State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
b.
University of Chinese Academy of Sciences, Beijing 100049, China
Received Date:
20 June 2024 Accepted Date:
29 August 2024 Revised Date:
02 August 2024 Available Online:
15 October 2025
Abstract:
For a long time, researchers have been fascinated by the structurally diverse and high-performance characteristics of polyoxometalates (POMs). Modifying POMs with various types and properties of metals has broadened their applications in fields such as magnetism, luminescence, and catalysis. However, despite the discovery of numerous POM structures doped with transition metal ions, the development of aluminum (Al) as a ⅢA group metal in the POM field has been slow. Aluminum, the most abundant metal in nature, offers innate electron-deficient properties that, when combined with highly charged POMs, could introduce novel structures and excellent functionalities like proton conduction to this field. Therefore, this review will address the gap in summarizing Al-containing POMs by categorizing and summarizing the synthesis, structural characteristics, and properties of Al-containing POMs, aiming to provide a theoretical foundation for exploring POM structures doped with Al atoms. The review also analyzes and forecasts the prospects in this field.
Polyoxometalate (POM), consisting predominantly of the early transition metal (TM) ions Mo, W, V, Nd, Ta and various types of transition, main group, lanthanide (Ln) and actinide (An) metals as bridging groups, as a major branch of inorganic chemistry, has given rise to a myriad of rich and colorful structures containing various metals after almost 200 years of prosperous development (Fig. 1a) [1-3]. The types of metal elements in POMs encompass nearly the entire periodic table, with transition metals possessing various excellent properties occupying the majority of the polyoxometalate structures (Fig. 1b). For example, in recent years, transition metal-substituted POMs (TM-POMs) such as noble metal ions, Fe3+, Co2+, Ni2+ have been extensively synthesized and reported in large quantities [4-9]. These TM-POMs structures can be used to adjust the solubility, redox properties of the structures by modifying the type of counter cations such as alkali metal ions, quaternary ammonium salts and doped TM ions species, which has broadened the way for the application of POMs in various fields, such as catalysis, biology, and energy [10-15]. In the rapid development of TM-POMs, with a clear structure, the group ⅢA metal elements Al, Ga, and In substitution of POMs due to its very small proportion and a lot of space for development, is also gradually showing a thriving trend (Fig. 1b) [16-21].
Figure 1
Figure 1.
An inclusive statistical summary of all metal elements present in current POM structures. (a) A simplified periodic table analysis of the elements contained in POMs: including the base elements that form the POMs (blue), the other elements contained (the main group metals: red; the transition metals: green; the lanthanide and actinide metals: orange; and the Al element: yellow), which is the focus of this manuscript. The elements are highlighted and classified in different colors. (b) The proportion of POM structures containing various elements.
Aluminum is the most abundant metallic element in the earth's crust, but Al-related crystalline materials are still underdeveloped due to its hydrolytic properties. In 1996, Powell and Heath made a systematic compilation of Al complexes crystal materials and their applications in biology [22]. Professor Casey conducted a comprehensive classification and kinetics of aluminum hydroxide macromolecules {Al13}, {Al30}, etc., also known as polyoxoaluminates (POAls), in aqueous solution in 2005 [23]. As a transition, Johnson et al. in 2012 revealed the research potential of some "missing" structures by summarizing the synthesis of inorganic aluminum hydroxide and organic ligand-supported structures [24]. It is only after the emergence of "induced aggregation, solvent regulation, and supercluster assembly" strategies in recent years [25] that many organic-inorganic-hybrid high-nuclearity aluminum oxo clusters (AlOCs) have gradually emerged [26-28]. In terms of induced aggregation, the lacunary POMs structure undoubtedly provides coordination sites for the directed addition of Al3+, which acts as strong nucleophilic reagents to induce targeted aggregation of Al3+ [29,30]. Therefore, the combination between POMs as Lewis bases and Al3+, POAls and AlOCs as Lewis acids has attracted the attention of a wide range of researchers working to provide more efficient functional materials for proton conduction, catalysis and other fields [18,31].
All Al-containing POMs materials are classified into two main categories based on coordination bonds and interionic interactions: (1) Aluminum-substituted POMs (Al-POMs) structures: the focus is on Al-substituted mono- to tetra-vacant Keggin or Dawson as well as a monomer to tetramer structures formed as AlOCs bridging ions; (2) POM-based ionic crystals based on charge interactions between POMs anions and POAls cations (POAls@POMs), including cationic POAls such as [AlO4Al12(OH)24(H2O)12]7+ (Al13), [(AlO4)2Al28(OH)56(H2O)26]18+ (Al30) and anionic POMs building blocks such as Anderson-type [Al(OH)6Mo6O18]3− (AlMo6), Keggin-type [H2W12O40]6− (W12), [CoW12O40]6− (CoW12), crystallized by electrostatic gravitational ordering between them. Given that the field is still in its early stages, there has yet to be a relevant review published. Therefore, our objective is to comprehensively review all known Al-containing POM structures, including both types of Al-POMs and POAls co-crystallized with POMs (Table 1). To explore the laws of development in the field of intersection between Al element and POM, and to provide the theoretical basis for more possibilities in the future. The structure of Al3+ as a central heteroatom in POMs will not be reviewed in this article.
Table 1
Table 1.
Information on the molecular formula of Al-containing POMs structures.
a Abbreviations: Bu4N: tetrabutylammonium; TBA: tetra- n-butylammonium; Cp: C5H5−; bpe=trans-1,2-di-(4-pyridyl)-ethylene; CD: γ-cyclodextrin. b CCDC: CCDC number.
2.
Structural features of Al-containing POMs
Up to now, the basic conformations of POMs have gradually increased to nearly 10 species, including Keggin, Well-Dawson, Lindqvist, Anderson, and others [32]. Utilizing structurally diverse and precise POM building blocks for crystal engineering to create functional materials with desirable properties through coordination bonds, hydrogen bonds, and interionic interactions is a challenging and rewarding task [33-35].
Currently, methods to obtain Al-containing POMs primarily include aqueous solution, hydrothermal, and solvothermal methods. Due to the uniqueness of the two types of structures, it is natural that each has its own style in the choice of synthesis strategy. In aiming to target Al3+ coordinated within POM building blocks, the straightforward yet less predictable one-pot reaction is gradually being replaced by a two-step strategy using POM precursors. To ensure the smooth coordination of Al3+ in POMs, the utilized POM precursors are usually vacant POM units containing multiple coordination sites, which can surround the Al3+ inside them to form clusters (Scheme 1A); In the case of POAls@POMs, to obtain structurally precise and regularly arranged ionic crystals, POAl and saturated POM building blocks solutions are usually prepared and mixed, followed by diffusion crystallization or volatilization to obtain co-crystal products rich in hydrogen bonding and ionic interactions (Scheme 1B). Until now, the synthesis of such structures is still concentrated in aqueous solutions, and hydrothermal or solvothermal methods with high-temperature and high-pressure conditions are still less frequently used. In the following, detailed descriptions and summaries of the structural characteristics and related properties of the two systems will be provided, aiming to explore the idea of function-oriented structural design.
Scheme 1
Scheme 1.
The reaction strategy can be chosen according to the target requirements: Strategy A: to obtain Al-POMs then vacant POM building blocks with coordination sites are required; Strategy B: to obtain POAls@POMs then saturated POM units and intact POAl clusters are required.
Among the few structures of Al-POMs currently known, almost all result from the substitution of Al3+ for mono- to hexa-vacant Keggin and Dawson-type building blocks. Since the discovery of the first POMs in 1826, it was not until 1933 that the classical Keggin-type building block was identified by X-ray diffraction (XRD) [36]. The Keggin configuration with the general formula [XM12O40]n− (XM12, M = Mo, W, V, Nd, Ta) exists in five classes of isomers, except for α-Keggin, which has tetrahedral symmetry, and the remaining β, γ, ε, and δ configurations being derived from the α-configuration by rotating one to four {M3O13} trimeric fragments [37]. In contrast to the more common Keggin type in this field, the α-Dawson conformation [X2M18O62]n− (X2M18) was not confirmed until 1953 and has seen less application [38]. For each conformation, there is a unique mode of lacunary, which also provides the basis for the substitution of Al3+ at the corresponding lacunary sites to stabilize the Al3+ while forming more diverse structures [39].
Classified according to the amount of Al3+ embedded in the POMs, the history of the development from mono-Al-substituted POMs to the formation of POM-based AlOC structures in the presence of the Keggin and Dawson POMs anion template is first described separately.
2.1.1
Mono-aluminum containing Keggin-type POMs
Most structures containing one Al3+ are formed by removing an {MO6} octahedron from a saturated α,β-Keggin-type group and chelating it to one Al3+ as a pentadentate ligand [40]. In the initial development of the POMs field, polyoxotungstates (POTs) occupy the majority, so all in this section are POTs [41]. As early as 1982, in order to investigate the Al3+, Ga3+, In3+ and Tl3+ of group ⅢA elements substituted POM structures, which were relatively rare at that time compared to divalent ions, Tourné et al. synthesized and summarized studies of α-XM(OH2)W11O39n− (X = B, Si, Ge, P, As, M3+ = Al, Ga, In, Tl) and X2M(OH2)W17O617− (X = P, As, M3+ = Al, Ga, In, Tl) in a full series of nearly 70 structures [42]. It was found that these mono-Al-POMs are more stable to acids and bases than the lacunary and saturated homologs. During the following 40 years, α,β-Keggin-type mono-Al-substituted POTs structures of main group elements P, Si, B, Al, Ge and transition metals Cr, Cu, Co, Fe, Zn, etc. as the central heteroatoms have successively appeared (Fig. 2a) [43-53]. Compared to α-Keggin with tetrahedral symmetry, the reduced symmetry caused by the rotation of the {M3O13} trimeric fragment in β-Keggin results in the existence of three isomers of the single-deficient β-Keggin as well (Fig. 2b). This issue was explored in depth by a researcher in 2001, who illustrated the stability trends of POTs with Al3+, for example, as the central heteroatom, through the integration of thermodynamic and kinetic conditions in the isomerization of Keggin-type Al-POTs [47].
Figure 2
Figure 2.
(a) The mono-Al-substituted α-Keggin-type POTs. Reproduced with permission [42]. Copyright 1982, American Chemical Society. (b) The mono-Al-substituted β1, β2, β3-Keggin-type POTs. Reproduced with permission [47]. Copyright 2001, American Chemical Society. (c) The di-Al-substituted γ-Keggin-type POTs. Reproduced with permission [55]. Copyright 2008, American Chemical Society. (d) The di-Al-substituted γ-Keggin-type POTs TBA3[(C5H5N)H][γ-SiW10O36{Al(C5H5N)}2(µ-OH)2]·2H2O. Reproduced with permission [55]. Copyright 2008, American Chemical Society. (e) Di-Al containing Keggin-type POTs [(n-C4H9)4N]6[α-PW11Al(OH)O39ZrCp2]2. Reproduced with permission [57]. Copyright 2013, The Royal Society of Chemistry. (f) The tri-Al-substituted α,β-Keggin-type POTs. Reproduced with permission [60]. Copyright 1992, The Royal Society of Chemistry. (g) The sandwich-type K6Na[(A-PW9O34)2{W(OH)(OH2)}{Al(OH)(OH2)}{Al(μ-OH)(OH2)2}2]·19H2O compound. Reproduced with permission [63]. Copyright 2010, The Royal Society of Chemistry. (h) The tetra-Al containing Keggin-type dimer POTs (TBA)4[Al4(H2O)10(β-XW9O33H)2]·4H2O (X = As, Sb). Reproduced with permission [31]. Copyright 2013, The Royal Society of Chemistry. (i) The AlOC-containing trimer TBA9[{γ-H2SiW10O36Al2(μ-OH)2(μ-OH)}3] POTs. Reproduced with permission [64]. Copyright 2011, American Chemical Society. (j) The tetramer K22[{α-Al3SiW9O34(μ-OH)6}4{Al4(μ-OH)6}]·3KCl·69H2O POTs. Reproduced with permission [65]. Copyright 2015, The Chemical Society of Japan.
For POMs containing two Al3+, two main categories exist, one of which is the addition of Al3+ in the classical di-vacant γ-Keggin-type unit. The formation of the di-vacant γ-Keggin-type may be attributed to the dissociation of two W6+ due to the decrease in the stability of the two co-edge {WO6} octahedron by a 60° rotation of two pairs of neighboring {W3O13} units [54]. In 2008, Mizuno's group obtained the di-Al-substituted silicotungstate TBA3H[γ-SiW10O36{Al(OH2)}2(µ-OH)2]·4H2O by reacting in an acidic environment (Fig. 2c), further reaction with pyridine revealed that two of the pyridine N sites are coordinated axially to the Al3+, demonstrating that the two Al3+ add strong Lewis acid sites to the POMs cluster, while the presence of the pyridine cations reflects the Bronsted acidity of the structure (Fig. 2d) [55]. The Lewis acid cyclization of citronellal derivatives catalyzed by the two compounds, combined with density functional theory (DFT) calculations as well as experimental results, fully responded to the superior catalytic performance and selectivity of the bare Al3+ compared to the Al3+ sites coordinated by pyridine. In addition, to investigate the optical nonlinear response law of this kind of structure, Zhongmin Su's group in 2013 carried out a theoretical analysis using DFT [56]. The static second-order polarizability, i.e., zero-frequency hyperpolarizability β0, exhibits a pattern of decreasing with increasing atomic radius and increasing with increasing electronegativity (P > Si, As > Ge) in the same period for the change of atomic radius corresponding to the central heteroatom species as well as electronegativity.
The second class consists of mono-Al-substituted building blocks obtained by bridging bis(cyclopentadienyl)zirconium complexes to organic-inorganic dimeric structures discovered and studied by Kato et al., which still contain two Al3+ overall (Fig. 2e) [57-59]. This addition of metal ions to vacant POMs that can be further grafted with metal ions and organic complex sites remains a major route to expanding POM chemistry.
2.1.3
Tri-aluminum containing Keggin-type POMs
Common tri-vacant POM precursors were used for the two-step fabrication of tri-coordinated Al-substituted POMs α,β-Keggin-type compounds α,β-[SiW9O39(Al(H2O)}3]7− [60], α,β-[GeW9Al3O37(H2O)3]7− [61], (H2bpe)H4.5[AlW8.5Al0.5{Al(OH2)}3(OH)3O34]·4H2O (Fig. 2f) [62] and the unique sandwich type K6Na[(A-PW9O34)2{W(OH)(OH2)}{Al(OH)(OH2)}{Al(μ-OH)(OH2)2}2]·19H2O (Fig. 2g) [63]. Here the Al3+ is co-occupied with W6+. The coordination of Al3+ in the cavity as an electron-deficient group considerably stabilizes the POMs with a very high negative charge. In addition to this, protonated organic cations as an anti-balance cation can likewise stabilize the negatively polymerized anion clusters through effective intermolecular interactions.
2.1.4
Tetra-aluminum containing Keggin-type POMs
As the number of Al-containing increases, the monomeric compounds are insufficient to support the construction of the Al3+ framework, leading the structure to gradually transition from monomers to dimers. For example, the structure of arsenotungstate and antimoniotungstate containing four Al3+ in the center of the dimeric sandwich obtained in 2013 (Fig. 2h) [31]. During the reaction, the precursor [α-XW9O33]9− (X = As3+, Sb3+) undergoes a high-temperature transformation to [β-XW9O33]9−, effectively encapsulating the four Al3+ within the cavity. Side by side, while the POMs protect the Al3+, the Al3+ also plays a role in facilitating the dimer linkage and stabilizing the overall structure, as well as providing multiple exposed Lewis acid sites for the catalytic reaction and further structural expansion.
2.1.5
AlOC-containing Keggin-type POMs
Until 2011, based on the theoretical support provided by Kikukawa following the synthesis of {SiW10Al2}, the typical structural motifs of simple Al-POMs were realized in anhydrous solvents by a trimeric condensation, resulting in TBA9[{γ-H2SiW10O36Al2(μ-OH)2(μ-OH)}3] (Fig. 2i) [64]. From then on, this field gradually developed towards higher polymerization numbers, where Al3+ no longer merely occupies vacancies but stabilizes multiple POM building blocks through bridging oxygens outwardly. The first example of trimeric structure, under vacancy-guided assembly, can be seen as three di-coordinated {SiW10Al2} moieties linked by three μ-OHs, with six central Al3+ forming an AlOC in tandem with each other. Four years later, in 2015, even more tetrameric high-nucleation Al16-containing POM compounds emerged, with an {Al4} tetrahedron acting as a central hub in the central of the four tri-Al-substituted Keggin-type subunits, diverging outwards through three μ-OHs in each of the four directions respectively to give K22[{α-Al3SiW9O34(μ-OH)6}4{Al4(μ-OH)6}]·3KCl·69H2O (Fig. 2j) [65].
2.1.6
Aluminum containing Dawson-type POMs
In contrast to the mono- to tri-vacant present in the XM12 Keggin-type POMs building blocks with the ratio of 1:12, the 2:18 Dawson-type configuration features a unique hexa-vacant mode, implying nearly 12 coordination sites that can be modified in a controllable manner.
Mono-Al-substituted Dawson-type structure was improved in 1982 by Kato et al. through a scheme to obtain K7[α2-P2W17{Al(OH2)}O61]·14H2O in 2013 and a systematic investigation of the catalytic properties was carried out (Fig. 3a) [42,50]. A case of mono-Al coordinated to two tri-vacant positions obtained by using diffusion crystallography has been reported in 2019 dimers [(n-C4H9)4N]7[H14Al(B-α-P2W15O56)2] in Dawson configuration [66]. This also demonstrates that by changing the reaction conditions, Al3+ can not only be embedded in the original vacant site but also extend outward through bridging oxygen to form higher polymeric Al-containing POM materials. Of course, the tri-vacant Dawson-type unit is still preferred to the in situ addition of three Al3+ to obtain the K6[B-α-H3P2W15O59{Al(OH2)}3]·14H2O crystal (Fig. 3b) [67]. Besides, based on the multiple coordination sites in the triple vacancies, the dimeric POT structure containing four Al3+ was first obtained by Liu and Meng in 1995 [68], and the epoxidation catalytic reaction was investigated one year later [69].
Figure 3
Figure 3.
(a) The mono-Al-substituted Dawson-type POTs. Reproduced with permission [42]. Copyright 1982, American Chemical Society. (b) The tri-Al-substituted Dawson-type POTs. Reproduced with permission [67]. Copyright 2014, American Chemical Society. (c) The tetra-Al-substituted crescent-shaped POTs. Reproduced with permission [70]. Copyright 2016, Wiley-VCH. (d) a novel 16-AlⅢ-32-oxo cluster embedded {P8W48} archetype polyanion, {[Na(NO3)(H2O)]4[Al16(OH)24(H2O)8(P8W48O184)]}16−. Reproduced with permission [18]. Copyright 2018, Wiley-VCH.
Interestingly, Dawson crescent-shaped {Si2W18} building blocks with openings on one side were discovered in 2016, which can be viewed as open Dawson-type pockets and are capable of adjusting the opening bite angle just enough to fit ions of different radii, such as Al3+, into them (Fig. 3c) [70]. Such open-Dawson POMs are reported to be emerging POMs formed by suppressing the assembly of conventional Dawson through electrostatic repulsion from high charge. This open structure allows coordination of multiple ions and the opening angle increases with the number and ionic radius. In fact, the structure should be counted as a transition from Al3+ to AlOCs, with four Al3+ already forming complete closed tetragonal AlOCs via Al-O-Al bonds.
In 2018, Y. Peng et al. integrated the host-guest chemistry theory of molecular containers and ingeniously applied the {P8W48} molecular ring with hexa-vacant Dawson as the structural basis as the main body and embedded the ordered {Al16} guest in the buffer solution (Fig. 3d) [18]. This strategy not only solves the problem of pH change caused by the easy hydrolysis of Al3+, but also utilizes the Lewis acid-base theory, combining the acid-base properties of POMs and AlOCs, and makes use of their interactions to compensate for the charge migration, thereby facilitating the direct proton transfer via the Grotthuss mechanism and enhancing the proton conductivity.
From the above summary, it can be seen that the POMs substituted by high-nuclear AlOCs are still in their infancy and the exploration of this field has only gradually sprouted in the last decade. Fortunately, the regularities of such structures have begun to emerge preliminarily, which can support the establishment of structural development rules as theoretical guidance for future exploration. As it stands, the construction of more high-nuclear and configuration-rich structures can be considered in two main aspects: Firstly, the number and position of the lacunary sites. Throughout, mono-vacant building blocks limit aggregation between building blocks by only directing the addition of individual Al3+, which is limited by fewer active sites and higher repulsive forces due to higher charge density, among other reasons. The di- and tri-vacant units are relatively more open, with one side exclusively occupied by the Al3+ and providing a more active coordination site that directs its outward expansion. Secondly, for the POMs building block species, compared with the more developed structures based on the Keggin configuration, the diverse lacunary modes and abundant coordination sites of the Dawson-type structures have already demonstrated their excellent nucleophilicity in {P8W48} in 2018, eagerly awaiting further expansion at a later stage.
2.2
POAls@POMs
Ionic crystals based on inter-ionic forces and hydrogen bonding interactions combine the advantages of anions and cations, and through synergistic interactions between the components, they are endowed with more excellent properties such as proton conduction and photoelectrocatalysis and adsorption [71,72]. In the realm of POM structures, the very common Na+, K+, Cs+, etc. have been extensively reported as counter-cations (Fig. 1b) [73-76]. However, the molecular ionic structures with a certain size and number of charges as the countercations form an ordered alternating arrangement of ionic crystals, which has a higher porosity and practical significance [77]. The first example of a POM-based ionic crystal structure was obtained by gel crystallization technique in 2000, with the counter-cation being the matured POAls {Al13} structure [78]. Subsequently, researchers have found that inter-ionic forces can be utilized for constructing novel solid-state crystalline materials. Therefore, this chapter reviews the structures constructed based on anionic and hydrogen bonding interactions between POAls and POMs, such as {Al13}, {Al30}, and their derivatives, in order to provide theoretical support for the design, construction, and application of related crystalline materials in the future.
2.2.1
POAl structures
Before that, we will give a brief background on the study of POAls commonly applied in ionic crystal constructions. More interestingly, the aggregation of POAls in an aqueous solution exhibits a classical Keggin configuration similar to POMs. Reports on {ɛ-Al13} date back to as early as 1960, and its single crystal was obtained 10 years later [79,80]. The synthesis of {ɛ-Al13} was completed in 1991 using AlCl3 and aluminum foil in aqueous solution [81]. Concurrently, monitoring of NMR spectroscopy revealed the process of transformation of {ɛ-Al13} to the {δ-Al13} intermediate as well as to the dimer {Al30} at elevated temperatures, though the structure was not accurately predicted at that time. It was not until 2000, following characterization by XRD and 27Al NMR, that the structure of {Al30} was fully defined [82,83]. The structure of {Al30} has been reviewed and interpreted in detail by Liu et al. [84].
2.2.2
Al13@POMs
In fact, before the discovery of the first POMs-based ionic crystal structure in 2000, Kwon's research group had already initially explored the assembly technique of {Al13} and {V10} in the previous year and obtained the nanocomposite gel material Na0.3[AlO4Al12(OH)24(H2O)12][V10O28]Cl1.3(H2O)35.5 [85]. Building upon this, using gel crystallization technique, {Mo7} solution was slowly added to {Al13} solution to obtain {Al13} and {AlMo6} ionic single crystals with well-defined structures [AlO4Al12(OH)12(H2O)24][Al(OH)6Mo6O18]2·(OH)·29.5H2O (Fig. 4a) [78]. The authors suggested that this mode of ion stacking with certain channels could potentially play a unique role in adsorption, catalysis, and other fields. Since then, composite ionic crystals based on POMs building blocks such as Keggin, Linquist, and clusters of {Al13} have sprung up. In 2003, two cases of {δ-Al13} isolated from the fragmentation of homopolytungstates and heteropolytungstates reacting with {Al30} were constructed to form molecular ionic compounds via face-to-face hydrogen bonding interactions and electrostatic interactions. [δ-Al13O4(OH)24(H2O)12][H2W12O40](OH)·nH2O and [δ-Al13O4(OH)24(H2O)12][CoW12O40](OH)·nH2O, with pore energies of more than 40% (Fig. 4b) [86]. This face-to-face hydrogen bonding interaction was shown even more vividly in a work by Kwon immediately following [87]. In this study, octahedral Lindqvist-type [V2W4O19]4− (V2W4) clusters with truncated tetrahedral Keggin-type [ɛ-Al13O4(OH)24(H2O)12]7+ with four triangular faces that perfectly fit to maximize hydrogen bonding and electrostatic interactions, constituting a honeycomb structure with hexagonal cavities (Fig. 4c). In the following five years, porous nanocomposites of {Al13} and Anderson-type {XMo6} (X=Al3+, Co3+, Cr3+, V5+) have emerged and catalytic properties have been explored [88-90]. In 2016, Mizuno et al. also utilized {CoW12} and a novel {SiV3W9} with {ɛ-Al13} to obtain needle-type crystals and dissolution recrystallized plate-type crystals, which not only enhanced the stability of the crystals but also acted as non-homogeneous solid acid catalysts to catalyze phenol rearrangement (Fig. 4d) [91]. Very similar to it, ionic crystals of {PV3W9} and {δ-Al13} were obtained by a researcher in 2022 by synthesis, which was applied for solid catalyst performance comparison [92].
Figure 4
Figure 4.
(a) The structure of [AlO4Al12(OH)12(H2O)24][Al(OH)6Mo6O18]2·(OH)·29.5H2O. Reproduced with permission [78]. Copyright 2000, American Chemical Society. (b) The structure of [δ-Al13O4(OH)24(H2O)12][H2W12O40](OH)·nH2O. Reproduced with permission [86]. Copyright 2003, American Chemical Society. (c) The structure of [ε-Al13O4(OH)24(H2O)12]2[V2W4O19]3(OH)2·27H2O [87]. Copyright 2004, American Chemical Society. (d) The structure of [ε-Al13O4(OH)24(H2O)12][α-1,2,3-HSiV3W9O40]·35H2O. Reproduced with permission [91]. Copyright 2016, American Chemical Society. (e) The structure of {Al13}{[Mo6Bri8Cl6a]@2CD}Cl5·60H2O. Reproduced with permission [93]. Copyright 2020, The Royal Society of Chemistry. (f) The structure of [W2Al28O18(OH)48(H2O)24][H2W12O40]2·55H2O. Reproduced with permission [86]. Copyright 2003, American Chemical Society. (g) The structure of [V4Al28O20(OH)52(H2O)22][PW9V3O40]2·55H2O. Reproduced with permission [92]. Copyright 2022, The Royal Society of Chemistry.
In addition, the use of host-guest stabilizers to confine groups with specific properties in inert groups to achieve precise bottom-up tunability of the structure is an important approach for the design and synthesis of functionally oriented materials. Falaise et al. integrated the highly luminescent {Mo6} building blocks into an inert heterogeneous substrate polymerized with natural, non-toxic and inexpensive γ-cyclodextrins and [Al13O4(OH)24(H2O)12]7+, driven by the unique optical activity of the molybdenum clusters, thus designing luminescent compounds (Fig. 4e) [93].
2.2.3
Derivative of Al30@POMs
The {Al30} configuration typically undergoes slow dissociation and reassembly into corresponding derivatives due to the presence of the anti-cluster anions. This was demonstrated in 2003 by both [W2Al28O18(OH)48(H2O)24][H2W12O40]2·55H2O obtained from the reaction of the {Al30} feedstock with W12 (Fig. 4f) [86]. The {W2Al28} fragment was the first example of a transition metal substituting for an aluminum polycation. After almost 20 years, another example structure [V4Al28O20(OH)52(H2O)22][PW9V3O40]2·55H2O is also finally available. The {Al30} waist in this structure is not only replaced by four V atoms, but also two additional Al3+ are grafted on both sides (Fig. 4g) [92]. The DFT calculations have shown that the girdle is still Lewis acid, a conclusion validated through practical applications as a solid catalyst and basic molecular probe.
In summary, POAl@POM still has sufficient tunable variability and great room for exploration. For instance, POM building blocks with electron transport properties can be invoked to give ionic crystals excellent photoelectric activity. In addition to that, extending the range of Al sources to AlOCs may give organic-inorganic hybrid ionic crystal materials more unexpected results to appear.
3.
Properties of Al-containing POMs materials
The collision and fusion of two different fields can often produce unexpected excellent structures and special properties, achieving a synergy where 1 + 1 > 2 [94,95]. This is especially true in the field of POMs, where metal ions with very different characteristics bring many excellent properties to POM systems [96-99]. Similarly, the combination of Al clusters and POMs has shaped a class of structures with catalytic, proton conductive, antibacterial, and other functionalities (Fig. 5).
Figure 5
Figure 5.
(a) A review of the application of Al-substituted di-vacant [γ-SiW10O34(H2O)2]4− in heterogeneous catalysis. Reproduced with permission [100]. Copyright 2011, Springer. (b) Photocatalytic hydrogen evolution properties of α-[AlSiW11(H2O)O39]5− materials. Reproduced with permission [49]. Copyright 2012, Elsevier. (c) Theoretical study on NLO properties of Al-POMs. Reproduced with permission [56]. Copyright 2013, Elsevier. (d) A series of Al-POM is used to produce hydrogen in visible water in a novel photocatalytic system. Reproduced with permission [51]. Copyright 2015, Springer. (e) The application of mono-aluminum-substituted dimeric silicotungstate in organic catalysis. Reproduced with permission [58,59]. Copyright 2016, 2017, Springer, Elsevier. (f) Application of {P8W48} and {Al16} in the field of proton conduction. Reproduced with permission [18]. Copyright 2018, Wiley-VCH. (g) The photochromic properties of sandwich type mono-Al-POM complexes. Reproduced with permission [66]. Copyright 2019, MDPI. (h) The catalytic performance of aluminum substituted tungstoborate for pyrolysis of polyethylene waste to petrochemical feedstock. Reproduced with permission [52]. Copyright 2020, Elsevier. (i) The antibacterial activity of aluminum-substituted Keggin germanotungstate. Reproduced with permission [53]. Copyright 2021, American Chemical Society.
In a 2011 review of V, Cu, Al-substituted di-vacant [γ-SiW10O34(H2O)2]4− by Mizuno et al. it was noted that Al3+, as an electron-deficient ion and its related compounds, which usually exhibit unsaturated coordination, have the potential to serve as Lewis acid catalysts for organocatalytic reactions such as catalysis of cyclisation reactions (Fig. 5a) [100]. Typically, in this type of structure, there is a terminally coordinated H2O molecule on Al3+ that can be replaced by many other ligands [101]. Efficient photocatalytic hydrogen production from water can be achieved by substituting this weakly attached water using the Eosin Y (EY) sensitizer (Fig. 5b) [49]. It is worth mentioning that the chemical interactions between Al3+ and sensitizers enhanced the stability of EY, which is important for the application of dye sensitization. In addition to the field of catalysis, the static second-order nonlinear optical (NLO) response of a series of Al-POMs has been investigated using DFT, and the theoretical calculations have provided a theoretical basis for subsequent NLO-related experimental studies (Fig. 5c) [56]. From 2015 to 2017, C. N. Kato et al. published several papers reporting the catalytic activity of Al-POM structures in catalytic reactions such as photocatalytic hydrogen production, Meerwein-Ponndorf-Verley reduction reaction and Heterogeneous esterification reaction. They systematically summarized the extensive applications of these structures in Lewis acid catalyzed reactions (Figs. 5d and e) [51,58,59]. Furthermore, the presence of Lewis acid-rich centers, crystallization, coordinated water molecules, and hydroxyl groups have prompted researchers to investigate their proton conductivity properties (Fig. 5f) [18]. It is well known that POMs have good photochromic properties [102]. However, photochromism achieved by structural transformation of POMs is very rare in structures Al-POMs, which helps to further investigate the role of components in influencing photochromism (Fig. 5g) [66]. Furthermore, in the field of catalysis, aside from some fundamental reactions, these Al-POM structures can also catalyze the cracking of polyethylene to yield recyclable petrochemical products (Fig. 5h) [52]. More novelly, the antibacterial activity of [Al(H2O)GeW11]4− was investigated by Tanuhadi et al. in 2021, who found that the redox activity of POMs is a key determinant of their overall antibacterial efficacy (Fig. 5i) [53].
4.
Summary and prospective
The structure of Al-containing POMs undoubtedly represents a burgeoning class of POM materials with boundless exploration potential. In recent years, these structures have shown significant research potential in fields such as optoelectronics and organic catalysis, garnering widespread attention. In this paper, the development of two major categories of Al-containing POM structures is summarized in terms of synthetic strategies, structural diversity and properties.
From the standpoint of synthetic strategies, the one-pot approach, while unrestricted, reveals drawbacks in structural uncontrollability. In contrast, the two-step method effectively addresses this issue by incorporating Al3+ controllably into vacant sites using lacunary POM precursor, thereby achieving vacant-driven synthesis goals. Additionally, mixed crystallization using two solutions containing POAl and POM is an effective means of achieving an ordered arrangement between different clusters by supramolecular interactions. Current synthesis efforts primarily focus on the aqueous solution method, limiting the aggregation of Al3+ and resulting in predominantly purely inorganic structures. Future synthetic strategies could therefore break through primarily in the following aspects: (1) Exploring the chemical behaviors of Al3+ and POM under high-temperature and high-pressure conditions via hydrothermal and solvothermal methods; (2) Conducting reactions in mixed solvents or organic solvents to induce diverse aggregation modes of Al3+ while employing solvent modulation and organic ligand protection strategies, thus guiding the field towards organic-inorganic hybridization; (3) Expanding the selection of lacunary POM building blocks to enhance flexibility in coordination sites.
Furthermore, there remains limited exploration of performance aspects in this field. Future efforts should leverage the advantages of Al and POMs to achieve breakthroughs in optoelectronic catalysis, water treatment, dye removal, adsorption, antimicrobial activity, proton conduction, and other domains. Regarding the Al-POM system, firstly, vacant POMs serve as inorganic ligands, facilitating the exploration of Al3+ hydrolysis behavior in water using a host-guest strategy. Secondly, the significant development space in structural design has brought endless possibilities for the design and synthesis of organic-inorganic hybrid functional materials; POAls@POMs ionic crystals typically have good porosity and are highly likely to achieve excellent results in dye adsorption and water purification. The rich hydrogen bonding and electrostatic forces within their structures not only provide new insights for supramolecular research but also play a crucial role in intermolecular proton conduction, which is expected to be applied in the field of energy. It is hoped that this review will have a positive impact on further research in the realms of Al-POM and POAl@POM. We apologize for any omissions of significant literature in this article.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was supported by National Natural Science Foundation of China (No. 22371278), Funding of Fujian Provincial Chemistry Discipline Alliance, Natural Science Foundation of Fujian Province (No. 2021J06035) and Youth Innovation Promotion Association CAS (No. Y2018081).
N. Mizuno, K. Yamaguchi, K. Kamata, Catal. Surv. Asia 15 (2011) 68-79. doi: 10.1007/s10563-011-9111-2
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H. Liu, C.J. Gómez-García, J. Peng, et al., Dalton Trans. (2008) 6211-6218. doi: 10.1039/b805714a
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Figure 1
An inclusive statistical summary of all metal elements present in current POM structures. (a) A simplified periodic table analysis of the elements contained in POMs: including the base elements that form the POMs (blue), the other elements contained (the main group metals: red; the transition metals: green; the lanthanide and actinide metals: orange; and the Al element: yellow), which is the focus of this manuscript. The elements are highlighted and classified in different colors. (b) The proportion of POM structures containing various elements.
Scheme 1
The reaction strategy can be chosen according to the target requirements: Strategy A: to obtain Al-POMs then vacant POM building blocks with coordination sites are required; Strategy B: to obtain POAls@POMs then saturated POM units and intact POAl clusters are required.
Figure 2
(a) The mono-Al-substituted α-Keggin-type POTs. Reproduced with permission [42]. Copyright 1982, American Chemical Society. (b) The mono-Al-substituted β1, β2, β3-Keggin-type POTs. Reproduced with permission [47]. Copyright 2001, American Chemical Society. (c) The di-Al-substituted γ-Keggin-type POTs. Reproduced with permission [55]. Copyright 2008, American Chemical Society. (d) The di-Al-substituted γ-Keggin-type POTs TBA3[(C5H5N)H][γ-SiW10O36{Al(C5H5N)}2(µ-OH)2]·2H2O. Reproduced with permission [55]. Copyright 2008, American Chemical Society. (e) Di-Al containing Keggin-type POTs [(n-C4H9)4N]6[α-PW11Al(OH)O39ZrCp2]2. Reproduced with permission [57]. Copyright 2013, The Royal Society of Chemistry. (f) The tri-Al-substituted α,β-Keggin-type POTs. Reproduced with permission [60]. Copyright 1992, The Royal Society of Chemistry. (g) The sandwich-type K6Na[(A-PW9O34)2{W(OH)(OH2)}{Al(OH)(OH2)}{Al(μ-OH)(OH2)2}2]·19H2O compound. Reproduced with permission [63]. Copyright 2010, The Royal Society of Chemistry. (h) The tetra-Al containing Keggin-type dimer POTs (TBA)4[Al4(H2O)10(β-XW9O33H)2]·4H2O (X = As, Sb). Reproduced with permission [31]. Copyright 2013, The Royal Society of Chemistry. (i) The AlOC-containing trimer TBA9[{γ-H2SiW10O36Al2(μ-OH)2(μ-OH)}3] POTs. Reproduced with permission [64]. Copyright 2011, American Chemical Society. (j) The tetramer K22[{α-Al3SiW9O34(μ-OH)6}4{Al4(μ-OH)6}]·3KCl·69H2O POTs. Reproduced with permission [65]. Copyright 2015, The Chemical Society of Japan.
Figure 3
(a) The mono-Al-substituted Dawson-type POTs. Reproduced with permission [42]. Copyright 1982, American Chemical Society. (b) The tri-Al-substituted Dawson-type POTs. Reproduced with permission [67]. Copyright 2014, American Chemical Society. (c) The tetra-Al-substituted crescent-shaped POTs. Reproduced with permission [70]. Copyright 2016, Wiley-VCH. (d) a novel 16-AlⅢ-32-oxo cluster embedded {P8W48} archetype polyanion, {[Na(NO3)(H2O)]4[Al16(OH)24(H2O)8(P8W48O184)]}16−. Reproduced with permission [18]. Copyright 2018, Wiley-VCH.
Figure 4
(a) The structure of [AlO4Al12(OH)12(H2O)24][Al(OH)6Mo6O18]2·(OH)·29.5H2O. Reproduced with permission [78]. Copyright 2000, American Chemical Society. (b) The structure of [δ-Al13O4(OH)24(H2O)12][H2W12O40](OH)·nH2O. Reproduced with permission [86]. Copyright 2003, American Chemical Society. (c) The structure of [ε-Al13O4(OH)24(H2O)12]2[V2W4O19]3(OH)2·27H2O [87]. Copyright 2004, American Chemical Society. (d) The structure of [ε-Al13O4(OH)24(H2O)12][α-1,2,3-HSiV3W9O40]·35H2O. Reproduced with permission [91]. Copyright 2016, American Chemical Society. (e) The structure of {Al13}{[Mo6Bri8Cl6a]@2CD}Cl5·60H2O. Reproduced with permission [93]. Copyright 2020, The Royal Society of Chemistry. (f) The structure of [W2Al28O18(OH)48(H2O)24][H2W12O40]2·55H2O. Reproduced with permission [86]. Copyright 2003, American Chemical Society. (g) The structure of [V4Al28O20(OH)52(H2O)22][PW9V3O40]2·55H2O. Reproduced with permission [92]. Copyright 2022, The Royal Society of Chemistry.
Figure 5
(a) A review of the application of Al-substituted di-vacant [γ-SiW10O34(H2O)2]4− in heterogeneous catalysis. Reproduced with permission [100]. Copyright 2011, Springer. (b) Photocatalytic hydrogen evolution properties of α-[AlSiW11(H2O)O39]5− materials. Reproduced with permission [49]. Copyright 2012, Elsevier. (c) Theoretical study on NLO properties of Al-POMs. Reproduced with permission [56]. Copyright 2013, Elsevier. (d) A series of Al-POM is used to produce hydrogen in visible water in a novel photocatalytic system. Reproduced with permission [51]. Copyright 2015, Springer. (e) The application of mono-aluminum-substituted dimeric silicotungstate in organic catalysis. Reproduced with permission [58,59]. Copyright 2016, 2017, Springer, Elsevier. (f) Application of {P8W48} and {Al16} in the field of proton conduction. Reproduced with permission [18]. Copyright 2018, Wiley-VCH. (g) The photochromic properties of sandwich type mono-Al-POM complexes. Reproduced with permission [66]. Copyright 2019, MDPI. (h) The catalytic performance of aluminum substituted tungstoborate for pyrolysis of polyethylene waste to petrochemical feedstock. Reproduced with permission [52]. Copyright 2020, Elsevier. (i) The antibacterial activity of aluminum-substituted Keggin germanotungstate. Reproduced with permission [53]. Copyright 2021, American Chemical Society.
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