COO)₄Cl·47H₂O (Co₁₁(Co₈P₈W₄₈)_∞), which were characterized by FTIR, thermogravimetric analysis, elemental analysis, and magnetic measurements.¹²³ In Co₅{Co_5.5P₈W₄₈}_∞ and Co₅{Co₇P₈W₄₈}_∞, four external cobalt(II) ions are observed to link adjacent polyanions, resulting in two-dimensional networks, while Co₁₁{Co₈P₈W₄₈}_∞ is observed to form three-dimensional networks. Co₁₁{Co₈P₈W₄₈}_∞ exhibits the largest cobalt(II) containing P₈W₄₈ to date when counterions are also considered.

Kortz's group reported the polyanions Mn₄P₈W₅₀ and Ni₄P₈W₅₀ with four Mn²⁺/Ni²⁺ ions bound in the cavity of P₈W₄₈ as the Co²⁺ ions in Co₄P₈W₄₈, and two additional {WO₆} octahedral units disordered over the four equivalent positions perpendicular to the plane of the Mn²⁺/Ni²⁺ ions, resulting in a polyanion with C_2h point group symmetry. The “extra” tungsten centers in Mn₄P₈W₅₀ and Ni₄P₈W₅₀ are coordinated to oxygens of the P₈W₄₈ wheel just like the Mn²⁺/Ni²⁺ ions, but in a trans-related fashion. The average M–O(W) distance for the Mn²⁺ centers in Mn₄P₈W₅₀ and for the Ni²⁺ centers in Ni₄P₈W₅₀ is 2.13(2) Å and 2.02(2) Å, respectively. The average Mn²⁺–O(aqua) bond in Mn₄P₈W₅₀ and the Ni²⁺–O(aqua) bond in Ni₄P₈W₅₀ are 2.20(2) and 2.06(2) Å, respectively.

Cronin and coworkers, as well as Proust and coworkers, have reported Mn-based P₈W₄₈ derivatives, which differ in the number and location of the Mn²⁺ ions and their network arrays. The Cronin group reported an open framework nanocube-based, [Mn₈(H₂O)₄₈P₈W₄₈O₁₈₄]²⁴⁻ ({Mn₈(H₂O)₄₈P₈W₄₈}_∞) and a multidimensional framework [Mn₁₄(H₂O)₃₀P₈W₄₈O₁₈₄]¹²⁻ ({Mn₁₄P₈W₄₈}_∞) polyanion.^21,76 Each P₈W₄₈ fragment is linked by Mn–O–W coordination bonds, which form a higher-order packing arrangement. Proust and coworkers have reported two new Mn^II derivatives of P₈W₄₈: [Mn₈(H₂O)₂₆(P₈W₄₈O₁₈₄)]²⁴⁻ (Mn₈(H₂O)₂₆P₈W₄₈) and [Mn₆(H₂O)₂₂(P₈W₄₈O₁₈₄){WO₂(H₂O)₂}_1.5]²⁵⁻ (Mn₆{WO₂(H₂O)₂}_1.5P₈W₄₈).⁷⁷ In Mn₈(H₂O)₂₆P₈W₄₈, six Mn²⁺ centers are observed to be located inside the P₈W₄₈ cavity, while two other Mn²⁺ centers are coordinated to the outer rim of P₈W₄₈. The internal six Mn²⁺ ions are distributed among the eight hinges between the {P₂W₁₂} subunits. Four of the sites are fully occupied by the four Mn²⁺, which is the orthogonal plain to the main {P₈W₄₈}, and the remaining two Mn²⁺ centers are disordered over the four other positions, which resembles the Co²⁺ complex reported by the Cronin group.¹²² In Mn₆{WO₂(H₂O)₂}_1.5P₈W₄₈, four Mn²⁺ centers are located inside the P₈W₄₈ cavity, while two other Mn²⁺ centers are coordinated to the outer rim of P₈W₄₈, as in the former structure.

Müller and coworkers have also studied the interaction of P₈W₄₈ with VO²⁺ and Mo^VI in acetate buffer. They have successfully isolated the mixed-valent vanadium(IV/V) containing polyanion [P₈W₄₈O₁₈₄{V^V₄V^IV₂O₁₂(H₂O)₁₂}₂]³²⁻ ((V^V₄V^IV₂)₂P₈W₄₈) (Fig. 15a),⁹⁴ and the mixed-valent molybdenum(V/VI) [{P₈W₄₈O₁₈₄}{Mo^VIO₂}₄{(H₂O)(O=)Mo^V(μ₂-O)₂(O=)Mo^V(μ₂-H₂O)(μ₂-O)₂Mo^V(=O)(μ₂-O)₂Mo^V(=O)(H₂O)}₂]³²⁻ (Mo^VI₄Mo^V₄P₈W₄₈), the first examples of mixed-valent complexes incorporated in P₈W₄₈.¹²⁴ In (V^V₄V^IV₂)₂P₈W₄₈, two {V^V₄V^IV₂O₁₂(H₂O)₂}⁴⁺ units are observed to be trapped inside the cavity of the polyanion. The {V^V₄V^IV₂O₁₂(H₂O)₂}⁴⁺ unit consists of two octahedrally-coordinated V^IV and four tetrahedrally-coordinated V^V centers. The oxidation of V^IV to V^V occurs in situ due to air. Such type of oxidation has also been observed for (VO₂)₄P₈W₄₈, as reported by Kortz and coworkers.¹²¹ Wu's and Bi's groups have reported the reduction of Au³⁺ in the presence of (V^V₄V^IV₂)₂P₈W₄₈, acting as a stabilizing and reducing agent, forming the bamboo joint-like gold microstructure in aqueous medium at ambient temperature.¹²⁵

Fig. 15 Combined polyhedral/ball-and-stick representations of (a) [P₈W₄₈O₁₈₄{V^V₄V^IV₂O₁₂(H₂O)₁₂}₂]³²⁻, and (b) [{Mo₄O₄S₄(H₂O)₃(OH)₂}₂(P₈W₄₈O₁₈₄)]³⁶⁻. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), Mo (turquoise), V (green), S (yellow), O (red).^94,126

Mo^VI₄Mo^V₄P₈W₄₈ was observed to consist of two neutral tetranuclear {Mo^V₄O₁₀(H₂O)₃} and four {Mo^VIO₂}²⁺ units connected to the P₈W₄₈ ring via Mo–O–W bonds. Furthermore, the {Mo^V₄O₁₀(H₂O)₃} unit contains two of the well-known diamagnetic {Mo^V₂O₄}²⁺-type units. The four trapped {Mo^VIO₂}²⁺ units bind to two oxygen atoms of adjacent P₂W₁₂ units, resulting in tetrahedral coordination of the Mo atoms. The ³¹P and ¹⁸³W NMR data fully support the solid-state structure.

Cadot and coworkers have reported molybdenum oxothiocation complexes with the cyclic P₈W₄₈.¹²⁶ The reaction of the [Mo₂S₂O₂(H₂O)₆]²⁺ oxothiocation with P₈W₄₈ in an aqueous acidic medium resulted in two new molybdenum oxothiocation based compounds: [K₄{Mo₄O₄S₄(H₂O)₃(OH)₂}₂(WO₂)(P₈W₄₈O₁₈₄)]³⁰⁻ ((Mo₄O₄S₄)₂(WO₂)P₈W₄₈) and [{Mo₄O₄S₄(H₂O)₃(OH)₂}₂(P₈W₄₈O₁₈₄)]³⁶⁻ ((Mo₄O₄S₄)₂P₈W₄₈) (Fig. 15b). In (Mo₄O₄S₄)₂(WO₂)P₈W₄₈, the two disordered [Mo₄O₄S₄(OH)₂(H₂O)₃]²⁺ oxothiomolybdenum clusters are observed to be grafted on both sides of the cyclic P₈W₄₈ surface, resulting in two geometrical isomers where the two {Mo₄O₄S₄(H₂O)₃(OH)₂}²⁺ groups are arranged either in a perpendicular or parallel mode. The structure also comprises of a {WO₂}²⁺ group, which is disordered over four positions in P₈W₄₈. The polyanion (Mo₄O₄S₄)₂(WO₂)P₈W₄₈ is closely related to the compound Mo^VI₄Mo^V₄P₈W₄₈,¹²⁴ where the oxocation {Mo₂O₄}²⁺ exhibits a similar mode of connectivity as is usually observed for oxothio {Mo₂O₂S₂}-based polyanions. In Mo^VI₄Mo^V₄P₈W₄₈, the neutral core [Mo^V₄O₁₀(H₂O)₃] is observed to be formed by connections of two dinuclear units through a double oxo-bridge. In contrast, in the oxothio derivative (Mo₄O₄S₄)₂WP₈W₄₈, such connections are through a double hydroxo bridge. The polyanion (Mo₄O₄S₄)₂P₈W₄₈ is observed to be composed of the same two disordered [Mo₄O₄S₄(OH)₂(H₂O)₃]²⁺ oxothiomolybdenum clusters but without the extra {WO₂}²⁺ group. Both compounds were characterized in the solid-state by XRD and solution by NMR.

Kortz and coworkers have further investigated the reactivity of the cyclic P₈W₄₈ with 4d transition metal ions in a buffer solution. Interaction of [Ru(p-cymene)Cl₂]₂ with P₈W₄₈ in lithium buffer solution at pH 6.0 resulted in the polyanion [{K(H₂O)}₃{Ru(p-cymene)(H₂O)}₄P₈W₄₉O₁₈₆(H₂O)₂]²⁷⁻ (Ru₄P₈W₄₉) (Fig. 16).¹²⁷ The structure of the polyanion Ru₄P₈W₄₉ reveals that it has four {Ru(p-cymene)(H₂O)}²⁺ groups covalently attached to the inner rim of the cyclic P₈W₄₈ unit, resulting in a structure with C_i symmetry. Each organoruthenium group is bound to P₈W₄₈ via two Ru–O(W) bonds involving belt oxygens of each of two adjacent, hexalacunary P₂W₁₂ building blocks and the extra tungsten atom, resulting in Ru₄P₈W₄₉, which has been observed previously for other related polyanions.¹²¹

Fig. 16 Combined polyhedral/ball-and-stick representation of [{K(H₂O)}₃{Ru(p-cymene)(H₂O)}₄P₈W₄₉O₁₈₆(H₂O)₂]²⁷⁻. (a) Front-view, (b) side-view. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), Ru (sky blue), O (red), C (grey), H (white).¹²⁷

Pope and coworkers have investigated the interaction of the P₈W₄₈ ion with early lanthanide metal ions. They have synthesized and structurally characterized a family of four new lanthanide-substituted polyoxotungstates, [K⊂P₈W₄₈O₁₈₄(H₄W₄O₁₂)₂Ln₂(H₂O)₁₀]²⁵⁻ (Ln₂(W₄O₁₂)₂P₈W₄₈) (Ln = La, Ce, Pr, Nd) (Fig. 17) and all polyanions were characterized by infrared spectroscopy, ³¹P NMR, and X-ray crystallography.¹²⁸ The structural elucidation of the polyanions reveals that the central cavity of P₈W₄₈ is occupied by two additional {W₄O₁₂} groups, along with four lanthanides and two potassium ions, each with an occupancy of 50%. Therefore, the polyoxotungstate shell comprises two P₂W₁₆ and two P₂W₁₂ subunits, with equivalent ones facing each other.

Fig. 17 Combined polyhedral/ball-and-stick representation of [K⊂P₈W₄₈O₁₈₄(H₄W₄O₁₂)₂Ln₂(H₂O)₁₀]²⁵⁻. Color code: WO₆ octahedra (green and brown), PO₄ tetrahedra (pink), O (red), Ce (turquoise), K (grey).¹²⁸

In 2015, Kögerler and coworkers reported the reactivity of the main group element Sn²⁺ with P₈W₄₈ in aqueous solution and isolated the Sn²⁺-containing [K_4.5⊂(ClSn)₈P₈W₄₈O₁₈₄]^17.5− ((ClSn)₈P₈W₄₈) (Fig. 18).¹²⁹ Interestingly, a color change from bright-orange (reduction of W^VI to W^V) to brown and then to dark-green (air oxidation of W^V to W^VI) was observed during the reaction. In the polyanion (ClSn)₈P₈W₄₈, all eight Sn²⁺ atoms are incorporated in the central cavity of P₈W₄₈, occupying the eight equivalent vacant hinge-positions of the P₈W₄₈ moiety. The rate of evaporation of the reaction solution, temperature, and concentration of Sn²⁺ play a crucial role in the formation of this polyanion. The structure of (ClSn)₈P₈W₄₈ comprises eight {ClSn} groups, with each Sn²⁺ atom in a trigonal-pyramidal coordination geometry and the chloride ligand of {ClSn} pointing towards the center of the P₈W₄₈ cavity. The bond distance of Sn–Cl in (ClSn)₈P₈W₄₈ is 2.515(6) Å, which is comparable with Cs[SnCl₃] (2.523 Å) and [SnCl₂(H₂O)]·H₂O (2.595 Å). Similarly, the Sn–O bond is in good agreement with the Sn–O bond lengths in [SnCl₂(H₂O)]·H₂O (2.169 Å) and other Sn²⁺-containing polyoxotungstates.¹³⁰

Fig. 18 Combined polyhedral/ball-and-stick representation of [K_4.5⊂(ClSn)₈P₈W₄₈O₁₈₄]^17.5−. Color code: WO₆ octahedra (green), PO₄ tetrahedra (pink), O (red), Sn (brown), K (grey), Cl (green).¹²⁹

Recently, Kögerler and coworkers have reported aromatic organoarsenate-functionalized P₈W₄₈, [(RAs^VO)₄P^V₈W^VI₄₈O₁₈₄]³²⁻ [R = C₆H₅ or p-(H₂N)C₆H₄] ((RAsO)₄P₈W₄₈, R = C₆H₅ or p-(H₂N)C₆H₄).¹³¹ Recrystallization of the K⁺/Li⁺/dimethylammonium salt of ((p-(H₂N)C₆H₄AsO)₄P₈W₄₈) from 4 M LiCl solution was observed to yield a further functionalized product, [(H₃NC₆H₄AsO)₃P₈W₄₈O₁₈₄H_x{WO₂(H₂O)₂}_0.4]^{(30.2−x)−}, revealing dissociation of the organoarsonate groups in slightly acidic aqueous solution followed by their rearrangement within the inner polyanion cavity.¹³¹

In 2018, Khashab and coworkers isolated two main group 3 metal-substituted P₈W₄₈ in aqueous acidic solution, {[Na(NO₃)(H₂O)]₄[Al₁₆(OH)₂₄(H₂O)₈(P₈W₄₈O₁₈₄)]}¹⁶⁻ (Al₁₆P₈W₄₈) and its Ga analogue [Ga₁₆(OH)₃₂(P₈W₄₈O₁₈₄)]²⁴⁻ (Ga₁₆P₈W₄₈).¹³² The connectivity of Al³⁺/Ga³⁺ in Al₁₆P₈W₄₈ and Ga₁₆P₈W₄₈ is similar to that of Fe₁₆P₈W₄₈ reported by Kortz and coworkers (Fig. 13a).¹¹⁷ The incorporated “{Al₁₆} ring” comprises eight pairs of structurally equivalent, edge-shared AlO₆ octahedra that are interconnected via corners. The degree of protonation is different in the cationic aluminum-hydroxo core {Al₁₆(OH)₂₄(H₂O)₈}²⁴⁺ compared to the isostructural Ga³⁺ analogue {Ga₁₆(OH)₃₂}¹⁶⁺. This is due to different reaction pH (pH 4.0 for Al₁₆P₈W₄₈ vs. pH 5.0 for Ga₁₆P₈W₄₈). The bond distances of Al^III–O, Ga^III–O, and Fe^III–O fall in the range of 1.796(10)–2.044(9), 1.896(5)–2.068(5), and 1.895(12)–2.153(12) Å, respectively, and the corresponding Al–O–Al, Ga–O–Ga, and Fe–O–Fe angles are similar within the respective range of 92.7(4)°–148.0(5)°, 96.4(2)°–143.9(3)°, and 94.2(5)°–139.6(7)°, respectively.

In 2019, Wang and coworkers introduced selenium into the cavity of P₈W₄₈ and isolated a mixed potassium–lithium salt, K₂₆Li₆[(SeO)₄P₈W₄₈O₁₈₄]·98H₂O (Se₄P₈W₄₈).¹³³ Four [SeO₃]²⁻ ions were grafted in the cavity of the crown-shaped P₈W₄₈, like for Co₄P₈W₄₈, resulting in a structure with D_2h point group symmetry.

Sokolov and coworkers reported the incorporation of {NbO}³⁺ units into the cyclic P₈W₄₈ ion via reaction with the Nb–O_x reagent in different ratios (4 : 1, 8 : 1, and 16 : 1) and different concentrations. They have isolated mixed salts of different compounds with differing numbers of coordinated {NbO(H₂O)}³⁺ groups, disordered over the eight equivalent binding sites. The formulas suggest that the compounds are likely mixtures of two or more polyanions: K_25.7Li₅(NH₄)₅[(HP₈W₄₈O₁₈₄)(NbO(C₂O₄)(H₂O))_3.3]·73H₂O ((NbO(C₂O₄)(H₂O))_3.3P₈W₄₈), K_30.8Li_3.5(NH₄)₃[(P₈W₄₈O₁₈₄)(NbO(C₂O₄)(H₂O))_1.7]·74.5H₂O ((NbO(C₂O₄)(H₂O))_1.7P₈W₄₈), K_21.6Li₅(NH₄)₈H_6.8[(P₈W₄₈O₁₈₄)(NbO(H₂O))_4.4(C2O₄)_1.5]·66H₂O ((NbO(C₂O₄)_1.5(H₂O))_4.4P₈W₄₈), K_24.4Li₅(NH₄)_5.5[(HP₈W₄₈O₁₈₄)(NbO(C₂O₄)(H₂O))_3.1]·59H₂O ((NbO(C₂O₄)(H₂O))_3.1P₈W₄₈), K_26.7Li₄(NH₄)_5.5H_2.6[(P₈W₄₈O₁₈₄)(NbO(H₂O))_3.8(C₂O₄)_2.5]·55.5H₂O ((NbO(C₂O₄)_2.5(H₂O))_3.8P₈W₄₈).¹³⁴

Duval and coworkers have introduced the uranyl cation into the cavity of the cyclic P₈W₄₈ polyanion, resulting in the salt K_11.3Li_8.1Na₂₂[(UO₂)_7.2(HCOO)_7.8(P₈W₄₈O₁₈₄)Cl₈]·89H₂O ((UO₂)_7.2(HCOO)_7.2P₈W₄₈) (Fig. 17).¹³⁵ This is the first time that actinide elements were incorporated in the cavity of P₈W₄₈. The structure of (UO₂)_7.2(HCOO)_7.2P₈W₄₈ revealed that the 7.2 uranyl cations are disordered over eight positions, suggesting the presence of mixtures of two or more polyanions in the material.¹²⁶ Interestingly, Kortz and coworkers first introduced the peroxouranium containing wheel-shaped P₈W₄₈, resulting in peroxouranium complex K₁₈Li₂₂[(UO₂)₈(O₂)₈(P₈W₄₈O₁₈₄)]·133H₂O ((UO₂)₄(O₂)₄P₈W₄₈).¹³⁶ The polyanion ((UO₂)₄(O₂)₄P₈W₄₈) consists of four peroxo groups, and each one is connected to two uranium cations. The {(UO₂)₄(O₂)₄} unit comprises neutral four uranyl atoms and four peroxo groups, connecting to each other by side-on peroxo bridges, which is similar to previously reported (Li(UO₂)₄(O₂)₄(PO₃OH)₂P₆W₃₆) by the same group.⁹²

In 2019, Ibrahim et al. reported an isopoly tetratungsten-oxo cluster incorporated inside the rim of P₈W₄₈ along with six internal and four external Mn²⁺ ions, resulting in [(P₈W₄₈O₁₈₄)(W₄O₁₆)K₁₀Li₄Mn₁₀Na(H₂O)₅₀Cl₂]¹⁵⁻ (Mn₁₀W₄P₈W₄₈).¹³⁷ The Mn–O bond lengths are in the range of 2.087–2.315 Å, while Mn–Cl is 2.365 Å. The oxidation states of the Mn ions were checked by bond valence sum analysis,^138–140 and all were shown to be Mn²⁺. The outer Mn²⁺ ions assist the formation of a 3D network through intermolecular Mn–O(W) bonding together with potassium ions. Interestingly, the origin of the one Na⁺ ion in the compound is ambiguous.

Kögerler and coworkers studied the isomerization of the four {α-P₂W₁₂O₄₈} units comprising the P₈W₄₈ wheel in the presence of Cu²⁺ ions in 0.66 M acetate buffer at pH 5.2. They were able to isolate K₇Li₂Na₂₇[αγαγ-P₈W₄₈O₁₈₄{Cu(H₂O)}₂]·78H₂O (Cu₂-αγαγ-P₈W₄₈), K_7.5Na₁₇Cu_2.425(WO₂)_1.325[γγγγ-P₈W₄₈O₁₈₄{Cu(H₂O)_0.5}₄]·102H₂O (Cu₄-γγγγ-P₈W₄₈), and K₇Li₂Na_19.5Cu_1.75(WO₂)[αγγγ-P₈W₄₈O₁₈₄{Cu(H₂O)}₃]·72H₂O (Cu₃-αγγγ-P₈W₄₈).¹⁴¹ The molar ratio of Cu²⁺ to P₈W₄₈, temperature, and reaction time played a crucial role when trying to prepare the three compounds. The synthesis of Kortz's Cu₂₀P₈W₄₈,⁷⁸ and Mialane's Cu₂₀(N₃)₆P₈W₄₈ ⁹⁰ took 1 h and 15 min, respectively, at 80 °C, whereas Cu₂-αγαγ-P₈W₄₈ was isolated after 2 h reaction time at 95 °C, in order to transform two P₂W₁₂ units from α to γ in the P₈W₄₈ wheel. According to the authors, Li⁺ ions also play a crucial role in such isomeric transformation.

In the same year, Suzuki, Yamaguchi, and coworkers synthesized a series of P₈W₄₈ ions with eight incorporated 3d metal ions from mixed organic solvent. The products were isolated as tetra-n-butylammonium (TBA) salts, TBA₁₄H₂-[{M^II₂(OH₂)₂}₂{M^II(OH₂)₂}₄P₈W₄₈O₁₇₆(OCH₃)₈]·nH₂O·mCH₃CN, where M^II = Mn, Co, Ni, Cu, Zn (M₈P₈W₄₈O₁₇₆).¹⁴² Edge-shared bis(square pyramidal) metal-aqua sites {(μ-O)₂(M-OH₂)(μ₃O)₂(M-OH₂)(μ-O)₂} were incorporated in the cavity of P₈W₄₈. Interestingly, a new type of α,γ,α,γ-type P₈W₄₈ was observed after the reaction with the 3d metal ions, with two of the four α-P₂W₁₂ units having been transformed to γ-P₂W₁₂. The Co²⁺ ions are disordered over two of the four P₂W₁₂ units. However, with increased methanol concentration in the reaction, the Co²⁺ ions fully occupied each of the four P₂W₁₂ units. The Mn²⁺ and Ni²⁺-containing P₈W₄₈ polyanions exhibit the first examples of edge-shared bis(square pyramidal)manganese-aqua and nickel-aqua complexes. The M–O axial bond length increased for the metal ion M from left to right in the periodic table. For example, in the Mn²⁺ derivative Mn₈P₈W₄₈O₁₇₆, the axial Mn–O bond lengths of 2.17–2.22 Å (Mn3–O13, Mn3–O60, Mn4–O14, Mn4–O59) were similar to the equatorial bond lengths of 2.10–2.24 Å (Mn3–O3, Mn3–O4, Mn3–O39, Mn3–O42; Mn4–O5, Mn4–O6, Mn4–O40, Mn4–O41). On the other hand, for the Co²⁺ and Ni²⁺ derivatives, the axial bonds (2.13–2.18 Å) are longer than the equatorial ones (1.99–2.07 Å). As expected, for the Cu²⁺ derivative, the Cu–O axial bonds (2.22–2.27 Å) are much longer than the equatorial bonds (1.98–2.08 Å). For the Mn²⁺ and Co²⁺ derivatives, a decrease in magnetic susceptibility was observed upon cooling, implying antiferromagnetic interactions between the 3d metal ions. However, ferromagnetic interactions were observed for the Ni²⁺ and Cu²⁺ derivatives. The same authors have also reported two high-nuclear manganese derivatives of P₈W₄₈, (C₂₄PH₂₀)₁₇H₃₇[Mn₁₈P₈W₄₈O₂₁₄]·16H₂O·4CH₃CN (Mn₁₈P₈W₄₈O₂₁₄) and (C₁₆H₃₆N)₁₂H₁₆[Mn₂₀P₈W₄₈O₂₁₆]·4C₂H₃N·C₂Cl₂H₄ (Mn₂₀P₈W₄₈O₂₁₆).¹⁴³ The Mn₁₈P₈W₄₈O₂₁₄ polyanion is a mixed-valent species with 18 Mn^2+/3+ ions in the cavity of P₈W_48. Bond valence sum calculations revealed the presence of 8 Mn²⁺ and 10 Mn³⁺ ions. The four P₂W₁₂ units isomerized from α- to γ-type in situ in the presence of the transition metal ions, which was observed previously.^98,138 The Mn₂₀P₈W₄₈O₂₁₆ polyanion was obtained by reacting P₈W₄₈ with 20 equivalents of Mn(OAc)₃ in acetonitrile medium. This polyanion was also found to be mixed-valent as per bond valence sum calculations, showing the presence of 12 Mn³⁺ and 8 Mn⁴⁺ ions. All 20 Mn ions are bound in the cavity of P₈W₄₈ and all four units of P₂W₁₂ remained as α-type. The connectivity of the Mn ions in Mn₂₀P₈W₄₈O₂₁₆ is different from Mn₁₈P₈W₄₈O_214. In the former, four Mn³⁺ ions are bound to each α-P₂W₁₂ unit, and four Mn ions are bound at the hinges of the four α-P₂W₁₂ units_. Eight Mn ions of oxidation state either +3 or +4 are bound to the vacant sites within the cavity of the cyclic P₈W₄₈, which are disordered over eight positions together with K⁺ ions.⁸⁸ Two Mn ions occupy the middle part of opposing P₂W₁₂ units, and four Mn ions are at the hinges of the four P₂W₁₂ units. Very recently, Suzuki, Yamaguchi, and coworkers have reported a series of multi-nuclear copper(II)-containing P₈W₄₈ derivatives, which were synthesized in organic solvent. The four compounds thus reported were formulated as TBA₁₁H₁₃[Cu₄(H₂O)₄P₈W₄₈O₁₇₆(OCH₃)₈]·28H₂O·3CH₃NO₂ (Cu₄P₈W₄₈), TBA₁₄H₂[Cu₈(H₂O)₁₂P₈W₄₈O₁₇₆(OCH₃)₈]·24H₂O·CH₃CN (Cu₈P₈W₄₈), TBA₁₄H₂[Cu₁₂(H₂O)₁₆P₈W₄₈O₁₈₄]·4H₂O (Cu₁₂P₈W₄₈), and TBA₁₆H₈[Cu₁₆(OH)₁₆(H₂O)₄P₈W₄₈O₁₈₄]·12H₂O·C₃H₆O (Cu₁₆P₈W₄₈), respectively.¹⁴⁴ Interestingly, the authors were able to obtain the high nuclearity polyanions from the reaction of low nuclearity polyanions with copper(II) salt, for example, Cu₈P₈W₄₈ from Cu₄P₈W₄₈, Cu₁₂P₈W₄₈ from Cu₈P₈W₄₈, and Cu₁₆P₈W₄₈ from Cu₁₂P₈W₄₈, respectively. Moreover, in the case of Cu₄P₈W₄₈ and Cu₈P₈W₄₈, two P₂W₁₂ units with copper ions connected transformed from α to γ-type isomers by 60° rotation of the central {PO₄} hetero groups. The reactive sites of the remaining two {α-P₂W₁₂} units are occupied by methoxy groups. This result was similar to the previously reported cobalt-containing P₈W₄₈ work by the same group where eight cobalt(II) ions were introduced without affecting the presence of the methoxy groups,¹⁴⁵ suggesting that they act as a protecting organic ligands being essential for metal incorporation inside the cavity of P₈W₄₈ without disorder. In Cu₁₂P₈W₄₈ and Cu₁₆P₈W₄₈ comprising the same γ,γ,γ,γ-type P₈W₄₈ framework, the copper(II) coordination geometry differs from each other. The arrangements and connectivity of the copper(II) ions in these structures are also different from the Cu₂₀P₈W₄₈ polyanion.⁹⁴ Very recently, the same group has demonstrated the H₂-based reduction of copper(II) ions in the cavity of P₈W₄₈, resulting in a catalyst which is active for the catalytic hydrogenation of several organic substrates, such as alkenes, alkynes, as well as carbonyl- and nitro-containing compounds.¹⁴⁶

In 2019, Cronin and coworkers reported that P₈W₄₈ self-assembled into inorganic frameworks in the presence of silver ions, enabling interaction with the POM wheel and linking them together. It was observed that P₈W₄₈ was highly reactive towards silver ions, resulting in the formation of fragments as seen in the compounds Li₈K_9.5Ag₂₁[H₁₆P₁₀W₆₆O₂₅₁]_0.5[H₁₄P₉W₆₃O₂₃₅]·0.5Cl₂·50H₂O (Ag₂₁P₉W₆₃O₂₃₅), Li₈K₁₃Ag₁₃[H₁₂P₈W₅₁O₁₉₆]·50H₂O (Ag₁₃P₈W₅₁O₁₉₆), and Li₁₀K₁₂Ag₄[H₁₄P₈W₄₈O₁₈₄]·170H₂O (Ag₄P₈W₄₈O₁₄₈), respectively.¹⁴⁷ The species Ag₂₁P₉W₆₃O₂₃₅ revealed two cocrystallized P₈W₄₈ units connected by 10 Ag⁺ ions, forming a “POMzite” framework. In Ag₁₃P₈W₅₁O₁₉₆, the P₈W₄₈ units are linked forming a framework with 9 Ag⁺ ions per formula unit. Further tuning of the reaction conditions yields Ag₄P₈W₄₈O₁₄₈, where 4 Ag⁺ ions are linked to P₈W₄₈, resulting in a cubic array, and surprisingly, no Ag⁺ ions were detected in the cavity of P₈W₄₈. Very recently, Suzuki, Yamaguchi, and coworkers have reported a (Ag₃₀) cluster within P₈W₄₈, TBA₁₇H[Ag₃₀(P₈W₄₈O₁₈₄)]·10DMF·30H₂O (Ag₃₀P₈W₄₈), which was synthesized from a {Ag₁₆} cluster-containing P₈W₄₈ derivative.¹⁴⁵ The Ag₃₀P₈W₄₈ nanocluster possesses an additional 14 silver atoms that were introduced into the Ag₁₆ cavity, resulting in an Ag₃₀ nanocluster with distorted body-centered-cubic atom arrangements inside the P₈W₄₈ polyanion, revealed by single crystal X-ray crystallography. The Ag₃₀P₈W₄₈ exhibits high and selective catalytic activity towards reducing nitrobenzene and aromatic aldehydes under mild conditions. Very recently, Suzuki and coworkers have reported the mixed-metal silver–gold derivative of P₈W₄₈, wherein a [Au₈Ag₂₆] cluster is embedded within the P₈W₄₈ core, (Bu₄N)₁₆H₈[Au₈Ag₂₆(P₈W₄₈O₁₈₄)] (Au₈Ag₂₆P₈W₄₈).¹⁴⁸ In the silver–gold cluster, six out of eight Au atoms form an octahedral {Au₆} assembly, while in another one Au atom replaces the Ag site at the center of a hexagonal {Ag₇}, forming the {Ag₆Au} assembly. The 26 Ag atoms were thus observed to surround Au atoms to form the nano-cluster Au₈Ag₂₆P₈W₄₈.

Recently, Yang and coworkers were able to incorporate As^IIIO₃ and Sb^IIIO₃ in the cavity of P₈W₄₈, resulting in [{As^III₅O₄(OH)₃}₂(P₈W₄₈O₁₈₄)]³²⁻ (As₁₀P₈W₄₈), [(Sb^IIIOH)₄(P₈W₄₈O₁₈₄)]³²⁻ (Sb₄P₈W₄₈), and [(Sb^IIIOH)₈(P₈W₄₈O₁₈₄)]²⁴⁻ (Sb₈P₈W₄₈).¹⁴⁹ Very recently, the same group has reported the 3d–4f iron(III)-cerium(III) derivative [{Fe^III₈Ce^III₄O₂(OH)₁₂(H₂O)₈(PO₄)₂}(P₈W₄₈O₁₈₄₎]²⁶⁻ (Fe₈Ce₄P₈W₄₈), comprising an iron(III)–cerium(III)–phosphato moiety {Fe^III₈Ce^III₄O₂(OH)₁₂(H₂O)₈(PO₄)₂} encapsulated in the P₈W₄₈ cavity, resulting in a polyanion with idealized D_2h symmetry and it exhibited high sensitivity and specificity to detect ascorbic acid (Table 1).¹⁵⁰ In Table 1, the structural characteristics and component building blocks of the compounds presented in this review are summarized.

Table 1 Structural characteristics and component building blocks of tungstophosphate-based compounds

Sl. no.	Formula	Abbreviation	Brief description	Ref.
1	Li5.5K3H3.5[P2W12(NbO2)6O56]·H2O	P2W12(Nb-O2)6	Six {Nb–O2} occupy six vacant positions of {P2W12}. The six Nb atoms are connected by four η^2-O atoms, one η^4-bridging O atom (cap sites or triply-bridging O atom on belt sites), and one terminal η^2-coordinated peroxo unit.	53
2	K7[Fe(OH2)P2W12Mo5O61]	FeP2W12	All six-vacant positions of {P2W12} are filled up by five Mo^VI ions in the belt and one cap position and one cap position by a Fe^3+ ion.	54
3	K7[α1-Fe(OH2)P2W13Mo4O61] and K8[α2-Cu(OH2)-P2W13Mo4O61]	α1-FeP2W13Mo4, α2-CuP2W13Mo4	Four Mo^6+, one Fe^3+/Cu^2+ ion, and one extra tungsten atom fill the six vacancies of {P2W12}, generating a {P2W13} moiety.	54
4	K8[α2-Cu(OH2)P2W12Mo5O61]	α2-CuP2W12Mo5	The vacant positions of {P2W12} are occupied by five Mo^VI ions in the two belts and a cap and a Cu^2+ ion in the other cap.	54
5	Li2K4[H4P2W12Fe9O56(OAc)7]·34H2O	Fe9(OAc)7P2W12	The six vacancies of {P2W12} are occupied by Fe^3+ ions, forming a {P2W12Fe6} unit to which three additional Fe^3+ atoms are coordinated.	58
6	K6Na10[H12P4W28Fe8O120]·34H2O	Fe8P4W28	Dimeric clusters with four extra tungsten atoms in the cap position and eight iron atoms occupying the belt position.	59
7	K12[{M(H2O)4}2{H12P4W28Fe8O120}]·30H2O (M = Co^2+, Mn^2+, Ni^2+)	M2Fe8P4W28 (M = Co^2+, Mn^2+, Ni^2+)	Two {P2W12} units with four extra tungsten atoms in the cap positions and eight Fe^3+ atoms in the belt positions. Two such {Fe4P2W14} units are bridged by Co^2+, Ni^2+, or Mn^2+ ions.	59
8	K3Na17[{W2Co2O8(H2O)2}(P2W12O46)2]·30H2O	W2Co2O8(P2W12)2	The dimeric structure consists of two {P2W12} units fused via four W–O–W bonds and two W^VI and Co^2+ atoms are bound in the vacant positions.	62
9	K4Na4[H6P2W12Nb4O59(NbO2)2]2·48H2O	{P2W12Nb4(NbO2)2}2	Six Nb atoms occupy two cap and four belt sites of the lacunary {P2W12} precursor. Two such {P2W12Nb4(NbO2)2} units are dimerized via Nb–O–Nb bonds.	63
10	K7[H13{Nb6(O2)4P2W12O57}2]·31H2O	{P2W12Nb6(O2)4}2	The Wells–Dawson dimer consists of two {P2W12} units linked by two Nb–O–Nb bridges and the six vacant sites of each {P2W12} unit are filled by a Nb6(O2)4 group.	67
11	(NH4)16[H14{P2W12Nb7O63(H2O)2}4{Nb4O4(OH)6}]·16H2O	{(P2W12)Nb7}4Nb4	This polyanion comprises an adamantine-like {Nb4O6} core encapsulated by two [Nb6P2W12O61]^10− units (without any Nb-peroxo groups).	67
12	K3.5Li8[(CH3)2NH2]4.5[(PhXO)2P4W24O92]·nH2O (X = P, n = 35 and X = As, n = 40)	PhXOP4W24, X = P, As	The structure consists of a {P4W24O29} unit capped by two phenyl-phosphonate or -arsonate ligands.	68
13	KLi3[(CH3)2NH2]10[(o-H2N-C6H4AsO3)4P4W24O85]·17H2O·LiCl, KHLi2[(CH3)2NH2]6[Mn(H2O)4]2[{Mn(H2O)4}(o-H2N-C6H4-AsO)2P4W24O92]·25H2O·0.2KCl·0.4LiCl, K1.5Li2[(CH3)2NH2]6.5-[Co(H2O)4]2[{Co(H2O)4}(o-H2N-C6H4-AsO)2P4W24O92]·24H2O·0.5LiCl, and K1.5Li2[(CH3)2NH2]6.5[Ni(H2O)4]2[{Ni(H2O)4}(o-H2N-C6H4-AsO)2P4W24O92]·24H2O·0.2LiCl.	(o-NH2-C6H6-AsO)4P4W24O92M(o-NH2-C6H6-AsO)2P4W24 (M = Co^2+, Mn^2+, Ni^2+)	The introduction of divalent transition metal ions (Mn^2+, Co^2+, Ni^2+) in the reaction mixture containing (o-NH2-C6H6-AsO)4P4W24O92 resulted in 1D coordination polymers [{M(H2O)4}P4W24O92(C6H6AsNO)2]^14− (M = Mn^2+, Co^2+, Ni^2+) (M(o-NH2-C6H6-AsO)2P4W24).	69
14	K4Na15[K3⊂{Mn(H2O)4}2{WO2(H2O)2}2{WO(H2O)}3(P2W12O48)3]·77H2O	Mn2(P2W12)3{WO2(H2O)2}{WO(H2O)}3	Three {P2W12} units are connected by three WO(H2O) hinges forming a cyclic P6W39 assembly which accommodates two W^VI and two Mn^2+ guest atoms.	71
15	K3Na7Li5.5Ni0.25[Na3⊂{Ni3.5(H2O)13}{WO2(H2O)2}2{WO(H2O)}3(P2W12O48)3]·64H2O	Ni3.5(P2W12)3{WO2(H2O)2}2{WO(H2O)}3	Two W^VI and four Ni^2+ ions are incorporated in the cyclic P6W39 host. One of the Ni^2+ ions is disordered with a Na ion.	71
16	K6Na11[Na3⊂{Cu3(H2O)9}{WO2(H2O)2}2{WO(H2O)}3(P2W12O48)3]·47H2O	Cu3(P2W12)3{WO2(H2O)2}2{WO(H2O)}3	Two W^VI and three Cu^2+ ions are incorporated in the cyclic P6W39 host.	71
17	Na15[Na3⊂{Co(H2O)4}6{WO(H2O)}3(P2W12O48)3]·109H2O and Na15[Na3⊂{Ni(H2O)4}6{WO(H2O)}3(P2W12O48)3]·110H2O	Co6(P2W12)3{WO(H2O)} and Ni6(P2W12)3{WO(H2O)}3	Six Co^2+ or Ni^2+ ions are incorporated in the cyclic P6W39 assembly.	72
18	K3Na8[K3⊂{GdMn(H2O)10}{HMnGd2(tart)O2(H2O)15}{P6W42O151(H2O)7}]∞·44H2O	{(MnGd)(HMnGd2P6W42)}∞	The trimeric crown-shaped P6W39 encapsulates a Mn^2+ and two Gd^3+ ions in the cavity. In addition, a Mn^2+ and Gd^3+ ion are present outside the polyanion, resulting in a two-dimensional solid-state framework.	73
19	K3Na10[K3⊂{GdCo(H2O)11}2{P6W41O148(H2O)7}]·43H2O	(GdCoP6W41)∞	The structure comprises a trimeric, crown-shaped P6W39 ion encapsulating two Co^2+ and a Gd^3+ ion as well as two W^VI atoms. The second Gd^3+ ion is outside P6W39, linking polyanions to a one-dimensional chain, where this Gd^3+ ion shows a square-antiprismatic coordination geometry.	73
20	K4[H23(Cr(H2O)2)3(H2P2W12O48)3]·34H2O	Cr3(P2W12)3	Trimeric, cyclic assembly of {P2W12} units joined by three Cr^III ions.	74
21	Na16K12[H56P8W48Fe28O248]·20H2O	Fe28P8W48	The tetrameric polyanion comprises four {P2W12Fe6} units, and each is bridged through Fe–O–Fe linkages to a {Fe4O6} cluster core. The linking is through pairs of three Fe–O–Fe bridges, which involve the three outer Fe atoms.	55–57
22	Na16K10[H55P8W49Fe27O248]·yH2O	Fe27P8W49	Similar structure as Fe28P8W48 with one of the sites having an occupancy of 25% W and 75% Fe.	55–57
23	K4Na22{[Co(H2O)2Cl][Co(H2O)3]2[Co(H2O)5]1.5[Co(H2O)3H4P8W49O187(H2O)]}·2NaCl·41.5H2O, and, Na30{[Ni(H2O)3]2[{Ni(H2O)3}1.5H3P8W49O187(H2O)]}·41.5H2O	Co5.5P8W49, and Ni3.5P8W49	The transformation of four {P2W12} ions to the cyclic P8W49 happens in situ via self-condensation, performed in aqueous acidic medium in the presence of Co^2+/Ni^2+ ions. The extra W atom originates from partial decomposition of {P2W12}.	75
24	(NH4)36[{Nb4O6(OH)4}{Nb6P2W12O61}4]·16H2O	(Nb4O6)(Nb6P2W12)4	The polyanion comprises a {Nb4O6} core which is surrounded by four Nb6P2W12 units.	78
25	Na24[Mn8(H2O)32P8W48O184]·58H2O, K4Na16H4[Co8(H2O)32P8W48O184]·76H2O, and Na20H4[Ni8(H2O)32P8W48O184]·72H2O	(M8P8W48, M = Mn, Co, Ni)	Eight divalent 3d metal ions are incorporated in the cyclic P8W48 host.	79
26	K10[H123Nb36P12W72Mn^III12Mn^II3NaO424]·26H2O	Mn15(Nb6P2W12)6	The structure consists of six {Nb6P2W12} units which are connected alternately by four Mn^2+ ions and four trinuclear {Mn^III3} moieties.	80
27	[(n-C4H9)4N]20·5H21.5[{γ-P2W12O48Mn4(C5H7O2)2(CH3CO2)}6]·35H2O	{P2W12O48Mn4}6	The polyanion P2W12 reacts with Mn(acac), forming a hexameric polyanion in an organic medium. Two types of manganese coordination sites are present in each unit of manganese-substituted {P2W12}, and each unit is connected to the other unit of manganese-substituted P2W12 unit.	81
28	[(n-C4H9)4N]16.6H7.4[{γ-P2W12O48Mn4(H2O)6}4(H2O)4]·8H2O	{P2W12O48Mn4}4	The tetrameric polyanion forms after removing the organic capping in the manganese cation from the hexameric complex.	81
29	([(n-C4H9)4N]5[γ-P2W12O44M2(OAc)(CH3CONH)2]·nH2O·mCH3CN; M = Mn^2+, Co^2+, Ni^2+, Cu^2+, or Zn^2+; OAc = acetate)	(γ-P2W12O44M2(OAc))	The polyanion comprises a central edge-shared bis(square-pyramidal) {O2M(μ3-O)2(μ-OAc)MO2} group bound to the belt area of {γ-P2W12} and two acetamide (CH3CONH2) groups are coordinated to the vacant cap positions.	82
30	K12H2[Ce4(OH2)9(OH)2(α1,α1-P2W16O59)2]·48H2O	Ce4(α1α1-P2W16)2	The polyanion has a dimeric structure composed of two {α1,α1-P2W16O59} units connected by four cerium(III) ions.	60
31	K16[{La(CH3COO)(H2O)2(α2-P2W17O61)}2]·36H2O	La(OAc)(α2-P2W17)2	The polyanion consists of two [α2-P2W17O61]^10− units connected by two lanthanum acetate dimers (La2(CH3COO)2(H2O)4)^4+, resulting in a head-on transoid dimer. In an acidic medium, P2W12 quickly transforms into the monovacant [α2-P2W17O61]^10−.	84
32	K4Na10[α1-CuP2W17O60(OH)]2·∼58H2O	Cu2P4W34	The dimeric polyanion cluster is formed from two units each of α1-{CuP2W17}, connected through the W-OH-Cu bonds resulting in the dimeric cluster.	86
33	Na2[H2en][H2hn]0.5[Cu(en)2]4.5[α1-CuP2W17O60(OH)]2·∼43H2O	{Cu(en)2CuP2W17}∞	The dimeric polyanion assembly [α1-CuP2W17O60(OH)]2 is linked to an extended network by {Cu(en)2}^2+ units.	86
34	Na3[H2hn]2.5[P2W17O60Cu(OH)2]·∼14H2O	{(H2hn)(CuP2W17)2}∞	The polyanion possesses a 3-D hybrid supramolecular framework with 1-D tunnels.	86
35	Na8H2L(H2enMe)4[Mn(H2O)2(W4Mn4O12)(P2W14O54)2]·17H2O	{W4Mn4(MnP2W14)2}∞	The 1-D inorganic polymer building blocks comprise multi Mn^2+-substituted Wells–Dawson ions and Mn^2+ linkers with the idealized C2 symmetry, further connected into a 3-D supramolecular network via extensive hydrogen-bonding interactions.	87
36	K56Li74H14[Mn40P32W224O888]·ca. 680 H2O	(P8W48)(Mn4P2W14)4(Mn3P2W15)8	The polyanion consists of {P2W14}, {P2W15} and {P8W48} corner-sharing Wells–Dawson type units with no Mn^2+ ions found inside the cavity of P8W48.	88
37	[K(H2O)2]4[K4(μ-H2O)4(H2O)4]2{[Ln2(μ-OH)4(H2O)x]2(H24P8W48O184)}·yH2O (Ln = Nd, Sm, Tb)	Ln4P8W48	Eight lanthanide ions occupy the cavities of P8W48, with each lanthanide ion being bridged through hydroxyl groups and having a 50% occupancy in each position.	89
38	[K(H2O)2]4[K4(μ-H2O)8]2[K(H2O)]8{[Mn8(H2O)16](H4P8W48O184)}	K8Mn8P8W48	Eight Mn^2+ and eight K^+ ions are incorporated in the cavity of P8W48.	89
39	[cis-(P2W15Nb3O61)2]^14− and two phases of [trans-(P2W14.7Nb3.3O61)2]^14.6−	P2W15Nb3	Nb^V ions are taken up by P2W12 in acidic media resulting in the dimer [(P2W15Nb3O61)2]^14− and a mixture of derivatives with an average formula [(P2W14.7Nb3.3O61)2]^14.6−, indicating the coexistence of species with different composition, such as {(P2W15Nb3)2} (70%) and {(P2W14Nb4)2}(30%).	90
40	[Fe48(OH)76(H2O)16(HP2W12O48)8]^36−	Fe48(P2W12)8	This structure contains 48 Fe^3+ ions surrounded by eight P2W12 ions, comprising eight equivalent {Fe6P2W12} subunits, linked to each other via Fe–O–Fe/W bonds.	91
41	K17Li11[{Sn(CH3)2}4(H2P4W24O92)2]·51H2O	{Sn(CH3)2}4(P4W24)2	The structure consists of two P4W24 units linked through four dimethyltin groups, resulting in a dimeric hybrid inorganic–organic polyanion cluster.	44
42	K6Li19[Li(H2O)K4(H2O)3{(UO2)4(O2)4(H2O)2}2(PO3OH)2P6W36O136]·74H2O	Li(UO2)4(O2)4(PO3OH)2P6W36	The structure consists of three P2W12 units encapsulating two independent, neutral symmetrical uranium-peroxo [(UO2)(O2)]4 units in the central cavity, resulting in a U-shaped {P2W12}3 assembly for the first time.	92
43	K12Rb3[Rb3⊂{V^VV^IV3O7(H2O)6}2{H6P6W39O147(H2O)3}]·63H2O	(V^VV^IV3O7)2P6W39	The polyanion cluster is composed of three α-{P2W12} subunits, which form a macrocyclic template of P6W39, capped by two mixed-valent {(V^V=O)(V^IV=O)3(μ2-O)3(H2O)6}^3+ ({V^VV^IV3}) groups.	93
44	Na12K8H4[K8⊂{V^V4V^IV2O12(H2O)2}2{P8W48O184}]·80H2O	(V^V4V^IV2O12)2P8W48	In this polyanion, two {V^V4V^IV2O12(H2O)2}^4+ units are observed to be trapped inside the cavity of P8W48. The {V^V4V^IV2O12(H2O)2}^4+ unit consists of two octahedral V^IV and four tetrahedral V^V centers. The oxidation of V^IV to V^V occurs due to air.	94
45	K12Li13[Cu20Cl(OH)24(H2O)12(P8W48O184)]·22H2O	Cu20ClP8W48	This structure was the first example demonstrating that d-metal ions can be incorporated in the cavity of P8W48. Here, twenty Cu^2+ ions are grafted in P8W48, and the coordination geometry of Cu^2+ ranges from octahedral to square-pyramidal and square-planar, with a chloride ion encapsulated in the center of the structure.	96
46	K12Li13[Cu20Br(OH)24(H2O)12(P8W48O184)]·60H2O and K12Li13[Cu20I(OH)24(H2O)12(P8W48O184)]·50H2O	Cu20BrP8W48, and Cu20IP8W48	Identical structure to Cu20ClP8W48 but with bromide or iodide acting as the central template rather than instead of chloride.	97
47	LiK14Na9[P8W48O184Cu20(N3)6(OH)18]·60H2O	Cu20(N3)6P8W48	The polyanion comprises two {Cu5(OH)4}^6+ and two {Cu5(OH)2(μ1,1,3,3-N3)}^7+ units incorporated in the cavity of P8W48.	116
48	Li4K16[P8W48O184Fe16(OH)28(H2O)4]·66H2O·2KCl	Fe16P8W48	The polyanion contains a cationic {Fe16(OH)28(H2O)4}^20+ cluster incorporated in the cavity of P8W48. The 16-iron(III)-hydroxo core consists of eight pairs of structurally equivalent, edge-shared {Fe2O12} units.	117
49	K9LiNa[Fe16O2(OH)23(H2O)9P8W49O189Gd4(H2O)19]·50H2O and K8.5Na0.5Li0.5Eu0.5[Fe16O2(OH)23(H2O)9P8W49O189Eu4(H2O)19]·70H2O	Fe16Ln4P8W49 (Ln = Gd^3+, Eu^3+)	The Fe16Ln4P8W49 (Ln = Gd^3+, Eu^3+) polyanion can be described as a derivative of Fe16P8W48 with four Ln^3+ ions grafted on the iron-oxo core and the P8W48 wheel is cleaved with an extra tungsten atom being bound right there.	118
50	K12Li16Co2[Co4(H2O)16P8W48O184]·60H2O	Co4P8W48	The structure has four Co^2+ ions bound at two opposite hinge sites of P8W48.	121
51	K12Li10Mn3[Mn4(H2O)16(P8W48O184)(WO2(H2O)2)2]·67H2O and K14Li8Ni3[Ni4(H2O)16(P8W48O184)(WO2(H2O)2)2]·44H2O	Mn4P8W50 and Ni4P8W50	This structure is identical to Co4P8W48, except that two additional W^VI ions in the form of {WO2(H2O)2} groups are grafted in the cavity, one each at the two hinges which are not occupied by M^2+ (M = Mn, Ni) ions, resulting in a cyclic P8W50 unit.	121
52	K20Li16[(VO2)4(P8W48O184)]·48H2O	(VO2)4P8W48	Four V^VO groups are coordinated in the cavity of P8W48.	121
53	K15Li5[{Co10(H2O)34(P8W48O184)}]·54 H2O	{Co10(H2O)34P8W48}∞	This polyanion structure is similar to that of Co4P8W48, except that an additional four external Co^2+ ions link adjacent polyanions resulting in 1D chains.	122
54	K8Li12[{Co10(H2O)44(P8W48O184)}]·60H2O	{Co10(H2O)44P8W48}∞	Similar to Co4P8W48, except an additional four external Co^2+ ions link the adjacent polyanions, resulting in 3D networks.	122
55	Na8Li8Co5[Co5.5(H2O)19P8W48O184]·60H2O	Co5{Co5.5P8W48}∞	Five external Co^2+ ions are observed to link adjacent polyanions of Co5.5P8W48, resulting in two-dimensional chains.	123
56	K2Na4Li11Co5[Co7(H2O)28P8W48O184]Cl·59H2O	Co5{Co7P8W48}∞	Co7P8W48 is linked via 5 external Co^2+ ions are observed to link adjacent polyanions, resulting in two-dimensional chains.	123
57	K2Na4LiCo11[Co8(H2O)32P8W48O184](CH3COO)4Cl·47H2O	Co11{Co8P8W48}∞	Co8P8W48 is linked via 11 external Co^2+ ions into a three-dimensional network.	123
58	K18Li6[Mn8(H2O)48P8W48O184]·108H2O	{Mn8(H2O)48P8W48}∞	The Mn8(H2O)48P8W48 polyanion forms an open framework nanocube structure with each P8W48 fragment linked by Mn–O–W coordination bonds, which forms the higher order packing arrangement.	21 and 76
59	K12[Mn14(H2O)30P8W48O184]·111H2O	{Mn14P8W48}∞	A solid-state framework formed by P8W48 units linked by external Mn^2+ ions.	21 and 76
60	K13Li11[Mn8(H2O)26(P8W48O184)]·60H2O	Mn8(H2O)26P8W48	In this polyanion, six Mn^2+ ions are observed to be located inside the P8W48 cavity, while two other Mn^2+ ions are coordinated to the outer rim of P8W48.	77
61	K12Li13[Mn6(H2O)22(P8W48O184){WO2(H2O)2}1.5]·75H2O	Mn6{WO2(H2O)2}1.5P8W48	Four Mn^2+ ions are located in the cavity of P8W48, whereas two other Mn^2+ centers are attached to the surface of the wheel. In addition, one or two {WO2(H2O)2} groups are grafted in the cavity, leading to a mixture of products.	77
62	K10Na14[{P8W48O184}{Mo^VIO2}4{(H2O)(O=)Mo^V(μ2-O)2(O=)Mo^V(μ2-H2O)(μ2-O)2Mo^V(=O)(μ2-O)2Mo^V(=O)(H2O)}2]·80H2O	Mo^VI4Mo^V4P8W48	Observed to consist of two unprecedented neutral tetranuclear {Mo^V4O10(H2O)3} and four {Mo^VIO2}^2+ units connected to the P8W48 ring via Mo–O–W bonds.	124
63	K20Li6H4[K4{Mo4O4S4(H2O)3(OH)2}2(WO2)(P8W48O184)]·95H2O	(Mo4O4S4)2(WO2)P8W48	Two cationic [Mo4O4S4(OH)2(H2O)3]^2+ groups are grafted on both sides of P8W48. Different bonding modes of the oxothiomolybdenum groups result in two geometrical isomers.	126
64	K26Li2H8[{Mo4O4S4(H2O)3(OH)2}2(P8W48O184)]·90H2O	(Mo4O4S4)2P8W48	This polyanion is composed of the same two disordered {Mo4O4S4} oxothiomolybdenum clusters observed in (Mo4O4S4)2(WO2)P8W48 but without the extra {WO2}^2+ group.	126
65	K16Li11[{K(H2O)}3{Ru(p-cymene)(H2O)}4P8W49O186(H2O)2]·87H2O	Ru4P8W49	Four {Ru(p-cymene)(H2O)}^2+ groups are covalently grafted to the cavity of P8W48 via two Ru–O(W) bonds.	127
66	Ln4(H2O)28K6Li7[K⊂P8W48O184(H4W4O12)2Ln2(H2O)10]·87H2O (Ln = La^3+, Ce^3+, Pr^3+, Nd^3+)	Ln2(W4O12)2P8W48	The cavity of P8W48 contains four lanthanide ions and two {W4O12} groups, along with two potassium ions.	128
67	K10Li17.5[K4.5⊂(ClSn)8P8W48O184]·50H2O	(ClSn)8P8W48	Eight Sn^IICl groups are incorporated in the cavity of P8W48.	129
68	K8Li17[(CH3)2NH2]7[(C6H5AsO)4P8W48O184]·130H2O, K10.5Li14[(CH3)2NH2]3.5[(H3NC6H4AsO)4P8W48O184]·92H2O [R = C6H5 or p-(H2N)C6H4]	(RAsO)4P8W48, R = C6H5 or p-(H2N)C6H4	Four {RAsO3} units are bound covalently to the cavity of P8W48 through As–O(W) bonds.	131
69	K7.2Li23−x[(H3NC6H4AsO)3P8W48O184Hx{WO2(H2O)2}0.4]·nH2O	(H3NC6H4AsO)3P8W48(WO2)0.4	This compound has the (RAsO)4P8W48 structure, but in addition 0.4 equivalents of tungsten atoms are incorporated in the cavity, which indicates the presence of a compound mixture.	131
70	K8Na3Li5{[Na(NO3)(H2O)]4[Al16(OH)24(H2O)8(P8W48O184)]}·66H2O	Al16P8W48	The polyanion contains a cationic {Al16(OH)24(H2O)8}^24+ hydroxo-cluster inside the cavity of P8W48. The 16-aluminium-hydroxo unit comprises eight pairs of edge-shared AlO6 units connected via corners.	132
71	K11Li9(NH4)4[Ga16(OH)32(P8W48O184)]·112H2O	Ge16P8W48	The polyanion contains the cationic nanocluster {Ga16(OH)32}^16+ incorporated in the cavity of the crown-shaped P8W48. The 16-gallium-hydroxo core {Ga16(OH)32}^16+ comprises eight pairs of structurally equivalent, edge-shared GaO6 units interconnected via corners, and all bridging oxygens are monoprotonated.	132
72	K26Li6[(SeO)4P8W48O184]·98H2O	Se4P8W48	The crown-shaped P8W48 polyanion has four [SeO3]^2− ions inside the cavity in such a way that each Se atom is located inside the cavity perpendicular to the main plan of P8W48.	133
73	K25.7Li5(NH4)5[(HP8W48O184)(NbO(C2O4)(H2O))3.3]·73H2O	[(NbO(C2O4)(H2O))3.3P8W48]	The {NbO(H2O)}^3+ groups in the cavity of P8W48 have two different types of coordination environments.	134
74	K30.8Li3.5(NH4)3[(P8W48O184)(NbO(C2O4)(H2O))1.7]·74.5H2O	[(NbO(C2O4)(H2O))1.7P8W48]	The {NbO(H2O)}^3+ groups in the cavity of P8W48 have two different types of coordination environments.	134
75	K21.6Li5(NH4)8[(P8W48O184)(NbO(C2O4)1.5(H2O))4.4]·66H2O	[(NbO(C2O4)1.5(H2O))4.4P8W48]	The {NbO(H2O)}^3+ groups in the cavity of P8W48 have two different types of coordination environments.	134
76	K24.4Li5(NH4)5.5[(HP8W48O184)(NbO(C2O4)(H2O))3.1]·59H2O	[(NbO(C2O4)(H2O))3.1P8W48]	The {NbO(H2O)}^3+ groups in the cavity of P8W48 have two different types of coordination environments.	134
77	K26.7Li4(NH4)5.5H2.6[(P8W48O184)(NbO(C2O4)2.5(H2O))3.8]·55.5H2O	[(NbO(C2O4)2.5(H2O))3.8P8W48]	The {NbO(H2O)}^3+ groups in the cavity of P8W48 have two different types of coordination environments.	134
78	K11.3Li8.1Na22[(UO2)7.2(HCOOH)7.8(P8W48O184)Cl8]·89H2O	[(UO2)7.2(HCOO)7.2P8W48]	The 7.2 uranyl groups are disordered over eight positions, suggesting a mixture of compounds.	135
79	K18Li22[(UO2)8(O2)8(P8W48O184)]·133H2O	(UO2)4(O2)4P8W48	The polyanion contains four peroxo groups connected to two uranium ions. The connectivity of each peroxo-group is similar to the previously reported polyanion (Li(UO2)4(O2)4(PO3OH)2P6W36).	136
80	K3Li8Mn2[(P8W48O184)(W4O16)K10Li4Mn10Na(H2O)50Cl2]·62H2O	Mn10W4P8W48	The cavity of P8W48 contains six Mn^2+ ions and a tetratungstate unit {W4O16}^8−, and four additional, external Mn^2+ ions acting as linkers of the polyanions resulting in an extended network.	137
81	K7Li2Na27[αγαγ-P8W48O184{Cu(H2O)}2]·78H2O	(Cu2-αγαγ-P8W48)	The molar ratio of Cu^2+ and P8W48, temperature, and reaction time were crucial for obtaining this compound.	141
82	K7.5Na17Cu2.425(WO2)1.325[γγγγ-P8W48O184{Cu(H2O)0.5}4]·102H2O	(Cu4-γγγγ-P8W48)	The molar ratio of Cu^2+ and P8W48, temperature, and reaction time were crucial for obtaining this compound.	141
83	K7Li2Na19.5Cu1.75(WO2)[αγγγ-P8W48O184{Cu(H2O)}3]·72H2O	(Cu3-αγγγ-P8W48)	The molar ratio of Cu^2+ and P8W48, temperature, and reaction time were crucial for obtaining this compound.	141
84	[(n-C4H9)4N]14H2[{M2(OH2)2}2{M(OH2)2}4P8W48O176(OCH3)8]·nH2O·mCH3CN	M8P8W48O176 (M^II = Mn, Co, Ni, Cu, Zn)	Reaction of divalent 3d metal ions with α-P2W12 resulted in a partial transformation to the γ-P2W12 isomer, yielding a new type of α,γ,α,γ-type P8W48 ring, and the incorporation of eight metal ions M^II.	142
85	Li8K9.5Ag21[H16P10W66O251]0.5[H14P9W63O235]0.5Cl2·50H2O and Li8K13Ag13[H12P8W51O196]·50H2O and Li10K12Ag4[H14P8W48O184]·170H2O	Ag21P9W63O235, Ag13P8W51O196, Ag4P8W48O148	Heating P8W48 in the presence of Ag^+ ions at a high concentration (1 : 30). Ag13P8W51O196 forms in a similar procedure with a lower concentration of silver ions (1 : 12) and at a lower temperature. The Ag4P8W48O148 formed at even lower concentration of silver ions at room temperature.	147
86	(C24PH20)17H37[Mn18P8W48O214]·16H2O·4CH3CN and (C16H36N)12H16[Mn20P8W48O216]·4C2H3N·C2Cl2H4	Mn18P8W48O214, Mn20P8W48O216	The Mn18P8W48O214 polyanion contains 18 Mn ions of oxidation state +2 or +3 in the cavity of P8W48 and the four P2W12 units were transformed from α- to γ-isomer in the course of the reaction. In the Mn20P8W48O216 ion, the 20 Mn ions have an oxidation state either +3 or +4 and are aligned at the inner rim of P8W48.	143
87	[(n-C4H9)4N]11H13[Cu4(H2O)4P8W48O176(OCH3)8]·28H2O·3CH3NO2, and [(n-C4H9)4N]14H2, [Cu8(H2O)12P8W48O176(OCH3)8]·24H2O·CH3CN, and [(n-C4H9)4N]14H2[Cu12(H2O)16P8W48O184]·4H2O, and [(n-C4H9)4N]16H8[Cu16(OH)16(H2O)4P8W48O184]·12H2O·C3H6O	Cu4P8W48, Cu8P8W48, Cu12P8W48, Cu16P8W48	In Cu4P8W48 and Cu8P8W48 the two middle P2W12 units to which the copper(II) ions are connected, had transformed from from α- to γ-isomer with a 60° rotation of the PO4 hetero groups. In Cu12P8W48 and Cu16P8W48 the same γ,γ,γ,γ-type P8W48 framework is present, but the copper coordination geometry differs from each other.	144
88	[(n-C4H9)4N]17H[Ag30(P8W48O184)]·10DMF·30H2O	Ag30P8W48	This nanocluster material has exposed silver surfaces and interfaces with metal oxides, and it is highly stable despite the exposed silver surfaces, acting as a catalytically active sites for the selective reduction of organic substrates using H2 under mild reaction conditions.	145
89	(Bu4N)16H8[Au8Ag26(P8W48O184)]	Au8Ag26P8W48	The polyanion comprises {Au6} as well as {Ag6Au} clusters embedded in the cavity of P8W48.	148
90	(Me2NH2)13K7Na2Li10[{As^III5O4(OH)3}2(P8W48O184)]·32H2O, K20Li12[(Sb^IIIOH)4(P8W48O184)]·52H2O, and (Me2NH2)8K6Na5Li5[(Sb^IIIOH)8(P8W48O184)]·65H2O	As10P8W48, Sb4P8W48, Sb8P8W48	Ten As^III ions or four/eight Sb^III ions are grafted in the cavity of P8W48.	149
91	K22Li4[{Fe^III8Ce^III4O2(OH)12(H2O)8(PO4)2}(P8W48O184)]·106H2O	Fe8Ce4P8W48	The polyanion comprises an iron(III)–cerium(III)–phosphato moiety {Fe^III8Ce^III4O2(OH)12(H2O)8(PO4)2} in the cavity of P8W48.	150

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6. Summary and outlook

Over the years, many tungstophosphates have been synthesized and structurally characterized. This review focuses on the structural family P₂W₁₂, P₄W₂₄, and P₈W₄₈ and their interaction with metal ions during the last 20 years emphasizing synthetic and structural aspects. We have discussed the formation and stability of these polyanions in different reaction conditions, including pH, temperature, solvent, concentration, counterions, and ionic strength. Although P₂W₁₂, P₄W₂₄, and P₈W₄₈ all contain the P₂W₁₂ unit as a key building block, their stability and reactivity with metal ions is vastly different, and hence, unique products are observed. While the monomeric P₂W₁₂ is the least stable amongst the three in solution, the cyclic P₈W₄₈ (comprising four P₈W₄₈ units) is the most stable.

The reactivity, particularly the large, crown-shaped P₈W₄₈ with transition metal ions, has been systematically developed only since 2005. All three derivatives, P₈W₄₈, P₄W₂₄, and P₂W₁₂, are multilacunary, containing six or more vacant sites, which in principle can accommodate multiple transition metal ions. The literature in this area has expanded dramatically in the last two decades, reflecting the synthesis and property studies of a wide variety of compounds.

Subsequently, we discussed the versatile nature of these three polyanions, and their chemistry with an emphasis on their reactivity towards d and f-block metal ions, including mixed d/f derivatives, leading to discrete monomeric, dimeric, trimeric, and tetrameric structures, or extended solid state frameworks. Furthermore, gaining extra tungsten atoms in situ (arising from trace decomposition of the parent polyanion) provides additional degrees of structural flexibility. Other interesting features of P₂W₁₂, P₄W₂₄, and P₈W₄₈ are the multi-functional ways to accommodate high nuclearity transition metal oxo/hydroxo/aqua clusters according to their needs. Several novel and unexpected compounds have been isolated depending on the reaction conditions (e.g., type of transition metal, reaction temperature, solution pH, solvent, ionic strength, ratio and concentration of reagents, and countercations), which are all important parameters in synthetic POM chemistry. Several compounds have shown attractive properties in homogeneous and heterogeneous catalysis, as well as in magnetic studies. Researchers are still searching for new materials based on P₂W₁₂, P₄W₂₄, and P₈W₄₈, which are yet to be explored and examined and can benefit their associated properties and potential applications.

Data availability

This is a review paper and hence no new data is presented.

Only scientific publications that can be accessed via the usual academic routes have been cited.

Conflicts of interest

The authors declare no competing conflicts of interest.

Acknowledgements

S. M. thanks the Alexander von Humboldt Foundation for a renewed research stipend. A. B. thanks VNIT, Nagpur for research support. U. K. thanks the German Science Foundation (DFG) and Jacobs University (now Constructor University) for continuous support over the years. The polyanion structure figures were prepared using Diamond, version 3.2 (copyright Crystal Impact GbR).

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