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Introduction to Polyoxometalates

Daniela Flores and Carlos M. Granadeiro

University of Porto, LAQV/REQUIMTE, Department of Chemistry and Biochemistry, Faculty of Sciences, Rua do Campo Alegre, s/n, Porto 4169-007, Portugal

1.1 Introduction

Polyoxometalates (POMs) represent a captivating and unique class of nanoscale metal-oxide clusters, boasting remarkable structural and chemical versatility, making them a sought-after choice across diverse scientific domains [1-3], namely biomedicine, catalysis, colloid science, electronic or magnetic devices, functional materials, nanotechnology, sensors and surfaces [4, 5]. The exploration of these materials dates back to the 1826 discovery by Berzelius of the ammonium salt of [PMo12O40]3-. However, it was not until 1933, when Keggin conducted the first structural determination of the tungsten analogue [PW12O40]3- over a century later, that their structural intricacies began to be unveiled [6]. The scientific community has devoted considerable effort and time to the structural characterization of these intriguing compounds, and so the potential applications of POMs only began to be truly explored after the late 1970s.

POMs are inorganic clusters composed of metallic centres, frequently in their highest oxidation state (M = V, Mo, W, Nb and Ta), named the addenda atoms, connected by bridging or terminal oxygen atoms. Currently, there are a large number of known POMs structures with diverse architectures and distinct compositions. In order to establish correlations between their highly symmetric structures, physical properties and reactivity, POMs are typically divided into two main categories: isopolyanions and heteropolyanions (Figure 1.1) [7].

Isopolyanions, with the general formula [MxOy]m-, are composed of a metal-oxide framework where the addenda atom is a singular transition-metal ion from group V or VI surrounded by oxygen atoms. Heteropolyanions, represented by the general formula [XzMxOy]n- with z = x, contain an additional atom (X) named heteroatom. In POM structures, heteroatoms are typically elements from the p-block (X = P, Si, Al, Ga, Ge) but also from the d-block (X = Fe, Co, Ni, Zn), which are located at the centre of a MxOy shell [8].

The spatial arrangement of POMs can be visualized as packed arrays of polyhedral MOx units, typically MO6 octahedra or, more rarely, MO5 pyramidal units. These units act as building blocks in the construction of multiple architectures by connecting between themselves and sharing edges, corners or faces (Figure 1.2) [10]. Based on this, the oxygen atoms in a POM structure can be classified according to their position (Figure 1.3). The oxygen atom connected to the heteroatom is denoted as Oa, Ob and Oc represent corner- and edge-sharing atoms, while the unshared (terminal) oxygen atom is denoted as Od [12].

Figure 1.1 Structural representation of the most common POM categories: the Lindqvist isopolyanion ([M6O19]n-), and the heteropolyanions Keggin-([XM12O40]m-), Wells-Dawson-([X2M18O62]w-) and Anderson-type ([XM6O24]n-) anions. MOx: light blue; X: purple; O: red.

Source: Granadeiro et al. [4]/with permission of Elsevier.

Figure 1.2 Polyhedral and ball-and-stick representation of the different types of MO6 connectivity in POMs: (a) corner-shared, (b) edge-shared and (c) face-shared octahedra.

Source: Bijelic and Rompel [9]/Springer Nature/CC BY 4.0.

Figure 1.3 Ball-and-stick representation of the Keggin anions with the different type of oxygen atoms: Oa: heteroatom-connected oxygen; Ob: corner-shared oxygen; Oc: edge-shared oxygen; Od: terminal oxygen.

Source: Zheng et al. [11]/The Royal Society of Chemistry/CC BY 3.0.

The vast number of POM structures is further broadened by the use of more than one type of M addenda atoms (mixed-addenda anions) or by structural modification through the removal of one or more MOx groups (lacunary anions). The formation of lacunary POM anions (Figure 1.4) is achieved by varying the experimental conditions, such as temperature, pH or precursors, which are frequently used to achieve enhanced reactivity and superior mechanical properties [14, 15]. The obtained empty lacuna with free oxygen atoms leads the anion to act as a tetradentate or pentadentate ligand capable of coordinating to any electrophilic species, typically lanthanide or transition-metal ions [5]. By doing so, unprecedented structures have been obtained by the connection of two (or more) known anions, including the formation of supramolecular POM structures.

Figure 1.4 Polyhedral and ball-and-stick representations of the formation of mono- and trilacunary (vacant) Keggin-type POMs.

Source: Bao et al. [13]/with permission of Elsevier.

1.2 Polyoxometalate Structures

1.2.1 Synthetic Methodologies

The synthesis of POM structures is relatively straightforward, typically involving an acidic solution containing relevant metal oxide anions. This can be achieved by following two main methodologies: (i) one-pot synthesis and (ii) building block method. In the one-pot synthesis, a condensation reaction takes place between simple metal salts and heteroanions. However, this approach demands the precise control over several key parameters, namely the choice of reducing agent, concentration and type of metal oxide anion, heteroatom type and concentration, ionic strength, pH, presence of additional ligands and reaction temperature [16]. In the building block method, lacunary anions, obtained by removal of MOx groups from parent POM anions, act as precursors to build bulkier and more complex POM structures [17]. The lacunary derivatives, typically prepared by controlled hydrolysis of parent POM solutions, contain nucleophilic oxygen centres and will act as multidentate ligands in the construction of discrete larger clusters or even polymeric networks through coordination to electrophilic species [18]. The more efficient and elegant approach of the building block method has led to the exponential growth of novel POM structures with increasingly complex arrangements, higher number of functionalities and distinct physico-chemical properties, enlarging the application fields of POMs [19-21].

1.2.2 Lindqvist Structure

The Lindqvist structure is the most representative example of isopolyanions with the general formula [M6O19]n- with M = W, Mo, Cr and Nb. The monolacunary Lindqvist polyoxotungstate unit [W5O18]6- has been extensively used as building block by coordination to lanthanide and transition-metal ions. In particular, decatungstates composed of two [W5O18]6- units coordinated to a central lanthanide ion [22] have shown peculiar properties motivating their synthesis and application in photoluminescence, catalysis, medical imaging and as single-molecule magnets (SMM) [23-27]. In these compounds, with the general formula [Ln(W5O18)2]9-, the lanthanide ion is coordinated to eight oxygen atoms exhibiting a square antiprismatic geometry [28]. The [Eu(W5O18)2]9- anion is among the most studied POMs due to its exceptionally high quantum yield arising from the highly efficient energy transfer process from the O W charge transfer band to the lanthanide ion [29].

1.2.3 Keggin Structure

The Keggin structure, with the general formula [XM12O40]n-, stands as the most well-known heteropolyanion featuring tetrahedrally coordinated heteroatoms and four trimetallic M3O3 groups arranged around a central XO4 tetrahedron [30]. The Keggin anion exhibits five rotational isomers (a, ß, ?, d and ?) resulting from consecutive 60° rotations of each M3O3 unit (Figure 1.5) [7].

Figure 1.5 Polyhedral representation of the rotational isomers of the Keggin anions.

Source: Bijelic and Rompel [9]/Springer Nature/CC BY 4.0.

Keggin anions, through controlled alkaline hydrolysis, are also able to form lacunary species by removing one or more MOx groups from the plenary structure. The monolacunary [XM11O39](n+4)- Keggin anion is able to coordinate to trivalent (or even tetravalent) metallic cations through the available oxygen atoms in the lacuna [4]. These monolacunary anions typically originate as 1 : 1 [XM11M´(L)O39]n- or 1 : 2 [M´(XM11O39)2]n- complexes. The 1 : 1 complexes are mainly obtained when the metallic cation (M´) is a transition-metal or p-group element. For these complexes, L represents a monodentate ligand (generally a water molecule) necessary to complete the octahedral coordination of M´. Bulkier metallic cations (e.g. lanthanide ions) tend to coordinate to two monolacunary units forming a 1 : 2 complex [31]. Nevertheless, a few examples can be found in the literature reporting 1 : 1 complexes with lanthanide ions [17, 19, 32]. The formation of 2 : 2 Keggin-type complexes is even rarer,...