Hervé- and Krebs-Type Magnetic Polyoxometalate Dimers

Abstract

Lacunary polyoxometalates (POMs) are negatively charged metal–oxo clusters, formally obtained from plenary topologies via fragment removal. Owing to the fragment removal, the lacunary POMs archetypes are rich in nucleophilic terminal oxo ligands, making them suitable for post-synthetic coordination with various heterometals. Trilacunary heteropolytungstates (hetero-POTs) based on bowl-shaped {W₉O₃₀} framework incorporating a central lone-pair containing {XO₃} hetergroup (X = As^III, Sb^III, and Bi^III) function as all-inorganic scaffolds that in the presence of d-block metal cations typically construct sandwich-like dimers of Hervé and/or Krebs. Herein we review the preparative approaches, as well as compositional and magnetic versatility of the constructed Hervé- and Krebs-type dimers and discuss prospective uses as POMtronics.

1. Introduction

The structural and chemical versatility of molecular assemblies allowing fine-tuning of the magneto chemical phenomena has been a driving force for the development of many magnetic chemicals at the nanoscale [1]. Magnetic molecules show the potential for quantum bit (i.e., “qubit”) information storage as a superposition of states, which opens an emerging application in quantum computing [2,3,4,5,6,7]. Considering existing challenges such as loss of quantum coherence, practical deployment is currently out of reach [8]; however, at the same time, the field shows many encouraging examples [9,10,11,12,13,14].

Polyoxometalates (POMs) represent a class of molecular metal oxides, typically comprised of early transition metals in high oxidation state(s) (mainly V, Mo, and W) [15,16]. POMs are one of the most complex of all-inorganic architectures [17], with broad applications in catalysis [18], materials science [19,20], and nanoelectronics [21,22]. Owing to their all-inorganic nature, high symmetry, and structural robustness, POMs have also been of high interest in molecular magnetism [23]. Plenary POMs exposing oxo-based cavities are typically used as scaffolds capable of stabilizing magnetic heterometallic cations, occasionally leading to single-ion magnets (SIMs) with spin qubit behavior [24,25]. Prominent examples are the lanthanide functionalized Preyssler POMs [23], and the cuboidal and pentagonal prismatic polyoxopalladates [21,26]. Some high symmetry, but partially or fully-reduced POMs, such as the semimetal-functionalized polyoxovanadates [27], mixed-metal Keplerates [28], are known to exhibit complex magneto-chemical behavior as well [29]. POMs with large internal cavities can exhibit multiple accessible oxo sites, successfully mimicking naturally occurring zeolites in incorporating multimetal magnetic nuclei [30], or even full metal–oxo clusters [31,32].

Low symmetry POMs derived by formal removal of metal–oxo fragments from the plenary POM archetypes are another form of synthetic scaffolds that can function as inorganic polydentate ligands [33,34]. These types of POMs can coordinate to various transition-metals, especially 3d [35], or rare-earth 4f-block metals [36], or a combination of 3d/4f magnetically active metal centers [37].

Owing to the good inertness of the W–O bonds, polyoxotungstates (POTs) exhibit the most studied class of lacunary POMs [38]. Most of the lacunary POTs are derived from the Kegginoidal archetype [39]. The Keggin type archetype is often viewed as a clathrate, where an {XO₄} guest unit is encapsulated within an otherwise highly symmetrical and neutral {W₁₂O₃₆} [39]. These shells can be of O_h or D_3h symmetry, assigned respectively as α- and β-Kegginoids (Figure 1). The formal removal of one [WO]⁴⁺ unit leads to so-called monovacant or monolacunary POMs, which depending on the shell of derivation, can be further assigned as α, and β_1–3 [40]. The formal removal of two or more metal centers leads to a complex configuration isomer problem considering many possibilities where the formal removal can occur [41,42]. However, the trilacunary Kegginoids derived by a formal removal of the {W₃O₆} moiety, from the α-/β-{W₁₂O₃₆}, are most studied. The formed {W₉O₃₀} bowl-like (C_3v) metal oxo shells are inherently isomeric and can incorporate tetrahedral {XO₄} or lone-pair containing trigonal pyramidal {XO₃} units. In the case of {XO₄}, the guest unit can point with one terminal oxo towards the bottom of the bowl (configuration A) or towards the exterior (i.e., opening) of the {W₉O₃₀} bowl (configuration B). Configuration B is commonly observed among lone pair units containing {XO₃}, where X = As^III, Sb^III and Bi^III. Other lone pair units such as {S^IVO₃} or {Se^IVO₃} have also been observed to be incorporated within the cavity. The formed POMs can dimerize to form [(XO₃)₂(WO₃)₁₈]ⁿ⁻ Wells–Dawson Type POMs [40,43].

The trilacunary [XW₉O₃₃]⁹⁻ ligands readily form by the condensation reaction of acidified aqueous mixtures of X₂O₃ (X = As^III, Sb^III and Bi^III.) and Na₂WO₄·2H₂O from an initial pH in the range between 7–9 [44,45,46]. These POMs represent one of the most utilized all-inorganic ligands for the preparation of heterometallic structures, occasionally agglomerating several hundred or even thousands of atoms [47]. However, among the most prominent structures produced by the [XW₉O₃₃]⁹⁻ ligands are the Hervé- [48] and Krebs-type dimeric structures [46]. The lone pair-containing [XW₉O₃₃]⁹⁻ POMs have different reactivity in contrast to their related non-lone-pair containing [XW₉O₃₄]ⁿ⁻ types, that adopt {XO₄} units with X = P^V, Si^IV, As^V, and Ge^IV [49]. The {XO₄} heterogroup in [XW₉O₃₄]^n– may adopt two different configurations [50], facilitating the formation of so-called Weakley [51], and Knoth-type dimer structures [52].

In this regard, the present review article focuses on sandwich-type complexes based on the trilacunary [XW₉O₃₃]⁹⁻ and their post-synthetic derivatization. In the first line, we review the electronic, structural, and reactivity properties of these lacunary [XW₉O₃₃]⁹⁻POMs and discuss some general trends among them. Next, we systematically survey the reported Hervé- and Krebs-type assemblies and their magnetic properties. We have given here the inter-atomic coupling constants whenever reported by the authors, with the following convention H = −JS_AS_B.

2. Sandwich-Type POM Archetypes

α-[XW₉O₃₃]⁹⁻ (X = As^III, Sb^III, and Bi^III) species exhibit six terminal oxo sites that are highly basic and able to coordinate to different hetero-metal centers. The oxo sites form a virtual distorted hexagonal plane normal to the 3-fold symmetric axis of rotation. The distortion can be traced to the arrangement of the central {XO₃} unit, which shares its three oxo ligands with three pairs of tungsten centers, respectively. In this regard, the interatomic distances between terminal oxo centers that derive from a pair of edge-sharing {WO₆} units (i.e., d_a(O∙∙∙O)) differ from that between corner-sharing {WO₆} units (i.e., d_b(O∙∙∙O)). Standard geometry optimizations logically indicate that the relative difference between d_a(O∙∙∙O) and d_b(O∙∙∙O) is dependent on the nature of the X centers (Figure 2). The different X centers can lead to different charge distributions within α-[XW₉O₃₃]⁹⁻. The molecular electrostatic potentials reveal that the terminal oxo atoms in α-[AsW₉O₃₃]⁹⁻ are the most basic, while the relative basicity lowers gradually in α-[SbW₉O₃₃]⁹⁻ and then in α-[BiW₉O₃₃]⁹⁻.

Another aspect of the reactivity of α-[XW₉O₃₃]⁹⁻ is how they connect and differ from other hetero-POTs comprised of the same building blocks. In order to discuss this aspect, we briefly focus on the chemistry of α-[AsW₉O₃₃]⁹⁻. In aqueous media α-[AsW₉O₃₃]⁹⁻ is relatively reactive and can bind to additional mononuclear tungstate building blocks. In the pH range between 3–4 and the presence of Na⁺ cations, α-[AsW₉O₃₃]⁹⁻ interconnects with cis-[WO₂]²⁺ bridging units, leading to tetrameric [Na⊆(WO₂)₄(α-AsW₉O₃₃)₄]²⁷⁻ polyanions [53]. On the other hand, when the pH reaches 6 and K⁺ cations are present, α-[AsW₉O₃₃]⁹⁻ interconnects with trans-[WO(H₂O)]⁴⁺ units leading to dimeric, sandwich-like [K(H₂O)_n{WO(H₂O)}(α-AsW₉O₃₃)₂]¹³⁻ species (see Section 3) [54]. These structures show that pH and the nature of the cations have immense effect(s) upon the synthetic outcome, although, to date, their exact role in the particular self-assembly process is not clear [55].

Interaction of α-[XW₉O₃₃]⁹⁻ with heterometals (M) can lead to a wide range of structures differing in terms of POM ligands and interconnecting heterometal centers. As the trilacunary α-[XW₉O₃₃]⁹⁻ are polydentate ligands, their combination with high-coordinate cations (e.g., lanthanides or actinides) can result in many possible connectives and occasionally in oligomeric POM assemblies with impressive tungsten nuclearities [56]. However, for improved synthetic and structural control, trilacunary α-[XW₉O₃₃]⁹⁻ have often been reacted with transition metal cations that have more modest coordination numbers, ultimately leading to a lower number of connecting possibilities.

If the metal cations are partially protected with non-labile ligands, they typically coordinate and saturate the lacunary site of α-[XW₉O₃₃]⁹⁻ POM, leading to metal-functionalized Janus-like nanomolecules. However, many transition metal cations deriving from simple metal salt precursors (e.g., chlorides, nitrates, etc.) can offer up to four coordination sites in square planar orientation and thus can interconnect two α-[XW₉O₃₃]⁹⁻ units in an archetype that, due to historical reasons, is referred to as Hervé-type POM sandwich [48]. For a more profound discussion here, we describe the Hervé-type sandwich as a linear POM assembly where a cationic {M_6−x} belt (x = 0–5) is sandwiched between two α-[XW₉O₃₃]⁹⁻ units (see Figure 3). The {M_6−x} belt connects to six formerly terminal–oxo ligands deriving from both α-[XW₉O₃₃]⁹⁻ units. The {M_6−x} is thus a part of a larger cyclic {M_6−xO₁₂} fragment in which when x = 0, all M-centered {MO₄} square planes or square pyramids {MO₄L} are in edge-sharing configuration with one another. In a scenario when the {M_6−x} belt is fully saturated (i.e., x = 0), one formally distinguishes between two configurational isomers obtained because of the orientation of the {XO₃} units. When both units {XO₃} are oriented in an eclipsed fashion, the edge-sharing {WO₆} octahedra from both α-[XW₉O₃₃]⁹⁻ units are also eclipsed (i.e., syn). The eclipsing provides differences in interatomic d_{(O∙∙∙O)} distances and enforces an ideal D_3h symmetry point group. The syn orientation is thus dominant among inter-POM bridging with three metal centers. However, the remaining sites in a “saturated M₆” scenario can often be occupied by metal cations with different coordination or ligand environments (e.g., alkali Na⁺ and K⁺).

When the two {XO₃} units are in staggered (i.e., anti-) orientation, the remaining edge-sharing {WO₆} octahedra are also staggered, and this enforces the lacunary sites to conform to a more “uniform” connectivity, which leads to an ideal {M₆O₁₂} hexagonal belts (Figure 3). This is a profound difference and favors complete belt saturation. However, between the two very distinct sandwich archetypes {M₃(α-XW₉O₃₃)₂} D_3h and {M₆(α-XW₉O₃₃)₂} D_3d, there are obviously other scenarios where 2, 4, and 5 metal centers are being sandwiched between α-[XW₉O₃₃]⁹⁻. In many of these configurations, partial occupancy or coordination changes induced by auxiliary ligands or other factors play a role that leads to different configurations, although from what we have reviewed here, the syn orientation is far more common than the anti. In most of the structures discussed, the linear alignment enforces closer proximity of the M centers, which in the case of spin-polarized centers can lead to interesting magnetic interactions and behavior.

The last profound change in the formation of complex assemblies is the isomerization of the α-[XW₉O₃₃]⁹⁻ to β-[XW₉O₃₃]⁹⁻ (see Figure 1). While in the presence of transition metal cations, α-[XW₉O₃₃]⁹⁻ condensates in linear sandwich assemblies, the condensation of β-[XW₉O₃₃]⁹⁻ favors an offset-sandwich assembly vis-à-vis {M₄O₁₀(XW₉O₃₃)₂}, commonly referred to as the Krebs-type POMs [46,57]. The offset Krebs-type POM creates a belt of four centers forming a virtual rhomboid described by two geometrically inequivalent metal centers (a-site and b-site) placed along three different interatomic distances d₁ connecting a-b centers, d₂ connecting a-a centers, and d₃ connecting b-b centers as shown in Figure 4. The two most distant corners (i.e., those connected along d₂) are typically occupied by 3d centers, while the remaining directly bridging the β-[XW₉O₃₃]⁹⁻ can be 3d transition metal or W-based. In the Krebs-type structures, the spin-polarized centers are often distant from one another and typically exhibit very weak or no magnetic coupling interactions.

3. Hervé-Type Dimers

3.1. Atypical Hervé Polyanions

In 2001, Kortz and coworkers reported the sandwich type [{M^II₂(H₂O)₂}{WO}(α-AsW₉O₃₃)₂]¹⁰⁻ where M = Mn^II, Co^II, Zn^II (Figure 5a) [58]. In contrast to the classical Hervé-type POMs, this derived assembly incorporates only two heterometals, while a tungsten center acts as another linking group between the two α-{As^IIIW₉O₃₉} units. Single crystal X-ray diffraction analysis revealed 79, 73, and 77% occupancy in the belt region for Mn, Co, and Zn, respectively. Elemental analysis revealed that, on average, two of the three belt positions of the dimeric polyanions are occupied by the first-row transition metal (Co, Mn, and Zn) and a tungsten atom occupies the remaining position. The M−O bond distances for the polyanions range from 1.977 to 2.343, 1.95 to 2.284, and 1.967 to 2.227 Å for Mn, Co, and Zn, respectively.

An “open-sandwich” structure has been reported by Hussain and coworkers, viz. [{Ni(H₂O)₄}₂{WO(H₂O)}(α-AsW₉O₃₃)₂]¹⁰⁻ (Figure 5b) [59]. The latter structure is based upon the {(WO(H₂O))(α-AsW₉O₃₃)₂} moiety, wherein two of the α-{As^IIIW₉O₃₉} units are joined at one corner with an octahedrally coordinated {WO₅(H₂O)}. The polyanion consists of two nickel atoms and one sodium atom coordinated to one of the α-{As^IIIW₉O₃₉} units. The sodium cation acts as a bridge between the two metal atoms. The other α-{AsW₉O₃₉} unit does not bond with nickel atoms but connects to the nickel coordinated α-{AsW₉O₃₉} Keggin unit via the {WO₅(H₂O)} group, thus having a nominal C_2V symmetry. The Ni−O distances in this polyanion range from 2.03 to 2.09 Å. Magnetic studies performed upon the polyanion using a vibrating-sample magnetometer show paramagnetic behavior with the effective magnetic moment value calculated from data to be 3.23 B.M. (Bohr Magneton).

3.2. Hervé Polyanions Sandwiching Three Heterometals

3.2.1. Aqua Terminating Ligands

In 1982, Hervé and coworkers reported the polyanion [Cu₃(H₂O)₂(As^IIIW₉O₃₉)₂]¹²⁻, consisting of a trimeric assembly of Cu^II atoms, arranged in the pattern of an isosceles triangle (Cu∙∙∙Cu distances = 4.669 and 4.707 Å), sandwiched between two α-{AsW₉O₃₉} units [48]. Interestingly, the two α-{As^IIIW₉O₃₉} subunits are not observed to face each other exactly. The three Cu^II atoms are not equivalent in the polyanion, with two of the Cu^II ions having a square planar geometry coordinated with oxygens from the polyanion units, while one Cu^II ion having a square pyramidal geometry with additional oxygen at the axial position. The Cu−O bond distances in this polyanion are in the range of 1.87–2.39 Å. The magnetic properties of this compound, reported in 1988, showed intramolecular anti-ferromagnetic coupling between the Cu atoms with J = −7.5 K (~−5.2 cm⁻¹) [60].

In 2001, the latter species were revisited when a group of [(α-XW₉O₃₃)₂M₃(H₂O)₃]¹²⁻, where M = Cu^II, Zn^II; X = As^III, Sb^III was reported (Figure 6a) [58]. The principal difference in the formation of [(M(H₂O)₃)₃(α-AsW₉O₃₃)₂]¹²⁻ (pH range = 6.2–6.9) versus [(M(H₂O))₂(WO(H₂O))(α-AsW₉O₃₃)₂]¹⁰⁻ (pH range = 3.9–4.8) is the pH maintained during synthesis. The Cu−O bond distances in this polyanion were also reported to be in the range of 1.87–2.39 Å. Analogous derivatives with Se^IV and Te^IV also with Cu^II, [(Cu(H₂O))₃(α-XW₉O₃₃)₂]¹⁰⁻ (X = Se^IV, Te^IV) were also reported in the same article. The sandwich-type assembly in this polyanion joining the two α-[XW₉O₃₃]⁹⁻ units are also made of three isolated Cu^II atoms. However, unlike the previously reported polyanions, in [{M(H₂O)₃}₃(α-AsW₉O₃₃)₂]¹²⁻ [58], all the transition metals possess a square-pyramidal coordination geometry having four equatorial oxygens from the base polyanion units and one terminally coordinated water molecule at the axis, which now results in an idealized D_3h symmetry for the title polyanions. The transition metals are now observed to be bridged with each other by sodium ions in the solid-state structure. Magnetic susceptibility data indicate a single anti-ferromagnetic spin-exchange constant J for the triangular {Cu₃} belts with J = −1.36 cm⁻¹ for [(Cu(H₂O))₃(α-AsW₉O₃₃)₂]¹²⁻ [61,62,63]. Representative magnetic susceptibility behavior has been shown in Figure 6b.

The antimony derivative [(Cu(H₂O))₃(α-SbW₉O₃₃)₂]¹²⁻ has been synthesized by reactions of α-[SbW₉O₃₃]⁹⁻ with CuCl₂·2H₂O, or alternatively with CuNO₃·3H₂O, in aqueous media under heating [64,65,66]. In the latter structure, the Cu^II centers are in square pyramidal coordination surrounded by four bridging oxo and a single terminal aqua ligand. As Cu^II is a d⁹ complex with one unpaired electron, magnetically this sandwich POM can be considered somewhat similar to [(V^IVO)₃(α-SbW₉O₃₃)₂]¹²⁻ derivative. The anti-ferromagnetic coupling constant J = −1.04cm⁻¹, while the ground state of the structure is S = 1/2. Magnetic hysteresis shows that the sandwich POM can be excited to the S = 3/2 level [61,67]. By formal exchange of Sb center with other p block elements, it has been shown that magnetization can be minorly altered [61,67]. The [(Cu(H₂O))₃(α-SbW₉O₃₃)₂]¹²⁻ has also been found to act as a modular component for the formation of hybrid inorganic−organic networks materials synthesized in the presence of 4-amino-1-hydroxy-butyl-1,1-bisphosphonate ligands [68].

In 2002, David and coworkers studied the bismuth derivatives and reported [(Cu(H₂O))₃(α-BiW₉O₃₃)₂]¹²⁻ and [(Mn(H₂O))₃(α-BiW₉O₃₃)₂]¹²⁻ derivatives [69]. Measurements of magnetochemical susceptibility of [(Cu(H₂O))₃(α-BiW₉O₃₃)₂]¹²⁻ have been carried out; however reported in the literature[70] and based on limited information and reference missing, it is not clear what model was used to fit the magnetic behavior. EPR measurements of [(Cu^II(H₂O))₃(α-BiW₉O₃₃)₂]¹²⁻ revealed the anti-ferromagnetic coupling between the Cu^II ions involve spin frustration, leading to a doubly degenerate S = 1/2 ground states (EPR inactive) and an S = 3/2 excited state (EPR active) [71]. In [(Mn(H₂O))₃(α-BiW₉O₃₃)₂]¹²⁻, the Mn∙∙∙Mn interatomic distances are in the range of 4.2–5.1 Å, which makes the dipolar interactions be smaller than the superexchange. The EPR spectra of [(Mn(H₂O))₃(α-BiW₉O₃₃)₂]¹²⁻ revealed that each Mn^II ion is antiferromagnetically coupled through either the α-{BiW₉O₃₃} units or through space bipolar interactions. At the excited state of S = 7/2, the isotopic exchange constant, estimated in the temperature range of 5−225 K, is J = −2.074 cm⁻¹ and the zero-field splitting parameters D = −0.381 cm⁻¹, E = 0.054 cm⁻¹ [69].

3.2.2. Oxo Terminating Ligands

Similarly to [(M(H₂O))₃(α-AsW₉O₃₃)₂]¹²⁻ (M = Mn^II, Co^II, Ni^II), Mialane and coworkers reported [(VO)₃(α-AsW₉O₃₃)₂]¹¹⁻, featuring three cationic {VO} moieties [65]. Titration of the polyanion with Ce^IV revealed that [(VO)₃(α-AsW₉O₃₃)₂]¹¹⁻ is a mixed-valent {V^IV₂V^V} species. The terminal oxo atoms lead to shorter metal–oxygen bond lengths ca. 1.6 Å, in contrast to when a terminal aqua ligand is bonded (ca. 2.0 Å). The vanadium derivative, however, showed significant disorder on two of the three metals linking the {α-AsW₉O₃₃} units. For these two vanadium atoms, the V = O bonds are directed alternatively towards the inside or the outside of the sandwich interior (Figure 7). Magnetic studies on the polyanions reveal anti-ferromagnetic interactions between the metal centers with coupling constants J = −2.9 cm⁻¹.

In 2001, Yamase reported the synthesis of the polyanion [(V^IVO)₃(α-SbW₉O₃₃)₂]¹²⁻ [72]. This POM was obtained from the reaction of [α-SbW₉O₃₃]⁹⁻ and VOSO₄∙5H₂O in aqueous sodium acetate buffer at pH 4.8. In the later structures, the vanadium centers appear as {VO₅} square pyramids, and in such form, they normally can undergo reversible redox processes between vanadium(IV) and vanadium(V). Based on circular voltammetry, it was indeed shown that [(V^IVO)₃(α-SbW₉O₃₃)₂]¹²⁻ can undergo three one-electron oxidations, that is, forming the diamagnetic [(V^VO)₃(α-SbW₉O₃₃)₂]¹²⁻ species. The cyclic voltammogram is quasi-reversible, which suggests that the POM retains its structural integrity in the solution. In the ground state, the sandwiched core of [(V^IVO)₃(α-SbW₉O₃₃)₂]¹²⁻ is virtually identical to that reported for the arsenic analogue (Figure 7). In the [(V^IVO)₃(α-SbW₉O₃₃)₂]¹²⁻ sandwich structures, the interatomic V^IV···V^IV distances are 5.4–5.5 Å. Owing to the geometric frustration of the three centers, the particular POM shows a ground state of S = 1/2. Based on magnetic hysteresis, it has been shown that magnetization of the POM can cross from S = 1/2 level to S = 3/2. The later step is associated with a half-step magnetization, which normally is expected for an anti-ferromagnetic spin triangle [73]. The Yamase group also studied the spin frustrated [(VO)₃(α-BiW₉O₃₃)₂]¹²⁻ [73,74]. The authors observed an unusual phenomenon of the magnetization of [(VO)₃(α-BiW₉O₃₃)₂]¹²⁻, which jumps with distinct hysteresis for the S = 1/2 ↔ S = 3/2 levels, crossing under fast sweeping pulsed magnetic field (~10³ T/s) at the low temperature of 0.5 K and shorter pulse field. This hysteresis is expected for an anti-ferromagnetic spin triangle with anti-symmetrical Dzyaloshinky–Moriya interactions. The level-crossing field estimated zero-field splitting energies of 5–7 K between S = 1/2 and S = 3/2 states for both polyanions for the magnetization.

3.2.3. Other Types of Terminating Ligands

When other N- or O-terminating organic ligands are present as part of the synthesis course, these ligands may coordinate with the three sandwiched heterometals. The coordinating ligand may or may not lay in the same plane as the rest of the three heterometal centers showing a form of conformational isomerism.

Termination with pyridine (pyr, C₅H₅N) has been manifested in the preparation of [(Ni(pyr))₃(α-AsW₉O₃₃)₂]¹²⁻ polyanion (Figure 8a) [75]. In this polyanion, the terminal Ni−N bond distance is 2.016 Å, while the pyridine ligands lay in the same plane as the three heterometals. Magnetic susceptibility measurements have shown that the Ni^II cations are not fully equivalent, leading to ferromagnetic Ni∙∙∙Ni exchange as depicted by J = 6.17 cm⁻¹. Similar to pyridine, functionalization with aminopyrazine (apyr) also forms products through direct coordination of nitrogen-atom with the metal, as manifested in the polyanion [(Ni(apyr))₃(α-SbW₉O₃₃)₂]¹²⁻ [76]. In the latter antimonate derivative, the derived coupling constants J₁ = −0.39 cm⁻¹, J₂ = −1.07 cm⁻¹ are small and negative, indicating that the Ni∙∙∙Ni exchange interactions are weak anti-ferromagnetic [76].

Derivatives of 1,2,4-triazole (taz) have been reported for a number of polyanions [(M(Htaz))₃(AsW₉O₃₃)₂]⁹⁻ where M = Mn^II, Co^II, Ni^II (Figure 8b) [77]. The latter polyanions show weak M^II∙∙∙M^II ferromagnetic exchange interactions with J_{Mn∙∙∙Mn} =−0.36 cm⁻¹, J_{Co∙∙∙Co} = 5.93 cm⁻¹, and J_{Ni∙∙∙Ni} = 10.32 cm⁻¹ [77]. Similarly, the binding of imidazole (imi) has been largely explored, leading to anti-ferromagnetic systems of [(M(imi))₃(α-SbW₉O₃₃)₂]¹²⁻ where M = Mn^II, Ni^II, Co^II, Fe^II, and Cu^II. The Mn^II, Ni^II and Co^II derivatives are obtained through the interaction of Na₂WO₄·2H₂O and SbCl₃∙6H₂O (or alternatively Sb₂O₃) with the transition heterometal salt in the presence of imidazole [78,79]. The latter strategy does not work for creating copper derivatives, [80] and there, the synthesis is based on the trilacunary α-[SbW₉O₃₃]⁹⁻ [81]. The Fe^II derivatives are also made using α-[SbW₉O₃₃]⁹⁻, but the iron comes as Fe⁰ from iron powder in sodium acetate buffer [82]. Imidazole derivatives are also known based on the α-[BiW₉O₃₃]⁹⁻ unit. The polyanion [(Co(imi))₃(α-BiW₉O₃₃)₂]¹²⁻ has been reported to appear as a product where copper-based {Cu(imi)₄} is grafted on the surface of the polyanion [83]. Magnetic susceptibility studies of [(Co(imi))₃(α-BiW₉O₃₃)₂]¹²⁻ shows J_{Co∙∙∙Co} = −19.2(4) cm⁻¹ assuming g = 2.005(2) and θ = −1.18(5) K, indicating anti-ferromagnetic behavior.

The use of 1-methylimidazole (mimi) has also led to the interesting derivatives, [{M(mimi)}₃(α-XW₉O₃₃)₂]¹²⁻  where M = Co^II, Mn^II and X = Sb and Bi [84,85]. These derivatives show flexibility in the orientation of the imidazole ring in respect to the heteroatom plain (dihedral angle in the range of 13.7° to 89.7°). Magnetic susceptibility in the temperature range 2−300 K for [(Co(mimi))₃(α-BiW₉O₃₃)₂]¹²⁻ showed J = −0.053 cm⁻¹ and g = 2.02, whole for [(Mn(mimi))₃(α-BiW₉O₃₃)₂]¹²⁻  J = −0.06 cm⁻¹ and g = 1.99) have shown anti-ferromagnetic interaction. Coordination of outer acetate ligands has been recorded in [(Mn(COOH))₃(α-AsW₉O₃₃)₂]¹⁵⁻ (Figure 8c) [86]. Magnetic susceptibility studies for these polyanions showed J = −0.07 cm⁻¹ indicating a weak anti-ferromagnetic coupling between the Mn^II centers [86].

3.3. Hervé Polyanions Sandwiching Two and Four Heterometals

Following the discussion on the syn- and anti- scenarios discussed in Figure 3 that apply for three and six heterometal sandwiching architectures, here we discuss the sandwiching of two and four heterometals. Considering a D_3d-symmetrized anti scenario, populating with two centers will lead to three different configurational isomers (see Figure 9). On the other hand, using the same topology, populating with four metal centers will similarly lead to three different isomeric configurations representing reversed populations. In case anti-isomerism is switched with syn-orientation, then distortion between the centers may occur.

The polyanion [Cu₂(α-SbW₉O₃₃)₂]¹⁴⁻, obtained by the reaction of Na₂WO₄·2H₂O, Sb₂O₃ and copper precursor in the presence of ethylenediamine, is an example of Hervé polyanion sandwiching two metal centers [87]. The involved ethylene diamine is not part of the Hervé motif but a chelating agent to Cu^II cations [87]. It is not clear to what extent ethylene diamine coordinating to free Cu^II affects the active concentrations leading to two heterometal sandwiches. The resolved crystal structure of [Cu₂Na₄(α-SbW₉O₃₃)₂]¹⁰⁻ indicates that sodium and copper cations are partially distributed around the six-metal belt, implying that the three isomer configurations may be possible. On the basis of magnetic susceptibility data for [Cu₂(α-SbW₉O₃₃)₂]¹⁴⁻ it has been concluded that there are dominant ferromagnetic coupling interactions [16].

The reversed heterometal populated sandwich [Cu₄(α-SbW₉O₃₃)₂]¹⁰⁻ has been under hydrothermal conditions in the presence of diethylenetriamine [88]. Crystallographic study indicates that in the latter POM, the scenario where pairs of {Cu₂O₆} units sandwiched opposite to one another is prevalent [88]. Magnetic susceptibility data of [Cu₄(α-SbW₉O₃₃)₂]¹⁰⁻ revealed weak anti-ferromagnetic coupling [88].

Using α-{As^IIIW₉O₃₉} and Cu^II ion, the polyanion [Cu₄(H₂O)₄(α-AsW₉O₃₃)₂]¹⁰⁻ has also been reported [89]. The latter POM exhibits syn orientation where three copper oxo bridged cations are separated by two potassium cations from the remaining copper centers (Figure 10). Owing to the syn orientation, in comparison to the more idealized scenario (Figure 9), the heterometallic core of [Cu₄(H₂O)₄(α-AsW₉O₃₃)₂]¹⁰⁻ appears slightly more distorted (Figure 10). However, the overall symmetry remains C_2V. The Cu−O bond lengths range from 1.91 to 2.31 Å within the {Cu₃O₈(H₂O)₉} triad and from 1.95 to 2.55 Å within the isolated {CuO₄(H₂O)} unit. Magnetic susceptibility data for the polyanion show the presence of both ferromagnetically and antiferromagnetically coupled Cu^II ions (J₁ = 2.78 ± 0.13 cm⁻¹, J₂ = −1.35 ± 0.02 cm⁻¹, and J₃ = −2.24 ± 0.06 cm⁻¹).

3.4. Hervé Polyanions Sandwiching Five Heterometals

Furnishing the synthesis with bipyridine ligands can enforce symmetry break leading to structures with a syn arrangement of the α-[SbW₉O₃₃]⁹⁻ units, which instead of three, encapsulate five metal centers. Two polyanions of the type [{Mn(bpy)}₂(MnCl)₃(AsW₉O₃₃)₂]¹¹⁻ and [{Mn(bpy)}₂(MnCl){Mn(OH₂)}₂(α-SbW₉O₃₃)₂]⁹⁻, where X = As and Sb and bpy is bipyridine, have been reported [90]. The manganese centers are connected to bipyridine, chloride, and aqua ligand (Figure 11a). The Mn−N bond distance in the arsenic derivative is 2.25 Å, and the Mn–O bond distances are in the range of 2.08–2.24 Å. Magnetic studies on the polyanion show ferromagnetic interaction between the Mn^II ions. On the other hand, imidazole can also lead to an unusual binding and formation of metal-carbon bonds between Co^II and Ni^II, leading to {Na_0.7M_5.3(OH₂)₂(imi)₂(Himi)(α-SbW₉O₃₃)₂} (Figure 11b), that has partial sodium occupancy [91].

Zhao and coworkers reported in 2013 a pentameric assembly of magnetically active metals with α-[AsW₉O₃₃]⁹⁻. The polyanion [Na⊆Cu₅Cl(OH₂)₃(α-AsW₉O₃₃)₂]⁹⁻ has a hexagonal shaped {Cu₅Na} assembly sandwiched between the two α-[AsW₉O₃₃]⁹⁻ units [92]. The metal ions in this sandwich, having identical square pyramidal and square planar geometry, are categorized into four distinct groups based upon their coordination environments, viz. two identical {CuO₄}, one {CuO₄(H₂O)}, two disordered {Cu/NaO₄(H₂O)}, and one {CuO₄Cl}. The Cu–O bond lengths in this polyanion range from 1.953 to 2.007 Å, while the Cu-Cl bond has a length of 2.5368 Å. Magnetic measurements for the polyanion revealed ferromagnetic exchange interactions within the {Cu₅} core mediated by the oxygen bridges.

3.5. Hervé Polyanions Sandwiching Six Heterometals

Saturated sandwich structures with six metal centers have been reported for [(MnCl)₆(α-XW₉O₃₃)₂]¹²⁻ (Figure 12a), [93] and [(CuCl)₆(α-XW₉O₃₃)₂]¹²⁻ [94]. These forms of sandwich structures are prepared using similar conditions as the trimetalate Hervé-type sandwiches; however, the synthesis is typically carried out at ambient conditions, which may enhance slower sandwich closing giving time coordination between heterometals to occur. Both sandwich structures exhibit ferromagnetic coupling where for the ground state and the coupling constant values for [(CuCl)₆(α-SbW₉O₃₃)₂]¹²⁻ is S = 3 and J = +29.5 cm⁻¹, and for [(MnCl)₆(α-SbW₉O₃₃)₂]¹²⁻ is S = 15 and J = +0.14 cm⁻¹ [94,95]. Further studies of the magnetic interactions for the polyanions [(CuCl)₆(α-AsW₉O₃₃)₂]¹²⁻ and [(MnCl)₆(α-SbW₉O₃₃)₂]¹²⁻ by Tsukerblat and coworkers gave coupling constants of J = 35 and 0.55 cm⁻¹, respectively, and have also emphasized the importance of axial anisotropy towards explaining the magnetic properties of such arrangements [96].

Linking of imidazole ligands produces [M₆(imi)₆(α-AsW₉O₃₃H₃)₂] where M = Mn^II, Ni^II, Zn^II (Figure 12b) [97]. Magnetic susceptibility data for the nickel gave J = +1.96 cm⁻¹, showing Ni^II∙∙∙Ni^II ferromagnetic coupling. When instead of chloride ligands, oxalate ligands are involved, there is a possibility for the POM to grow further in the same plane with the hexametalate transition metal core. Using the oxalate addition, the ferromagnetic [(Mn(ox))₃(Mn(H₂O)₃)₃(Mn(H₂O)₂)₂(Mn(H₂O))(α-SbW₉O₃₃)₂]⁶⁻ species has been isolated (Figure 12c) [98].

3.6. Hervé Polyanions Sandwiching Mixed-Metallic Cores

In addition to the homometallic trimeric assemblies interspersed by alkali metal cations, several heterometallic assemblies with only transition metals and mixed transition metal-lanthanides have also been reported. One example is [((V^IVO)₂Ln(H₂O)₄)(α-AsW₉O₃₃)₂]¹¹⁻, where Ln = Dy and Gd (Figure 13a) [99,100]. The polyanion shows an idealized C_2V symmetry. V−O bond lengths range from ~1.6 to 1.99 Å, and the Ln–O (Ln = Dy, Gd) bond lengths range from ~2.3 to 2.8 Å. The two V^IV atoms in both the polyanions possess square pyramidal geometry with a short apical V = O bond of distance ~1.6 Å and are via sodium counter cations. Ln centers in square antiprism coordination. Magnetic data for the polyanions showed weak anti-ferromagnetic interactions between the vanadium (J_VV = −2.55 cm⁻¹) atoms and weak ferromagnetic interactions between vanadium and the lanthanide (J_GdV = 0.6 cm⁻¹).

Another structure is [M(H₂O)(Ti^IVO)₂(α-As^IIIW₉O₃₃)₂]¹²⁻ where M = Co^II, Ni^II, Cu^II, and Zn^II (Figure 13b) [101]. The polyanion was synthesized by the reaction of the monovacant [(Ti^IVO)₂(α-As^IIIW₉O₃₃)₂]¹⁴⁻, [102] with the respective metal. Magnetic studies on these polyanions indicate anti-ferromagnetic interactions with the nearest neighbors for these compounds, with effective magnetic moments calculated to be 5.45, 3.11, and 2.12 Bohr magnetons (B.M.) per molecule for M = Co^II, Ni^II, and Cu^II respectively.

Another is the polyanion [Cu(H₂btp)Ln_0.5Na_1.5Cu₃(H₂O)₅(α-AsW₉O₃₃)₂]⁷⁻ where Ln = Ho, Yb, and btp is 1,3-bis[tris(hydroxymethyl)methylamino]propane (Figure 14) [103]. The copper-organic group is in distorted octahedral geometry binding to two O and two N units. The moiety connects to the middle pentagonal metallocycle {Ln_0.5Na_1.5Cu₃(H₂O)₅} cluster where one position is a shared [Ln_0.5/Na_0.5]²⁺ cation. The [Ln_0.5/Na_0.5]²⁺ cation binds to eight oxygen atoms from the {α-AsW₉O₃₃} unit as well as two water ligands, achieving a square antiprism. The Ho/Na-O bond lengths range from 2.290 to 2.781 Å. Magnetic susceptibility data for the Ho derivative between 2 and 300 K were fitted according to the Curie–Weiss law, resulting in C = 11.21 emu K mol⁻¹ and θ = 2.89 K. The small positive θ value confirms the presence of the weak Cu^II∙∙∙Ho^III ferromagnetic interactions.

4. Krebs-Type Dimers

4.1. Differentiation between Classical Krebs Dimers

Referring to Figure 4, we generally distinguish between a- and b- site octahedra in the Krebs structure. As noted, the interatomic distance between two a-sites (i.e., d₂) is ca. 10.0 Å, between two b-sites (i.e., d₃) it is ca. 5.6 Å, and between an a and b site (i.e., d₁) it is ca. 5.4 Å. Overall, d₁, d₂, and d₃ are relatively large interatomic distances, and thus none or only very weak magnetic exchange interactions occur in this POM archetype [104,105].

The diamagnetic Krebs structure [(WO₂OH)₂(WO₂)₂(β-SbW₉O₃₃)]¹²⁻ is synthesized by reacting α-[SbW₉O₃₃]⁹⁻ with sodium tungstate in aqueous media. Early studies have shown that the structure can be post-functionalized with 3d metal cations (e.g., Fe^III, Co^II, Mn^II, Ni^II, and Cu^II) [44,105]. The latter reactions lead to the formal substitution of the peripheral {WO₂OH} units with {M(H₂O)₃)₂} units, yielding {(M(H₂O)₃)₂(WO₂)₂(β-SbW₉O₃₃)} derivative species which can be doubly protonated [44,105].

In addition to the typical synthetic pathways, mixtures of iron powder, imidazole, and pre-synthesized Krebs precursor [(WO₂OH)₂(WO₂)₂(β-SbW₉O₃₃)]¹²⁻ in acetic buffer solution under heating leads to the formation of the Fe^II derivative [Fe^II₂(H₂O)₆(WO₂)₂(β-SbW₉O₃₃)₂]¹⁰⁻ [106]. The latter structure is shown to be potential as catalyst for Fenton reaction and electrochemical sensing of ascorbic acid [106].

The Krebs-like tetrametallic archetype [M₄(H₂O)₁₀(β-SbW₉O₃₃)₂]ⁿ⁻ has been reported for trivalent cations such as Fe^III, Cr^III, and Al^III (Figure 15) [107,108]. The Fe^III and Cr^III derivative has been obtained through a reaction of respective chloride salts with trilacunary precursors α-[XW₉O₃₃]⁹⁻ (X = As, Sb) in water at pH 3.0 under heating [107]. Owing to the Fe^III oxidation state, the polyanion [Fe^III₄(H₂O)₁₀(β-SbW₉O₃₃)₂]⁶⁻ is a subject to four electron reductions, which can lead to [Fe^II₄(H₂O)₁₀(β-SbW₉O₃₃)₂]¹⁰⁻ species. The latter interconversion is of relevance for catalytic oxygenation of catechol [109]. However, at the same time, it may be causing uncertainty in the accurate determination of the magnetic ground state of [Fe^III₄(H₂O)₁₀(β-SbW₉O₃₃)₂]⁶⁻. Based on the isotropic Heisenberg Hamiltonian model, it was proposed that the ground state of the POM salt supposedly containing [Fe^III₄(H₂O)₁₀(β-SbW₉O₃₃)₂]⁶⁻ has a total spin of S = 2 [110]; however, X-ray photoelectron Fe 2p spectra have revealed that the iron centers have Fe^II formal valence state [110]. This deviation from the contained Fe^III has been explained based on charge-transfer effects. [110]

Isolation of {Mn₄}-Krebs derivative has been achieved with the help of manganese carbonyl cations {Mn(CO)₃}, leading to the polyanions [{Mn₄(H₂O)₁₀}{Mn(CO)₃}₂(β-XW₉O₃₃)₂]⁸⁻ (X = Sb and Bi) [111]. Magnetic measurements of the latter compound at 1.8–300 K under a 2k Oe applied field suggest that the Mn^II ions are strongly antiferromagnetically coupled.

4.2. Partial Substitutions in Krebs Dimers

In the solid state, some Krebs-type polyanions may reflect compositions that deviate from the scenarios {(M(H₂O)₃)₂(WO₂)₂(β-SbW₉O₃₃)} and {M₄(H₂O)₁₀(β-XW₉O₃₃)₂} shown in Figure 15. However, currently, it is not clear if these partially substituted compositions reflect averages of {(M(H₂O)₃)₂(WO₂)₂(β-SbW₉O₃₃)} and {M₄(H₂O)₁₀(β-XW₉O₃₃)₂} in the crystallographic lattice, or individual POM entities with partial substitutions indeed being formed. Examples of such polyanionic motifs in the solid state are: the discrete [{Mn_3.5W_0.5(H₂O)₁₀}{Mn(CO)₃}₂(β-SbW₉O₃₃)₂]⁴⁻ [111], [Mn_2.5W_1.5(H₂O)₈(β-SbW₉O₃₃)₂]⁴⁻ [112], and the chain-type {(Co(H₂O)₂)₃W(H₂O)₂(β-SbW₉O₃₃)₂}⁶⁻ [112]. The latter two Mn-functionalized Krebs structures exhibit weak anti-ferromagnetic coupling (θ_c = −0.056 K in the range 50–300K) [112]. A number of related species to [(M(H₂O)₃)₂(M_0.5W_0.5O)₂(β-BiW₉O₃₃)₂)]^10– where M = Mn^II, Co^II, Ni^II have been produced in the presence of triethanolamine cations [113].

4.3. Ligand Substitution in Krebs Dimers

Decorated Krebs structures can be prepared when the aqua ligands are linked to the heterometal M in {(M(H₂O)₃)₂(WO₂)₂(β-XW₉O₃₃)} and {M₄(H₂O)₁₀(β-XW₉O₃₃)₂} are exchanged. An example of such exchange is illustrated by [Fe₂(DMSO)₈(WO)₂(β-XW₉O₃₃)₂]⁴⁻ which has been obtained by dissolving [(Fe(H₂O)₃)₂(WO₂)₂(β-SbW₉O₃₃)]⁸⁻ in DMSO, and recrystallizing it in the presence of ruthenium bipyridine cations [114]. Another strategy to exchange the aqua ligands may be by introducing suitable binding ligands in the synthesis process. For instance, by introducing pyridine-3,4−dicarboxylate (pdc), the polyanion [Fe₄(H₂O)₈(pdc)₂(β-SbW₉O₃₃)₂]⁶⁻ (Figure 16a) has been prepared instead of [Fe^III₄(H₂O)₁₀(β-SbW₉O₃₃)₂]⁶⁻ [115]. Field-dependent magnetization measurement for [Fe₄(H₂O)₈(pdc)₂(β-SbW₉O₃₃)₂]⁶⁻ at 2 K indicated S = 7 ground state [115]. The addition of oxalate (ox) can similarly lead to the formation of different products depending on the reaction pH. At pH = 3.0 discrete [Fe^II₄(ox)₄(H₂O)₂(β-SbW₉O₃₃)₂]¹⁴⁻ (Figure 16b) polyanions are isolated, [104] while at pH = 6.0 chains of {Fe^II₄(ox)₄(β-SbW₉O₃₃)₂}_n are formed [104]. The presence of ethylenediamine during the formation process also leads of chain-like structures made of {Fe^III₄(H₂O)₈(β-SbW₉O₃₃)₂} components [104].

If the added ligands are larger and have more than one binding site, then dimerization may be possible. By introducing N, O-chelating ligands such as pyrazine-2,3-dicarboxylate (pyzdc), [116] dimeric product [(pyzdc)₂{NaNi₂(H₂O)₄(WO₂)₂(β-SbW₉O₃₃)₂}₂]^22– containing two components of [{Ni(H₂O)₃}₂(WO₂)₂(β-SbW₉O₃₃)₂]^10– is obtained [117]. The dimerization is also prevalent among Krebs-type polyanions with central belt involving heterometallic centers such as Ni^II, Co^II, or Mn^II), [118] as or Krebs-type structures where the two tungstate metals at the a-sites are exchanged with tin heterogroups [119].

4.4. Grafting Metal Cations and Interconnecting Krebs Dimers

The incorporation of transition metals and lanthanides within a single POM stabilized structure has been seen as a viable method for designing 3d-4f complexes [120]. However, using the [α-SbW₉O₃₃]⁹⁻ polyanion incorporating both metal centers has been challenging. One reason is that the lanthanides have larger coordination spheres and often may lack directionality. In this regard, a number of structures containing Krebs-type complexes [Fe^II₄(H₂O)₁₀(β-SbW₉O₃₃)₂]^10– and lanthanide cations have been reported [121]. Such compounds have been obtained through the use of 2−picolinic acid (pic), leading to [Fe^II₄(H₂O)₂(pic)₄(β-SbW₉O₃₃)₂]^10– species and [Ln(H₂O)₅]³⁺ cations where Ln^III = La^III, Pr^III, Nd^III, Sm^III, Eu^III (Figure 17a) [121]. Using a similar strategy and threonine (thr) ligands, another series of [Fe^III₄(H₂O)₈(thr)₂(β-SbW₉O₃₃)₂]^6– species interconnected with [Ln(H₂O)₈]³⁺ cations where Ln = Pr^III, Nd^III, Sm^III, Eu^III, Gd^III, Dy^III, Lu^III have been prepared [122]. Among the latter series, the Eu^III derivatives have shown expected fluorescent emission.

Patzke and coworkers reported a series of 0-, 1-, 2-, and 3-dimensional copper-containing tungstobismuthate polyanions [70]. The reported complexes include the discrete [Cu₂(H₂O)₄Cl₂(β-BiW₁₀O₃₅)₂]¹²⁻ and [Cu₂(H₂O)₆(β-BiW₁₀O₃₅)₂]¹⁰⁻ polyanions. Two-dimensional networks of {Cu_0.5Cl[Cu₂(H₂O)₄(β-BiW₁₀O₃₅)₂]}¹⁰⁻ (Figure 17b), {Cu[Cu₂(H₂O)₄(β-BiW₁₀O₃₅)₂]}⁸⁻, [Cu₃(H₂O)₃(α-BiW₉O₃₃)₂]¹²⁻ and three-dimensional [(Cu₃Cl)(K_2.62Cu_0.38(H₂O)₃(α-BiW₉O₃₃)₂]⁹⁻ have also been reported. The copper centers in these series are relatively away from one another in the range of 8 to 11 Å.

5. Vision on POMtronics

POMtronics is a term coined in 2020 [41] that broadly refers to advanced applications of POMs deriving from their magnetic, electronic, and optical properties [123]. In a more narrow sense, POMtronics refers to POM-based nanoelectronics and spintronic devices [124]. A typical example may be a symmetrical and spherical POM sandwiched between two gold electrodes functioning as a current switch (Figure 18) [124]. Spin-polarized POMs are of special interest for molecular junction as their current transport can be influenced without the need to employ an external magnetic field [125].

Sandwich-type POMs exhibit an ellipsoidal shape, meaning that upon binding to electrodes, two or more distinguishable orientations can be adopted. A similar scenario appears for the ellipsoidal {(SO₃)₂W₁₈O₅₄}) POMs which have been used in POMtronics applications [126]. However, in contrast to the previous example, many Hervé dimers, as showcased in this work, have an enormous potential for broad rational derivatization and formation of magnetic POMs in their ground state. Unfortunately, to date, Hervé has rarely been used for functionalization of electronic nanosystems. In 2008, Charron et al. reported a successful deposition of {Co(H₂O)(WO)₂(AsW₉O₃₃)₂} on a single walled nanotube [127]. More recently, the group of Kögerler demonstrated that trimeric cobalt functionalized POTs be effectively deposited through simple incubation on freshly prepared gold substrates [125].

6. Conclusions

The objective of this article has been to provide a comprehensive overview of the two most common sandwich-type archetypes in POM chemistry—Hervé-type and Krebs-type. The Hervé-type POMs generally adopt two main arrangements of the lacunary units and are capable to incorporate between two and six magnetic heterometals. The close arrangement of the metal centers typically causes spin interactions leading to ferromagnetic and antiferromagnetic scenarios. The relative arrangement of the lacunary units also plays role in the number of incorporated centers. Considering the synthetic conditions, these types of POM dimers can be further functionalized with a variety of binding ligand and grafting cations. The Krebs-type polyanions appear to be more common for Sb- and Bi-based lacunary heteroPOMs. In the latter polyanion, the metal centers lay further apart from one another, and thus most of the interactions between magnetic heterocenters remain weakly antiferromagnetic. Both sorts of polyanions show stability to multielectron reductions, which is not only interesting in magnetism but in catalysis as well.

With the present review we thus aimed to provide a comprehensive structuring of magnetic POM sandwich dimers, which will be extended to other POM systems in future. Although currently it may be challenging to derive rules in which one can rationally design new sandwich POMs with desired magnetic properties, current applications of knowledge engineering in chemistry [128] show promise that such endeavor to be achieved in near future. In this regard, we also envision that magnetic POM dimers would be of high interest in the development of future POM-based nanoelectronics (i.e., POMtronics) [124,129].