Recent studies on the reaction of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions

Abstract

Polyoxoniobates (PONbs) and polyoxotantalates (POTas) are anionic metal–oxide clusters composed of Nb^V and Ta^V, which have higher charge–size ratios than polyoxotungstates and polyoxomolybdates. The synthesis of PONbs and POTas usually involves the use of [X₆O₁₉]⁸⁻ (X = Nb, Ta) as precursors, which confine the pH of aqueous solutions to be consistently confined to the alkaline region of the pH scale, rendering them incompatible with the reactivity of most metal cations. Due to the lack of negative analogues to MO₄²⁻ (M = Mo/W) that can be polymerized through acidification, the progress in the study of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions remains in its nascent stages. The methods reported to date for synthesizing PONb and POTa clusters can be categorized into two broad strategies: ligand protection under alkaline conditions and hydrogen peroxide (H₂O₂) utilization under acidic conditions. In the latter strategy, H₂O₂ is used to prevent the formation of Nb₂O₅ or Ta₂O₅ in an acidic solution by grafting peroxo groups onto the Nb or Ta atoms. The subsequent cleavage of the peroxo O–O bond enhances the reactivity of oxo intermediates, facilitating the prediction and rational design of novel reactions. In this review, we provide an overview of recent developments from the recent studies on the reaction of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions, spanning from the synthesis to the structural elucidation of PONb and POTa clusters. These developments fall into two main categories: mixed-addendum niobotungstates/tantalotungstates and hetero-peroxo-PONb/Ta clusters. Furthermore, we briefly introduce the properties and applications of some compounds. The concluding section is dedicated to discussing the prospects and essential guidance in this field.

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Haiying Wang

Haiying Wang received her M.S. from the College of Chemistry and Molecular Sciences at Henan University in 2017 and her Ph.D. from the College of Chemistry and Chemical Engineering at Xiamen University in 2021. She is currently a postdoctoral researcher at Henan University. Her research is focused on the synthesis and materials chemistry of polyoxometalates.

Zhijie Liang

Zhijie Liang was born in Henan, China. She received her M.S. and Ph.D. degrees in inorganic chemistry from Henan University in 2016 and 2019, respectively. Currently, she is working at Nantong University. Her research is focused on polyoxometalate-based composite materials.

Dongdi Zhang

Prof. Dongdi Zhang received her B.S. degree from Henan University (2002) and M.S. degree from Sun Yat-Sen University (2005). In 2005, she became a lecturer at the Pharmaceutical College of Henan University. There, she was promoted to Associate Professor before moving to the College of Chemistry and Molecular Sciences at the same university in 2014. She obtained her Ph.D. degree under the supervision of Prof. Jingyang Niu and joined the Niu Group as a researcher in 2014. Her research interests are focused on polyoxometalates. In particular, she is interested in the design of functional polyoxoniobate materials and the mechanistic elucidation of self-assembled polyoxometalate chemical systems.

Jingyang Niu

Prof. Jingyang Niu is the Leading Talent in basic research of the Thousand Talents Project in Henan Province, the President of Henan Chemical Society, a Member of the Inorganic Science Committee and Crystal Chemistry Committee of CCS, has been honored with the second prize and third prize of the Henan Science and Technology Progress Award, is an evaluation expert in the NSFC, and focuses on systematically exploring the chemistry of polyoxoniobates and polyoxotantalates, on constructing the non-hydrogen peroxide synthesis system of polyoxoniobates and polyoxotantalates, and on the conversion of carbon dioxide to cyclic organic carbonates catalyzed by POMs.

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1. Introduction

Polyoxometalates (POMs) represent a distinctive class of discrete anionic molecular metal oxide clusters. These structures result from the assembly of {MO_x}-type units, where M denotes Mo, W, V, Nb, and even Ta in their high oxidation states, through edge and corner-sharing. Despite the longstanding history of POM chemistry, it has attracted considerable interest not only for its unique structural characteristics but also for its potential applications across diverse fields, including catalysis, magnetism, medicine, materials science, and nanotechnology.^1–10 In particular, polyoxoniobates (PONbs) and polyoxotantalates (POTas) differ significantly from the renowned POMs involving Mo, W, and V. By utilizing key terms in the Web of Science database, it is evident that the count of PONb and POTa is relatively small when compared to polyoxotungstate (POT), polyoxovanadate (POV), and polyoxomolybdate (POMo) (Fig. 1). The reasons behind these notable distinctions are as follows (as the ionic radius of V⁵⁺ is small, which is different from those of Mo, W, Nb and Ta, it will not be discussed in this review): (i) the oxoanions of POT and POMo exist as monomers under acidic conditions and can readily self-assemble into polynuclear clusters via acidification. Conversely, PONbs and POTas exhibit significantly higher charge-to-ionic radius ratios than those observed in POT and POMo. [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions are assembled and stabilized only in highly alkaline solutions, commonly above pH 10.5, which is incompatible with the solubility of most metals, thus seriously hindering the study of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions. (ii) POT and POMo encompass a variety of isolated species, serving as transferable building blocks that can be reliably employed in the creation of novel architectures. For example, the plenary clusters can be readily converted to lacunary derivatives through hydrolysis by removing one or more MO₆ (M = Mo, W) octahedra. These preformed lacunary species possess geometric vacancies that facilitate the formation of larger metal moieties or the accommodation of additional metals, thereby establishing a robust building block strategy.⁶ However, the precursors for PONbs and POTas continue to be dominated by the isopolyoxoanions [Nb₆O₁₉]⁸⁻ and [Ta₆O₁₉]⁸⁻, as reported by Lindqvist and Tobias in 1953¹¹ and 1963,¹² respectively. The progress in the structural development of PONb chemistry began in the early part of this century with the groundbreaking discovery of the Keggin-type heteropolyniobate [(Ti₂O₂)(SiNb₁₂O₄₀)]¹²⁻, reported in Science in 2002.¹³ Significant milestones in PONb chemistry have been achieved in the following years. For instance, Zheng et al. reported the largest PONb cluster, {Nb₂₈₈O₇₆₈(OH)₄₈(CO₃)₁₂}, in 2018.¹⁴

Fig. 1 Comparative analysis of published articles involving various POMs.

From a synthetic perspective, the synthesis of PONb and POTa clusters has predominantly centered on alkaline conditions due to the alkaline nature and stability of the frequently employed precursors K₇[HNb₆O₁₉] and K₈[Ta₆O₁₉].¹⁵ Notably, Dabbabi et al. proposed that H₂O₂ can stabilize the Nb^V and Ta^V components through the attachment of peroxo groups onto the Nb or Ta atoms under acidic conditions,¹⁶ thus preventing the generation of Nb₂O₅ or Ta₂O₅ in an acidic solution. Upon the cleavage of the peroxo O–O bond, it substantially amplifies the reactivity of subsequently formed oxo intermediates, enabling the prediction and rational design of novel reactions, thereby offering new approaches for development. Specifically, the initial Nb/W POM, [Si₂W₁₈Nb₆O₇₇]⁸⁻, was synthesized through the reaction between the lacunary [SiW₉O₃₄]¹⁰⁻ unit and the generated oxo Nb intermediates under acidic conditions in the presence of H₂O₂. Subsequently, other researchers including Richard G. Finke, Craig L. Hill, Shuxia Liu, Jingyang Niu, Zhiming Zhang, and Bin Yue successfully reported various series of mixed-addendum POMs. Simultaneously, leveraging the protective effect of H₂O₂, metal peroxo species can be directly stabilized by hetero atoms, resulting in the formation of hetero-peroxo-PONbs (HPPONbs) and hetero-peroxo-POTas (HPPOTas). Notably, since 2011, prominent researchers such as May Nyman,^17–19 Enbo Wang,²⁰ Guoyu Yang,¹⁵ Shoutian Zheng²¹ and Jingyang Niu²² have systematically reviewed the progress in PONb/Ta development, offering guidance for ongoing exploration from diverse angles. However, it is important to highlight that no comprehensive review has been conducted on the synthesis and structural study of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions. In this review, we comprehensively discuss the recent studies on the reaction of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions and their compositions, structural diversities, and applications. We anticipate that this review will offer useful insights and promising avenues for exploring and discovering the synthesis and structure of PONb/Ta. Based on their structural features, the clusters presented in this review can be broadly categorized into two groups: (1) mixed-addendum PONb/Ta clusters and (2) hetero-peroxo-PONb/Ta clusters. Lastly, a summary encompassing prospects and challenges is offered.

2. Synthetic strategy of PONb/Ta clusters in acidic conditions

The precursors K₇[HNb₆O₁₉] and K₈[Ta₆O₁₉] demonstrate constrained pH stability in aqueous solutions, primarily favoring alkaline conditions, which makes it incompatible with the reactivity of metal cations. The addition of H₂O₂ is effective in preventing the formation of Nb₂O₅/Ta₂O₅ or gel-like substances in acidic solutions, consequently stabilizing Nb^V/Ta^V species at lower pH levels. Moreover, owing to the addition of H₂O₂, peroxide Nb or Ta-based polyoxoanions are formed. Besides, the subsequent cleavage of the peroxo O–O bond enhances the reactivity of oxo intermediates. This process facilitates the dissociation and recombination of cluster units, contributing to the predictable and rational design of novel reactions. It should be noted that pH and heat are indispensable in the synthesis process and complement each other. Indeed, maintaining an appropriate pH range is crucial to ensure the formation and stability of peroxy bonds while minimizing the risk of hydrolysis, typically controlled within the range of about 2.0–4.0. Additionally, heat serves to facilitate the cleavage of peroxy bonds, promote interunit aggregation, and facilitate the formation of transition metal or lanthanide derivatives.

From the structural standpoint, the reported mixed-addendum PONb/Ta clusters can be categorized into three subsets: Lindqvist-, Keggin-, and Wells–Dawson-type mixed-addendum PONb/Ta clusters. However, in the development of POMs, Keggin- and Wells–Dawson-type POMs have consistently garnered wide recognition. Typically, Keggin and Wells–Dawson clusters can readily transform into desired building block units by selectively removing MO_x (M = Mo, W) polyhedra under specific experimental conditions.²³ Lacunary polyoxotungstates represent a distinctive subclass of POMs characterized by diverse structural attributes and versatile physicochemical properties. They have found extensive utility as valuable inorganic polydentate ligands.^24–26 Employing lacunary POMs in conjunction with Nb or Ta atoms to assemble larger species through interactions represents a viable synthetic approach for exploring the reaction of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions. This concept is rooted in the thesis that the stability of structural motifs is augmented through the substitution of Nb or Ta. This is attributed to the fact that the replacement of Nb⁵⁺/Ta⁵⁺ for W⁶⁺ in the lacunary POM results in an elevation in charge density, bolstering the basicity of terminal O atoms. Furthermore, this substitution can engage in reactions with electrophilic cations (transition metals or rare earth elements) or self-assembly with Nb/Ta–O–Nb/Ta bridges, culminating in the formation of novel structures via the integration of building blocks. As expected, these obtained mixed-addendum PONb/Ta clusters manifest novel functionalities, including rich redox activity and exceptional catalytic prowess.

3. Structural characterization of mixed-addendum PONb/Ta clusters

3.1. Mixed-addendum niobotungstate clusters

3.1.1. Lindqvist-type niobotungstate clusters. The initial documentation of Nb/W mixed compounds exhibiting the Lindqvist structure dates back to 1977.²⁷ Dabbabi et al. reported the formation of [NbW₅O₁₉]³⁻ and [Nb₂W₄O₁₉]⁴⁻ isopolyanions through reactions of tungstate and hexaniobate in water/H₂O₂. In 2015, Nyman's group systematically reported several Lindqvist-based Nb/W and Ta/W compounds, including the polyanions [NbW₉O₃₂]⁵⁻ (NbW₉) (Fig. 2a), [TaW₉O₃₂]⁵⁻ (TaW₉) (Fig. 2a), [Ta₂W₈O₃₂]⁶⁻ (Ta₂W₈) (Fig. 2b), and [Ta₃W₃O₁₉]⁵⁻ (Ta₃W₃) (Fig. 2c).²⁸ Ta or Nb and W are disordered over some positions and they are established based on the At% ratio (EDX) and charge balance considerations according to the author's statement. The synthesis of these compounds involved the reaction of Na₂WO₄·2H₂O with the corresponding peroxide precursor, K₃[Nb(O₂)₄] or Cs₃[Ta(O₂)₄], in the presence of H₂O₂. Electrospray mass spectrometry has verified the stability of these clusters. In the gas phase, the spectra reveal the presence of NbW₉ and TaW₉, whereas only fragments are detected in the case of Ta₃W₃ and Ta₂W₈. In 2018 and 2019, Carbó et al. and O. Kholdeeva et al. verified the excellent catalytic activity and the heterolytic pathway of a series of NbW₅ compounds through theoretical calculations and experiment, respectively.^29,30

Fig. 2 Polyhedral representation of the Lindqvist-type mixed-PONb clusters: (a) [XW₉O₃₂]⁵⁻ X=Nb/Ta, (b) [Ta₂W₈O₃₂]⁶⁻, (c) [Ta₃W₃O₁₉]⁵⁻.

3.1.2. Keggin-type niobotungstate clusters. Keggin-type POM compounds have garnered extensive research attention. In 1984, Finke and colleagues made groundbreaking reports on Keggin-type niobotungstates,³¹ including the triniobium-substituted tungstosilicate monomer [SiW₉Nb₃O₄₀]⁷⁻, its organometallic rhodium complex [(C₅Me₅)RhSiW₉Nb₃O₄₀]⁵⁻, and the dimeric [Si₂W₁₈Nb₆O₇₇]⁸⁻ cluster. The synthesis of the dimer involved the reaction of K₇H[Nb₆O₁₉]·13H₂O, H₂O₂, and A-β-Na₉HSiW₉O₃₄·23H₂O. Notably, the addition of NaHSO₃ played a pivotal role in subsequent polymerization by destroying the peroxide and preventing peroxide-induced degradation. Subsequently, Keggin-type Nb/W mixed POMs garnered increasing attention. In 1994, Hill and colleagues reported mono- and tri-substituted peroxyniobium tungstosilicates, including [(CH₃)₃NH]₇[Si(NbO₂)₃W₉O₃₇], Cs₇[Si(NbO₂)₃W₉O₃₇], α-K₅[Si(NbO₂)W₁₁O₃₉] and α-[(CH₃)₃NH]₅[Si(NbO₂)W₁₁O₃₇].³² Subsequently, Beer and co-workers reported (n-Bu₄N)₄[PNbW₁₁O₄₀] and (n-Bu₄N)₄[PTaW₁₁O₄₀] utilizing the lacunary (n-Bu₄N)₄H₃[PW₁₁O₃₉] in conjunction with NbCl₅ or TaCl₅.³³ These structures were characterized via²⁹Si NMR, ¹⁸³W NMR, ⁵¹V, ³¹P, and IR spectroscopy, with X-ray single-crystal diffraction not being employed. From the current point of view, the reports were mainly concentrated on the trilacunary [XW₉O₃₄]⁹⁻ (X = Si, Ge, P, As) and monolacunary [XW₁₁O₃₉]⁷⁻ (X = Si, P).

Additionally, Hill et al. also made significant contributions in this field. They isolated the tri-peroxoniobium-substituted monomeric tungstophosphate (TBA)₄H₂[(NbO₂)₃PW₉O₃₇] (Fig. 3a) via a reaction involving K₇H[Nb₆O₁₉]·13H₂O, H₂O₂, and A-Na₉[PW₉O₃₄].³⁴ Remarkably, refluxing (TBA)₄H₂[(NbO₂)₃PW₉O₃₇] in acetonitrile for 24 h yields the corresponding peroxo-free monomeric analogue (TBA)₄H₂[Nb₃PW₉O₄₀], which can subsequently be converted into the dimeric (TBA₃)H₃[Nb₆P₂W₁₈O₇₇] species upon treatment with acetic acid. Besides, the dimeric species can revert to a monomer when subjected to hydroxide treatment.³⁴ In 1999, they reported the isostructural dimeric tungstosilicate A-α-[Si₂Nb₆W₁₈O₇₇]⁸⁻ (Fig. 3b) and the tetramer Cs₂₀[Nb₄O₆(α-Nb₃SiW₉O₄₀)₄] (Fig. 3c) based on [SiW₉Nb₃O₄₀]⁷⁻.^35,36 However, ¹⁸³W NMR and FT-IR spectroscopy indicated that these polyoxoanions are unstable at physiological pH, readily decomposing into the corresponding monomeric form, [SiNb₃W₉O₄₀]⁷⁻. Recognizing the significance of A-α-[Si(NbO₂)₃W₉O₃₇]⁷⁻, Hill et al. successfully obtained high-quality single crystals of peroxo-silicotungstate clusters [Si(NbO₂)₃W₉O₃₇]⁷⁻ for in-depth characterization.³⁷ The existence of Nb–O–Nb bonds connecting the two monomeric units implies that the NbO₂ groups within the monomeric peroxo [Si(NbO₂)₃W₉O₃₇]⁷⁻ can be reduced by several reagents in H₂O, which can undergo dimerization via Nb–O–Nb linkages. It is important to highlight that they observed reversible transformation between the monomer and dimer. Additionally, under pH control, they isolated four distinct dimeric analogues: a mono-Nb–μ-O–Nb-linked dimer (A-α-[Si₂Nb₆W₁₈O₇₉]¹²⁻), two isomers (syn and anti) di-μ-oxo-bridged dimers (A-α-[Si₂Nb₆W₁₈O₇₈]¹⁰⁻), and a tri-μ-oxo-bridged structure (A-α-[Si₂Nb₆W₁₈O₇₇]⁸⁻).³⁸

Fig. 3 Polyhedral and ball–stick representation of Keggin-type mixed-PONb clusters based on {XW₉O₃₄}: (a) (TBA)₄H₂[(NbO₂)₃PW₉O₃₇], (b) A-α-[Si₂Nb₆W₁₈O₇₇]⁸⁻, (c) Cs₂₀[Nb₄O₆(α-Nb₃SiW₉O₄₀)₄], (d) [Ge₂W₁₈Nb₈O₈₈]²⁰⁻, (e) [Nb₂K(H₂O)₄(A-α-SiW₉O₃₄)₂]⁹⁻.

Subsequently, many groups began to devote themselves to this area. In 2010, Liu's research group reported a controllable assembly and disassembly approach involving niobium-substituted germanotungstates. They first synthesized the monomeric peroxoniobium-substituted [GeW₉(NbO₂)₃O₃₇]⁷⁻ and its corresponding peroxo-free analogue [GeW₉Nb₃O₄₀]⁷⁻,³⁹ by which the dimeric [Ge₂W₁₈Nb₈O₈₈]²⁰⁻ was formed (Fig. 3d). Additionally, they observed the formation of tetrameric [Ge₄W₃₆Nb₁₆O₁₆₆]²⁰⁻, which could be formed through the self-aggregation of niobium centers incorporated into the trivacant Keggin-type germanotungstate structure. Intriguingly, monomeric [GeW₉(NbO₂)₃O₃₇]⁷⁻ and [GeW₉Nb₃O₄₀]⁷⁻ can also be obtained by disassembling the tetrameric [Ge₄W₃₆Nb₁₆O₁₆₆]²⁰⁻, achieved through the cleavage of Nb–O–Nb bonds. These results suggest that the conversion between [GeW₉(NbO₂)₃O₃₇]⁷⁻ and [GeW₉Nb₃O₄₀]⁷⁻ can be accomplished by the addition of H₂O₂ or NaHSO₃, respectively. However, the transformation between oligomers and monomers primarily relies on pH levels. In 2011, they achieved three niobium-substituted arsenotungstates for the first time. These compounds include peroxoniobium-containing [AsW₉(NbO₂)₃O₃₇]⁶⁻, peroxo-free [AsW₉Nb₃O₄₀]⁶⁻, and the tetramer [As₄W₃₆Nb₁₆O₁₆₆]¹⁶⁻.⁴⁰ Importantly, these structures are analogous to the germanotungstate structures discussed earlier. Besides, under acidic conditions, both monomers [AsW₉(NbO₂)₃O₃₇]⁶⁻ and [AsW₉Nb₃O₄₀]⁶⁻ can aggregate into the tetramer [As₄W₃₆Nb₁₆O₁₆₆]¹⁶⁻. Likewise, the tetramer [As₄W₃₆Nb₁₆O₁₆₆]¹⁶⁻ can transform into either the monomer [AsW₉(NbO₂)₃O₃₇]⁶⁻ or [AsW₉Nb₃O₄₀]⁶⁻ by the addition of H₂O₂ or adjustment of the pH.

Another strategy for enhancing structural diversity in PONbs involves incorporating additional transition metals or rare earth elements as linkers to guide the self-assembly process, in addition to the Nb–O–Nb bridges. A promising approach to combine POMs with lanthanides is through the utilization of mixed Nb/W addenda, based on the reactivity of peroxo-free Nb₃ clusters. Liu et al. investigated the influence of lanthanide cations on the assembly of niobotungstate-based derivatives, as discussed below. Alterations in reaction conditions and raw material ratios could yield compounds with entirely distinct structures, providing valuable insights for the synthesis of other compounds. For example, they conducted experiments involving {GeW₉Nb₃O₄₀} and lanthanide ions Eu³⁺ under different conditions, taking advantage of the nucleophilic properties of terminal oxygen O_t(Nb). As a result, they successfully isolated six Nb–O–Eu bridge-based POMs, including a dimer [(Ge₂W₁₈Nb₆O₇₈)Eu(H₂O)₄]⁷⁻ (Fig. 4a), three tetramers [(Ge₄W₃₆Nb₁₂O₁₅₆)Eu(H₂O)₃]¹⁷⁻ (Fig. 4b), [Cs(GeW₉Nb₃O₄₀)₄(SO₄)Eu₅(H₂O)₃₆]¹⁴⁻ (Fig. S1a†), and [Cs₂(GeW₉Nb₃O₄₀)₄Eu₄(H₂O)₂₂]¹⁴⁻ (Fig. S1b†), a one-dimensional compound [(GeW₉Nb₃O₄₀)₄Eu_5.5(H₂O)₂₆]^11.5− (Fig. 4d), and a two-dimensional compound [GeW₉Nb₃O₄₀Eu_1.25(H₂O)₁₂]^3.25− (Fig. 4c).⁴¹ In 2013, Su's group made the initial report of Keggin-type Nb/W-TM dimer derivatives, including [(Si₂W₁₈Nb₆O₇₈)Cr(H₂O)₄]⁷⁻ (Fig. 4f), [(Si₂W₁₈Nb₆O₇₈)Cr₂(H₂O)₈]⁴⁻ (Fig. 4g) and [(Si₂W₁₈Nb₆O₇₈)FeCl₂(H₂O)₂]⁹⁻ (Fig. 4h).⁴² Additionally, in 2014, they synthesized a Keggin-type Nb/W compound, [Nb₄O₆(SiW₉Nb₃O₄₀)₄]²⁰⁻, through hydrothermal processes by reacting Cs₆H[SiW₉(NbO₂)₃O₃₇]·8H₂O with KCl.⁴³ The polyanion [Nb₄O₆(SiW₉Nb₃O₄₀)₄]²⁰⁻ is composed of four {Nb₃SiW₉O₄₀} subunits and one central {Nb₄} nucleus, bearing similarity to the compound previously reported by Hill's group in 1999.³⁵ In 2017, they presented a 3D architectural arrangement built upon a La-containing Nb/W mixed-addendum POM, a polyanion denoted as [H₃La₈(H₂O)₃₂(C₆H₅NO₂)₆][SiW₉Nb₃O₄₀]₃, utilizing a steam-assisted method (Fig. 4e).⁴⁴ This compound comprises saturated Keggin-type {SiW₉Nb₃O₄₀} subunits and a ligand of pyridine-3-carboxylic acid, with La³⁺ centers that provide additional stabilization to the framework.

Fig. 4 Polyhedral and ball–stick representation of Keggin-type mixed-PONb Ln/TM-derived clusters based on {XW₉Nb₃O₄₀}: (a) [(Ge₂W₁₈Nb₆O₇₈)Eu(H₂O)₄]⁷⁻, (b) [(Ge₄W₃₆Nb₁₂O₁₅₆)Eu(H₂O)₃]¹⁷⁻, (c) [GeW₉Nb₃O₄₀Eu_1.25(H₂O)₁₂]^3.25−, (d) [(GeW₉Nb₃O₄₀)₄Eu_5.5(H₂O)₂₆]^11.5−, (e) [H₃La₈(H₂O)₃₂(C₆H₅NO₂)₆][SiW₉Nb₃O₄₀]₃, (f) [(Si₂W₁₈Nb₆O₇₈)Cr(H₂O)₄]⁷⁻, (g) [(Si₂W₁₈Nb₆O₇₈)Cr₂(H₂O)₈]⁴⁻, (h) [(Si₂W₁₈Nb₆O₇₈)FeCl₂(H₂O)₂]⁹⁻, (i) [(SiW₉Nb₃O₃₈)₃MnO₃(H₂O)₃]¹³⁻.

Niu and his colleagues also made substantial contributions to the field through their research on PONbs. In 2012, they published a study on a Nb/W mixed POM, [Nb₂K(H₂O)₄(A-α-SiW₉O₃₄)₂]⁹⁻, depicted in Fig. 3e.⁴⁵ This compound exhibited a sandwich structure comprising two trilacunary Keggin-type {SiW₉O₃₄} units and a central {Nb₂K} unit. They also synthesized a trimeric compound incorporating a transition metal, Mn, denoted as [(SiW₉Nb₃O₃₈)₃MnO₃(H₂O)₃]¹³⁻ (Fig. 4i).⁴⁶ This marked the first appearance of a trimeric structure in this series. This compound consisted of three {(NbO₂)₃SiW₉} units surrounding a central {MnO₆} fragment. Subsequently, they reported another trimeric compound, [(SiW₉Nb₃O₃₈)₃WO₃(OH)₃]¹²⁻, where the central unit featured {WO₆} (Fig. 4i).⁴⁷

In 2018, He and Yue, along with their colleagues, synthesized two organic–inorganic hybrid materials: [Cu^ICu^II₃(μ₃-OH)(H₂O)₆(trz)₃]₂(PW₉Nb₃O₄₀)·13H₂O and [Cu^ICu^II₃(μ₃-OH)(H₂O)₄(Htrz)(trz)₃]₂(PW₉Nb₃O₄₀)·13H₂O. These compounds feature mixed-addendum Nb/W {PW₉Nb₃O₄₀} with three-dimensional frameworks (Fig. 5).⁴⁸ The former demonstrates an (8,8)-connected (3⁶·4¹⁴·5⁷·6)₂(3⁶·4⁸·5¹⁴) topology, whereas the latter showcases a (4,8,8)-connected (3⁴·4¹⁰·5¹⁰·6⁴)(3⁴·4¹²·5⁸·6⁴)₂(4⁴·5²) topology. Both compounds feature tricopper {Cu₃(μ₃-OH)(trz)₃} units, with each {PW₉Nb₃O₄₀} subunit connecting to eight {Cu₃} units, resulting in an overall 3D structure.

Fig. 5 Polyhedral and ball–stick representation of [Cu^ICu^II₃(μ₃-OH)(H₂O)₆(trz)₃]₂(PW₉Nb₃O₄₀)·13H₂O (a) and [Cu^ICu^II₃(μ₃-OH)(H₂O)₄(Htrz)(trz)₃]₂(PW₉Nb₃O₄₀)·13H₂O (b).

3.1.3. Wells–Dawson-type niobotungstate clusters. In 1988, Finke's group introduced two Wells–Dawson-type Nb/W mixed-addendum POMs [H₄P₄W₃₀Nb₆O₁₂₃]¹²⁻ and [P₂W₁₅Nb₃O₆₂]⁹⁻ as well as organometallic derivatives [(C₅Me₅)RhP₂W₁₅Nb₃O₆₂]⁷⁻ and [(C₆H₆)RuP₂W₁₅Nb₃O₆₂]⁷⁻.⁴⁹ Subsequently, several analogous structures were documented by Hill's and Qu's groups, respectively. These compounds are synthesized through the combination of POM-based lacunary building blocks (mainly focused on trilacunary {P₂W₁₅O₅₆} or hexalacunary {P₂W₁₂O₄₈}) with K₇H[Nb₆O₁₉]·13H₂O in aqueous H₂O₂ solutions accompanied by subsequent pH adjustment. Initially, these compounds exhibited monomeric composition and structural features.^50–52 However, researchers subsequently concentrated on investigating their polymerization, leading to the formation of dimeric, tetrameric, hexameric, and derivative structures. As mentioned earlier, Nb/W mixed-addendum POMs exhibit enhanced nucleophilicity upon removal of the peroxy group, especially on the O_t(Nb). Consequently, the elimination of the peroxy group is frequently employed in Wells–Dawson-type Nb/W mixed-addendum POMs to serve as polydentate ligands for coordinating with electrophilic groups, such as lanthanide ions.^25,26,53

In 2012, Liu et al. published a groundbreaking study on the synthesis of two novel Wells–Dawson-type mixed POMs incorporating lanthanide ions, [Ln₆(H₂O)₃₈(P₂W₁₅Nb₃O₆₂)₄]¹⁸⁻ (Ln = Eu, Ce) (Fig. 6a).⁵⁴ These POMs exhibit one-dimensional structures, and detailed single crystal X-ray diffraction studies unveiled the preferential binding of lanthanide ions to the O_t(Nb) sites within {P₂W₁₅Nb₃O₆₂} fragments, underscoring the high nucleophilicity of O_t(Nb). Additionally, Su and collaborators synthesized two mixed-addendum Nb/W POMs incorporating Cr atoms, with polyanions as {Cr₃(H₂O)₁₂[P₂W₁₅Nb₃O₆₂]₂}⁹⁻ (Fig. 6b) and {Cr₄(H₂O)₁₂[P₂W₁₅Nb₃O₆₂]₄}²⁴⁻ (Fig. 6c).⁵⁵ Both compounds were synthesized via hydrothermal methods employing Cs₉[P₂W₁₅(NbO₂)₃O₅₉]·16H₂O ⁵⁵ as the precursor material.

Fig. 6 Polyhedral and ball–stick representation of Wells–Dawson-type mixed-PONb Ln/TM-derived clusters based on {P₂W₁₅Nb₃O₆₂}: (a) [Ln₆(H₂O)₃₈(P₂W₁₅Nb₃O₆₂)₄]¹⁸⁻, (b) {Cr₃(H₂O)₁₂[P₂W₁₅Nb₃O₆₂]₂}⁹⁻, (c) {Cr₄(H₂O)₁₂[P₂W₁₅Nb₃O₆₂]₄}²⁴⁻, (d) [Ag₂₅{C≡CC(CH₃)₃}₁₆(CH₃CN)₄(P₂W₁₅Nb₃O₆₂)].

Despite notable advancements, reproducing [(n-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂ with a high degree of purity remains an enduring challenge. In 2014, Finke and colleagues successfully attained a purity level of 90% through extensive experimental investigations and identified three pivotal factors contributing to successful synthesis. They also systematically optimized the synthesis process to achieve a purity of 91–94% and successfully reproduced the process with another researcher using a written method.⁵⁶ This established a foundation for future research.

Under the low-temperature condition, a compound containing high-nuclearity silver cluster, [Ag₂₅{C≡CC(CH₃)₃}₁₆(CH₃CN)₄(P₂W₁₅Nb₃O₆₂)], was synthesized by Tomoji Ozeki's research team (Fig. 6d).⁵⁷ In the polyoxoanion, 25 silver (Ag) atoms were arranged in four layers parallel to the plane defined by the {Nb₃} fragment of {P₂W₁₅Nb₃}. The first layer, situated away from the {Nb₃} fragment, comprised seven Ag atoms arranged in the form of a hexagon. Similarly, the second layer was also a hexagon but staggered in relation to the first layer. The third layer consisted of nine Ag atoms, connecting with the second layer through six C≡C^tBu groups. The fourth layer contained three Ag atoms.

Moreover, polynuclear compounds featuring diverse components have the potential to display synergistic effects and multifunctionality, arising from the amalgamation of diverse moieties. Chen's research team has pioneered the development of a family of organoboron-functionalized (Fig. 7a) POMs. Boron is highly oxophilic and electron-deficient, rendering it a valuable element for the functionalization of POMs. In 2019, they reported the synthesis of three novel lanthanide-POM derivatives of boric acid, accomplished through pH control using K₈H[P₂W₁₅(NbO₂)₃O₅₉]·12H₂O ⁵⁰ as a raw material. The polyanions in these compounds are designated as {Er₃[3-PyB(OH)₃]₄(H₂O)₁₂P₂W₁₅Nb₃O₆₂}⁴⁻ (Fig. 7c), {[Er₂3-PyB(OH)₂(H₂O)₁₁][P₂W₁₅Nb₃O₆₂]}³⁻ (Fig. 7b), and {[Eu₂3-PyB(OH)₂(H₂O)₁₁][P₂W₁₅Nb₃O₆₂]}³⁻ (Fig. 7b).⁵⁸ In contrast to Ln-POMs lacking boronic acid, the boronic acid-coordinated Eu^III center in {[Eu₂3-PyB(OH)₂(H₂O)₁₁][P₂W₁₅Nb₃O₆₂]}³⁻ resulted in a longer lifetime. Furthermore, the group successfully synthesized giant aggregates containing organo-boron moieties, employing [M₃P₂W₁₅O₆₂]⁹⁻ (M = Ta^V or Nb^V)⁵⁹ as a raw material. Density functional theory calculations indicated that nucleophilic metal oxo sites are crucial for the linkage of boronic acid in polyanions: {Na(3-PyB)₄[P₂W₁₅Ta₃O₆₂]₄}²⁷⁻, {Na(3-PyB)₄[P₂W₁₅Nb₃O₆₂]₄}²⁷⁻ (Fig. 7d), {K₄[P₂W₁₅Ta₃O₆₂]₁₂(5-PymB)₃(5-PymBOH)₁₂}⁸⁶⁻, and {K₄[P₂W₁₅Nb₃O₆₂]₁₂(5-PymB)₃(5-PymBOH)₁₂}⁸⁶⁻ (Fig. 7e).⁵⁹ In 2021, they presented the synthesis of an organoboronic acid-polymer, {[P₂W₁₅Nb₃O₆₂]₂(4PBA)₂((4PBA)₂O)}¹⁶⁻ (Fig. 7f),⁶⁰ which was synthesized utilizing K₈H[P₂W₁₅(NbO₂)₃O₅₉]·12H₂O as the raw material. This compound exhibits an isostructural arrangement akin to the previously reported compound containing 3-pyridylboronic acid (3PBA) species.⁵⁸ Similarly, in 2023, they obtained four novel tetrameric POM supramolecules, [(4PyB)₃O₂(OH)₃][Ln₂(H₂O)₁₂(P₂Nb₃W₁₅O₆₂)₄(4PyBOH)₄]²⁷⁻ (Ln = La, Ce, Pr, Eu) (Fig. 7g).⁶¹

Fig. 7 Polyhedral and ball–stick representation of Wells–Dawson-type mixed-PONb organoboron-functionalized POMs: (b) {[Ln₂3-PyB(OH)₂(H₂O)₁₁][P₂W₁₅Nb₃O₆₂]}³⁻, (c) {Er₃[3-PyB(OH)₃]₄(H₂O)₁₂P₂W₁₅Nb₃O₆₂}⁴⁻, (d) {Na(3-PyB)₄[P₂W₁₅Nb₃O₆₂]₄}²⁷⁻, (e) {K₄[P₂W₁₅Nb₃O₆₂]₁₂(5-PymB)₃(5-PymBOH)₁₂}⁸⁶⁻, (f) {[P₂W₁₅Nb₃O₆₂]₂(4PBA)₂((4PBA)₂O)}¹⁶⁻, (g) [(4PyB)₃O₂(OH)₃][Ln₂(H₂O)₁₂(P₂Nb₃W₁₅O₆₂)₄(4PyBOH)₄]²⁷⁻, and several transition metal-mixed addendum POM derivatives (h) [M₄(H₂O)_x(P₂W₁₅Nb₃O₆₂)₃]¹⁹⁻, (i) [(Cr(H₂O)₄)₃(P₂W₁₅Nb₃O₆₂)₂]⁹⁻, (j) [(Co(H₂O)₃)₂(C₁₀H₈N₂)₄(P₄W₃₀Nb₆O₁₂₃)]¹⁴⁻.

In addition, they endeavor to promote the advancement of transition metal-mixed addendum POM derivatives. For example, they achieved the successful synthesis of four novel trimers of Nb/W mixed-addendum POMs, [M₄(H₂O)_x(P₂W₁₅Nb₃O₆₂)₃]¹⁹⁻ (M = Cu, Co, Mn, Zn) (Fig. 7h).⁶² These compounds were synthesized through a solvothermal method employing a water–ethanol mixed solvent. Besides, in 2022, they prepared two dimeric Nb/W mixed-addendum POMs incorporating transition metals, [(Fe(H₂O)₄)₃(P₂W₁₅Nb₃O₆₂)₂]⁹⁻ and [(Cr(H₂O)₄)₃(P₂W₁₅Nb₃O₆₂)₂]⁹⁻ (Fig. 7i).⁶³ In 2022, Li et al. synthesized a novel organic–inorganic hybrid cluster with the polyanion [(Co(H₂O)₃)₂(C₁₀H₈N₂)₄(P₄W₃₀Nb₆O₁₂₃)]¹⁴⁻ using the solvothermal method (Fig. 7j),⁶⁴ similar to a previous report.⁵⁸

For hexalacunary {P₂W₁₂O₄₈}, in 2014, He and Yue's group reported the synthesis of a dimeric compound, [H₆P₂W₁₂Nb₄O₅₉(NbO₂)₂]₂⁸⁻ (Fig. 8a).⁶⁵ A noteworthy departure from previous reports is the utilization of an HCOOH/HCOONa reaction medium in this study. Within this dimeric structure, two {P₂W₁₂Nb₄O₅₉(NbO₂)₂} subunits are connected through two Nb–O–Nb bridges. The peroxoniobium-substituted [(NbO₂)₆P₂W₁₂O₅₆]¹²⁻, previously reported by Hill et al.,⁵¹ is an interesting subunit, but it has received little attention. Niu and Zhang et al. employed it as a secondary building block in their work, leading to the synthesis of various novel Wells–Dawson-type Nb/W mixed-addendum POMs. These include [H₂₄{Nb₄O₆(OH)₄}{Nb₆P₂W₁₂O₆₁}₄]¹²⁻ (Fig. 8b),⁶⁶ [H₁₃{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]⁷⁻ (Fig. 8c)⁶⁷ and [H₁₄{P₂W₁₂Nb₇O₆₃(H₂O)₂}₄{Nb₄O₄(OH)₆}]¹⁶⁻ (Fig. 8d).⁶⁷ These POM polyanions were synthesized employing a “one-pot” or self-assembly approach. Despite utilizing similar synthetic processes, distinct architectures were observed, highlighting the sensitivity of [(NbO₂)₆P₂W₁₂O₅₆]¹²⁻ to reaction conditions and its propensity for transformation and self-assembly. Specifically, the tetramer [H₂₄{Nb₄O₆(OH)₄}{Nb₆P₂W₁₂O₆₁}₄]¹²⁻ (Fig. 8b) is composed of two {P₄W₂₄Nb₁₂O₁₂₂} dimeric subunits interconnected by four Nb–O–Nb bridges after rotating 180°. The {P₄W₂₄Nb₁₂O₁₂₂} dimeric subunit can be viewed as two [Nb₆P₂W₁₂O₆₁] fragments connected through two Nb–O–Nb bridges (Fig. S2†). The dimeric structure was formed by the polyoxoanion [H₁₃{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]⁷⁻ (Fig. 8c), akin to two peroxoniobate-containing {Nb₆(O₂)₄P₂W₁₂O₅₇} subunits merged through two Nb–O–Nb bridges. The polyoxoanion [H₁₄{P₂W₁₂Nb₇O₆₃(H₂O)₂}₄{Nb₄O₄(OH)₆}]¹⁶⁻ (Fig. 8d) was also a tetramer composed of a {P₄W₂₄Nb₁₄O₁₂₆} subunit, and the {P₄W₂₄Nb₁₄O₁₂₆} subunit could be viewed as two {Nb₆P₂W₁₂O₆₁} moieties fused by two Nb–O–Nb bridges and two additional Nb atoms (Fig. S3†). Interestingly, adjusting the synthesis conditions led to variations in the products. Lower temperature and shorter reaction times resulted in peroxoniobium-substituted [H₁₃{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]⁷⁻ (Fig. 8c) with a yellow solution. In contrast, increasing the heating times and temperature led to peroxo-free niobium-substituted polyoxoanion [H₁₄{P₂W₁₂Nb₇O₆₃(H₂O)₂}₄{Nb₄O₄(OH)₆}]¹⁶⁻ (Fig. 8d) with a colorless solution. “One-pot” processes frequently involve intricate chemical reactions influenced by various experimental variables, including pH, temperature, reaction time, and the mole ratio of raw materials. Furthermore, even minor adjustments in the synthetic conditions can significantly influence the final product. Minor alterations in these variables can potentially trigger unforeseen side reactions or incomplete conversions, thereby reducing yields or introducing impurities into the product. Consequently, meticulous control and optimization of the synthetic conditions are imperative to achieve the desired results.

Fig. 8 Polyhedral and ball–stick representation of Wells–Dawson-type mixed-PONb and Ln/TM-derived clusters based on {Nb₆P₂W₁₂O₆₁}: (a) [H₆P₂W₁₂Nb₄O₅₉(NbO₂)₂]₂⁸⁻, (b) [H₂₄{Nb₄O₆(OH)₄}{Nb₆P₂W₁₂O₆₁}₄]¹²⁻, (c) [H₁₃{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]⁷⁻, (d) [H₁₄{P₂W₁₂Nb₇O₆₃(H₂O)₂}₄{Nb₄O₄(OH)₆}]¹⁶⁻, (e) [H₁₂₃Nb₃₆P₁₂W₇₂Mn^III₁₂Mn^II₃NaO₄₂₄]¹⁰⁻, (f) [H₅(Nb₆P₂W₁₂)₂Co₃(H₂O)₁₁O₁₂₁]⁷⁻, (g) {[Nb₄O₆(H₂O)₄Na₄(H₂O)₈][LnP₂W₁₂Nb₆O₆₁(H₂O)₇]₄}¹⁶⁻.

It is important to highlight that {Nb₆P₂W₁₂O₆₁} has emerged as a promising building block for the synthesis of sizable and intricate mixed-addendum POM clusters. Some researchers have combined these building blocks with lanthanide or transition metal ions, whereas others have adopted an in situ synthesis strategy employing simple compounds as raw materials. An illustration of a mixed-addendum POM synthesized through this approach is [H₁₂₃Nb₃₆P₁₂W₇₂Mn^III₁₂Mn^II₃NaO₄₂₄]¹⁰⁻ (Fig. 8e),⁶⁸ characterized by a hexameric structure with a central {Nb₃₆Mn₁₅Na} core. Another example includes [H₅(Nb₆P₂W₁₂)₂Co₃(H₂O)₁₁O₁₂₁]⁷⁻ (Fig. 8f),⁶⁹ which can be viewed as two reversed {Nb₆P₂W₁₂Co₂O_68.5} units interconnected through three Nb–O–Nb bridges. These units subsequently form a one-dimensional (1D) chain through Co ions. Remarkably, the 1D chain architecture disassembles into discrete dimeric clusters {Co₃(Nb₆P₂W₁₂)₂O₁₂₁} in water, and these charged clusters will self-assemble into blackberry-type structures in solution.

Recently, researchers achieved the successful synthesis of four isostructural POMs incorporating Wells–Dawson-type subunits with mixed Nb/W addenda and lanthanide ions. These polyanions are denoted as {[Nb₄O₆(H₂O)₄Na₄(H₂O)₈][LnP₂W₁₂Nb₆O₆₁(H₂O)₇]₄}¹⁶⁻, where Ln represents La, Ce, Pr, or Nd (Fig. 8g).⁷⁰ The synthesis of these POMs was conducted in an acidic solution, employing dimeric Wells–Dawson-type POMs, specifically K₄Na₄[H₆P₂W₁₂Nb₄O₅₉(NbO₂)₂]₂·48H₂O,⁶⁵ as the primary raw materials. Single-crystal X-ray diffraction analysis unveiled that the Wells–Dawson-type subunit {P₂W₁₂Nb₆O₆₁} forms the dimeric unit through connections with terminal oxygen atoms of Nb at equatorial sites. Two dimeric units are further connected via a {Nb₄O₆} core, resulting in the formation of tetrameric polyanions. Lanthanide ions are coordinated through the terminal oxygen atoms of Nb located at polar sites within the Wells–Dawson-type subunits. The successful synthesis of these POMs highlights the viability of employing dimeric Wells–Dawson-type POMs as initial building blocks for the construction of intricate POMs featuring mixed addenda and lanthanide ions. Otherwise, the intricate mixed Nb/W addendum POMs can also be constructed by one-pot synthesis from simple raw materials. In 2022, Zhang and colleagues achieved the successful synthesis of a series of isomorphic compounds via a one-pot method, subsequently investigating their high proton release capacity.⁷¹

3.2. Mixed-addendum tantalotungstate clusters

The lanthanide contraction results in the atomic and ionic radii of niobium and tantalum being highly similar. Their similarity contributes to their similar chemical properties and the ability to form isostructural compounds, such as Lindqvist K₇[HNb₆O₁₉] and K₈[Ta₆O₁₉]. Nevertheless, despite these similarities, there exist distinctions in their behavior. For instance, in the solution of K₈[Ta₆O₁₉], the dominant interaction is solvent-separated and/or solvent-shared ion association,⁷² whereas, in the case of K₇[HNb₆O₁₉], contact ion association prevails.⁷³ Furthermore, tantalum chemistry exhibits a higher degree of inertness compared to niobium chemistry. Despite considerable efforts, the progress in the development of POTas has been relatively sluggish. Nyman and colleagues proposed that Ta/W POMs complexes could serve as promising building blocks to advance the field of POTa chemistry.²⁸ In recent years, numerous POTs with tantalum substitutions in classic structures like Lindqvist (as described in section 3.1.1), Keggin, and Wells–Dawson derivatives have been synthesized. However, the quantity of reported compounds in this category significantly lags behind that of niobium.

3.2.1. Keggin-type tantalotungstate clusters. So far, Keggin-type tantalotungstate clusters have been documented based on the {SiW₉Ta₃O₄₀} subunit, showing various degrees of polymerization, ranging from dimerization to tetramerization and even hexamerization. In 2012, Liu and colleagues published findings on two Keggin-type polytantalotungstates:⁷⁴ [SiW₉(TaO₂)₃O₃₇]⁷⁻ and a tetramer, [Ta₄O₆(SiW₉Ta₃O₄₀)₄]²⁰⁻ (Fig. 9a). The former, which contains peroxide bonds, represents a tris-substituted Keggin-type Ta/W POM, whereas the tetramer comprises four peroxy-free {SiW₉Ta₃O₄₀} units linked by a {Ta₄O₆} core (Fig. 9a). In 2014, Wang and collaborators reported two tetramerizations involving transition metals: [Cu(bpy)(H₂O)₃]₃{[Cu(bpy)₂]₂[Cu(bpy)(H₂O)₂]₃[Ta₄O₆(SiW₉Ta₃O₄₀)₄]}⁴⁻ (Fig. 9b) and {[Ta₄O₆(SiW₉Ta₃O₄₀)₄][Cu(apy)(H₂O)₂]₄}¹²⁻ (Fig. 9c).⁷⁵ These compounds were synthesized through the hydrothermal method, involving the reaction of the peroxy-precursor Cs₃K_3.5H_0.5[SiW₉(TaO₂)₃O₃₇]·9H₂O (as reported by Liu et al. in 2012),⁷⁴ CuCl₂·2H₂O, and ligands. Each compound can be understood as a tetrameric cluster, wherein four Keggin units are interconnected by a central {Ta₄} fragment and the polyoxoanion is linked via Cu-ligand complexes.

Fig. 9 Polyhedral and ball–stick representation of Keggin-type mixed-POTa and Ln/TM-derived clusters based on {SiW₉Ta₃O₄₀}: (a) [Ta₄O₆(SiW₉Ta₃O₄₀)₄]²⁰⁻, (b) [Cu(bpy)(H₂O)₃]₃{[Cu(bpy)₂]₂[Cu(bpy)(H₂O)₂]₃[Ta₄O₆(SiW₉Ta₃O₄₀)₄]}⁴⁻, (c) {[Ta₄O₆(SiW₉Ta₃O₄₀)₄][Cu(apy)(H₂O)₂]₄}¹²⁻, (d) [(Si₂W₁₈Ta₆O₇₈)Cr(H₂O)₄]⁷⁻, (e) [(Si₂W₁₈Ta₆O₇₈)FeCl₂(H₂O)₂]⁹⁻, (f) [Ta₁₂Si₄W₃₇O₁₅₈]¹⁸⁻, (g) [MnTa₁₈Si₆W₅₄O₂₃₁]²²⁻.

Su et al. synthesized several Keggin-type mixed-POTa clusters based on the {SiW₉Ta₃O₄₀} subunit, including two dimeric aggregates, [(Si₂W₁₈Ta₆O₇₈)Cr(H₂O)₄]⁷⁻ (Fig. 9d) and [(Si₂W₁₈Ta₆O₇₈)FeCl₂(H₂O)₂]⁹⁻ (Fig. 9e);⁷⁶ a tetramer, [Ta₁₂Si₄W₃₇O₁₅₈]¹⁸⁻ (Fig. 9f);⁷⁷ and a hexamer, [MnTa₁₈Si₆W₅₄O₂₃₁]²²⁻ (Fig. 9g).⁷⁸ Each of the dimeric aggregates, [(Si₂W₁₈Ta₆O₇₈)Cr(H₂O)₄]⁷⁻ and [(Si₂W₁₈Ta₆O₇₈)FeCl₂(H₂O)₂]⁹⁻, comprised two {SiW₉Ta₃O₄₀} clusters connected by two Ta–O–Ta bridges and one O_t–Cr(Fe)–O_t bridge. These compounds were prepared through the conventional aqueous method by reacting Cs₃K_3.5H_0.5[SiW₉(TaO₂)₃O₃₇]·9H₂O (reported by Liu et al. in 2012⁷⁴) with Cr³⁺ or Fe³⁺. [Ta₁₂Si₄W₃₇O₁₅₈]¹⁸⁻ can be depicted as three {SiW₉Ta₃O₄₀} subunits connected by two Ta–O–Ta bridges and one Ta–O–W bridge, forming a triangle {Si₃Ta₉W₂₈O₁₂₀} unit (Fig. S4†). Additionally, one {SiW₉Ta₃O₄₀} subunit links to the {Si₃Ta₉W₂₈O₁₂₀} unit through two Ta–O–W bridges (Fig. 9f). In addition, [MnTa₁₈Si₆W₅₄O₂₃₁]²²⁻ is assembled from six {SiW₉Ta₃O₄₀} building units interconnected by six (Ta)O_t–Mn–O_t(Ta) bridges, three W–O–W bridges, and six Ta–O–Ta bridges (Fig. 9g).

3.2.2. Wells–Dawson-type tantalotungstate clusters. Generally speaking, Wells–Dawson-type mixed-POTa clusters consist of {P₂W₁₅O₅₆} units interconnected by Ta-clusters. Occasionally, transition metals (TM), lanthanides (Ln), and other electrophilic reagents or linkers are introduced to enhance the structures. These clusters are synthesized employing the peroxide K₅Na₄[P₂W₁₅O₅₉(TaO₂)₃]·17H₂O as a reactive precursor, which was initially described by Liu et al. in 2012.⁷⁴ It represents a tris-substituted Wells–Dawson-type Ta/W POM. Additionally, another compound, K₈Na₈H₄[P₈W₆₀Ta₁₂(H₂O)₄(OH)₈O₂₃₆]·42H₂O, was reported simultaneously, featuring a tetrameric structure (Fig. 10a). In the tetramer, the [P₈W₆₀Ta₁₂(H₂O)₄(OH)₈O₂₃₆]²⁰⁻ polyanion can be envisaged as lacunary Wells–Dawson {P₂W₁₅O₅₆} fragments interconnected by a {Ta₁₂} cluster. Subsequently, Chen et al. reported the synthesis of a similar structure through an ion-exchange approach, leading to H₂₀[P₈W₆₀Ta₁₂(H₂O)₄(OH)₈O₂₃₆]·125H₂O.⁷⁹ These architectural templates have served as a basis for the synthesis of various Wells–Dawson-type Ta/W POMs bearing Ln or TM derivatives.

Fig. 10 Polyhedral and ball–stick representation of Wells–Dawson-type mixed-POTa and Ln/TM-derived clusters based on {P₂W₁₅Ta₃O₆₂}: (a) [P₈W₆₀Ta₁₂(H₂O)₄(OH)₈O₂₃₆]²⁰⁻, (b) [Ta₁₈P₁₂W₉₀(OH)₆(H₂O)₂O₃₆₀]³⁶⁻, (c) [Yb₂Ta₁₈P₁₂W₉₀(OH)₆(H₂O)₁₆O₃₆₀]³⁰⁻, (d) [Ln₃(H₂O)₂₂][P₂W₁₅Ta₃O₆₂], (e) {Cr₃[Ta₃P₂W₁₅O₆₂]₂(H₂O)₁₂}⁹⁻, (f) {Cr₄[Ta₃P₂W₁₅O₆₂]₄(H₂O)₁₂}²⁴⁻, (g) [K₃(H₂O)₃(P₂W₁₅Ta₃O₆₂)₆(Mo₂O₄CH₃CO₂)₃(MoO₃)₂]⁴⁸⁻.

In 2016, Su et al. reported the synthesis of two Wells–Dawson-type Ta/W mixed-addendum POMs: [Ta₁₈P₁₂W₉₀(OH)₆(H₂O)₂O₃₆₀]³⁶⁻ (Fig. 10b) and, resulting from lanthanide substitution, [Yb₂Ta₁₈P₁₂W₉₀(OH)₆(H₂O)₁₆O₃₆₀]³⁰⁻ (Fig. 10c),⁸⁰ using a hydrothermal method. [Ta₁₈P₁₂W₉₀(OH)₆(H₂O)₂O₃₆₀]³⁶⁻ and the lanthanide counterpart [Yb₂Ta₁₈P₁₂W₉₀(OH)₆(H₂O)₁₆O₃₆₀]³⁰⁻ exhibit hexameric structures formed by the fusion of {P₂W₁₅O₅₆} units through {Ta₁₈} and {Ta₁₈Yb₂} clusters, respectively.

Chen's research group obtained lanthanide-containing POMs through the reaction of the Wells–Dawson-type Ta/W mixed-addendum POM K₅Na₄[P₂W₁₅O₅₉(TaO₂)₃]·17H₂O⁷⁴ with lanthanide ions, resulting in the 1D chain [Ln₃(H₂O)₂₂][P₂W₁₅Ta₃O₆₂], where Ln represents Y, La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu (Fig. 10d).^81,82 The Ln^III cations are positioned at the polar sites of the Wells–Dawson {P₂W₁₅Ta₃O₆₂} subunit (Fig. S5†), connected by three Ta–O–Ln bridges. Su et al. presented Cr-containing dimeric and tetrameric POMs, polyanions denoted as {Cr₃[Ta₃P₂W₁₅O₆₂]₂(H₂O)₁₂}⁹⁻ (Fig. 10e) and {Cr₄[Ta₃P₂W₁₅O₆₂]₄(H₂O)₁₂}²⁴⁻ (Fig. 10f),⁸³ respectively. The former is composed of {P₂W₁₅Ta₃O₆₂} clusters connected by three {CrO₆} octahedra, whereas the latter features a {Cr₄Ta₁₂} core enveloped by four {P₂W₁₅O₅₆} units. Interestingly, they additionally first documented a Mo/Ta/W ternary mixed-addendum POM, [K₃(H₂O)₃(P₂W₁₅Ta₃O₆₂)₆(Mo₂O₄CH₃CO₂)₃(MoO₃)₂]⁴⁸⁻ (Fig. 10g),⁸⁴ characterized by six {P₂W₁₅Ta₃O₆₂} subunits connected via eighteen Mo–O–Ta bridges, involving three {Mo^V₂O₄(OOCCH₃)⁺} and two {Mo^VIO₃} units. The Mo/Ta/W ternary mixed-addendum POM is prepared using N₂H₄·H₂SO₄ as a reducing agent, causing a conversion of some Mo^VI to Mo^V; meanwhile, the peroxo-containing [P₂W₁₅O₅₉(TaO₂)₃]⁹⁻ is transformed into peroxo-free [P₂W₁₅Ta₃O₆₂]⁹⁻.

4. Structural characterization of hetero-peroxo-PONb/Ta clusters

4.1. Hetero-peroxo-PONb clusters

Hetero-peroxo-PONb clusters were synthesized in an acidic environment in the presence of H₂O₂, among which [Nb₆O₁₉]⁸⁻ tend to decompose. The O_t(Nb) of recombined units exhibits elevated electronegativity and reactivity, facilitating the formation of hetero-peroxo-PONb clusters with VA-P, VA-As, and VIA-Se as heteroatoms. These clusters were further modified by adjusting the reaction materials and pH to obtain TM or Ln derivatives. Research in this domain has been predominantly carried out by the teams of Niu and Casey.

In 2014, Niu et al. synthesized the first example of a hetero-peroxo-PONb cluster, {As₂Nb₄(O₂)₄O₁₄}⁶⁻ (Fig. 11a), by employing K₇H[Nb₆O₁₉]·13H₂O as a precursor through a combination of conventional aqueous solution and diffusion methods.⁸⁵ {As₂Nb₄(O₂)₄O₁₄}⁶⁻ can be described as two NbO₆ octahedra in [Nb₆O₁₉]⁸⁻ being replaced by AsO₄ tetrahedra, with each Nb atom forming a peroxo bond in the presence of H₂O₂. In 2015, Casey et al. successfully synthesized isostructural polyanions [HNb₄P₂O₁₄(O₂)₄]⁵⁻ (Fig. 11a) by utilizing Nb₂O₅·nH₂O as the raw material, with H₃PO₄ serving as a pH regulator, and heteroatoms. Meanwhile, decreasing the pH value of the solution, [H₇Nb₆P₄O₂₄(O₂)₆]³⁻ was yielded. The polyanion {P₄Nb₆} can be viewed as a condensation of two Nb₄P₂ units, by sharing one corner Nb(O2) site (Fig. 11b).⁸⁶ Besides, [HNb₄P₂O₁₄(O₂)₄]⁵⁻ and [H₇Nb₆P₄O₂₄(O₂)₆]³⁻ can be converted into each other by modifying the pH. In 2017, Niu et al. synthesized [P₄Nb₆(O₂)₆O₂₄]¹⁰⁻ and its TM and Ln derivatives using K₇H[Nb₆O₁₉]·13H₂O as precursors (Fig. 11c–f).^87,88 The Ln derivatives [Ln^III(H₂O)₆]₂[H₄(NbO₂)₆P₄O₂₄] (Ln = Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) exhibit 2D structures (Fig. 11f). Additionally, the TM derivative, {[M(H₂O)_4.5]₂[P₄Nb₆(O₂)₆O₂₄]}⁶⁻ (M=Co, Ni, Zn), exhibits 0D structure (Fig. 11c), [{Cd(H₂O)₄}{P₄Nb₆(O₂)₆O₂₄}]⁸⁻ exhibits a 1D structure when the pH is adjusted to 3.8 (Fig. 11d). However, when the pH is adjusted to 2.6, the TM derivative [{Cd(H₂O)₄}₂{P₄Nb₆(O₂)₆O₂₄}]⁶⁻ exhibits a different 1D aggregation pattern (Fig. 11e). Subsequently, in 2019, Niu et al. expanded this system by introducing two different heteroatoms, Se and P (Fig. 11g).⁸⁹ The structure of [P₂Se₂Nb₆(O₂)₆O₂₂]⁸⁻ closely resembles that of [P₄Nb₆(O₂)₆O₂₄]¹⁰⁻, with the upper and lower PO₄ groups replaced by SeO₃. Mass spectrometry results confirmed that the structure of [P₂Se₂Nb₆(O₂)₆O₂₂]⁸⁻ remained intact in aqueous solution. In 2022, they reported the in situ synthesis of the Ln derivatives of [P₂Se₂Nb₆(O₂)₆O₂₂]⁸⁻ polyanions,⁹⁰ formed as {[Ln(H₂O)₆][H_2.5P₂Se₂Nb₆(O₂)₆O₂₂]}₂⁵⁻ [Ln = Dy, Tm, Yb, Lu]; {[Ln(H₂O)₄][H₄P₂Se₂Nb₆(O₂)₆O₂₂]}₂²⁻ [Ln = Ce, Pr, Sm, Eu, Gd]; and {[Ln(H₂O)₆][P₂Se₂Nb₆(O₂)₆O₂₂]}₂{[Ln_x(H₂O)_y]₂[P₂Se₂Nb₆(O₂)₆O₂₂]}^(7+m)− [Ln = Ho, Er, Tm, Yb, Lu], and represented them as one-dimensional (Fig. 11h) and two-dimensional (Fig. 11i) architectures and a one-dimensional chain structure containing double polyanions (Fig. 11j), respectively.

Fig. 11 Polyhedral and ball–stick representation of hetero-peroxo-PONb clusters and Ln/TM-derived clusters: (a) {X₂Nb₄(O₂)₄O₁₄}⁶⁻ X=As/P, (b) [H₇Nb₆P₄O₂₄(O₂)₆]³⁻, (c) {[M(H₂O)_4.5]₂[P₄Nb₆(O₂)₆O₂₄]}⁶⁻, (d) [{Cd(H₂O)₄}{P₄Nb₆(O₂)₆O₂₄}]⁸⁻, (e) [{Cd(H₂O)₄}₂{P₄Nb₆(O₂)₆O₂₄}]⁶⁻, (f) [Ln^III(H₂O)₆]₂[H₄(NbO₂)₆P₄O₂₄], (g) [P₂Se₂Nb₆(O₂)₆O₂₂]⁸⁻, (h) {[Ln(H₂O)₆][H_2.5P₂Se₂Nb₆(O₂)₆O₂₂]}₂⁵⁻, (i) {[Ln(H₂O)₄][H₄P₂Se₂Nb₆(O₂)₆O₂₂]}₂²⁻, (j) {[Ln(H₂O)₆][P₂Se₂Nb₆(O₂)₆O₂₂]}₂{[Ln_x(H₂O)_y]₂[P₂Se₂Nb₆(O₂)₆O₂₂]}^(7+m)−.

4.2. Hetero-peroxo-POTa clusters

While Nb and Ta share similar chemical properties, Ta is recognized for its greater chemical inertness, resulting in the development of hetero-peroxo-POTa clusters that have progressed at a slightly slow pace. Over the past decade, significant progress has been achieved in the in-depth study of H₂O₂ protection under acidic conditions. In this field, Niu et al. made exceptional contributions. In 2017, they made the initial report of two hetero-peroxo-POTa compounds with polyanions: [H₄P₄Ta₆(O₂)₆O₂₄]⁶⁻ (trans-P₄Ta₆(O₂)₆) (Fig. 12a) and [H₉P₄Ta₆(O₂)₆O₂₅]³⁻ (cis-P₄Ta₆(O₂)₆) (Fig. 12b).⁹¹ These compounds were synthesized with precise pH control and cation selection. Both polyanions comprise two {P₂Ta₃(O₂)₃} subunits, and their cis- or trans- configurations arise from the connection of these subunits at distinct angles. The structure of trans-P₄Ta₆(O₂)₆ closely resembles that of [P₄Nb₆(O₂)₆O₂₄]¹⁰⁻. The stability of these compounds was verified through ³¹P MAS NMR analysis, both in the solid state and in solution. Subsequent studies in 2021 reported a series of TM derivatives built upon trans-P₄Ta₆(O₂)₆ (Fig. 12c and d).⁹² Furthermore, the Ni derivative [Ni₂(H₂O)₄{P₄Ta₆(O₂)₆O₂₄}]⁶⁻ exhibits a discrete structure (Fig. 12c), whereas the Zn and Cd derivatives [Zn/Cd(H₂O)₄{P₄Ta₆(O₂)₆O₂₄}]⁸⁻ display 1D aggregation (Fig. 12d).

Fig. 12 Polyhedral and ball–stick representation of hetero-peroxo-POTa clusters and Ln/TM-derived clusters: (a) [H₄P₄Ta₆(O₂)₆O₂₄]⁶⁻, (b) [H₉P₄Ta₆(O₂)₆O₂₅]³⁻, (c) [Ni₂(H₂O)₄{P₄Ta₆(O₂)₆O₂₄}]⁶⁻, (d) [TM(H₂O)₄{P₄Ta₆(O₂)₆O₂₄}]⁸⁻, (e) [Ln(H₂O)₆{H₄(TaO₂)₆As₄O₂₄}]³⁻, (f) [(TaO₂)₆Se₄(OH)₄O₁₇]⁴⁻, (g) [Ln(H₂O)₆(TaO₂)₆Se₄(OH)₃O₁₈]²⁻, (h) [HSe₂(TaO₂)₆(OH)₄(H₂O)₂O₁₃]³⁻, (i) [Se₄(TaO₂)₆(OH)₃O₁₈]⁵⁻.

Besides VA-P, the synthesis system also incorporates two other heteroatoms, VA-As and VIA-Se, similar to the hetero-peroxo-PONb system. In 2019, Niu et al. meticulously adjusted the synthesis conditions, leading to the production of the peroxo-polyoxoarsenotungstate Ln derivatives, [Ln(H₂O)₆{H₄(TaO₂)₆As₄O₂₄}]³⁻ (Ln = Sm, Eu, Tb, Dy, Er, Tm, Yb, Lu) (Fig. 12e).⁹³ The {(TaO₂)₆As₄O₂₄} (Fig. S6†) subunit differs significantly from cis-P₄Ta₆(O₂)₆ and trans-P₄Ta₆(O₂)₆ in that one “half-unit” {As₂Ta₃(O₂)₃O₁₄} rotates to some extent, resulting in an intriguing arrangement. [Ln(H₂O)₆{H₄(TaO₂)₆As₄O₂₄}]³⁻ forms a 1D structure by linking Ln atoms with {(TaO₂)₆As₄O₂₄} subunits. Time-resolved emission spectroscopy (TRES) reveals the energy transfer process from the POTa fragment to the Eu³⁺ ion. In 2020 and 2022, Niu et al. reported the [(TaO₂)₆Se₄(OH)₄O₁₇]⁴⁻ polyanion (Fig. 12f)^94,95 and its Ln derivatives, [Ln(H₂O)₆(TaO₂)₆Se₄(OH)₃O₁₈]²⁻ (Ln = Tb, Dy, Ho, Er, Tm, Yb) (Fig. 12g).⁹⁴ The structure of [(TaO₂)₆Se₄(OH)₄O₁₇]⁴⁻ closely resembles that of cis-P₄Ta₆(O₂)₆. Notably, the {(TaO₂)₆Se₄} subunit in [Ln(H₂O)₆(TaO₂)₆Se₄(OH)₃O₁₈]²⁻ (Fig. 12g) exhibits a slight difference from the independent [(TaO₂)₆Se₄(OH)₄O₁₇]⁴⁻ (Fig. 12f). In the independent [(TaO₂)₆Se₄(OH)₄O₁₇]⁴⁻, two SeO₃ units in the equatorial position are oriented in opposite directions, while in [Ln(H₂O)₆(TaO₂)₆Se₄(OH)₃O₁₈]²⁻, they are oriented in the same direction. Intriguingly, two years later, Zhang et al. documented another independent polyanion [Se₄(TaO₂)₆(OH)₃O₁₈]⁵⁻, where the equatorial position SeO₃ is oriented in the same direction (Fig. 12i),⁹⁶ possibly influenced primarily by pH. Furthermore, the cluster [HSe₂(TaO₂)₆(OH)₄(H₂O)₂O₁₃]³⁻ is achieved by adjusting the pH from 3.2 to 4.9 (Fig. 12h).

5. Application

Based on the above, it is evident that under acidic conditions, structurally diverse mixed-addendum PONb/Ta clusters and hetero-peroxo-PONb/Ta clusters have been synthesized through self-assembly principles, crystal engineering, and molecular design strategies. The rich structure has also prompted researchers to pursue applied research. Considering the properties of POMs, mixed-addendum PONb/Ta clusters and hetero-peroxo-PONb/Ta clusters hold promising potential in many fields, including photocatalysis, thermocatalysis, proton conductivity,^{60,79,81,92,97} magnetic properties,^48,68 luminescence properties,^41,58,93 and various other applications. Among them, photoelectrocatalysis has been extensively researched, including electrocatalytic reduction of nitrite^40,54 and bromate ions,⁷⁰ photocatalytic hydrogen evolution,^{42,43,47,67,74–77,80,83,85} organocatalysis,^{30,34,62–64,82,84,96,97} and the degradation of RhB.^45,94 In this section, only a selection of representative results will be introduced.

The POMs with polyanions [Ta₁₈P₁₂W₉₀(OH)₆(H₂O)₂O₃₆₀]³⁶⁻ and [Yb₂Ta₁₈P₁₂W₉₀(OH)₆(H₂O)₁₆O₃₆₀]³⁰⁻ have demonstrated remarkable UV photocatalytic water splitting activity, yielding 8065.7 and 6486.8 μmol h⁻¹ g⁻¹, respectively.⁸⁰ These findings open new opportunities for the development of highly efficient photocatalysts and the advancement of polytantalotungstate chemistry. Moreover, numerous POMs have demonstrated efficient catalytic performance in diverse reactions, including cyanosilylation,^62,82 primary amine coupling,⁸⁴ Knoevenagel condensation,⁹⁸ amidation,⁹⁶ imidation,⁹⁵N-aryl-tetrahydroisoquinoline synthesis,⁶⁴ and visible-light-mediated aerobic benzylic C–H oxidations.⁶³

Moreover, owing to their capability to store and release electrons, POMs stand as promising candidates in the design of electrode materials. Recently, Su's research team investigated the charge and discharge capacity of Nb/W and Ta/W mixed-addendum POMs. For example, {Cr₃[Ta₃P₂W₁₅O₆₂]₂(H₂O)₁₂}⁹⁻ exhibited an initial discharge capacity of 1371 mA h g⁻¹ and a charge capacity of 400 mA h g⁻¹,⁵⁵ while the compound with the [MnTa₁₈Si₆W₅₄O₂₃₁]²²⁻ polyanion demonstrated an initial discharge capacity of 1115.5 mA h g⁻¹ and a charge capacity of 829.9 mA h g⁻¹.⁷⁸ Both compounds displayed exceptional stability, attributed to the reduced impedance value that promotes the formation of a stable Li-ion transport channel.

Additionally, the compound with the polyanion {[P₂W₁₅Nb₃O₆₂]₂(4PBA)₂((4PBA)₂O)}¹⁶⁻ demonstrated significant bulk proton conductivity, achieving 1.59 × 10⁻¹ S cm⁻¹ at 90 °C and 98% relative humidity, owing to its extensive hydrogen-bonding network within the crystal lattice.⁶⁰

Furthermore, mixed-addendum POM derivatives containing transition or rare-earth metals may exhibit intriguing magnetic and optical properties. As an illustration, Niu and colleagues explored the single-molecule magnet properties of [H₁₂₃Nb₃₆P₁₂W₇₂Mn^III₁₂Mn^II₃NaO₄₂₄]¹⁰⁻, revealing that each {Mn^III₃} subunit functions as an independent single-molecule magnet (SMM).⁶⁸ In addition, the photoluminescence properties unveil an energy transfer phenomenon from the {(TaO₂)₆As₄O₂₄}⁹³ and {P₂Se₂Nb₆}⁹⁰ segments to the Ln^III center, effectively enhancing emissions from the Ln³⁺ center. And, as Casey et al. reported, the [H₇Nb₆P₄O₂₄(O₂)₆]³⁻ cluster enables the deposition of high-quality patterned thin films from simple aqueous solutions – a first step in evaluating their potential as functional materials for the semiconductor industry.⁸⁶

Hill's group found that Cs₇[Si(NbO₂)₃W₉O₃₇] effectively combats human immunodeficiency virus type 1 (HIV-1), demonstrating an EC₅₀ of 1.0 μM.³² Subsequently, they additionally verified the efficacy of four Wells–Dawson-type compounds against HIV-1, showing significant activity, minimal toxicity, and selective inhibition of purified HIV-1 protease.⁵² Motivated by these findings, Niu's research team ascertained that [H₂₄{Nb₄O₆(OH)₄}{Nb₆P₂W₁₂O₆₁}₄]¹²⁻ exhibits notable efficacy against human breast cancer MCF-7 cells.⁶⁶

6. Outlook

Owing to their remarkable diversity of structures and prominent applications in catalysis, photology, and magnetic properties, PONbs and POTas have attracted considerable attention. Currently, acid synthesis methods are under exploration to design novel architectures with unique functionalities. This review provides a systematic summary of the advancements in this evolving field, encompassing the recent studies on the reaction of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions, synthetic methodologies, structural features, and potential applications. The objective is to provide a comprehensive overview of the acid synthesis of PONbs and POTas, offering valuable guidance to researchers in this field.

It is noteworthy that the acid chemistry of PONbs and POTas is at an early stage, offering both challenges and opportunities. Limited comprehension of solution behavior and molecular assembly has complicated the development of PONbs and POTas chemistry, impeding their potential applications. Nevertheless, incorporating auxiliary materials could facilitate solution control and the development of new building block libraries. Furthermore, progress in size control and controlled synthesis could broaden their applications in novel fields. In summary, the progress achieved over the past decades underscores the intrinsic excellence and practical significance of this research. We anticipate that this work will provide valuable assistance to fellow researchers engaged in the synthesis chemistry of PONbs and POTas.

Author contributions

Haiying Wang and Zhijie Liang contributed equally and collaborated closely to co-write this review. Dongdi Zhang and Jingyang Niu provided valuable input on the overall conceptual framework.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22071045), the Excellent Youth Science Fund Project of Henan Province (202300410042), the Natural Science Foundation of Henan Province (232300420372), and Henan University. We thank Zongfei Yang from Xuzhou University of Technology and Rimsha Rehman from Henan University for providing valuable suggestions for the final version of the manuscript.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi00899e

‡ These authors contributed equally to this work.

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