Semimetal-functionalised polyoxovanadates

Abstract

Polyoxovanadates (POVs), known for their wide applicability and relevance in chemical, physical and biological sciences, are a subclass of polyoxometalates and usually self-assemble in aqueous-phase, pH-controlled condensation reactions. Archetypical POVs such as the robust [V^IV₁₈O₄₂]¹²⁻ polyoxoanion can be structurally, electronically and magnetically altered by heavier group 14 and 15 elements to afford Si-, Ge-, As- or Sb-decorated POV structures (heteroPOVs). These main-group semimetals introduce specific chemically engineered functionalities which cause the generally hydrophilic heteroPOV compounds to exhibit interesting reactivity towards organic molecules, late transition metal and lanthanoid ions. The fully-oxidised (V^V), mixed-valent (V^V/V^IV and V^IV/V^III), “fully-reduced” (V^IV) and “highly-reduced” (V^III) heteroPOVs possess a number of intriguing properties, ranging from catalytic to molecular magnet characteristics. Herein, we review key developments in the synthetic and structural chemistry as well as the reactivity of POVs functionalised with Si-, Ge-, As- or Sb-based heterogroups.

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Kirill Monakhov received his Dr rer. nat. degree in 2010 with Prof. Gerald Linti (Heidelberg University, Germany). After two years as a postdoctoral fellow of the German Research Foundation and the Cercle Gutenberg with Prof. Pierre Braunstein (University of Strasbourg, France), and after being awarded the Academia Europaea Burgen Scholarship in 2011, he returned to Germany in 2013 with a DFG postdoctoral reintegration fellowship to join the group of Paul Kögerler at RWTH Aachen University. In 2015 he received a DFG Emmy Noether fellowship and now leads a junior research group at the Institute of Inorganic Chemistry at RWTH Aachen.

Wolfgang Bensch received his Dr rer. nat. with Prof. Eberhard Amberger at the Ludwig-Maximilians-University of Munich (Germany) in 1983. He was a postdoctoral fellow at the University of Zurich (Switzerland) and joined Siemens Company (Munich) in 1986. In 1990 he started his habilitation at the Johann Wolfgang Goethe University Frankfurt (Germany) which was finished in 1993. He was appointed as full professor for Inorganic Solid State Chemistry at the Christian-Albrechts-University Kiel in 1997.

Paul Kögerler graduated with a Dr rer. nat. degree with Prof. Achim Müller at the University of Bielefeld (Germany) in 2000, followed by a postdoctoral research stay at the Department of Physics and Astronomy at Iowa State University (USA). In 2003, he was appointed as a tenured Associate Scientist at the U.S. DOE Ames Laboratory, before returning to Germany in 2006 as Professor of Chemistry at the Institute of Inorganic Chemistry at RWTH Aachen University and Group Leader for Molecular Magnetism at the Peter Grünberg Institute, Research Centre Jülich.

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1. Introduction

1.1 General remarks

Although vanadium is best known for its use in the large-scale industrial production of alloys and steels, this group 5 transition metal also plays a significant role in biological systems and bioinorganic chemistry.^1,2 Vanadium shows common oxidation states between +2 and +5 revealed by characteristic colours such as lilac/purple (V²⁺), green (V³⁺), blue ({V^IVO}²⁺), and yellow ({V^VO₂}⁺ and {V^VO₃}⁻) and exhibits a particularly rich coordination chemistry in aqueous solutions.^3,4 It has a high tendency to form oxovanadium ions whose nuclearity, structural motifs and net charge are strongly influenced by the specific reaction conditions such as stoichiometries and concentration of the reactants, pH (alkaline vs. acidic aqueous solutions), temperature, pressure, and reaction time. In general, vanadium oxide compounds find a wide range of applications in catalysis,⁵ biochemistry,^1,2,6 sol–gel chemistry,⁷ gas sensing,⁸ geochemistry,⁹ sorption¹⁰ and intercalated layered material,¹¹ surface and nano sciences¹² and perform their role as secondary electrode materials for advanced lithium ion batteries and vanadium redox-flow batteries.¹³ Even in photosynthesis the role of vanadate and vanadyl citrate compounds was investigated.¹⁴ The chemistry of polyoxovanadates (POVs),¹⁵ the latter being a subclass of polynuclear molecular early transition metal oxides known as polyoxometalates (POMs),¹⁶ is a very fast growing area of research, mainly due to the versatile redox activities of POVs,¹⁷ their current application and further perspectives in various branches of chemical, physical and biological sciences.

1.2 Structure and characterisation of conventional POVs

In aqueous and organic¹⁸ solutions, POVs are formed in pH-dependent condensation reactions in which small [VO_n]^Q− fragments aggregate to form a large variety of high- and low-nuclearity cluster structures with diverse coordination geometries of the vanadium cations. POVs usually exhibit cage-, sphere-, hollow-, basket-, belt-, and barrel-like structural motifs,¹⁹ which are constructed of a number of polyhedra fused and/or linked through a common vertex (corner) and/or polyhedral edges and faces. These polyhedra consist of the homo- or heterovalent V atoms showing e.g. square-pyramidal²⁰ [V^VO₅]⁵⁻ and [V^IVO₅]⁶⁻, octahedral [V^VO₆]⁷⁻ and [V^IVO₆]⁸⁻ and tetrahedral [V^VO₄]³⁻ coordination geometries. Thus, the adjustable coordination behaviour of V in POVs contrasts with the predominantly octahedral coordination environments of Mo and W ions in their POM structures. One of the most studied and structurally characterised POVs is the orange-coloured decavanadate ion,²¹ [V₁₀O₂₈]⁶⁻ (Fig. 1), that demonstrates the ability to build supramolecular assemblies²² and shows fascinating biological activity.²³ This polyanion, reported for the first time in 1956, has also been found in minerals (see, e.g. lasalite, Na₂Mg₂[V₁₀O₂₈]·20H₂O).²⁴

Fig. 1 Polyhedral representation of structurally characterised fully-oxidised POVs with different topologies involving V ions in the formal oxidation states of +5. Oxygen positions are shown as small red spheres, the V^VO_x coordination polyhedra in orange colour.

POVs can be divided into four general families: fully-oxidised (V^V), mixed-valent (V^V/V^IV or V^IV/V^III), “fully-reduced” (V^IV) and “highly-reduced” (V^III) species. The following crystallographically characterised POVs [V₂O₇]⁴⁻,²⁵ [V₃O₉]³⁻,²⁶ [V₄O₁₂]⁴⁻,²⁷ [V₅O₁₄]³⁻,²⁸ [V₁₀O₂₈]⁶⁻,²⁹ [V₁₂O₃₂]⁴⁻,³⁰ [V₁₃O₃₄]³⁻,³¹ [V₁₅O₄₂]⁹⁻,³² [V₁₆O₄₂]^4− 33 constitute the class of fully-oxidised vanadium species (Fig. 1). X-ray single-crystal structures have been determined for [V^IV₂V^V₈O₂₆]⁴⁻,³⁴ [V^IV₈V^V₇O₃₆]⁵⁻,³⁵ [V^IV₁₁V^V₅O₃₈]⁷⁻,³⁶ [V^IV₃V^V₁₃O₄₂]⁷⁻,³⁷ [V^IV₅V^V₁₂O₄₂]⁴⁻,³⁸ [V^IV₁₆V^V₂O₄₂]¹⁰⁻,³⁹ [V^IV₁₀V^V₈O₄₂]⁴⁻,³⁹ [V^IV₈V^V₁₀O₄₄]⁶⁻,⁴⁰ [V^IV₆V^V₁₃O₄₉]⁹⁻,⁴¹ [V^IV₈V^V₁₄O₅₄]⁶⁻,⁴⁰ [V^IV₁₆V^V₁₈O₈₂]^10− 42 (disregarding encapsulated supramolecular guest species) from the class of the mixed-valent V^V/V^IV species (Fig. 2). The mixed-valent V^IV/V^III-POVs and the “highly-reduced” V^III-POVs consist mostly of a variety of vanadium alkoxide structures.^43,44 The most renowned representative in the class of the “fully-reduced” POVs is the [V^IV₁₈O₄₂]¹²⁻ polyoxoanion whose chemical and structural characterisation were reported for the first time by Johnson and Schlemper in 1978 (Fig. 3).⁴⁵ The {V₁₈O₄₂} architecture constructed from the edge-sharing square-pyramidal {O=VO₄} units represents an archetypal structure with a diameter of ca. 11 Å which may adopt idealised T_d and D_4d symmetries. The strong infrared stretching frequencies of terminal V=O groups characteristically appear in the range 940–1000 cm⁻¹.

Fig. 2 (a) [V₁₀O₂₆]⁴⁻ with two V^IV (caps) and eight V^V atoms. (b) [V₁₇O₄₂]⁴⁻ with five V^IV (four peripheral and one central) and twelve V^V atoms. Colour code: O, red; V^IVO_x, sky-blue polyhedra; V^VO_x, light-orange polyhedra.

Fig. 3 Schematic decomposition of the “fully-reduced” [V₁₈O₄₂]¹²⁻ POV into the {V^IV₁₈} skeleton and the {O₂₄} polyhedron of bridging oxygen sites (rhombicuboctahedron). Colour code: O, red; V^IV, sky blue; V^IVO_x, sky-blue polyhedra.

The presence of heterovalent vanadium atoms in some POVs occasionally complicates the assignment of the positions of the individual V atoms. This is due to the multifaceted nature of the POV structures, which often afford similar coordination geometries (e.g. square-pyramidal) around the V^V/V^IV ions and feature charges which are delocalised over the vanadium centres. Generally, there are several approaches to assign the oxidation states of V atoms in the entire POV structure: (i) overall charge balance, (ii) calculation of bond valence sums from determined bond lengths,⁴⁶ (iii) redox titration and (iv) X-ray photoelectron spectroscopy (XPS). Elemental, thermal (TG-DTA),⁴⁷ X-ray powder diffraction and electron microprobe (EMP) analyses, infrared (IR) spectroscopy, electron paramagnetic resonance spectroscopy (EPR), ¹H, ¹⁷O and ⁵¹V nuclear magnetic resonance spectroscopy (NMR),⁴⁸ energy-dispersive X-ray spectroscopy (EDXS), ultraviolet-visible (UV-vis) spectroscopy, electrospray ionisation mass spectrometry (ESI-MS),⁴⁹ cyclic voltammetry (CV), and inelastic neutron scattering (INS) have been used for the chemical and structural characterisation of POVs. Temperature- and field-dependent magnetic susceptibility measurements were routinely applied to evaluate the magnetic characteristics of reduced POVs. A study combining density functional theory (DFT) calculations and ⁵¹V NMR experiments for [V₁₀O₂₈]⁶⁻ was published some time ago.⁵⁰ More recently, joint experimental and computational studies have been performed on positively-charged vanadium oxide clusters in the gas phase and in solution. These involved e.g. the combination of laser desorption time-of-flight mass spectrometry, UV-vis and IR spectroscopy, and DFT calculations,⁵¹ or analysis by ESI-MS, collision-induced dissociation experiments and DFT.⁵²

1.3 Relation of POVs to classical POMs

The POV structures are usually derived or deduced from transferable vanadium oxoanion building blocks and host–guest aggregates, but also from small stable POM archetypes. Although the redox behaviour of the POVs differs from that of POMs comprised of group 6 transition metals (M = Mo, W), some of the construction principles of the metal-oxide core frameworks are very similar between these two classes of compounds. The POVs have thus been shown to adopt the structures of the classical Lindqvist-type [M₆O₁₉]^Q− polyanions,⁵³ as exemplified by the mixed-valent [V₆O₁₉]^Q− polyoxoanions,⁵⁴ and to exhibit expanded Keggin-type [YM₁₂O₄₀]^Q− structures (Y: heteroatom).⁵⁵ A first representative in the latter family, a fully-oxidised [PV₁₄O₄₂]⁹⁻ POV, which is composed of an α-Keggin [PV₁₂O₄₀]¹⁵⁻ polyoxoanion capped by two VO³⁺ moieties, was reported in 1980.⁵⁶ A mixed-valent bicapped polyoxomolybdate Keggin-type polyoxoanion with two {VO}²⁺ caps,⁵⁷ [PMo^V₆Mo^VI₆O₄₀(V^IVO)₂]⁵⁻, is of interest due to its isostructural relations with the fully-oxidised [V₁₅O₄₂]⁹⁻ isopolyoxovanadate. So-called superkeggin structures based on the {V₁₈O₄₂} constituents have been found for the mixed-valent complexes [V^IV₁₂V^V₆O₄₂(SO₄)]⁸⁻,⁵⁸ [V^IV₁₈O₄₂H₉(V^VO₄)]⁶⁻,⁵⁸ and [V^IV₁₄V^V₄O₄₂(PO₄)]¹¹⁻.⁵⁹ Note that the structure of the archetypal “fully-reduced” [V^IV₁₈O₄₂]¹²⁻ polyoxoanion with D_4d symmetry can formally be derived stepwise from the fully-oxidised α-Keggin-type framework [V^V₁₂O₃₆]¹²⁻, as illustrated in Fig. 4.

Fig. 4 Schematic structural evolution of the fully-oxidised α-Keggin-type framework [V₁₂O₃₆]¹²⁻ towards the “fully-reduced” [V₁₈O₄₂]¹²⁻ POV via formation of the bicapped Keggin structure [V₁₄O₃₈]⁶⁻ and the hypothetical [V^V₁₈O₄₂]⁶⁺ structure, the latter here assumed as isostructural to [V^IV₁₈O₄₂]¹²⁻. Colour code as in Fig. 2 and 3.

1.4 ‘Host–guest’ complexation in POV chemistry

The cage-, basket-, barrel- and sphere-like shape of high-nuclearity POVs enables them to entrap small guest species⁶⁰ in their central voids, in a manner that is structurally reminiscent of the “molecular container” behaviour of e.g. fullerenes. The case of a discrete (small) number of water molecules enclosed in a molecular container generally is of great interest: Jung and coworkers e.g. emphasise that “a single water molecule within an isolated space is a hot research topic in molecular science, owing to potential applications to delicate functions such as proton transfer, tautomerism, recognition, and biological systems”.^61,62Fig. 5 illustrates that not only Pd(II) cage-type coordination complexes⁶¹ and fullerene C₆₀,⁶³ but also the hydrophilic POVs are able to host an individual H₂O molecule.⁶⁴ In the following, for reasons of clarity we indicate the supramolecular encapsulation state of a guest species by the “@” notation when specifying the composition of a discrete polyanion, whereas in full formulas for POV-based compounds the guest species is simply set in brackets.

Fig. 5 The H₂O-endohedral (@) complexes of C₆₀ (a),⁶⁵ the “fully-reduced” [V₁₈O₄₂]¹²⁻ POV (b),^45,64 and [Pd₃L₂]⁶⁺ (c).⁶¹ C, grey; N, blue; O, red; V^IV, sky blue; Pd, brown.

The prototypical fully-oxidised, mixed-valent and “fully-reduced” POV shells (e.g., bowl-like {V₁₂O₃₂}, nearly spherical D_4d {V₁₈O₄₂}, ellipsoidal D_2h {V₁₈O₄₄}, bulb-shaped D_2d {V₂₂O₅₄}) were found to enclose not only water molecules but also anions (e.g., Cl⁻, Br⁻, I⁻, HCOO⁻, MeCOO⁻, CN⁻, CO₃²⁻, NO⁻, NO₂⁻, N₃⁻, PO₄³⁻, SH⁻, SCN⁻, SO₃²⁻, SO₄²⁻, ClO₄⁻, and VO₄³⁻) within the vanadium oxide shells.^39,66 Larger neutral guests (methanol, ethanol, ethane-1,2-diol) have furthermore been reported to exist in phosphate- and diaminopropane-capped {V^III₃V^IV₁₈}-type cluster shells.⁶⁷ In all of these compounds, the encapsulated halide, pseudohalide, inorganic or organic guest anion interacts with the anionic host shell solely via weak electrostatic and van der Waals forces at distances of ca. 3.5–4.0 Å. In this regard, the POV host structures⁶⁸ can also be regarded as vanadium oxide matrices that isolate unstable guests with short lifetimes or high reactivities. The formation of POV host shells can also be controlled by templating effects of small guest anions present in solution.^40,69 Interestingly, also certain cationic groups such as dimethylammonium that weakly bind to polyoxovanadate shell structures can assume the role of an interchangeable template as well, for example in the POV [Cl@V^V₁₂O₃₂(H₂NMe₂)₂]³⁻ where one or both dimethylammonium groups can subsequently be exchanged by first-row transition metal cations.⁷⁰ Macrocyclic, highly-oxidised POV structures [PdV^V₆O₁₈]⁴⁻, [Cu₂V^V₈O₂₄]⁴⁻, and [Ni₄V^V₁₀O₃₀(OH)₂(H₂O)₆]⁴⁻ incorporating transition-metal cationic templates (Pd²⁺, Cu²⁺, and Ni²⁺) are also known.^15,71

The far-reaching consequences for the properties of the host matrix are exemplified by a family of purely inorganic host–guest assemblies [X@HV^IV₈V^V₁₄O₅₄]⁶⁻ (X = VO₂F₂⁻, SCN⁻, and ClO₄⁻).⁷² Here the nature of the guest (X) anion defined the magnetic and electrochemical characteristics of the virtually isostructural, mixed-valent V^V/V^IV-POV host matrices. Similarly, template-dependent reactivity has also been observed for the photooxidation of an organic dye by nearly isostructural [X@(Bi(dmso)₃)₂V^V₁₂O₃₃]⁻-type POVs (X = Cl⁻, Br⁻).⁷³ In general, POVs exhibiting host–guest relationships consist of transferable substructures and exist in a variety of reduction (and protonation) states. In the words of Müller et al.: “The template syntheses of new cluster compounds utilizing intrinsic host–guest relationships open up new possibilities for chemists to create nanosized host or cluster structures with novel magnetic properties”.⁷⁴

1.5 Magnetism of POVs

The POVs can be magnetically functionalised either via their reduction towards the mixed-valent, “fully-reduced”, and “highly-reduced” structures comprising vanadyl {V^IVO}²⁺ (isotropic spin-1/2) and {V^IIIO}⁺ (spin-1) groups or via incorporation of paramagnetic transition metal or lanthanoid cations. The last few decades have shown that such POVs are ideal objects for magnetochemical studies⁷⁵ and the exploration of nanoscale molecular magnetism⁷⁶ due to their intriguing magnetic features, ranging from geometrical spin frustration to single-molecule magnet characteristics.⁷⁷ Most commonly, the mixed-valent (V^V/V^IV or V^IV/V^III), “fully-reduced” (d¹-V^IV) and “highly-reduced” (d²-V^III) vanadium spin-centres are coupled antiferromagnetically via bridging oxygen atoms, although ferromagnetic coupling between the vanadium ions is also possible, as has been forecasted in a quantum-chemical study of the synthetically readily accessible mixed-valent alkoxo-hexavanadates of the [V^IV₄V^V₂O₇(OR)₁₂] type.⁷⁸ A comparative theoretical study of the magnetic properties of a homovalent [V^IV₁₈O₄₂]¹²⁻ POV with 18 fully localised valence electrons and its structurally identical mixed-valent derivative [V^IV₁₀V^V₈O₄₂]⁴⁻ with 10 partially delocalised electrons explored the role of electron transfer processes and magnetic exchange interactions in these structures.⁷⁹ The preparation of the first water-soluble salt-inclusion solid [Cs₁₁Na₃(V₁₅O₃₆)Cl₆] containing the mixed-valent [V^IV₁₁V^V₄O₃₆Cl]⁹⁻ building block is an interesting result in view of the development of “quantum magnetic solids within extended systems”.^80,81 Notably, the chemical reactivity and acid-dissociation constants of fully and partly reduced POVs can in principle be influenced by the extent of charge delocalisation.

1.6 Catalysis with POVs

The catalytic activity of the POV-based compounds was demonstrated by a number of authors. Gao and Hua reported that K₇[Ni^IVV₁₃O₃₈]·16H₂O containing the fully-oxidised [V^V₁₃O₃₈]¹¹⁻ building block catalyses oxidative mineralisation of p-nitrophenol and p-chlorophenol into CO₂, NO₃⁻, and Cl⁻ using 30% aqueous H₂O₂ as oxidant under mild conditions.^82,83 Khan and coworkers investigated the selective catalytic reduction of nitrogen oxides into N₂ by propylene as the reductant using the mixed-valent heterometallic compounds Li₆[M₃(H₂O)₁₂V₁₈O₄₂(YO₄)]·24H₂O (M = Mn^II, Ni^II; Y = V, S) and [M₃(H₂O)₁₂V₁₈O₄₂(YO₄)]·24H₂O (M = Fe^II, Co^II; Y = V, S) as precursors.⁸⁴ The catalytic oxidative dehydrogenation of propane by the nanostructured compounds [M₃(H₂O)₁₂V₁₈O₄₂(YO₄)]·24H₂O (M = Fe, Co, Mn; Y = V, S) has also been studied.⁸⁵ Wu et al. showed that the compound (NH₄)₂[{Mn(salen)(H₂O)}₆V₆O₁₈](NO₃)₂·30H₂O (salen²⁻ = N,N′-(ethylene)bis(salicylideneiminate)), the structure of which comprises a cyclic POV anion decorated with six Mn^III–salen Schiff-base complex groups, acts as photocatalyst for organic dye degradation.⁸⁶ Chakrabarty and Banerjee studied the decomposition of H₂O₂ into O₂ and H₂O, which is catalysed by the [Mn^IVV^V₁₃O₃₈]⁷⁻ polyoxoanion in aqueous acetate buffer.⁸⁷ The compound (NH₄){[Zn₄(dach)₇(H₂O)₃][V₃V₁₈P₆O₆₀(dach)₃]}·19H₂O (dach = (±)-trans-1,2-cyclohexanediamine = C₆H₁₄N₂) containing the nanoscale, highly-reduced [V^III₃V^IV₁₈P₆O₆₀(dach)₃]⁹⁻ building block was shown to selectively catalyse styrene oxidation in the presence of H₂O₂.⁴⁴ (N_nBu₄)₄[V₁₆O₃₈(Br)] containing the mixed-valent [Br@V^IV₇V^V₉O₃₈]⁴⁻ building block exhibited catalytic activity in the oxidative bromination reactions of aromatic substrates under aerobic conditions.⁸⁸ The mixed-valent (N_nBu₄)[V^IV₅V^V₁O₇(OMe)₁₂] compound was demonstrated to catalyse photoinduced water oxidation.⁸⁹

1.7 Scope of the review

Embedding heteroelements into POV structures⁹⁰ allows us to alter the electronic and magnetic properties, as well as the electrical conductivities⁹¹ of the resultant heteropolyoxovanadate (heteroPOV) spin structures (reminiscent of “doping” semiconducting materials) through the interplay between stoichiometries, heavy atom effects and redox potentials as well as steric and electronic environment around the terminal V=O groups. Herein, we place emphasis on the structural variety of silicato-, germanato-, arsenato-, and antimonato-derivatised polyoxovanadates and their hybrid compounds with middle and late transition metals and discuss interesting molecular features and application perspectives of these heteroPOVs. Great interest in the polynuclear molecular vanadium oxides decorated with the heavier group 14 and 15 elements (E) is generated by the specific synthetic, structural, host–guest and vast redox chemistry and extraordinary magnetic characteristics of these compounds, involving e.g. geometrical spin-frustration.⁹² Remarkably, the structurally, electronically and spin density-modified heteroPOVs introduce different functionalities linked to the main-group metalloid atoms (E = Si, Ge, As, Sb) that may act as bonding mediators. The steric requirements around terminal V=O groups in the heteroPOVs differ between the building blocks comprising E = Si^IV, Ge^IV and those with E = As^III, Sb^III due to the presence of lone electron pairs at the latter elements. The fully-oxidised (V^V), mixed-valent (V^V/V^IV and V^IV/V^III), “fully-reduced” (V^IV), and “highly-reduced” (V^III) heteroPOVs with various point-group symmetries and different isomeric structures of α and β types show an astonishing propensity for organic and transition metal/lanthanoid functionalization. This grants access to multifunctional inorganic–organic supramolecular materials⁹³ and the closely-related chemistry of metal–organic frameworks (MOFs)⁹⁴ and thus offers potential for applications of heteroPOVs in catalysis, surface science, and information technology.

2. Group 14 (Si, Ge) element-functionalised POVs

2.1 Preview

The chemistry of heteroPOVs incorporating the most common {E₂O₇} groups and more rare {E₂O₅S₂} ones with E = Si or Ge (SiPOVs and GePOVs) is still underdeveloped. In their molecular structures, the E^IV atoms usually favour four-fold coordination, thus yielding tetrahedral {EO₄} geometries. However, an example of a heteroPOV with a Ge^IV centre adopting a six-fold {GeO₆} coordination was also described. Interestingly, the Si–O and Ge–O bond lengths (d_E–O = 1.56–1.79 Å) are comparable to the bond lengths of terminal V=O groups (d_V=O = 1.56–1.64 Å), but shorter when compared to the single V–O bonds (d_V–O = 1.88–2.28 Å). The {E₂O₇}- or {E₂O₅S₂}-decorated POV structures are typically viewed as being formally derived from the [V^IV₁₈O₄₂]¹²⁻ archetype. The family of polyoxovanadatosilicates (SiPOVs) is represented by the assemblies with nuclearity V₁₅, V₁₇ and V₁₈. The polyoxovanadatogermanates (GePOVs) display a series of assemblies classified as V₆, V₉, V₁₂, V₁₄, V₁₅ and V₁₆. The SiPOVs and GePOVs usually encapsulate discrete water molecules or halide ions and can exhibit negative charges up to 12−. The SiPOVs (Table 1) and GePOVs (Table 2) are usually synthesised by hydrothermal or solvothermal reactions using V₂O₅, VOSO₄ and NH₄VO₃ as precursors.

Table 1 Selected details of synthesis and characterisation of SiPOV-based compounds

Formula	Colour	Characteristics of crystal structure^a	Reactants	Reaction conditions	Yield	Characterised via	Ref.
a Dimensionality resulting from hydrogen bonding networks is not considered.
SiPOV-based compounds
V15
(H2pdn)3(Hpdn)[V15Si6O42(OH)6(Cl)]·nH2O (n = 7–10)	Brown	Close-packed layer aggregate	V2O5, H2O, pdn, tetraethyl orthosilicate (teos), HCl	180 °C, 6 d	72% based on V2O5	EA, IR, TGA, EDXS, XPS, powder X-ray, single-crystal XRD, magnetometry	98
(H2pdn)(Hpdn)2[H6V15Si6O48][Co(pdn)2(H2O)]·9H2O	Red brown	1D infinite chain	V2O5, pdn, H2O, teos, EtOH, Co(OAc)2·4H2O, HCl	180 °C, 6 d	35% based on V	EA, IR, XPS, single-crystal XRD, magnetometry	96
V17
(H1.5pdn)6[{H2V15Si6O48(Cl)}(VO2)2]·4H2O	Brown	1D zig-zag chain	V2O5, pdn, H2O, teos, EtOH, HOAc, HCl	180 °C, 6 d	83% based on V	EA, IR, XPS, single-crystal XRD, magnetometry	96
V17–18
Cs10.5[(V16O40)(V1.5Si4.5O10)]·3.5H2O	Dark brown	1D extended chain	VOSO4·3H2O, SiO2, H2O, CsOH	240 °C, 3 d		EMP, IR, single-crystal XRD	95
V18
[H4V18O46(SiO)8(dab)4(H2O)]·4H2O	Dark green	Cross-linked 3D network	V2O5, SiO2, dab, H2O	180 °C, 5 d	61% based on SiO2	EA, IR, powder XRD, single-crystal XRD	97

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Table 2 Selected details of synthesis and characterisation of GePOV-based compounds

Formula	Colour	Characteristics of crystal structure^a	Reactants	Reaction conditions	Yield	Characterised via	Ref.
a Dimensionality resulting from hydrogen bonding networks is not considered.
V6
[V6Ge5O21(heda)6]·3H2O	Pale purple	Layer-like arrangement	NH4VO3, GeO2, Hheda, H2O	160 °C, 3 d	65% based on NH4VO3	EA, IR, EPR, TGA, powder XRD, single-crystal XRD, optical diffuse reflection	109
V9
(NH4)2[H2V9Ge6O26(L)6]·0.65H2O	Blue		NH4VO3, GeO2, H2O, HF, ethylene glycol (H2L)	170 °C, 3 d	58% based on Ge	EA, IR, TGA, EPR, single-crystal XRD	103
V12
K5[H8V12Ge8O48(SO4)]·10H2O	Brown	Channels	VOSO4, GeO2, KOH	180 °C, 4 d		IR, single-crystal XRD	100
[Cd(en)2]2[Cd2(en)2V12O40(GeOH)8(H2O)]·6H2O	Black	1D sinusoidal chain	NH4VO3, GeO2, CdCl2·2.5H2O, H3BO3, en, H2O	170 °C, 5 d	80% based on GeO2	EA, IR, TGA, EDXS, powder XRD, single-crystal XRD, magnetometry	110
V14
K2Na6[V14GeO40]·10H2O	Blue black		NaVO3, GeBr2, NaOAc, KCl	50 °C (1 h) → r.t.	64%	EA, IR, UV-vis, ^51V-NMR, XPS, electrochemistry, EPR, single-crystal XRD, magnetometry	104
(H2ppz)4(Hppz)4[V14Ge8O50(H2O)]	Olive brown		VOSO4, GeO2, ppz, H2O	170 °C, 4 d	45% based on V	EA, IR, TGA, single-crystal XRD	100
(H2ppz)4(Hppz)4[V14Ge8O50(H2O)]	Black		NH4VO3, GeO2, Cu(NO3)2·3H2O, aep, H2O	>150 °C, 9 d		EA, single-crystal XRD	101
(H3aep)4[V14Ge8O50]·2(aep)·13H2O	Dark brown	Layer-like arrangement	NH4VO3, GeO2, Cu(NO3)2·3H2O, aep, H2O	<150 °C, 9 d	73% based on GeO2	EA, IR, DTA-TG, single-crystal XRD, magnetometry	101
(H2dab)4[V14O44(GeOH)8]·6H2O	Dark green		V2O5, GeO2, dab, H2O	170 °C, 5 d	69% based on GeO2	EA, IR, single-crystal XRD	97
(H3dien)4[V14Ge8O42S8]·5H2O	Black	Layer-like arrangement	NH4VO3, Ge, S, dien, H2O	160 °C, 7 d	60% based on Ge	EA, DTA-TG-MS, powder X-ray, single-crystal XRD	99
(H3aep)4[V14Ge8O42S8]	Black	Layer-like arrangement	NH4VO3, Ge, S, aep, H2O	170 °C, 7 d	60% based on Ge	EA, DTA-TG-MS, powder X-ray, single-crystal XRD, magnetometry	99
[{Cd(en)}4V10Ge8O46(H2O)[V(H2O)2]4(GeO2)4]·8H2O	Red	3D framework	NH4VO3, GeO2, CdCl2·2.5H2O, H3BO3, en, H2O	170 °C, 4 d, pH = 9.7–10	36% based on GeO2	EA, IR, EDXS, XPS, TGA, powder XRD, single-crystal XRD, magnetometry	112
[{Cd(enMe)}4V10Ge8O46(H2O)[V(H2O)2]4(GeO2)4]·8H2O	Red	3D framework	NH4VO3, GeO2, CdCl2·2.5H2O, H2C2O4·2H2O, enMe, H2O	170 °C, 4 d, pH = 9.7–10	52% based on GeO2	EA, IR, EDXS, XPS, TGA, powder XRD, single-crystal XRD, magnetometry	112
V15
(H2tren)2(H3tren)[V15Ge6O42(OH)6(Cl)]·2H2O	Brown	1D channel	NH4VO3, GeO2, H2O, tren, HCl	170 °C, 5 d	53% based on NH4VO3	EA, IR, TGA, EDXS, XPS, powder X-ray, single-crystal XRD	98
[Zn2(enMe)3][Zn(enMe)]2[V15Ge6O48(H2O)][Zn(enMe)2(H2O)]2·3H2O	Brown	2D layered network	V2O5, GeO2, ZnSO4, enMe, H2O	170 °C, 3 d	78% based on GeO2	EA, IR, TGA, EDXS, powder XRD, single-crystal XRD, magnetometry	110
[{Ni(tren)}4(H2tren)2V15Ge6O48(H2O)]·2H2O	Dark brown	2D layer	NH4VO3, GeO2, Ni(NO3)2·6H2O, tren, H2O	130 °C, 7 d	43% based on V	EA, IR, TGA, powder XRD, single-crystal XRD, magnetometry	114
[Co(tren)(H2tren)]2[{Co(tren)}2V15Ge6O42S6(H2O)]·9H2O	Dark red brown	Channels	NH4VO3, Ge, Co, S, tren, H2O	140 °C, 7 d	60% based on V	EA, IR, TG-DTA-MS, single-crystal XRD, magnetometry	102
[{Mn(tren)(H2tren)}{Mn(tren)}4V15Ge6O48(H2O)0.5]·(tren)·2H2O	Dark brown	Helical 1D strand	NH4VO3, GeO2, MnCl2·2H2O, tren, H2O	130 °C, 7 d	41% based on V	EA, IR, TGA, powder XRD, single-crystal XRD, magnetometry	114
V16
Cs8[V16Ge4O42(OH)4]·4.7H2O	Brown	Layer-like arrangement	VOSO4, GeO2, H2O, CsOH	170 °C, 3 d	30% based on V	IR, EMP, TGA, single-crystal XRD	100
[Cd3(dien)2(Hdien)2(H2O)2][V16Ge4O42(OH)4(H2O)]·2H2O	Black	3D open framework	V2O5, GeO2, CdCl2·2.5 H2O, dien, ethylene glycol, H2O	170 °C, 3 d	90% based on GeO2	EA, IR, TGA, EDXS, powder XRD, single-crystal XRD, magnetometry	110
[Co(enMe)2]3[Co2(enMe)4][V16Ge4O44(OH)2(H2O)]·5H2O	Brown	3D open framework	V2O5, GeO2, Co(OAc)2·4H2O, H2O, enMe	170 °C, 5 d	86% based on V	EA, IR, XPS, TGA, powder XRD, single-crystal XRD, magnetometry	113
[Co2(en)3][Co(en)2]2[Co(en)2(H2O)][V16Ge4O44(OH)2(H2O)]·10.5H2O	Brown	3D open framework	V2O5, GeO2, Co(OAc)2·4H2O, H2O, en	170 °C, 5 d	63% based on V	EA, IR, XPS, TGA, powder XRD, single-crystal XRD, magnetometry	113

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2.2 Polyoxovanadatosilicates (SiPOVs)

Only a few studies describing the isolation of SiPOVs have been performed so far. These SiPOVs constitute a series with the general formula {V_18−zSi_2zO_42+2z} and are characterised by antiferromagnetic coupling between the spin-1/2 vanadyl {VO}²⁺ moieties.

2.2.1 SiPOVs with one-dimensional (1D) structures.
Vanadyl(V)-extended SiPOVs. The first report about the spherical POV chemically modified with Si^IV dates back to 2001 when Jacobson and coworkers described the [V^IV₁₆Si₄O₄₆]¹²⁻ polyoxoanion (Fig. 6a) that is a component of the compound Cs_10.5[(V₁₆O₄₀)(V_1.5Si_4.5O₁₀)]·3.5H₂O synthesised under hydrothermal conditions (Table 1).⁹⁵ The crystal structure of this hybrid compound shows a 1D linear chain of the “fully-reduced” [V^IV₁₆O₄₀]¹⁶⁻ POVs interlinked by vanadosilicate six-membered rings with the composiiton {V^V_1.5Si_4.5O₁₀}^5.5+ (Fig. 7a). [V₁₆O₄₀]¹⁶⁻ is hypothetically derived from the α-Keggin {V₁₂O₃₆} archetype or, accordingly, the {V₁₈O₄₂} archetype (Fig. 4). Replacing two vanadyl {VO}²⁺ groups on two opposite sides of the [V₁₈O₄₂]¹²⁻ isopolyoxoanion by two silicate {Si₂O₃} subunits, the spherical [V^IV₁₆Si₄O₄₆]¹²⁻ dodecaanion is formed. A water molecule is located in the void of the polyoxoanion.

Fig. 6 Polyhedral representations of the “fully-reduced” SiPOVs [V^IV₁₆Si₄O₄₆]¹²⁻ (a), α-[H₄V^IV₁₄O₄₄(SiO)₈]¹²⁻ (b), and [V^IV₁₅Si₆O₄₂(OH)₆]⁶⁻ (c). Encapsulated H₂O (a and b), Cl⁻ (c), and hydrogen atoms are not shown. Colour code: Si, violet; O, red spheres; V^IVO_x, sky-blue polyhedra.

Fig. 7 Representation of the extended structures of SiPOVs covalently linked by inorganic groups.

Later on, two extended chains based on the “fully-reduced” {V^IV₁₅Si₆O₄₈} building blocks⁹⁶ were obtained from reactions under hydrothermal conditions where the pdn molecules (pdn = 1,3-propanediamine) acted as reductant for V^V (Table 1). These two inorganic–organic hybrid compounds can be assigned to the classes of vanadyl-extended and transition metal complex (TMC)-supported SiPOVs.

1D zig-zag chains of the SiPOVs doubly bridged by the fully-oxidised {V^VO₂}⁺ moieties through (Si–)O–V–O(–Si) bonds with d_V–O = 1.79 Å and 1.81 Å (Fig. 7b) were observed in the crystal of the compound (H_1.5pdn)₆[{H₂V₁₅Si₆O₄₈(Cl)}(VO₂)₂]·4H₂O.⁹⁶ The partially protonated pdn molecules compensate the high negative charge of the mixed-valent [{Cl@H₂V^IV₁₅Si₆O₄₈}(V^VO₂)₂]⁹⁻ assembly and reside in the voids between the zig-zag chains, along with H₂O solvate molecules.

TMC-supported SiPOV. One-dimensional infinite chains of [H₆V^IV₁₅Si₆O₄₈]⁶⁻ polyoxoanions that are singly bridged by [Co(pdn)₂(H₂O)]²⁺ fragments through (V–)O–Co–O(–Si,V) linkages (Fig. 7c) were found in the crystal structure of (H₂pdn)(Hpdn)₂[H₆V₁₅Si₆O₄₈][Co(pdn)₂(H₂O)]·9H₂O.⁹⁶

2.2.2 SiPOV with three-dimensional (3D) structures. In 2003, Clearfield and coworkers described a 3D cross-linked structure consisting of the [H₄V₁₄O₄₄(SiO)₈]¹²⁻ building blocks bridged by {(VO)O₂N₂} pyramids that are covalently interconnected by bidentate dab ligands (dab = 1,4-diaminobutane) through the V^IV ions (Fig. 7d).⁹⁷ The “fully-reduced”, α-[H₄V^IV₁₄O₄₄(SiO)₈]¹²⁻ polyoxoanion (Fig. 6b) with idealised D_2d symmetry is a component of the hydrothermally prepared compound [H₄V₁₈O₄₆(SiO)₈(dab)₄(H₂O)]·4H₂O (Table 1). This SiPOV shows a modified [V₁₈O₄₂]¹²⁻ structure where four {VO₅} square pyramids are substituted by four handle-like {Si₂O₇} silicate units. Each of these units is made up of two corner-sharing [SiO₄]⁴⁻ tetrahedra. The POV shell is constructed of an octagonal ring sharing two crescent-type {VO₅} chains fused to each side of the former and exhibits non-bonding V⋯V distances of 2.83–3.09 Å.

2.2.3 Discrete SiPOV within hydrogen bonding networks. The formal replacement of three {VO₅} square pyramids in the {V₁₈O₄₂} archetypal structure with three partially protonated handle-like {Si₂O₅(OH)₂} groups results in the D₃-symmetrical [V^IV₁₅Si₆O₄₂(OH)₆]⁶⁻ polyoxoanion (Fig. 6c) found as the central building block in the hydrothermally prepared compound (H₂pdn)₃(Hpdn)[V₁₅Si₆O₄₂(OH)₆(Cl)]·nH₂O (n = 7–10) with a solid-state close-packed layer aggregate structure (Table 1).⁹⁸ Here, the discrete SiPOV [Cl@V^IV₁₅Si₆O₄₂(OH)₆]⁷⁻ exhibits closest intracluster V⋯V distances of ca. 3.0 Å and hosts a Cl⁻ anion. In the crystal structure, extensive N–H⋯O hydrogen bonding interactions were observed, which interlink the pdn groups and the “fully-reduced” SiPOVs. Notably, these N–H⋯O hydrogen bonds are crucial in the formation of the supramolecular network structure.

2.2.4 Corollary for SiPOVs. All above-mentioned SiPOVs can be regarded as derivatives of the [V₁₈O₄₂]¹²⁻ archetype (Fig. 3), as a specific number (n = 2–4) of its [VO₅]⁶⁻ square pyramids are formally replaced with an equal number of handle-like silicate [Si₂O₇]⁶⁻ iso-anionic or partly protonated [Si₂O₅(OH)₂]⁴⁻ groups. All discussed “fully-reduced” SiPOV building blocks were shown to form diverse networks (Fig. 7) in the solid state principally due to the following points: (i) the chemical composition and structures of the inorganic linkers and the nature of the metal ions, which played a crucial role in the self-assembly behaviour of the spherical SiPOVs; (ii) the presence of the silicate groups in SiPOVs which are involved in the covalent bonding to the linking groups. As it was noted, “the interconnection of polyoxometal building blocks like polyoxovanadates by means of covalent bonds could create new porous materials with ultralow framework densities and high porosity”.⁹⁹ In view of the structural arrangement and organisation of the extended solid-state structures discussed above, it appears highly desirable to explore the role of the SiPOV-based networks as microporous molecular materials resembling zeolites or molecular sieves, e.g., for catalytic performance, gas storage, and ion exchange.

2.3 Polyoxovanadatogermanates (GePOVs)

2.3.1 Discrete GePOVs with {Ge₂O₇} and partially protonated germanate groups. The chemical and structural characterisation of the first GePOVs dates back to 2003, when Jacobson and coworkers described the compounds Cs₈[V₁₆Ge₄O₄₂(OH)₄]·4.7H₂O, (H₂ppz)₄(Hppz)₄[V₁₄Ge₈O₅₀(H₂O)] (ppz = piperazine, C₄H₁₀N₂) and K₅[H₈V₁₂Ge₈O₄₈(SO₄)]·10H₂O, which were synthesised under hydrothermal conditions (Table 2).¹⁰⁰ These compounds are based, respectively, on the “fully-reduced” [V^IV₁₆Ge₄O₄₂(OH)₄]⁸⁻ (Fig. 8a) and α-[V^IV₁₄Ge₈O₅₀]¹²⁻ polyoxoanions (Fig. 8b) and the mixed-valent [H₈V₁₂Ge₈O₄₈(SO₄)]⁵⁻ polyoxoanion that encloses a sulfate template anion in the centre of a spherical GePOV shell. The structures of [V₁₆Ge₄O₄₂(OH)₄]⁸⁻ and [H₈V₁₂Ge₈O₄₈(SO₄)]⁵⁻ are derived from the “fully-reduced” [V^IV₁₈O₄₂]¹²⁻ shell (i.e., Keggin-type [V^IV₁₂O₃₆]²⁴⁻ + 6{V^IVO}²⁺ caps) on substitution of {VO}²⁺ caps in the latter by a certain number of {Ge^IV₂O(OH)₂}⁴⁺ dimeric units (two and four, respectively). To get to the final composition of [H₈V₁₂Ge₈O₄₈(SO₄)]⁵⁻, two further {VO}²⁺ caps in [V^IV₁₈O₄₂]¹²⁻ are formally omitted. Similarly, replacing four {VO}²⁺ caps in the archetypal structure by four {Ge^IV₂O₃}²⁺ groups affords the polyoxoanion cage of the composition [V₁₄Ge₈O₅₀]¹²⁻. It is interesting to note that the [V₁₄Ge₈O₅₀]¹²⁻ and [H₈V₁₂Ge₈O₄₈(SO₄)]⁵⁻ constituents are the structural analogues to the family of the polyoxovanadoarsenates with the general formula [V_18−zAs_2zO₄₂(Y)]^m− (Y = SO₃, SO₄, Cl; z = 3, 4), which are discussed below. In contrast to the structures of [V₁₄Ge₈O₅₀]¹²⁻ and [H₈V₁₂Ge₈O₄₈(SO₄)]⁵⁻ displaying rhombicuboctahedral topology, the molecular structure of [V₁₆Ge₄O₄₂(OH)₄]⁸⁻ reveals an elongated square gyrobicupola topology.

Fig. 8 Polyhedral representations of the “fully-reduced” GePOVs [V^IV₁₆Ge₄O₄₂(OH)₄]⁸⁻ (a), α-[V^IV₁₄Ge₈O₅₀]¹²⁻ (b), α-[V^IV₁₄Ge₈O₄₂S₈]¹²⁻ (c), and [H₂V^IV₉Ge₆O₂₆(L)₆]²⁻ (d). Encapsulated water molecules and hydrogen atoms are not shown. Colour code: C, grey; Ge, green; O, red spheres; V^IVO_x, sky-blue polyhedra.

The compound (H₂dab)₄[V₁₄O₄₄(GeOH)₈]·6H₂O was obtained by Clearfield and coworkers from the hydrothermal reaction between V₂O₅, GeO₂ and 1,4-diaminobutane (dab) in water (Table 2).⁹⁷ The “fully-reduced”, protonated α-[V^IV₁₄O₄₄(GeOH)₈]⁸⁻ polyoxoanion is isostructural to the aforementioned α-[H₄V^IV₁₄O₄₄(SiO)₈]¹²⁻ polyoxoanion (Fig. 6b). In contrast to [H₄V₁₈O₄₆(SiO)₈(dab)₄(H₂O)]·4H₂O, the Ge(IV)-containing compound contains doubly protonated H₂dab²⁺ countercations.

A GePOV structural analogue to the [Cl@V^IV₁₅Si₆O₄₂(OH)₆]⁷⁻ polyoxoanion (Fig. 6c) was also reported. The main structural difference between the two compounds containing the [Cl@V₁₅E₆O₄₂(OH)₆]⁷⁻ component is the packing of the structural building blocks. The compound with E = Si shows a close-packed layer structure, whereas the one with E = Ge exhibits a 1D chain structure.⁹⁸ The crystal structure of the GePOV-based compound (Table 2) with the formula (H₂tren)₂(H₃tren)[V₁₅Ge₆O₄₂(OH)₆(Cl)]·2H₂O is furthermore characterised by the N–H⋯O hydrogen bonds formed between the oxygen atoms of the “fully-reduced” polyoxoanions and the organic tris(2-aminoethyl)amine (tren) countercations.

The compound (H₃aep)₄[V₁₄Ge₈O₅₀]·2(aep)·13H₂O (aep = 1-(2-aminoethyl)piperazine = C₆H₁₅N₃) and the already described (H₂ppz)₄(Hppz)₄[V₁₄Ge₈O₅₀(H₂O)] compound¹⁰⁰ with the “fully-reduced” α-[V^IV₁₄Ge₈O₅₀]¹²⁻ components were formed in solvothermal reactions involving Cu(NO₃)₂·3H₂O below 150 °C and above 150 °C, respectively (Table 2).¹⁰¹ Since Cu²⁺ was reduced to metallic Cu during the syntheses, the linking of the GePOV building blocks through Cu²⁺-centred complexes into extended structures was unsuccessful. Interestingly, (H₃aep)₄[V₁₄Ge₈O₅₀]·2(aep)·13H₂O could also be produced without utilising the copper salt Cu(NO₃)₂·3H₂O. However, the presence of copper in the reaction mixture turned out to be crucial for the crystallisation of (H₂ppz)₄(Hppz)₄[V₁₄Ge₈O₅₀(H₂O)]. The high negative charge of [V^IV₁₄Ge₈O₅₀]¹²⁻ (Fig. 8b) is compensated by the protonated amine molecules. Weak (>2 Å) and strong (<2 Å) N–H⋯O hydrogen bonds between the organic ammonium countercations and the exposed oxygen atoms of the GePOVs in the crystals of both compounds result in 3D H-bonded networks. The preliminary analysis of the magnetic properties of (H₃aep)₄[V₁₄Ge₈O₅₀]·2(aep)·13H₂O indicated strong antiferromagnetic exchange interactions between the spin-1/2 vanadyl {VO}²⁺ moieties through the near-linear V–μ-O–V and bent V(–μ-O–)₂V bridges.

2.3.2 GePOVs with {Ge₂O₅S₂} groups.
Discrete GePOV. The structural diversity of the GePOVs was explored towards their sulfur-functionalised analogues. Two polyoxothio compounds (H₃dien)₄[V₁₄Ge₈O₄₂S₈]·5H₂O (dien = diethylenetriamine) and (H₃aep)₄[V₁₄Ge₈O₄₂S₈] consisting of the “fully-reduced” [V^IV₁₄Ge₈O₄₂S₈]¹²⁻ GePOVs and protonated amines as countercations were synthesised hydrothermally (Table 2).⁹⁹ The α-[V^IV₁₄Ge₈O₄₂S₈]¹²⁻ polyoxoanion (Fig. 8c) with a diameter of ca. 7.4 Å shows structural, but not functional similarities to α-[V^IV₁₄Ge₈O₅₀]¹²⁻ (Fig. 8b). The {V₁₄}-nuclearity building block of the thio-modified GePOV is composed of fourteen condensed {VO₅} square pyramids and eight tetrahedral {GeO₃S} units. Thus, eight terminal oxygens sites of four {Ge₂O₇} groups were formally exchanged for eight sulfur atoms to form four {Ge₂O₅S₂} groups.
Cobalt–GePOV hybrid. Another example of thio-functionalised GePOV, [H₂O@V^IV₁₅Ge₆O₄₂S₆]¹²⁻, composed of six slightly distorted {GeO₃S} tetrahedra (d_Ge–S = 2.09–2.14 Å), fifteen {VO₅} square pyramids and an encapsulated central H₂O molecule was obtained under hydrothermal conditions using metavanadate as precursor and elemental Ge, Co and S (Table 2).¹⁰² The spherical [V^IV₁₅Ge₆O₄₂S₆]¹²⁻ structure is constructed of three {V₇Ge₂O₂₄S₂} rings which are perpendicular to each other. This polyoxoanion is formally derived from the {V₁₈O₄₂} archetype by substituting three {VO₅} pyramidal units of [V₁₈O₄₂]¹²⁻ by three handle-like {Ge₂O₅S₂} groups (for comparison, see [V^IV₁₅Si₆O₄₂(OH)₆]⁶⁻ in Fig. 6c). This “fully-reduced” GePOV is further expanded by two {Co(tren)}²⁺ complexes through Co–S bonds, thus featuring the Co–S=Ge–O–V connections. These two trigonal-bipyramidal Co(II) complexes are attached to the terminal sulfide sites (S²⁻) and, along with two [Co(tren)(H₂tren)]⁴⁺ countercations, reduce the high negative charge of the polyoxoanion to give the compound [Co(tren)(H₂tren)]₂[{Co(tren)}₂V₁₅Ge₆O₄₂S₆(H₂O)]·9H₂O where tren molecules act as mono- and tetradentate ligands. The presence of the N–H⋯O hydrogen-bonding pattern in the crystal structure of this compound results in two differently oriented channels. Similar to other {V^IV₁₅E₆}-type structures (see also Section 3.2.5), the magnetic structure of the GePOV building block can be described as being composed of a geometrically frustrated, antiferromagnetically coupled equilateral {V₃} triangle that is sandwiched between the two {V₆} hexagons characterised by strong antiferromagnetic nearest-neighbor coupling.

2.3.3 Discrete GePOV with alkoxo ligands. Wang and colleagues synthesised and characterised the first polyoxoalkoxovanadatogermanate (hereafter referred to as alkoxoGePOV). The compound (NH₄)₂[H₂V₉Ge₆O₂₆(L)₆]·0.65H₂O (H₂L = HOCH₂CH₂OH, ethylene glycol) was obtained from a solvothermal reaction¹⁰³ where ethylene glycol functioned as a reducing agent for NH₄VO₃ that was used as the vanadium source (Table 2). The “fully-reduced” [H₂V^IV₉Ge₆O₂₆(L)₆]²⁻ species shows the cage-like structure consisting of six edge- and corner-sharing {VO₆} octahedra, three edge-sharing {VO₅} square pyramids, and six corner-sharing {GeO₄} tetrahedra (Fig. 8d). The six ethylene glycol oxygen-donor groups act as bridging alkoxide ligands. This alkoxoGePOV features the spherical {V₉Ge₆O₃₈} shell with approximate D_3h symmetry and experiences non-bonding V⋯V distances of ca. 3.07 Å. EPR studies performed on (NH₄)₂[H₂V₉Ge₆O₂₆(L)₆]·0.65H₂O at room temperature revealed the presence of a partial delocalisation of nine unpaired 3d¹ (V^IV) electrons in the alkoxoGePOV building block.

2.3.4 Discrete GePOVs with two-electron reduced structures.
GePOV with a {GeO₆} group. A mixed-valent [V^IV₂V^V₁₂GeO₄₀]⁸⁻ polyoxoanion with an octahedrally coordinated Ge^IV heteroatom occupying a central geometrical position within the GePOV structure was isolated as the compound K₂Na₆[V₁₄GeO₄₀]·10H₂O.¹⁰⁴ The compound was obtained by reaction of GeBr₂ and NaVO₃ in aqueous acidic solution where GeBr₂ acted as a reducing agent for the V^V source (Table 2). This D_4h-symmetric GePOV consists of a central {Ge^IVO₆} octahedron and fourteen {VO₆} octahedra, all of which are edge-shared (Fig. 9). Interestingly, this polyoxoanion is the structural analogue to the [V^IV₂V^V₁₂Al^IIIO₄₀]^9− 105 and [V^IV₂V^V₁₂As^VO₄₀]^7− 106 polyoxoanions; the Al-containing heteroPOV is found in the mineral sherwoodite, Ca_4.5[V₁₄AlO₄₀]. It is worth mentioning that the [V^IV₂V^V₁₂GeO₄₀]⁸⁻ polyoxoanion extends the family of heteroPOVs where the main-group heteroatoms (E = Al^III, Ge^IV, As^V) reside within the {V₁₄O₄₀} cores. The dicapped cuboctahedral topology was ascribed to the {V^IV₂V^V₁₂EO₄₀} building blocks. Magnetic studies of K₂Na₆[V₁₄GeO₄₀]·10H₂O showed that two apical spin-1/2 V^IV ions, which are separated from each other by 8.52 Å via one O–Ge–O bond, are weakly antiferromagnetically coupled. X-band and Q-band EPR measurements indicated very weak spin–spin exchange coupling between these V^IV ions. The magnetic properties of the mixed-valent [V₁₄GeO₄₀]⁸⁻ were theoretically studied by Suaud, Coronado and coworkers on the basis of model Hamiltonian calculations and wave function theory as well as an electric field approach.¹⁰⁷ In particular, it was shown that an external electric field can induce a reversible transition from a paramagnetic (triplet) to an antiferromagnetic (singlet) ground state configuration of [V₁₄GeO₄₀]⁸⁻, meaning that this polyoxoanion can in principle act as a molecular switch. For a discussion about the relevance of [V₁₄GeO₄₀]⁸⁻ to the field of molecular spintronics, see the review by Coronado and coworkers.⁷⁵ Moreover, the [V^IV₂V^V₁₂GeO₄₀]⁸⁻ polyoxoanion was reported to be a two-electron catalyst for the oxidation of the coenzyme NADH at pH = 8. Note that POMs usually act as one-electron catalysts.¹⁰⁸

Fig. 9 Structure of the mixed-valent [V^IV₂V^V₁₂GeO₄₀]⁸⁻ polyoxoanion, as present in K₂Na₆[V₁₄GeO₄₀]·10H₂O. Colour code: O, red; Ge, green; V^IVO_x, sky-blue polyhedra; V^V, light orange.

Neutral GePOV. Another two-electron reduced GePOV was reported by Bian, Dai and colleagues.¹⁰⁹ The mixed-valent compound [V^IV₂V^V₄Ge₅O₂₁(heda)₆]·3H₂O, in which the deprotonated amine ligands [Hheda = N-(2-hydroxyethyl)ethylenediamine] coordinate covalently to the GePOV shell in three different coordination modes (monodentate and chelating for V centres, and multi-chelating for Ge centres), was obtained by solvothermal synthesis using Hheda as a reducing agent for V^V (Table 2). The pale purple colour of this low-nuclearity compound was shown to be due to the absorption bands at 2.18 eV (568 nm) and 1.46 eV (856 nm) assigned to d–d transitions of the vanadium ions in quasi-octahedral coordination. The central fragment of [V₆Ge₅O₂₁(heda)₆]·3H₂O is a C₂ symmetrical GePOV, which is not structurally related to the {V₁₈O₄₂} archetype like the above-mentioned “fully-reduced” alkoxoGePOV [H₂V^IV₉Ge₆O₂₆(L)₆]^2− 103 and the mixed-valent [V^IV₂V^V₁₂GeO₄₀]⁸⁻ polyoxoanion.¹⁰⁴ The structure of the neutral [V₆Ge₅O₂₁(heda)₆] compound is composed of two {VO₅} square pyramids, two {VO₆} octahedra and two {VO₅N} octahedra and involves three tetrahedral {GeO₄} and two octahedral {GeO₄N₂} moieties. According to the optical diffuse reflectance spectrum, [V₆Ge₅O₂₁(heda)₆]·3H₂O possesses an energy gap of 2.88 eV.

2.3.5 TMC-supported GePOVs.
Zinc– and cadmium–GePOV hybrids. Yang and coworkers succeeded in the hydrothermal synthesis of a series of the inorganic–organic hybrid materials [Cd(en)₂]₂[Cd₂(en)₂V₁₂O₄₀(GeOH)₈(H₂O)]·6H₂O (en = ethylenediamine = C₂H₈N₂), [Zn₂(enMe)₃][Zn(enMe)]₂[V₁₅Ge₆O₄₈(H₂O)][Zn(enMe)₂(H₂O)]₂·3H₂O (enMe = 1,2-diaminopropane), and [Cd₃(dien)₂(Hdien)₂(H₂O)₂][V₁₆Ge₄O₄₂(OH)₄(H₂O)]·2H₂O (Table 2).¹¹⁰ The adjustment of the pH value to the alkaline media (pH = 8.8–12.5) was crucial for isolation of these products. Their crystal structures show the “fully-reduced” polyoxoanions [Cd₂(en)₂V^IV₁₂O₄₀(GeOH)₈(H₂O)]⁴⁻ (Fig. 10), [H₂O@V^IV₁₅Ge₆O₄₈]¹²⁻ and [H₂O@V^IV₁₆Ge₄O₄₂(OH)₄]⁸⁻ (Fig. 8a) which are interconnected by transition metal cations [Cd(en)₂]²⁺, [Zn(enMe)]²⁺/[Zn₂(enMe)₃]⁴⁺ and [Cd₃(μ-dien)₂(Hdien)₂(H₂O)₂]⁸⁺, respectively. These bind to the GePOV building blocks through the O atoms of {VO₅} square pyramids or the apical O atoms of {GeO₄} tetrahedra, thus forming Cd–O_term–V and Zn–O_term–Ge connectivities. Interestingly, the [Cd₂(en)₂V^IV₁₂O₄₀(GeOH)₈]⁴⁻ polyoxoanion illustrated in Fig. 10 incorporates two [Cd(en)]²⁺ units in the dilacunary-type α-[V^IV₁₂O₄₀(GeOH)₈]⁸⁻ backbones. The 1D sinusoidal chains established for [Cd(en)₂]₂[Cd₂(en)₂V₁₂O₄₀(GeOH)₈(H₂O)]·6H₂O with d_V⋯V = 2.86–3.03 Å are additionally arranged through N–H⋯O and O–H⋯O hydrogen bonds to afford a 3D supramolecular framework with pseudocircular channels. The interchain distance was found to be ca. 10.65 Å. The {V₁₅}- and {V₁₆}-nuclearity GePOVs are the representatives of the class of polynuclear molecular mixed-metal oxides with the general formula [V_18−zGe_2zO₄₂(O,OH)_2z] (z = 2, 3) and show interesting structural relationships to other reported GePOVs and SiPOVs. Whereas [V^IV₁₅Ge₆O₄₈]¹²⁻ and [V^IV₁₅Si₆O₄₂(OH)₆]⁶⁻ (Fig. 6c) are isostructural analogues, the polyoxoanions [V^IV₁₆Ge₄O₄₂(OH)₄]⁸⁻ (Fig. 8a) and [V^IV₁₆Si₄O₄₆]¹²⁻ (Fig. 6a) are conformational isomers differing by the geometrical positions of the handle-like {E₂O₇} groups in the POV structure. While the [Cd₂(en)₂V^IV₁₂O₄₀(GeOH)₈(H₂O)]⁴⁻ and [V^IV₁₅Ge₆O₄₈]¹²⁻ polyoxoanions exhibit strong antiferromagnetic coupling, the magnetic behaviour of the [V^IV₁₆Ge₄O₄₂(OH)₄]⁸⁻ polyoxoanion was reported to exhibit intramolecular ferrimagnetic coupling, which is rare for such POMs.¹¹¹

Fig. 10 Schematic design of the TMC-supported, “fully-reduced” hybrid polyoxoanion [Cd₂(en)₂V^IV₁₂O₄₀(GeOH)₈]⁴⁻ (right). This tetraanion is formed when two {VO}²⁺ caps situated between the {Ge₂O₇} groups in α-[V^IV₁₄O₄₂(GeOH)₈]⁴⁻ (left) are replaced with two [Cd(en)]²⁺ fragments. Encapsulated water molecule and hydrogen atoms are not shown. Colour code: Ge, green; N, blue; O, red; V^IVO_x, sky-blue polyhedra; Cd, brown.

Tetra-Cd^II-substituted GePOVs were also reported. Two isomorphic, mixed-valent compounds with the general formula [{CdL}₄V₁₀Ge₈O₄₆(H₂O)[V(H₂O)₂]₄(GeO₂)₄]·8H₂O (L = en, enMe) were synthesised under hydrothermal conditions using the weak acids H₃BO₃ or H₂C₂O₄·2H₂O and the alkaline media at pH = 9.7–10 (Table 2).¹¹² Their crystal structures exhibit 3D 10-connected, inorganic–organic frameworks (point-symbol of 3¹²·4²⁸·5⁵ for the bct topology) which are constructed from the D_4h-symmetric, “fully-reduced” [{CdR}₄V^IV₁₀Ge₈O₄₆(H₂O)]¹²⁻ hybrid polyoxoanions covalently connected to each other via the planar, “highly-reduced” [V^III₄O₂(H₂O)₈]⁸⁺ units (d_V⋯V = 2.47 Å) and the bridging {GeO₄} tetrahedra (an unusual situation in POM chemistry, however observed for {SiO₄} units in Cs_10.5[(V₁₆O₄₀)(V_1.5Si_4.5O₁₀)]·3.5H₂O⁹⁵). The Cd²⁺ ions are incorporated into the backbones of the tetralacunary β-isomeric GePOV building block composed of ten {VO₅} square pyramids and thus display trigonal prismatic geometries (CdO₄N₂), similarly to the “fully-reduced” [Cd₂(en)₂V^IV₁₂O₄₀(GeOH)₈]⁴⁻ hybrid polyoxoanion (Fig. 10). An attempt to replace these Cd²⁺ ions coordinated in a circular {{CdR}₄Ge₈O₂₈}¹⁶⁻ constituent of the dodecaanion by Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺ or Zn²⁺ under applied reaction conditions (Table 2) has failed. This type of Cd^II-substituted GePOVs exhibit antiferromagnetic properties.

Cobalt–GePOV hybrids. Further two {V₁₆Ge₄}-type polyoxoanions now supported by bridging Co^II complexes were described by Xu, Hu and colleagues.¹¹³ The compounds [Co(enMe)₂]₃[Co₂(enMe)₄][V₁₆Ge₄O₄₄(OH)₂(H₂O)]·5H₂O and [Co₂(en)₃][Co(en)₂]₂[Co(en)₂(H₂O)][V₁₆Ge₄O₄₄(OH)₂(H₂O)]·10.5H₂O with extended 3D frameworks in their crystal structures were obtained hydrothermally in alkaline solutions at pH = 10.2–10.8 (Table 2). The structures of these compounds consist of the “fully-reduced” [H₂O@V^IV₁₆Ge₄O₄₄(OH)₂]¹⁰⁻ anions charge-balanced by Co²⁺ centred amine complexes and exhibit different structural topologies. Whereas [Co(enMe)₂]₃[Co₂(enMe)₄][V₁₆Ge₄O₄₄(OH)₂(H₂O)]·5H₂O adopts a NaCl-type network composed of different TMC linkers and [H₂O@V^IV₁₆Ge₄O₄₄(OH)₂]¹⁰⁻ polyoxoanions as 6-connected nodes, [Co₂(en)₃][Co(en)₂]₂[Co(en)₂(H₂O)][V₁₆Ge₄O₄₄(OH)₂(H₂O)]·10.5H₂O is characterised by a (4,6)-connected network [Schläfli symbol (4⁶·6⁷·8²)₂(4²·6⁴)], which was previously not known for heteroPOVs. Two types of relatively short interatomic distances d_V⋯V = 2.80–3.07 Å and d_Co⋯V = 3.38–3.60 Å were observed. According to the variable temperature susceptibility measurements, these two compounds feature ferrimagnetic characteristics.
Manganese– and nickel–GePOV hybrids. The solvothermally prepared [{Mn(tren)(H₂tren)}{Mn(tren)}₄V₁₅Ge₆O₄₈(H₂O)_0.5]·tren·2H₂O and [{Ni(tren)}₄(H₂tren)₂V₁₅Ge₆O₄₈(H₂O)]·2H₂O compounds are characterised by the {V₁₅Ge₆}-type polyoxoanions which could be expanded into the high-nuclearity GePOV networks.¹¹⁴ The structural analysis of these charge-neutral helical 1D strands and 2D layers revealed different coordination environments of unique, in situ-formed Mn^II and Ni^II complexes as well as different interconnection modes of the spherical, “fully-reduced” [V^IV₁₅Ge₆O₄₈]¹²⁻ polyoxoanions with encapsulated water guest molecules. Interestingly, the reaction conditions were very similar in each case except the nature and amount of transition metal starting materials (Table 2). The central {V₁₅Ge₆O₄₈(H₂O)_0.5} structural motif of [{Mn(tren)(H₂tren)}{Mn(tren)}₄V₁₅Ge₆O₄₈(H₂O)_0.5]·tren·2H₂O, which is stable up to 350 °C, is coordinated by five crystallographically independent Mn^II complexes via Mn–O(–V/Ge) bonds. The compound exhibits a three layer solid-state structure, which is composed of the following subfragments: (i) six corner- and edge-sharing square pyramidal {VO₅} units and a [MnO(H₂tren)(tren)]²⁺ complex; (ii) a {V₃Ge₆O₂₈} ring, [Mn(tren)]²⁺ and [Mn₂O₂(tren)₂]⁴⁺ complexes; (iii) six corner- and edge-sharing {VO₅} square pyramids. The crystal structure of this GePOV-based inorganic–organic hybrid displays [Mn(tren)]²⁺ and [Mn(tren)(H₂tren)]⁴⁺ moieties and a rare dinuclear [Mn₂O₂(tren)₂]⁴⁺ fragment that bridges two terminal O atoms of the handle-like {Ge₂O₇} groups from the adjacent GePOVs. In contrast to [Mn₂O₂(tren)₂]⁴⁺, the [Mn(tren)(H₂tren)]⁴⁺ complex is bound to the surface of the GePOV shell via a terminal O atom of a pyramidal {VO₅} unit. The crystal structure of [{Ni(tren)}₄(H₂tren)₂V₁₅Ge₆O₄₈(H₂O)]·2H₂O comprises binuclear [{Ni(tren)}(H₂tren){Ni(tren)}]⁶⁺ complexes, which coordinate only to the {VO₅} square pyramids of the polyoxoanion and bridge the neighbouring {V₁₅Ge₆O₄₈} building blocks through V=O–Ni bonds in a manner to allow for the formation of “rhombic windows” in which crystal water molecules reside. The magnetochemical analysis of these two compounds indicated that neither Mn^II nor Ni^II ions affect the spin structure of the [V₁₅Ge₆O₄₈]¹²⁻ polyoxoanion showing the typical “V₆–V₃–V₆” layer structure with V⋯V distances ranging from 2.84 to 3.10 Å. The compounds feature weak antiferromagnetic interactions between the spin-1/2 vanadyl {VO}²⁺ moieties of the {V₁₅Ge₆O₄₈} building block and the adjoined Mn^II or Ni^II complexes.

2.3.6 Corollary for GePOVs. “Fully-reduced” alkoxoGePOV and TMC-supported GePOVs as well as a number of structurally isolated “fully-reduced” and mixed-valent GePOVs have been published so far. These polyoxoanions include tetrahedral Ge^IV heteroatoms, with only one example of octahedral Ge^IV inclusion and are characterised by a range of different (approximate) symmetries, e.g. D_3h for [H₂V^IV₉Ge₆O₂₆(L)₆]²⁻, D_2d for [(en)₂Cd₂V^IV₁₂O₄₀(GeOH)₈]⁴⁻ and [V^IV₁₄O₄₄(GeOH)₈]⁸⁻, D_4h for [V^IV₂V^V₁₂GeO₄₀]⁸⁻ and [{CdR}₄V^IV₁₀Ge₈O₄₆(H₂O)]¹²⁻, S₄ for α-[V^IV₁₄Ge₈O₅₀]¹²⁻ and α-[V^IV₁₄Ge₈O₄₂S₈]¹²⁻, D₃ for [V^IV₁₅Ge₆O₄₈]¹²⁻ and C₂ for [V^IV₁₆Ge₄O₄₂(OH)₄]⁸⁻. Similarly to the SiPOV building blocks, the GePOVs were connected by TMC bridging groups to afford network structures of different dimensionality. Unlike the SiPOVs, the GePOVs were found to incorporate secondary TMCs in the backbones of the polyoxoanion shells (Fig. 10). Furthermore, unusual handle-like {Ge₂O₅S₂} groups were identified in the [V^IV₁₄Ge₈O₄₂S₈]^12− 99 (Fig. 8c) and [H₂O@V^IV₁₅Ge₆O₄₂S₆]^12− 102 polyoxoanions. The {V₁₆Ge₄}-type GePOVs were found to exhibit ferrimagnetic properties.^110,113

3. Group 15 (As, Sb) element-functionalised POVs

3.1 Preview

One of the key differences between the SiPOVs/GePOVs and the polyoxovanadatoarsenates (AsPOVs)/polyoxovanadatoantimonates (SbPOVs) is the absence of the effective electron lone pairs at Si^IV/Ge^IV in the former and their presence at As^III/Sb^III in the latter. Note that these electron lone pairs provide different steric hindrances around the E and terminal oxygen atoms (O_term) in the heteroPOVs, thus influencing the reactivity of the E sites towards the TMC complexes and organic ligands. In comparison to SiPOVs and GePOVs, the chemistry of Sb- and, especially, As-incorporating POVs is more widely developed and resulted in a variety of discrete and multidimensional structures with interesting chemical and physical properties. The AsPOVs and SbPOVs are frequently prepared under hydrothermal or solvothermal conditions (Tables 3 and 4). The AsPOVs known thus far are characterised by both the low-nuclearity and high-nuclearity assemblies, which are classified as V₅, V₆, V₁₀, V₁₂, V₁₃, V₁₄, V₁₅, V₁₆, V₂₀ and V₂₄ series. SbPOVs show assemblies with nuclearity V₁₄, V₁₅, V₁₆ and V₂₀. Some of their most common skeletons are shown in Fig. 11.

Table 3 Selected details of synthesis and characterisation of AsPOV-based compounds

Formula	Colour	Characteristics of crystal structure^a	Reactants	Reaction conditions	Yield	Characterised via	Ref.
a Dimensionality resulting from hydrogen bonding networks is not considered.
V5
Na5[V5O9(O3AsC6H4-4-NH2)4]·20.5H2O·3DMF	Green		NaVO3, NaN3, p-arsanilic acid, H2O, DMF, N2H4·H2O, HCl	70 °C, pH = 7.3	82%	EA, IR, UV-vis, single-crystal XRD	166
V6
(NnBu4)4[V6As8O26]	Bright green		NaVO3, H2O, NaAsO2, N2H4, nBu4NBr	r.t.	50% based on As	EA, IR, TGA, single-crystal XRD, magnetometry	137
(NnBu4)2[H2{V6O10(O3AsPh)6}]·2H2O	Dark green		PhAsO3H2, (NH4)2Na2K2[V10O28], (TBA)Br, MeOH, MeCN, pentane, Et2O, isopropanol	Reflux, 40 min → 4 d	65%	EA, IR, UV-vis, electrochemistry, magnetometry, single-crystal XRD	161
V10
[V10As2O26(H2O)]·8H2O	Dark green	3D network	V2O5, H3AsO4, H2C2O4, H2O, en	160 °C, 3 d, pH = 8	40% based on V	EA, IR, TGA, EPR, powder XRD, single-crystal XRD, magnetometry	118
H5[V10O18(O3AsC6H4-4-NH2)7(DMF)2]·7DMF·5H2O	Green	Hexagonal packing arrangement	NaVO3, (p-aminophenyl)arsonic acid, H2O, DMF, HNO3, N2H4·H2O	70 °C, 30 min, pH = 4	35%	EA, IR, DTA-TG, powder XRD, single-crystal XRD	165
(NnBu4)2(NH4)2[V10O24(O3AsC6H4-4-NH2)3]	Red		(TBA)Br, arsanilic acid, MeOH, (NH4)2Na2K2[V10O28]	Reflux, 20 min → 4 °C, 20 h	45%	EA, IR, UV-vis, ^51V NMR, EPR, electrochemistry, single-crystal XRD	161
V12
(NH4)4[V12As8O40(H2O)]·4H2O	Dark green		NH4VO3, As2O3, ethyl ether (or EtOH), H2O	80 °C, 7 d	20% based on V	EA, IR, ESI-MS, TG-MS, UV-vis, powder XRD, single-crystal XRD	122
(NHEt3)4[V12As8O40(H2O)]·H2O	Dark blue		NaVO3, As2O3, NHEt3Cl, H2O, N2H4·HCl, HCl	24 °C, 2–3 d, pH = 6.0	29% based on V	IR, magnetometry, powder XRD, single-crystal XRD, INS	121
(NHEt3)2(NH2Me2)[V12As8O40(HCO2)]·2H2O, Na5[V12As8O40(HCO2)]·18H2O	Deep green, deep blue		NaVO3·2H2O, As2O3, N2H4·HCl, N,N-dimethylformamide			EA, IR, UV-vis, TGA, magnetometry, single-crystal XRD	119
Na4[V12As8O40(H2O)]·23H2O	Dark blue		NaVO3, As2O3, H2O, N2H4·H2SO4, HCl	4 °C, 2–3 d, pH = 6.0	53% based on V	IR, magnetometry, powder XRD, single-crystal XRD, INS	121
Na4[V12As8O40(D2O)]·16.5D2O			NaVO3, As2O3, D2O, N2H4·H2SO4, DCl	4 °C, 2–3 d, pH = 6.0		IR, magnetometry, powder XRD, single-crystal XRD, INS	121
K3[H12V12O36(AsO)2(AsO4)]·12H2O	Dark green		NaVO3, As2O3, H2O, H2NC(CH2OH)3, KSCN, H2SO4	70 °C, 22 h, pH = 4.6	25% based on V	EA, IR, TGA, single-crystal XRD	116
K6[H3KV12As3O39(AsO4)]·8H2O	Black grey	Zig-zag chain	KVO3, H2O, As2O5·5H2O, KSCN, H2SO4	1 h, 90 °C, pH = 3		EA, IR, EPR, magnetometry, single-crystal XRD	115
(NEt4)2[V12O12(OH)2(H2O)4(O3AsPh)10(HO3AsPh)4]·6H2O	Light green		(NnBu4)3[H3V10O28], PhAsO3H2, Et4NCl, MeOH/H2O	100 °C, 17 h → 120 °C, 3 d	30% based on V	EA, IR, TGA, single-crystal XRD	162
(H7O3)2(NnBu4)2[(MeOH)2V12O14(OH)4(O3AsPh)10]·H2O	Light green		(NnBu4)3[H3V10O28], PhAsO3H2, MeOH/H2O	100 °C, 17 h → 120 °C, 3 d	20% based on V	EA, IR, TGA, single-crystal XRD	162
Na4(H2O)10[{V12O14(OH)4(H2O)3(O3AsC6H4-4-NH2)10}]·1.5DMF·1.25H2O	Blue		NaVO3, NaN3, p-arsanilic acid, H2O, DMF, N2H4·H2O, HCl	70 °C, pH = 4.9	43%	EA, IR, UV-vis, single-crystal XRD	166
[Zn(en)2]2[Zn2(bpe)2V12As8O40(H2O)]	Brown	1D straight chain	NH4VO3, As2O3, ZnCl2·7H2O, bpe, en, HNO3, H2O	170 °C, 7 d	59% based on V	EA, FT-IR, TGA, CV, single-crystal XRD	125
{[Zn(dien)]2(dien)2[Zn2V12As8O40(0.5H2O)]}2·6H2O		Cluster dimers	V2O5, As2O3, H2O, Zn(OAc)2·4H2O, dien	160 °C, 3 d	36% based on V2O5	EA, IR, single-crystal XRD	124
[{Zn(enMe)2}2(enMe)2{Zn2V12As8O40(H2O)}]·4H2O	Brown		V2O5, As2O3, enMe, Zn(OAc)2·4H2O, H2O	180 °C, 7 d	76% based on V	EA, IR, TGA, EPR, magnetometry, single-crystal XRD	123
[Zn(enMe)2]2[(4,4′-bipy)Zn2V12As8O40(H2O)]	Brown	1D straight chain	NH4VO3, As2O3, ZnCl2·7H2O, 4,4′-bipy, enMe, HNO3, H2O	170 °C, 5 d	60% based on V	EA, FT-IR, TGA, CV, magnetometry, single-crystal XRD	126
[Zn(en)2(H2O)][Zn(en)2(4,4′-bipy)Zn2V12As8O40(H2O)]·3H2O	Brown	1D sinuate chain	NH4VO3, As2O3, ZnCl2·7H2O, 4,4′-bipy, en, HNO3, H2O	170 °C, 5 d	30% based on V	EA, FT-IR, TGA, CV, single-crystal XRD	126
[{Zn(en)3}2{Zn2V12As8O40(H2O)}]·4H2O·0.25bipy	Brown	Eight-shaped chiral helix	NH4VO3, As2O3, ZnCl2·7H2O, 4,4′-bipy, en, HNO3, H2O	170 °C, 5 d	39% based on V	EA, FT-IR, TGA, CV, single-crystal XRD	126
[Zn2(en)5][{Zn(en)2}{(bpe)HZn2V12As8O40(H2O)}2]·7H2O	Brown	2D network	NH4VO3, As2O3, ZnCl2·7H2O, bpe, en, HNO3, H2O	170 °C, 5 d	37% based on V	EA, FT-IR, TGA, CV, UV-Vis, magnetometry, single-crystal XRD	126
[Cd(en)2]2[Cd2(en)2V12As8O40]	Brown	1D chain	V2O5, As2O3, en, H2O, Cd(OAc)2·2H2O	180 °C, 6 d	52% based on V	EA, IR, TGA, EPR, magnetometry, powder XRD, single-crystal XRD	127
[Cd(enMe)3]2[(enMe)2Cd2V12As8O40(0.5H2O)]·5.5H2O	Brown	Layer	V2O5, As2O3, H2O, enMe, Cd(OAc)2·2H2O	170 °C, 3 d	42% based on V	EA, IR, TGA, UV-Vis, magnetometry, powder XRD, single-crystal XRD	128
[Cd(enMe)2]2[Cd2(enMe)2V12As8O40(0.5H2O)]	Brown	1D linear chain	V2O5, As2O3, H2O, enMe (excess), Cd(OAc)2·2H2O	170 °C, 3 d	32% based on V	EA, IR, TGA, UV-Vis, magnetometry, powder XRD, single-crystal XRD	128
V13
[Zn(2,2′-bipy)3]4[(ppz){{Zn(tepa)}2ZnV13As8O41(H2O)}2][V14As8O42(0.5H2O)]2·4H2O	Brown	3D network	V2O5, As2O3, H2O, dien, Zn(OAc)2·4H2O, 2,2′-bipy	160 °C, 3 d, pH = 7.45 → 8.35	39% based on V2O5	EA, IR, TGA, magnetometry, powder XRD, single-crystal XRD	124
[Cd(en)3][Cd(phen)(en)(H2O)2][Cd(en)V13As8O41(H2O)]·1.5H2O	Brown		NH4VO3, As2O3, 1,10-phen, en, CdCl2·7H2O, HNO3, H2O	170 °C, 7 d	46% based on V	EA, FT-IR, TGA, CV, powder XRD, single-crystal XRD	125
[Cd(phen)2(en)]2[Cd(phen)V13As8O41(H2O)]·21H2O·phen	Brown		NH4VO3, As2O3, 1,10-phen, en, CdCl2·7H2O, HNO3, H2O	170 °C, 7 d	55% based on V	EA, FT-IR, TGA, CV, magnetometry, powder XRD, single-crystal XRD	125
[Cd(dien)2]2[Cd(dien)V13As8O41(H2O)]·4H2O	Brown		V2O5, As2O3, dien, H2O Cd(OAc)2·2H2O	180 °C, 6 d	61% based on V	EA, IR, EPR, TGA, magnetometry, single-crystal XRD	127
{[V13As8NiClO41][Ni(en)2(H2O)][Ni(en)2]}{[Ni(en)2(H2O)2]0.5}·4H2O	Black	1D chain	V2O5, As2O3, H2C2O4·2H2O, en, NiCl2·6H2O, H2O	160 °C, 3 d	68% based on V	EA, IR, TGA, magnetometry, single-crystal XRD	129
V14
(NH4)6[V14As8O42(SO3)]	Brown		NH4VO3, As2O3, H2O, Na2S2O4, NH3, NH4SCN	80 °C, 1–2 d (not stirred)		EA, IR, UV-vis, single-crystal XRD	130
(NH4)6[V14As8O42(SO4)]	Brown		NH4VO3, As2O3, H2O, N2H6SO4, NH3, NH4SCN	90 °C, 3 d (not stirred)		EA, IR, UV-vis, single-crystal XRD	130
(NMe4)4[V14As8O42(H2O)]	Dark green		NaVO3, As2O3, NMe4Cl, H2O, N2H5Cl	70 °C, 7 d (not stirred)		EA, IR, UV-vis, single-crystal XRD	130
(NMe4)4[V14As8O42(H2O)0.5]	Dark brown		V2O5, As2O3, Me4NOH, H2O	200 °C, 2 d	89% based on V	EA, IR, single-crystal XRD	131
(NH4)2(NMe4)4[V14As8O42(SO4)]	Black		NH4VO3, As2O3, NiSO4·6H2O, NMe4OH, H2O	160 °C, 5 d	54% based on V	EA, IR, TGA, single-crystal XRD	134
(NEt4)3[H6V12AsO40(VO)2]·20H2O	Dark red		NH4VO3, Na2HAsO4, NH2OH, HCl, H2O, Et4NBr	Reflux temperature (1.5 h) → r.t.		EA, single-crystal XRD	117
(H2en)3.5[V14As8O42(PO4)]·2H2O	Black		V2O5, As2O3, H3PO4, en, H2C2O4·2H2O, H2O	160 °C, 3 d	80% based on V	EA, IR, TGA, XPS, ESR, single-crystal XRD	135
(H2enMe)2[V14As8O42(H2O)]·3H2O	Black	Triple clusters	NH4VO3, As2O3, FeSO4·4H2O, H3PO4, enMe, H2C2O4·2H2O, H2O, NH3·H2O	160 °C, 3 d	80% based on V	EA, IR, UV-vis, XPS, EPR, powder XRD, single-crystal XRD, magnetometry	136
(Hen)2(H2en)[V14As8O42(H2O)]·2.33H2O	Black		V2O5, As2O3, CuCl2·2H2O, en, H2O, HCl	170 °C, 7 d, pH = 6.0	64% based on V	EA, IR, TGA, single-crystal XRD	140
[NH2(CH2)4NH2]4[V14As8O42(SO4)]·(HSO4)2	Black	Layer	VOSO4·H2O, As2O5, ppz, H2O	180 °C, 6 d		EA, IR, TGA, single-crystal XRD	132
[NH2(CH2CH2)2NH2]3[V14As8O42(SO4)]·6.5H2O	Brown		VOSO4·H2O, As2O5, HN(CH2CH2)2NH, H2O	180 °C, 6 d	76% based on V	EA, IR, TGA, single-crystal XRD, magnetometry	133
H6[Cl2V14O16(OH)8(O3AsC6H4-4-NH2)10]·8DMF·16H2O	Turquoise		NaVO3, (p-aminophenyl)arsonic acid, H2O, DMF, HCl, N2H4·H2O	70 °C, 30 min, pH = 4.5	19%	EA, IR, ESI-MS, UV-vis, single-crystal XRD	165
K7[V14AsO40]·12H2O	Dark blue		KVO3, As2O5·5/3H2O, KSCN, H2O, H2SO4, KOH	70–75 °C, 16 h, pH = 4.6		EA, IR, Raman, TGA, UV-vis, EPR, single-crystal XRD, magnetometry	106
Rb5[V14As8O42(Cl)]·2H2O	Brown		V2O5, V2O3, V (mesh), H5As3O10, RbOH, H2O	200 °C, 2.5 d	40% based on V	EA, IR, UV-vis, single-crystal XRD	116
[Zn(bbi)2]2[V14As8O42(H2O)]	Dark green	2D network	NH4VO3, bbi, ZnCl2·2H2O, NaOAc, H2C2O4·2H2O, tris(hydroxymethyl)amino-methane, Na3AsO4·H2O, H2O, HCl		88% based on V	EA, IR, powder XRD, magnetometry	148
[Zn(2,2′-bipy)2]2[V14As8O42(H2O)]·H2O	Brown	1D tubular chain	V2O5, As2O5, Zn(OAc)2·2H2O, 2,2′-bipy, NMe4OH, H2O	160 °C, 6 d	33% based on V	EA, IR, EPR, single-crystal XRD, magnetometry	144
[Zn(2,2′-bipy)3]2[V14As8O42(H2O)]·4H2O	Brown		V2O5, As2O3, Zn(OAc)2·2H2O, 2,2′-bipy, ppz, H2O	180 °C, 6 d	73% based on V	EA, IR, EPR, TGA, powder XRD, single-crystal XRD, magnetometry	138
[{(H2O)Zn(4,4′-bipy)}2{V14As8O42(H2O)}]·2H2O	Black	2D network	V2O5, As2O3, Zn(OAc)2·2H2O, 4,4′-bipy, H2O	160 °C, 7 d	23% based on V	EA, IR, XPS, TGA, CV, single-crystal XRD	143
[Zn(2,2′-bipy)(dien)]2[V14As8O42(H2O)]·2H2O	Brown		V2O5, As2O3, Zn(OAc)2·2H2O, 2,2-bipy, dien, H2O	180 °C, 6 d	68% based on V	EA, IR, EPR, TGA, powder XRD, single-crystal XRD, magnetometry	138
[{(H2O)Zn(1,10-phen)2}2{V14As8O42(H2O)}]·4H2O	Black	2D supermolecular array	V2O5, As2O3, Zn(OAc)2·2H2O, 1,10-phen, H2O	160 °C, 7 d	39% based on V	EA, IR, XPS, TGA, CV, single-crystal XRD	143
[Zn(phen)3]2[V14As8O42(H2O)]·4H2O	Brown	Layer-like arrangement	V2O5, As2O3, Zn(OAc)2·6H2O, phen, dien, H2O	180 °C, 3 d	62% based on V	EA, IR, TGA, single-crystal XRD, fluorescence	141
[Zn(en)2][(H2O)Zn(en)2V14As8O42(H2O)]·H2O	Black	Infinite 1D channels	NH4VO3, As2O3, Zn(NO3)2·6H2O, HNO3, en, H2O	160 °C, 7 d	45% based on V	EA, IR, TGA, CV, single-crystal XRD	143
[{(H2O)Zn(2,2′-bipy)2}{Zn(2,2′-bipy)2}V14As8O42(H2O)0.5]2[{(H2O)Zn(2,2′-bipy)2V14As8O42(H2O)0.5}2{Zn(2,2′-bipy)2}2]·3H2O	Black	Cluster dimers	V2O5, As2O3, Zn(OAc)2·2H2O, 2,2′-bipy, H2O	160 °C, 7 d	29% based on V	EA, IR, XPS, TGA, CV, single-crystal XRD	143
[Cd(phen)3]2[V14As8O42(H2O)]·2H2O	Brown	Layer-like arrangement	V2O5, As2O3, Cd(OAc)2·6H2O, phen, dien, H2O	150 °C, 5 d	63% based on V	EA, IR, TGA, single-crystal XRD, fluorescence	141
[Cd(1,10-phen)3]2[V14As8O42(H2O)0.5]·0.5H2O	Black		NaVO3·2H2O, As2O3, Cd(OAc)2, 1,10-phen, NaOAc/HOAc	175 °C, 5 d	50% based on V	EA, IR, EPR, TGA, CV single-crystal XRD	140
[Cd(2,2′-bipy)3][Cd(dien)V14As8O42(H2O)]	Brown	1D wave-like chain	V2O5, As2O3, CdSO4·8H2O, 2,2′-bipy, H2C2O4·2H2O, dien, H2O	160 °C, 3 d	51% based on V	EA, IR, EPR, single-crystal XRD, magnetometry	144
[Cu(en)2]2[V14As8O42(H2O)]·2.5H2O	Brown	1D chain	NH4VO3, As2O3, CuCl2·2H2O, en, HNO3, H2O	160 °C, 7 d	33% based on V	EA, IR, TGA, single-crystal XRD	140
[Cu(en)2]3[V14As8O42(CO3)]·10H2O	Black	2D layered network	V2O5, As2O3, CuCl2·2H2O, en, H2C2O4·2H2O, H2O	160 °C, 3 d	80% based on V	EA, IR, TGA, single-crystal XRD	149
[Cu(bbi)]4[V14As8O42(H2O)]	Green	3D network	NH4VO3, bbi, Cu(OAc)2·3H2O, NaOAc, H2C2O4·2H2O, tris(hydroxymethyl)amino-methane, Na3AsO4·H2O, H2O, HCl	170 °C, 5 d	62% based on V	EA, IR, single-crystal XRD	148
[Ni(bbi)2]2[V14As8O42(H2O)]	Dark green	2D network	NH4VO3, bbi, Ni(NO3)2·6H2O, NaOAc, H2C2O4·2H2O, tris(hydroxymethyl)aminomethane, Na3AsO4·H2O, H2O, HCl	170 °C, 5 d	82% based on V	EA, IR, powder XRD, single-crystal XRD, magnetometry	148
[Ni(en)2]3[V14As8O42(SO4)]·4.5H2O	Black	2D sinusoidal layer	V2O5, As2O3, en, H2C2O4·2H2O, H2O, NiSO4·6H2O	160 °C, 3 d	73% based on V	EA, IR, EPR, XPS, single-crystal XRD, magnetometry	145
[Ni(en)2]3[V14As8O42(HPO3)]·4H2O	Brown	2D puckery layer	V2O5, As2O3, H3PO3, en, Ni, Ni(OAc)2·4H2O, H2O	160 °C, 4 d	16% based on V	EA, IR, EPR, single-crystal XRD	144
(2,2′-bipy)[Ni(2,2′-bipy)3]2[V14As8O42(H2O)]·3H2O	Brown		V2O5, As2O3, H2O, Ni(OAc)2·4H2O, en, 2,2′-bipy	160 °C, 3 d	46% based on V	EA, IR, EPR, TGA, UV-vis, XPS, single-crystal XRD	139
[Ni(enMe)3]4[Ni(enMe)2][V14As8O42(NO3)]2·8H2O	Black	Cluster dimers	NH4VO3, As2O3, Ni(NO3)2·6H2O, bpp, enMe, HNO3, H2O	170 °C, 7 d	45% based on V	EA, IR, TGA, CV, powder XRD, single-crystal XRD, magnetometry	125
[{Ni(en)2}4(4,4′-bipy)4{Ni(H2O)2}]2[V14As8O42(NO3)]4·16H2O	Black	Box-like framework	NH4VO3, As2O3, Ni(NO3)2·6H2O, 4,4′-bipy, HNO3, en, H2O	170 °C, 5 d	70% based on V	EA, IR, TGA, CV, single-crystal XRD, magnetometry	146
[Ni(en)2(H2O)2]2[{Ni(en)2(H2O)}2V14As8O42(NO3)][{Ni(en)2}V14As8O42(NO3)]·6H2O	Black	1D chain	NH4VO3, As2O3, Ni(NO3)2·6H2O, 1,3-bis(4-pyridyl)propane, HNO3, en, H2O	170 °C, 5 d	10% based on V	EA, IR, single-crystal XRD	146
[Co(2,2′-bipy)2]2[V14As8O42(H2O)]·H2O	Black	1D tubular chain	VOSO4, As2O3, CoC2O4·2H2O, 2,2′-bipy, H2O, dien	160 °C, 9 d, pH = 5.5	30% based on V	EA, IR, TGA, single-crystal XRD, magnetometry	147
[Co(2,2′-bipy)3]2[V14As8O42(H2O)]·3H2O	Brown		V2O5, As2O3, H3PO3, H2O, H2C2O4·2H2O, Co(OAc)2·4H2O, en, 2,2′-bipy	160 °C, 3 d	52% based on V	EA, IR, TGA, UV-vis, XPS, EPR, single-crystal XRD	139
[Co(dien)2]2[V14As8O42(H2O)]·3.5H2O	Brown	Soft channels	V2O5, Na3AsO4, CoCl2·6H2O, dien, H2O, H2SO4	170 °C, 7 d, pH = 8.0	33% based on V	EA, IR, TGA, single-crystal XRD	140
[Co(bbi)2]2[V14As8O42(H2O)]	Dark green	2D network	V2O5, bbi, Co(OAc)2·4H2O, H2C2O4·2H2O, NaOAc, tris(hydroxymethyl)aminomethane, Na3AsO4·H2O, H2O, HCl	170 °C, 5 d	55% based on V	EA, IR, powder XRD, single-crystal XRD, magnetometry	148
[Co(en)3][Co(en)2V14As8O42(H2O)]·16H2O	Black	1D chain	NH4VO3, As2O3, Co(NO3)2·6H2O, 4,4′-bipy, en, HNO3, H2O	170 °C, 7 d	40% based on V	EA, IR, TGA, CV, powder XRD, single-crystal XRD	125
[Mn(1,10-phen)3]2[V14As8O42(H2O)0.5]·0.5H2O	Green		NH4VO3, As2O3, KMnO4, 1,10-phen, H2O	160 °C, 7 d	10% based on V	EA, IR, EPR, TGA, single-crystal XRD	140
[{La(H2O)6}2V14As8O42(SO3)]·8H2O	Brown	2D layered network	(NH4)6[V14As8O42(SO3)], La(NO3)3·6H2O, H2O	7 d	32% based on (NH4)6[V14As8O42(SO3)]	EA, IR, EPR, TGA, diffuse reflectance, powder XRD, single-crystal XRD	150
[{Ce(H2O)6}2V14As8O42(SO3)]·8H2O	Brown	2D layered network	(NH4)6[V14As8O42(SO3)], Ce(NO3)3·6H2O, H2O	7 d	32% based on (NH4)6[V14As8O42(SO3)]	EA, IR, EPR, TGA, diffuse reflectance, powder XRD, single-crystal XRD	150
[{Sm(H2O)6}2V14As8O42(SO3)]·8H2O	Brown	2D layered network	(NH4)6[V14As8O42(SO3)], Sm(NO3)3·6H2O, H2O	7 d	27% based on (NH4)6[V14As8O42(SO3)]	EA, IR, EPR, TGA, diffuse reflectance, powder XRD, single-crystal XRD	150
V15
K6[V15As6O42(H2O)]·8H2O	Brown	3D network	KVO3, As2O3, KSCN, KOH, H2O, N2H4·H2SO4	85 → 20 °C, pH = 8.4	55%	EA, IR, TGA, single-crystal XRD, magnetometry	151
[Zn(H2O)4]2[H2V15As6O42(H2O)]·2H2O	Black		V2O5, As2O3, H2C2O4·2H2O, en, Zn(OAc)2·2H2O, H2O	160 °C, 3 d	53% based on As	EA, IR, ESR, single-crystal XRD, magnetometry, third-order NLO	153
[Zn(en)2][Zn(en)2(H2O)2][{Zn(en)(enMe)}V15As6O42(H2O)]·4H2O	Black		V2O5, As2O3, en, enMe, Zn(OAc)2·2H2O, H2O	160 °C, 6 d, pH = 8.5	52% based on V	EA, IR, TGA, single-crystal XRD	154
[Zn2(enMe)2(en)3][{Zn(enMe)2}V15As6O42(H2O)]·4H2O	Black		V2O5, As2O3, en, enMe, Zn(OAc)2·2H2O, H2O	160 °C, 6 d	37% based on V	EA, IR, TGA, single-crystal XRD	154
(Hen)2[{{Zn(en)2}2V15As6O42(H2O)}2{Zn(en)2}]·3H2O	Black	Cluster dimers	VOSO4, As2O3, ZnO, en, HCl, H2O	160 °C, 4 d, pH = 7	58% based on V	EA, IR, TGA, single-crystal XRD	155
[Zn2(dien)3(H2O)2]0.5[{Zn2(dien)3}V15As6O42(H2O)]·2H2O	Brown	1D helical chain	V2O5, As2O3, dien, Zn(OAc)2·2H2O, H2O	160 °C, 3 d	57% based on V	EA, IR, TGA, EPR, single-crystal XRD	156
[Cu(en)2]1.5[H3V15As6O42(H2O)]·3H2O	Black	1D sinusoidal chain	V2O5, As2O3, H2C2O4·2H2O, CuSO4·5H2O, en, H2O	160 °C, 3 d	67% based on V	EA, IR, single-crystal XRD	158
[Cu(enMe)2]2.5[HV15As6O42(H2O)]·2H2O	Black	Brick-wall-like 2D layer	V2O5, As2O3, H2SO4, enMe, H2C2O4·2H2O, CuSO4·5H2O, H2O, NH3·H2O	160 °C, 3 d, pH = 10	51% based on V	EA, IR, TGA, UV-vis, EPR, XPS, magnetometry, single-crystal XRD	136
[Ni(2,2′-bipy)3]2[{Ni(en)2}V15As6O42(H2O)]·9.5H2O	Brown	1D straight chain	V2O5, As2O3, 2,2′-bipy, Ni(OAc)2·4H2O, en, H2O	160 °C, 3 d	65% based on V	EA, IR, TGA, EPR, single-crystal XRD	156
[Co(en)3][{Co(en)2}2V15As6O42]·4H2O	Black	1D chain	V2O5, As2O3, H3PO3, Co(OAc)2, en, H2O	160 °C, 3 d	78%	IR, single-crystal XRD, magnetometry	157
[Co(enMe)2]3[V15As6O42(H2O)]·2H2O	Brown	2D framework	V2O5, As2O3, enMe, H2C2O4·2H2O, CoSO4·4H2O, H2O	160 °C, 6 d	27% based on V	EA, IR, TGA, EPR, single-crystal XRD	156
V16
(HNEt3)2[{Br2(H2O)4}(VO)16(OH)8(O4AsPh)2(O3AsPh)8]·6MeCN	Blue		VBr3, PhAsO3H2, NEt3, MeCN	80 °C, 30 min	62% based on V	EA, IR, ESI-MS, powder XRD, single-crystal XRD	163
(HNEt3)2[{Cl2(H2O)4}(VO)16(OH)8(O4AsPh)2(O3AsPh)8]·2H2O	Blue		VCl3, PhAsO3H2, NEt3, MeCN	80 °C, 30 min	65% based on V	EA, IR, ESI-MS, powder XRD, single-crystal XRD	163
H5[{Cl4(H2O)2}(VO)16O16(O3AsPh)8]Cl·4H2O·3MeCN	Green		VCl3, Dy(NO3)3·xH2O, PhAsO3H2, NEt3, MeCN	80 °C, 30 min	12% based on V	EA, IR, ESI-MS, powder XRD, single-crystal XRD	163
[Zn2(dien)3][{Zn(dien)}2V16As4O42(H2O)]·3H2O	Brown	1D linear chain	V2O5, As2O3, H2O, dien, Zn(OAc)2·4H2O	170 °C, 4 d pH = 7.15 → 8.05	31% based on V2O5	EA, IR, TGA, magnetometry, single-crystal XRD	159
V20
[{Cl4(H2O)2}(VO)20O16(OH)4(O3AsPh)8]·7H2O·3MeCN	Green		VCl3, Dy(NO3)3·xH2O, PhAsO3H2, NEt3, MeCN	80 °C, 30 min	58% based on V	EA, IR, ESI-MS, powder XRD, single-crystal XRD	163
V24
H10[{Cl6}(VO)24O24(O3AsPh)8]Cl4·10H2O·2MeCN	Green		VCl3, Dy(NO3)3·xH2O, PhAsO3H2, NEt3, MeCN	80 °C, 30 min	52% based on V	EA, IR, ESI-MS, powder XRD, single-crystal XRD	163

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Table 4 Selected details of synthesis and characterisation of SbPOV-based compounds

Formula	Colour	Characteristics of crystal structure^a	Reactants	Reaction conditions	Yield	Characterised via	Ref.
a Dimensionality resulting from hydrogen bonding networks is not considered.
V14
(NH4)4[V14Sb8O42]·2H2O	Green brown	Layer-like arrangement	NH4VO3, Sb2O3, theed, H2O	150 °C, 14 d	40% based on Sb	Powder XRD, single-crystal XRD	185
(H2en)2[V14Sb8O42(H2O)]·3H2O	Black	Layer-like arrangement	NH4VO3, Sb2O3, N,N,N′,N′-tetramethylethylenediamine, H2O	180 °C, 7 d	37% based on Sb	EA, IR, single-crystal XRD	193
(H2ppz)2[V14Sb8O42(H2O)]	Black	Layer-like arrangement	NH4VO3, Sb2O3, 1-methylpiperazine, H2O	180 °C, 7 d		Powder XRD, SEM, single-crystal XRD	193
[(H2en)2{V14Sb8O42(H2O)}]·(en)·4H2O	Black	1D double chain	VOSO4, Sb2O3, H2O, C2N2H8 (en)	pH = 7.5, 175 °C, 4 d	61% based on Sb	EA, ICP, IR, single-crystal XRD	184
[V14Sb8(Haep)4O42(H2O)]·4H2O	Brown green	Chain	NH4VO3, Sb2O3, aep, H2O	180 °C, 7 d	24% based on Sb	EA, UV-vis, DTA-TGA, single-crystal XRD, magnetometry	188
[Zn(dien)2]2[{Zn(dien)}2(V14Sb8O42)2(H2O)]·4H2O	Brown	1D zig-zag chain	Sb2O3, V2O5, Zn(OAc)2·2H2O, dien, H2O, NaOH	pH = 9.5, 180 °C, 7 d	25% based on V	EA, IR, XPS, TGA, single-crystal XRD	189
[{Co(en)2}2V14Sb8O42(H2O)]·6H2O	Black	2D network	V2O5, Sb2O3, H2C2O4·2H2O, CoC2O4·2H2O, H2O, en	pH = 9.0, 160 °C, 9 d	46% based on Sb	EA, ICP, IR, single-crystal XRD	191
V15
(H3tren)2[V15Sb6O42]·0.33(tren)·nH2O (n = 3–5)	Brown greenish	Hexagonal layer	NH4VO3, Sb2O3, tren, H2O	150 °C, 7 d	70% based on Sb	EA, IR, UV-vis, Raman, DTA-TGA-MS, single-crystal XRD, magnetometry	186
(H2aep)2[V15Sb6(Haep)2O42(H2O)]·2.5H2O	Black	Rows	NH4VO3, Sb2O3, aep, H2O	160 °C, 7 d	64% based on Sb	EA, UV-vis, DTA-TGA, single-crystal XRD, magnetometry	188
[Ni(dien)2]3[V15Sb6O42(H2O)]·nH2O (n = 8, 12)	Brown	Layer-like arrangement	NH4VO3, Sb2O3, NiCl2·6H2O, dien, H2O	130 °C/150 °C, 7 d	36% based on Sb	EA, IR, DTA-TGA, single-crystal XRD, SEM	196
[Ni(aepda)2]2[{Ni(aepda)2}V15Sb6O42(H2O)]·8H2O	Brown	Double clusters	NH4VO3, Sb2O3, NiCl2·6H2O, aepda, H2O	150 °C, 7 d	60% based on Sb	EA, IR, DTA-TGA, powder XRD, single-crystal XRD, SEM, magnetometry	194
[Ni(Htren)2][Ni2(tren)3(V15Sb6O42(H2O)0.5)]2·H2O	Brown	1D double chain	NH4VO3, Sb2O3, NiCl2·6H2O, tren, H2O	150 °C, 5 d		EA, IR, UV-vis, powder XRD, single-crystal XRD	195
[Co(aepda)2]2[{Co(aepda)2}V15Sb6O42(H2O)]·5H2O	Brown	Double clusters	NH4VO3, Sb2O3, CoCl2·6H2O, aepda, H2O	130 °C, 7 d	45% based on Sb	EA, IR, DTA-TGA, single-crystal XRD, SEM, magnetometry	194
[Co(tren)(H2O)]3[V15Sb6O42(H2O)]·H2O	Dark brown	2D network	NH4VO3, Sb2O3, CoCl2·6H2O, tren (conc. 50%)	170 °C, 7 d	60% based on Sb	EA, IR, EDXS, DTA-TG, powder XRD, single-crystal XRD	192
[Co2(tren)3]2[Co(tren)(en)][{V15Sb6O42(H2O)(Co(tren)2)}V15Sb6O42(H2O)]·nH2O (n ≈ 11)	Brown	1D double chain	NH4VO3, Sb2O3, CoCl2·6H2O, tren (conc. 75%)	170 °C, 7 d	35% based on Sb	EA, IR, EDXS, DTA-TG, powder XRD, single-crystal XRD	192
[{Fe(dach)2}3{V15Sb6O42(H2O)}]·8H2O	Dark red	Porous 3D network	NH4VO3, Sb2O3, FeSO4·7H2O, dach, H2O	160 °C, 7 d	23% based on V	EA, IR, UV-vis, single-crystal XRD, magnetometry	197
V16
(H2aep)4[V16Sb4O42]·2H2O	Brown	Chain	NH4VO3, Sb2O3, aep, H2O	150 °C, 7 d		Single-crystal XRD	185
[Zn2(dien)3][{Zn(dien)}2V16Sb4O42(H2O)]·4H2O	Brown red	1D linear chain	Sb2O3, V2O5, Zn(OAc)2·2H2O, dien, H2O, NaOH	pH = 7.8–8.3, 180 °C, 7 d	75% based on V	EA, IR, XPS, TGA, single-crystal XRD	189
[Zn(tren)(H2tren)]2[V16Sb4O42(H2O)]·nH2O (n = 6–10)	Brown	Layer-like arrangement	NH4VO3, Sb2O3, Zn(NO3)2·6H2O, tren, H2O	pH = 12.5, 130 °C, 7 d	47% based on Sb	EA, IR, DTA-TGA, powder XRD, single-crystal XRD, SEM, magnetometry	190
[Ni(dien)2]4[V16Sb4O42(H2O)]	Black	Layer-like arrangement	NH4VO3, Sb2VO5, NiCl2·6H2O, dien, H2O	150 °C, 7 d		Powder XRD, single-crystal XRD, SEM	196
[Co(tren)(H2tren)]2[V16Sb4O42(H2O)]·6H2O	Brown	Layer-like arrangement	NH4VO3, Sb2O3, CoCl2·6H2O, tren (conc. 25%)	130 °C, 7 d	30% based on Sb	EA, IR, EDXS, DTA-TG, powder XRD, single-crystal XRD	192
V20
[V16Sb4O42(H2O){VO(dach)2}4]·(dach)·10H2O	Dark brown	Porous 3D network	Sb2O3, NH4VO3, dach, H2O	150 °C, 7 d	60% based on V	EA, IR, EDXS, UV-vis, single-crystal XRD, magnetometry	187

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Fig. 11 Vanadium skeletons found in some “fully-reduced” AsPOVs and SbPOVs.

As vanadium precursors for the synthesis of the AsPOVs (Table 3) and SbPOVs (Table 4), V₂O₅ and NH₄VO₃ were extensively used, whilst NaVO₃, VOSO₄ and vanadium halides were employed less frequently. The POV structures decorated with handle-like {E₂O₅} groups (E = As, Sb) are typically viewed as being derived from the {V₁₈O₄₂} archetype, with only some exceptions which are discussed in Section 3. In contrast to the Si^IV and Ge^IV atoms displaying tetrahedral {EO₄} geometries in the heteroPOV structures, the heavier group 15 elements, As and Sb, usually adopt trigonal pyramidal {EO₃} geometries and show the formal oxidation state +3; however, very rarely tetrahedral {AsO₄}³⁻ units and arsenic atoms in the formal oxidation state +5 were also identified. The As–O bonds (d_As–O = 1.52–2.02 Å) are usually longer than the terminal V=O bonds (d_V=O = 1.52–1.68 Å), but shorter than the bridging V–O bonds (d_V–O = 1.72–2.33 Å). The Sb–O bonds (d_Sb–O = 1.90–2.04 Å) fall in the range of V–O bond lengths, but these are remarkably longer than the V=O bonds.

3.2 Polyoxovanadatoarsenates (AsPOVs)

3.2.1 AsPOVs with a low proportion of As^V.
AsPOVs with the encapsulated AsO₄ units. In the early 1990s Müller, Gatteschi and colleagues described the synthesis and magnetic properties of the compound K₆[H₃KV₁₂As₃O₃₉(AsO₄)]·8H₂O exhibiting electron delocalisation effects.¹¹⁵ The central constituent of this compound obtained upon reduction of an aqueous solution of potassium metavanadate KVO₃ is the mixed-valent [H₃KV^IV₄V^V₈As^V₃O₃₉(As^VO₄)]⁶⁻ polyoxoanion with approximate C₃ symmetry (Table 3). The structure of this AsPOV consists of nine {VO₆} octahedra, three {VO₄} tetrahedra, and four {As^VO₄} tetrahedra. One of these arsenate units {AsO₄}³⁻ is covalently enclosed within the AsPOV shell (Fig. 12). The terminal O atoms of three other, peripheral {As^VO₄} groups are protonated. A potassium ion caps the monolacunary [H₃V₁₂As₃O₃₉(AsO₄)]⁷⁻ polyoxoanion. The electron population in this AsPOV was confirmed by bond valence sum analysis and manganometric titration. In the structure, K₆[H₃KV₁₂As₃O₃₉(AsO₄)]·8H₂O displays zig-zag chains in which the neighbouring [H₃KV^IV₄V^V₈As^V₃O₃₉(As^VO₄)]⁶⁻ polyoxoanions are linked through the K⋯O_term–V interactions.

Fig. 12 Polyhedral representation of the mixed-valent [H₃KV^IV₄V^V₈As^V₃O₃₉(As^VO₄)]⁶⁻ polyoxoanion resembling the ε-Keggin-type POM archetype. Hydrogen atoms are not shown. Colour code: As^V, rose; {As^VO₄}, rose tetrahedron in the centre; O, red; V^V/V^IVO_x, dark-grey polyhedra; K, turquoise.

Another potassium-containing compound, K₃[H₁₂V₁₂O₃₆(AsO)₂(AsO₄)]·12H₂O (Table 3), displays a structure that can be formally viewed as α-Keggin-type {V^IV₆V^V₆O₃₆(As^VO₄)} core with two of its six tetragonal {V₄O₄} faces additionally capped by two {As^VO} groups.¹¹⁶ As in the case of [H₃KV^IV₄V^V₈As^V₃O₃₉(As^VO₄)]⁶⁻, the tetrahedral {AsO₄}³⁻ unit is enclosed within the {V₁₂O₃₆} cage of the mixed-valent [H₁₂V^IV₆V^V₆O₃₆(AsO)₂(AsO₄)]³⁻ polyoxoanion. The twelve O sites of this AsPOV are protonated and three K⁺ cations then compensate the −3 net charge of the polyoxoanion. In the crystal structure, each of these K⁺ cations is coordinated by four water molecules and is involved in bonding to four adjoined [H₁₂V₁₂O₃₆(AsO)₂(AsO₄)]³⁻ polyoxoanions through (weak) K–O bonds.

Another type of a bicapped Keggin structure with two capping {V^VO} groups instead of two {As^VO} ones was found for the fully-oxidised [H₆V^V₁₂O₃₆(V^VO)₂(AsO₄)]³⁻ polyoxoanion isolated as (NEt₄)₃[H₆V₁₂AsO₄₀(VO)₂]·20H₂O (Table 3).¹¹⁷ The α-Keggin core of this AsPOV consists of twelve {VO₆} octahedra and the covalently enclosed, central {AsO₄} tetrahedron whose O atoms are shared by four {V₃O₁₃} groups of edge-sharing octahedra.

Neutral AsPOV with the capping AsO₄ units. The tetrahedral {AsO₄}³⁻ units capping tetragonal vanadium oxide faces were observed in hydrothermally synthesised [V^IV₈V^V₂As^V₂O₂₆(H₂O)]·8H₂O (Table 3).¹¹⁸ Its crystal structure shows an extended 3D network in which the neutral, mixed-valent POVs are joined through {AsO₄} bridging groups. Notably, this compound exhibited catalytic activity for phenol hydroxylation, offering high selectivity to hydroquinone.

3.2.2 {V₁₂As₈}-type polyoxoanions.
Discrete mixed-valent AsPOVs. The first polyoxoanions from this class of AsPOVs were presented by Müller and co-workers, in 1991.¹¹⁹ The formate-enclosing β-[HCO₂@V^IV₆V^V₆As₈O₄₀]³⁻ and β-[HCO₂@V^IV₈V^V₄As₈O₄₀]⁵⁻ components of the mixed-valent compounds (NHEt₃)₂(NH₂Me₂)[V₁₂As₈O₄₀(HCO₂)]·2H₂O and Na₅[V₁₂As₈O₄₀(HCO₂)]·18H₂O (Table 3), respectively, display different electron populations and different types of spin–spin interactions, albeit their AsPOV shells with encapsulated formiate ions are isostructural and possess D_4h symmetries. The strength of the spin–spin coupling in these polyoxoanions composed of four handle-like {As₂O₅} groups and twelve {VO₅} square pyramids was shown to be influenced by the number of V^IV centres: the larger number of these centres, the stronger coupling. Their magnetic properties were additionally studied by Gatteschi and colleagues.¹²⁰ Although the [HCO₂@V^IV₆V^V₆As₈O₄₀]³⁻ and [HCO₂@V^IV₈V^V₄As₈O₄₀]⁵⁻ polyoxoanions are isonuclear, they exhibit dissimilar magnetic properties due to the different AsPOV spin topologies and delocalisation effects of the d¹-V^IV electrons. According to the magnetic susceptibility data, the AsPOV trianion is characterised by ferromagnetic coupling between delocalised and localised V^IV ions and the AsPOV pentaanion features a singlet (S = 0) ground state because of strongly coupled, delocalised V^IV ions. Furthermore, it was stated that field-dependent relaxation effects may influence the EPR spectra (at X-band) of these dodecanuclear species.

About 10 years later, Güdel and colleagues conducted an INS study for three new, but related dodecanuclear β-AsPOVs isolated as Na₄[V^IV₈V^V₄As₈O₄₀(H₂O)]·23H₂O, the deuterated derivative Na₄[V^IV₈V^V₄As₈O₄₀(D₂O)]·16.5D₂O, and (NHEt₃)₄[V^IV₈V^V₄As₈O₄₀(H₂O)]·H₂O (Table 3).¹²¹ The goal of the study was to gain insights into magnetic exchange interactions in these mixed-valent spin structures possessing two outer (d_V⋯V = 3.410–3.450 Å) and one inner (or central) V₄ squares (d_V⋯V = 5.26–5.31 Å) bridged by [As₂O₅]⁴⁻ groups. The magnetic transitions (up to four) between the S = 0 ground state of the {V^IV₈V^V₄As₈} building block and its excited states were identified by INS. By this study, the authors confirmed “the essential correctness of an earlier model developed on the basis of magnetic and EPR measurements”¹²⁰ that were performed for the aforementioned [HCO₂@V^IV₈V^V₄As₈O₄₀]⁵⁻ polyoxoanion.

The family of the {V^IV₈V^V₄As₈}-type compounds^119–121 was extended by the compound (NH₄)₄[V₁₂As₈O₄₀(H₂O)]·4H₂O obtained under solvothermal conditions using the mixtures of ethyl ether/H₂O or ethanol/H₂O as reducing agents (Table 3).¹²² The building block of this compound is the mixed-valent β-[H₂O@V^IV₈V^V₄As₈O₄₀]⁴⁻ polyoxoanion with the shortest interatomic V⋯V distance of 3.02 Å and a diameter of ca. 11 Å. In the crystal of (NH₄)₄[V₁₂As₈O₄₀(H₂O)]·4H₂O, the AsPOVs are joined by weak intercluster As⋯O interactions with the shortest As⋯O distance of 3.05 Å. The H₂O molecules and NH₄⁺ countercations reside in the voids between the polyoxoanions. The strong hydrogen bonds formed between the NH₄⁺ ions and the terminal O atoms of the [H₂O@V^IV₈V^V₄As₈O₄₀]⁴⁻ polyoxoanion result in the formation of a 3D network. (NH₄)₄[V₁₂As₈O₄₀(H₂O)]·4H₂O exhibits a distinct solubility in H₂O, MeOH and DMF and shows an optical energy gap of 2.80 eV.

Zinc–AsPOV hybrids. A number of hydrothermally synthesised compounds based on the bis-transition metal-substituted AsPOVs of the type {M₂V₁₂As₈O₄₀}, which exhibit antiferromagnetic exchange interactions, were described. These hybrid polyoxoanions are formally derived from the α-{V₁₄As₈O₄₂} polyoxoanion (discussed in Section 3.2.4) in which two {VO}²⁺ caps situated between the mutually opposite {As₂O₅} groups are replaced by two M²⁺ ions. This kind of incorporation of transition metal cations into the backbones of dilacunary α-heteroPOV shells was also observed in the case of the aforementioned “fully-reduced” [Cd₂(en)₂V^IV₁₂O₄₀(GeOH)₈]⁴⁻ polyoxoanion¹¹⁰ (Fig. 10). One of these M₂-decorated AsPOVs was found in [{Zn(enMe)₂}₂(enMe)₂{Zn₂V₁₂As₈O₄₀(H₂O)}]·4H₂O¹²³ (Table 3). The main structural motif of this hybrid compound is a dilacunary {V₁₂As₈O₄₀} shell functionalised with two [Zn₂(enMe)₃]²⁺ complexes via Zn–O bonds (Fig. 13a). Each of two [Zn(enMe)₂]²⁺ moieties is linked to the [H₂O@Zn₂V^IV₁₂As₈O₄₀]⁴⁻ polyoxoanion by the enMe residuals through Zn–N bonds. Strong hydrogen bonds contribute to the formation of a 3D supramolecular array. The crystal structure of another similar compound {[Zn(dien)]₂(dien)₂[Zn₂V₁₂As₈O₄₀(0.5H₂O)]}₂·6H₂O¹²⁴ (Table 3) displays two crystallographically independent [0.5H₂O@Zn₂V^IV₁₂As₈O₄₀]⁴⁻ polyoxoanions each of which is decorated with two [Zn(dien)]²⁺ complexes through dien connecting groups. The two resulting [Zn(dien)]₂(dien)₂[Zn₂V₁₂As₈O₄₀(0.5H₂O)] constituents are linked to form a dimer by a weak Zn–O bond.

Fig. 13 (a) Polyhedral representation of [{Zn(enMe)₂}₂(enMe)₂{Zn₂V₁₂As₈O₄₀(H₂O)}] containing the “fully-reduced” [H₂O@Zn₂V^IV₁₂As₈O₄₀]⁴⁻ hybrid polyoxoanion. (b) A segment of the polymeric solid-state structure of [{Zn(en)₃}₂{Zn₂V₁₂As₈O₄₀(H₂O)}]·4H₂O·0.25bipy. (c) A segment of the polymeric solid-state structure of [Zn₂(en)₅][{Zn(en)₂}{(bpe)HZn₂V₁₂As₈O₄₀(H₂O)}₂]·7H₂O. Hydrogen atoms are omitted for clarity. Colour code: C, grey; N, blue; As, rose; O, red; V^IVO_x, sky-blue polyhedra; Zn, dark green.

The compound [Zn(en)₂]₂[Zn₂(bpe)₂V₁₂As₈O₄₀(H₂O)] (Table 3) is composed of the [(bpe)₂Zn₂V^IV₁₂As₈O₄₀(H₂O)]⁴⁻ polyoxoanion [bpe = 1,2-bis(4-pyridyl)ethylene] and two [Zn(en)₂]²⁺ countercations.¹²⁵ This hybrid polyoxoanion based on the α-AsPOV is constructed from twelve {VO₅} square pyramids, two square-pyramidal {ZnNO₄} units and four handle-like {As₂O₅} groups. The Zn²⁺ ions introduced into the backbones of the AsPOV building block are coordinated by the bpe ligands through Zn–N bonds. The two terminal O atoms on the opposite sides of the eight-membered, {VO₅}-composed ring of the [(bpe)₂Zn₂V^IV₁₂As₈O₄₀(H₂O)]⁴⁻ polyoxoanion are covalently connected to the [Zn(en)₂]²⁺ complexes through Zn–O bonds. This compound with two structurally exposed pendant pyridyl rings of the bpe ligand could probably be of relevance for surface deposition studies.

The compounds [Zn(enMe)₂]₂[(4,4′-bipy)Zn₂V₁₂As₈O₄₀(H₂O)], [Zn(en)₂(H₂O)][Zn(en)₂(4,4′-bipy)Zn₂V₁₂As₈O₄₀(H₂O)]·3H₂O, [{Zn(en)₃}₂{Zn₂V₁₂As₈O₄₀(H₂O)}]·4H₂O·0.25bipy and [Zn₂(en)₅][{Zn(en)₂}{(bpe)HZn₂V₁₂As₈O₄₀(H₂O)}₂]·7H₂O composed of zinc(II) amine complexes and the “fully-reduced”, bis-zinc-substituted α-isomeric AsPOVs with compositions [H₂O@Zn₂V^IV₁₂As₈O₄₀]⁴⁻ and its protonated [H₂O@HZn₂V^IV₁₂As₈O₄₀]³⁻ form were reported.¹²⁶ Crystalline samples of these inorganic–organic hybrid materials could only be obtained in alkaline solutions when the pH value was adjusted between 8 and 10, which allowed deprotonation of N-donor ligands and their easier coordination to the Zn²⁺ ions (Table 3). The different structural features of the organic ligands captured by the secondary Zn²⁺ ions were shown to have an effect on the formation and construction of the above compounds, thus resulting in the polyoxoanions that are packed in linear or staggered arrangements in the solid state. The crystal structure of [Zn(enMe)₂]₂[(4,4′-bipy)Zn₂V₁₂As₈O₄₀(H₂O)] exhibits linear chains of the [(4,4′-bipy)Zn₂V₁₂As₈O₄₀(H₂O)]⁴⁻ polyoxoanions charge-balanced by discrete [Zn(enMe)₂]²⁺ complexes occupying the interchain regions. The “fully-reduced” [H₂O@Zn₂V^IV₁₂As₈O₄₀]⁴⁻ polyoxoanions are linked by bridging 4,4′-bipy ligands through Zn–N bonds. It was also found that [Zn(enMe)₂]₂[(4,4′-bipy)Zn₂V₁₂As₈O₄₀(H₂O)] is electrocatalytically active in the reduction and oxidation of H₂O₂ and NO₂⁻ using bulk-modified carbon paste electrodes. The crystal structure of [Zn(en)₂(H₂O)][Zn(en)₂(4,4′-bipy)Zn₂V₁₂As₈O₄₀(H₂O)]·3H₂O displays 1D winding chains of the [(4,4′-bipy)Zn₂V₁₂As₈O₄₀(H₂O)]⁴⁻ hybrid clusters functionalised with the [Zn(en)₂]²⁺ complexes through Zn–O bonds; [Zn(en)₂(H₂O)]²⁺ complexes act as countercations. Similarly to the previous compound, the 4,4′-bipy ligands bridge the {Zn₂V₁₂As₈O₄₀} building blocks through Zn–N bonds. The Zn²⁺ cations are incorporated into the dilacunary-type AsPOV shell through Zn–O bonds. The extensive hydrogen bonding in the crystal lattice results in the formation of a 3D supramolecular architecture. The crystal structure of [{Zn(en)₃}₂{Zn₂V₁₂As₈O₄₀(H₂O)}]·4H₂O·0.25bipy is described as an eight-shaped chiral helical chain, where the “fully-reduced” [H₂O@Zn₂V^IV₁₂As₈O₄₀]⁴⁻ hybrid polyoxoanions are covalently bridged by the [Zn(en)₃]²⁺ complexes through Zn–N and Zn–O bonds, as shown in Fig. 13b. In contrast to the above-mentioned compounds, [Zn₂(en)₅][{Zn(en)₂}{(bpe)HZn₂V₁₂As₈O₄₀(H₂O)}₂]·7H₂O has a 2D layer structure with nanosized inner 1D rectangular cavities of 33.7 × 14.7 Å. These cavities are occupied by the lattice H₂O molecules and [Zn₂(en)₅]⁴⁺ cations. The connectivity of the “fully-reduced” [H₂O@HZn₂V^IV₁₂As₈O₄₀]³⁻ building blocks through the [Zn(en)₂]²⁺ complexes and the bidentate bpe ligands (Fig. 13c), yielding a single-stranded (right- and left-handed) helical chain, resembles that found in the crystal structure of [Zn(en)₂]₂[Zn₂(bpe)₂V₁₂As₈O₄₀(H₂O)].¹²⁵

Cadmium–AsPOV hybrids. Cd²⁺–en complexes were incorporated into the dilacunary β-{V₁₂As₈O₄₀}-type structure through Cd–O bonds to form the “fully-reduced” [Cd₂(en)₂V^IV₁₂As₈O₄₀]⁴⁻ polyoxoanion, which is charge-balanced by two [Cd(en)₂]²⁺ complexes.¹²⁷ This compound with composition [Cd(en)₂]₂[Cd₂(en)₂V₁₂As₈O₄₀] was prepared under hydrothermal conditions (Table 3) and displays a 1D chain structure where neighbouring [Cd₂(en)₂V^IV₁₂As₈O₄₀]⁴⁻ polyoxoanions are doubly bridged via [Cd(en)₂]²⁺ groups. The interface between these hybrid building blocks is characterised by eight-membered rings involving Cd²⁺ ions from the TMC linkers and terminal and bridging O atoms from the connected polyoxoanions. Hydrogen bonds extend the 1D chains into a 3D supramolecular network.

Two other compounds, [Cd(enMe)₃]₂[(enMe)₂Cd₂V₁₂As₈O₄₀(0.5H₂O)]·5.5H₂O and [Cd(enMe)₂]₂[Cd₂(enMe)₂V₁₂As₈O₄₀(0.5H₂O)], were found to contain α- and β-isomeric [Cd₂(enMe)₂V^IV₁₂As₈O₄₀(0.5H₂O)]⁴⁻ constituents, respectively (Table 3).¹²⁸ Whereas the former compound can be viewed as an isolated (0D) inorganic–organic hybrid, the latter compound shows an infinite, 1D linear chain structure due to the [Cd(enMe)₂]²⁺ linkages and is furthermore isomorphic with the above-mentioned example [Cd(en)₂]₂[Cd₂(en)₂V₁₂As₈O₄₀],¹²⁷ with the exception of organic ligands coordinated to the Cd(II) centres and a half H₂O molecule encapsulated by the polyoxoanion shell. In both compounds, the [Cd(enMe)]²⁺ complexes are attached to the AsPOV building blocks through Cd–O bonds. According to the diffuse reflectance UV-vis spectra, these compounds are characterized by optical energy gaps of ca. 2 eV.

3.2.3 {V₁₃As₈}-type polyoxoanions. The structural chemistry of this class of AsPOVs is still underdeveloped. To date, only the results reported by the groups of Yang, Xu and Wang are available. When a {VO}²⁺ group situated in between the {As₂O₅} groups in α-[V^IV₁₄As₈O₄₂]⁴⁻ (discussed in Section 3.2.4) is substituted by a divalent transition metal ion, a polyoxoanion of general composition [M^IIV^IV₁₃As₈O₄₁]⁴⁻ results. Thus, the cluster structure of this monosubstituted hybrid polyoxoanion is comparable to that of the aforementioned di-substituted [M₂V^IV₁₂As₈O₄₀]⁴⁻ polyoxoanion whose two {VO}²⁺ groups were exchanged by two divalent transition metal ions. The monolacunary {V₁₃As₈}-nuclearity polyoxoanion was found to incorporate Zn²⁺ or Cd²⁺ ions as well as Ni²⁺ ion in the way presented above.
Zinc–AsPOV hybrid with a mixed V₁₃/V₁₄-structure. The compound [Zn(2,2′-bipy)₃]₄[(ppz){{Zn(tepa)}₂ZnV₁₃As₈O₄₁(H₂O)}₂][V₁₄As₈O₄₂(0.5H₂O)]₂·4H₂O (tepa = tetraethylenepentamine)¹²⁴ displays two different types of AsPOV constituents, namely the Zn-monosubstituted [H₂O@ZnV^IV₁₃As₈O₄₁]⁴⁻ hybrid and the [0.5H₂O@V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanion. The latter belongs to the class of {V₁₄As₈}-based compounds which are discussed in the next section. The ppz and tepa organic groups in this compound were in situ-formed from dien molecules used in the hydrothermal reaction (Table 3) and coordinate to the Zn²⁺ cations to result in the complexes [Zn₂(ppz)]⁴⁺ and [Zn(tepa)]²⁺. The [Zn₂(ppz)]⁴⁺ unit bridges the two neighbouring α-{V₁₃As₈}-type AsPOVs, each of which is decorated with two [Zn(tepa)]²⁺ moieties through Zn–O bonds (Fig. 14a). The Zn²⁺ ions of the [Zn₂(ppz)]⁴⁺ complex are integrated into the backbones of the [{Zn(tepa)}₂V^IV₁₃As₈O₄₁(H₂O)]²⁻ polyoxoanions to form [{Zn(tepa)}₂ZnV^IV₁₃As₈O₄₁(H₂O)], which are thus held together by ppz linkers via Zn–N bonds.

Fig. 14 (a) The “fully-reduced” [{Zn(tepa)}₂ZnV^IV₁₃As₈O₄₁(H₂O)] building blocks connected via ppz ligand. (b) Polyhedral representation of the “fully-reduced” [(dien)CdV^IV₁₃As₈O₄₁(H₂O)]⁴⁻ hybrid polyoxoanion. (c) Polyhedral representation of the “fully-reduced” AsPOV dimer, {Cl@NiV^IV₁₃As₈O₄₁}₂, expanded by two octahedral [Ni(en)₂(H₂O)] fragments. Hydrogen atoms are omitted for clarity. Colour code: C, grey; N, blue; As, rose; O, red; Cl, lime; V^IVO_x, sky-blue polyhedra; Ni, bright green; Zn, dark green; Cd, brown.

Cadmium–AsPOV hybrid. The “fully-reduced” [Cd(dien)V^IV₁₃As₈O₄₁(H₂O)]⁴⁻ polyoxoanion (Fig. 14b) with the seven-coordinate Cd²⁺ ion was hydrothermally isolated as the compound [Cd(dien)₂]₂[Cd(dien)V₁₃As₈O₄₁(H₂O)]·4H₂O (Table 3) showing strong antiferromagnetic interactions between the spin-1/2 vanadyl {VO}²⁺ moieties.¹²⁷ The other two hydrothermally prepared compounds [Cd(en)₃][Cd(phen)(en)(H₂O)₂][Cd(en)V₁₃As₈O₄₁(H₂O)]·1.5H₂O and [Cd(phen)₂(en)]₂[Cd(phen)V₁₃As₈O₄₁(H₂O)]·21H₂O·phen (phen = 1,10-phenanthroline) feature the [Cd(en)V^IV₁₃As₈O₄₁(H₂O)]⁴⁻ and [Cd(phen)V^IV₁₃As₈O₄₁(H₂O)]⁴⁻ structures, where the Cd²⁺ ions coordinated by en and phen ligands are incorporated into the monolacunary α-type heteroPOV shells (Table 3).¹²⁵ In contrast to [Cd(dien)V₁₃As₈O₄₁(H₂O)]⁴⁻, the antiferromagnetic [Cd(en)V^IV₁₃As₈O₄₁(H₂O)]⁴⁻ and [Cd(phen)V^IV₁₃As₈O₄₁(H₂O)]⁴⁻ hybrid polyoxoanions comprise six-coordinate Cd²⁺ ions. In all these Cd-containing compounds, the [Cd(dien)₂]²⁺, [Cd(en)₃]²⁺, [Cd(phen)(en)(H₂O)₂]²⁺, and [Cd(phen)₂(en)]²⁺ complexes act as countercations.
Nickel–AsPOV hybrid. The “fully-reduced” [NiV^IV₁₃As₈O₄₁]⁴⁻ polyoxoanion with the Ni^II-filled monolacunary POV structure was isolated as the compound {[V₁₃As₈NiClO₄₁][Ni(en)₂(H₂O)][Ni(en)₂]}{[Ni(en)₂(H₂O)₂]_0.5}·4H₂O under hydrothermal reaction conditions (Table 3).¹²⁹ Its crystal structure displays the host–guest [Cl@NiV^IV₁₃As₈O₄₁]⁵⁻ building blocks linked to each other through Ni–O–As bonds to form a dimeric assembly (Fig. 14c). The latter is further connected to the neighbouring dimer via bridging [Ni(en)₂]²⁺ complexes (corner-sharing Ni–O_term–V interactions) to result in an infinite 1D chain. The [Cl@NiV^IV₁₃As₈O₄₁]⁵⁻ structure can be described as being constituted of thirteen square-pyramidal {VO₅} moieties, four handle-like {As₂O₅} groups, and one square-pyramidal {ClNiO₄} entity with the strong Ni–Cl bonding interaction. The [Ni(en)₂(H₂O)]²⁺ and [Ni(en)₂]²⁺ complexes and a half [Ni(en)₂(H₂O)₂]²⁺ complex compensate the negative charge of this hybrid polyoxoanion. The compound exhibits antiferromagnetic properties.

3.2.4 {V₁₄As₈}-type polyoxoanions.
Discrete AsPOVs. This class of AsPOVs offers a large number of discrete polyoxoanions. In 1991, Müller and Döring reported a series of the “fully-reduced” α-AsPOVs with the general formula [Y@V_18–zAs_2zO₄₂]^m− where Y = SO₃²⁻, SO₄²⁻ or H₂O and z = 4. These host–guest polyoxoanions were isolated as crystal solvent-free ammonium compounds (NH₄)₆[V₁₄As₈O₄₂(SO₃)], (NH₄)₆[V₁₄As₈O₄₂(SO₄)] and (NMe₄)₄[V₁₄As₈O₄₂(H₂O)].¹³⁰ The authors highlighted that the V/As ratio, pH, and concentration of the reducing agents such as hydrazine sulfate (N₂H₆SO₄), hydrazine chloride (N₂H₅Cl) und sodium dithionite (Na₂S₂O₄) play an important role in the formation of these {Y@V₁₄As₈O₄₂}-based compounds (Table 3) where the inner voids of the POV shells are occupied by statistically disordered small anionic or neutral guest (Y) species. Here, the D_2d-symmetrical AsPOVs are formally derived from the corresponding [Y@V₁₈O₄₂]^m− structures by replacing four {V^IVO}²⁺ vanadyl groups in the latter with four {As^III₂O}⁴⁺ moieties.

Also in 1991, Huan et al. presented a spherical [(H₂O)_0.5@V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanion that formed as the compound (NMe₄)₄[V₁₄As₈O₄₂(H₂O)_0.5] under hydrothermal conditions (Table 3).¹³¹ This “fully-reduced” AsPOV consists of fourteen condensed {VO₅} square pyramids and four handle-like {As₂O₅} groups and is characterised by a α-{V₁₄As₈O₄₂} shell of rhombicuboctahedral topology (Fig. 15a). This cluster shell encapsulates a disordered H₂O guest molecule. The β-isomer (Fig. 15b) encapsulating a statistically disordered SO₄²⁻ ion was identified in the hydrothermally prepared compound [NH₂(CH₂)₄NH₂]₄[V₁₄As₈O₄₂(SO₄)]·(HSO₄)₂ (Table 3).¹³² In contrast to the α-isomer with D_2d symmetry, the “fully-reduced” β-[SO₄@V^IV₁₄As₈O₄₂]⁶⁻ polyoxoanion exhibits D_4h symmetry which results from the rotation of the three-membered vanadium oxide arc and two {As₂O₅} groups over an eight-membered ring in α-[V^IV₁₄As₈O₄₂]⁴⁻ by 90° around the S₄ axes (Fig. 15a and b). In the structure crystal the (C₄N₂H₁₂)²⁺ piperazine cations, two HSO₄⁻ anions and terminal O atoms of the β-AsPOV building blocks are involved in strong hydrogen bonding interactions, thus generating a 3D network structure.

Fig. 15 Polyhedral representation of the discrete “fully-reduced” α-[V^IV₁₄As₈O₄₂]⁴⁻ (a) and β-[V^IV₁₄As₈O₄₂]⁴⁻ (b) polyoxoanions. Colour code: As, rose; O, red; V^IVO_x, sky-blue polyhedra.

Other polyoxoanions with the same or a similar elemental composition were also reported. The antiferromagnetic compound [NH₂(CH₂CH₂)₂NH₂]₃[V₁₄As₈O₄₂(SO₄)]·6.5H₂O was obtained by the hydrothermal reaction in acidic solution at pH = 3 (Table 3).¹³³ Yang and colleagues showed that protonated [NH₂(CH₂CH₂)₂NH₂]²⁺ amine molecules not only compensate the negative charge of the “fully-reduced” α-[SOV^IV₁₄As₈O₄₂]⁶⁻ polyoxoanion but also assume space-filling and structure-directing roles. The strong hydrogen bonds between the AsPOV building blocks, organic amine molecules, and crystal water molecules result in the formation of a 3D supramolecular array. The “fully-reduced” β-[SO₄@V^IV₁₄As₈O₄₂]⁶⁻ isomer was synthesised as the compound (NH₄)₂(NMe₄)₄[V₁₄As₈O₄₂(SO₄)] under hydrothermal conditions.¹³⁴ The hydrothermal synthesis and characterisation of two other compounds (H₂en)_3.5[V₁₄As₈O₄₂(PO₄)]·2H₂O¹³⁵ and (H₂enMe)₂[V₁₄As₈O₄₂(H₂O)]·3H₂O¹³⁶ based on the “fully-reduced” α-polyoxoanions with encapsulated PO₄³⁻ and H₂O guests, respectively, were described as well (Table 3).

Discrete AsPOV with rubidium countercations. The above series of discrete, host–guest AsPOVs also includes the “fully-reduced” [Cl@V^IV₁₄As₈O₄₂]⁵⁻ polyoxoanion, which was shown to exhibit elongated square gyrobicupola topology that is also observed for [V^IV₁₆Ge₄O₄₂(OH)₄]⁸⁻ (Fig. 8a). This AsPOV pentaanion is the central component of the hydrothermally prepared compound Rb₅[V₁₄As₈O₄₂(Cl)]·2H₂O with a chain-like structure.¹¹⁶ Interestingly, the [Cl@V^IV₁₄As₈O₄₂]⁵⁻ polyoxoanion could only be isolated in the presence of Rb⁺ cations (Table 3) and not with Na⁺, K⁺ or Cs⁺.
Transformation of {V₁₄As₈} into {V₆As₈}-type AsPOV. It is worth mentioning that, in the general case, the [V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanion can be formally converted into the lower-nuclearity [V^IV₆As₈O₂₆]⁴⁻ polyoxoanion by removal of the central eight-membered ring composed of edge-sharing {VO₅} square pyramids.¹³⁷ The “fully-reduced” [V^IV₆As₈O₂₆]⁴⁻ structure thus consists of four handle-like {As₂O₅} groups and six distorted {VO₅} square pyramidal units with the shortest V^IV⋯V^IV distance of ca. 4.5 Å. Furthermore, this AsPOV contains more As than V atoms, thus giving a high As/V ratio of 4/3, a very unusual situation in the structural chemistry of heteroPOVs discussed herein. The [V^IV₆As₈O₂₆]⁴⁻ polyoxoanion crystallised under reducing conditions at room temperature as the compound (N_nBu₄)₄[V₆As₈O₂₆] (Table 3) and shows antiferromagnetic behaviour.
TMC–AsPOV hybrids without specific cation–anion interactions. The two antiferromagnetic compounds [Zn(2,2′-bipy)₃]₂[V₁₄As₈O₄₂(H₂O)]·4H₂O and [Zn(2,2′-bipy)(dien)]₂[V₁₄As₈O₄₂(H₂O)]·2H₂O were prepared hydrothermally at pH = 7 using organic amines as reducing agents for the starting V₂O₅ material (Table 3).¹³⁸ These compounds are composed of the “fully-reduced” α-[H₂O@V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanions with approximate D_2d symmetries and [Zn(2,2′-bipy)₃]²⁺ and [Zn(2,2′-bipy)(dien)]²⁺ countercations, respectively. In contrast to [Zn(2,2′-bipy)₃]²⁺, the [Zn(2,2′-bipy)(dien)]²⁺ complexes are involved in extensive hydrogen bonding interactions with the oxygen atoms of the AsPOVs, thus yielding a densely packed 3D network in the crystal lattice of the [Zn(2,2′-bipy)(dien)]₂[V₁₄As₈O₄₂(H₂O)]·2H₂O compound.

The nickel(II) and cobalt(II) analogues (2,2′-bipy)[Ni(2,2′-bipy)₃]₂[α-V₁₄As₈O₄₂(H₂O)]·3H₂O and [Co(2,2′-bipy)₃]₂[α-V₁₄As₈O₄₂(H₂O)]·3H₂O of the [Zn(2,2′-bipy)₃]₂[α-V₁₄As₈O₄₂(H₂O)]·4H₂O compound were also hydrothermally synthesised (Table 3).¹³⁹ Similarly, the inorganic–organic hybrid compounds [Cu(en)₂]₂[V₁₄As₈O₄₂(H₂O)]·2.5H₂O, [M(1,10-phen)₃]₂[V₁₄As₈O₄₂(H₂O)_0.5]·0.5H₂O (M = Mn, Cd) and [Co(dien)₂]₂[V₁₄As₈O₄₂(H₂O)]·3.5H₂O were obtained by pH-controlled hydrothermal syntheses.¹⁴⁰ Notably, the α-[H₂O@V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanions in the crystal structure of [Co(dien)₂]₂[V₁₄As₈O₄₂(H₂O)]·3.5H₂O are linked by van der Waals forces into channels where the [Co(dien)₂]²⁺ complexes and H₂O molecules act as space-fillers.

The excitation and emission spectra of other hydrothermally prepared compounds [Zn(phen)₃]₂[V₁₄As₈O₄₂(H₂O)]·4H₂O and [Cd(phen)₃]₂[V₁₄As₈O₄₂(H₂O)]·2H₂O (Table 3) were reported that indicate fluorescence in the UV region with an emission peak at ca. 300 nm.¹⁴¹ Although the authors claimed that these compounds feature a new type of the “fully-reduced” [V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanion, namely the γ isomer with d_V⋯V = 3.01–3.04 Å, their structures are in fact characterised by co-crystallised polyoxoanions of α and β types with occupation factors of 0.5. For the rotational isomerism, electronic structures and acidity/basicity properties of “fully-reduced” [V^IV₁₄E₈O₅₀]¹²⁻ heteroPOVs and their chalcogenide-substituted [V^IV₁₄E₈O₄₂X₈]¹²⁻ derivatives (E = Si, Ge, Sn; X = S, Se, Te), see the theoretical work of Kondinski et al.¹⁴² For comparison, Fig. 16 illustrates the α-, γ-, and β-isomeric vanadium oxide skeletons of the {V₁₄As₈}-nuclearity AsPOVs.

Fig. 16 The “fully-reduced” vanadium oxide skeletons of α-/γ-/β-[V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanions. The {As₂O} moieties are not shown. Colour code: O, red; V^IVO_x, sky-blue polyhedra.

TMC–AsPOV hybrids with specific cation–anion interactions. A large series of {V₁₄As₈O₄₂}-type polyoxoanions charge-balanced and covalently coordinated with various TMC complexes were presented by Yang, Xu, Wang, Peng, Das and coworkers. The inorganic–organic hybrid compounds with the α-[V^IV₁₄As₈O₄₂]⁴⁻ building blocks pillared by Zn(II) complexes [Zn(en)₂][(H₂O)Zn(en)₂V₁₄As₈O₄₂(H₂O)]·H₂O, [{(H₂O)Zn(1,10-phen)₂}₂{V₁₄As₈O₄₂(H₂O)}]·4H₂O, [{(H₂O)Zn(2,2′-bipy)₂}{Zn(2,2′-bipy)₂}V₁₄As₈O₄₂(H₂O)_0.5]₂[{(H₂O)Zn(2,2′-bipy)₂V₁₄As₈O₄₂(H₂O)_0.5}₂{Zn(2,2′-bipy)₂}₂]·3H₂O, and [{(H₂O)Zn(4,4′-bipy)}₂{V₁₄As₈O₄₂(H₂O)}]·2H₂O were all synthesised under hydrothermal conditions.¹⁴³ These compounds display differing extended structures as well as different coordination modes of the organodiamine ligands (en, 1,10-phen, 2,2′-bipy, and 4,4′-bipy) used in the synthesis reactions (Table 3). Notably, the formation of the crystalline products was shown to be strongly dependent on the acidic pH range (3–6), which is quite different from the pH (alkaline media) for reactions resulting in the TMC-supported GePOVs, and was seemingly not influenced by the reaction temperatures in the range 130–170 °C. The layered crystal structure of [Zn(en)₂][(H₂O)Zn(en)₂V₁₄As₈O₄₂(H₂O)]·H₂O features the [(H₂O)Zn(en)₂V₁₄As₈O₄₂(H₂O)]²⁻ anion, in which a single [Zn(en)₂(H₂O)]²⁺ moiety is attached to a terminal O atom from the eight-membered ring of the AsPOV, and the [Zn(en)₂]²⁺ complex is the counteraction. The [{(H₂O)Zn(1,10-phen)₂}₂{V₁₄As₈O₄₂(H₂O)}]·4H₂O compound shows the “fully-reduced” [V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanion whose central eight-membered ring is doubly decorated with the [(H₂O)Zn(1,10-phen)₂]²⁺ complexes through covalent Zn–O bonds. The crystal structure is, furthermore, characterised by a 2D supramolecular array formed due to an extended hydrogen bond network. The crystal structure of [{(H₂O)Zn(2,2′-bipy)₂}{Zn(2,2′-bipy)₂}V₁₄As₈O₄₂(H₂O)_0.5]₂[{(H₂O)Zn(2,2′-bipy)₂V₁₄As₈O₄₂(H₂O)_0.5}₂{Zn(2,2′-bipy)₂}₂]·3H₂O displays two crystallographically independent motifs, namely [{(H₂O)Zn(2,2′-bipy)₂V₁₄As₈O₄₂(H₂O)_0.5}₂{Zn(2,2′-bipy)₂}₂] and [{(H₂O)Zn(2,2′-bipy)₂}{Zn(2,2′-bipy)₂}V₁₄As₈O₄₂(H₂O)_0.5]. The structure of the former, neutral dimer consists of two [(H₂O)Zn(2,2′-bipy)₂V₁₄As₈O₄₂(H₂O)_0.5]²⁻ hybrids, in which each AsPOV building block is coordinated with one [(H₂O)Zn(2,2′-bipy)₂]²⁺ complex, and two [Zn(2,2′-bipy)₂]²⁺ complexes function as bridging groups between these two dianions. The structure of another neutral hybrid compound [{(H₂O)Zn(2,2′-bipy)₂}{Zn(2,2′-bipy)₂}V₁₄As₈O₄₂(H₂O)_0.5] consists of the [(H₂O)Zn(2,2′-bipy)₂]²⁺ and [Zn(2,2′-bipy)₂]²⁺ moieties covalently attached to the AsPOV building block via Zn–O bonds. In the structure, the [(H₂O)Zn(2,2′-bipy)₂]²⁺ complex is involved in bonding to a terminal O atom from the eight-membered ring of [V^IV₁₄As₈O₄₂]⁴⁻ and the [Zn(2,2′-bipy)₂]²⁺ complex to that of a {VO₅}-composed trimer capping the AsPOV ring. The crystal structure of [{(H₂O)Zn(4,4′-bipy)}₂{V₁₄As₈O₄₂(H₂O)}]·2H₂O displays a 2D network where the adjacent AsPOVs are linked by two [(H₂O)Zn(4,4′-bipy)]²⁺ fragments through covalent Zn–O bonds. Thus, each polyoxoanion is surrounded by four bridging Zn(II) complexes coordinated to the terminal O atoms of the eight-membered vanadium oxide ring. The 2D network of this compound is further expanded into a 3D supramolecular structure via hydrogen bonds between water molecules and terminal oxygen positions of [V^IV₁₄As₈O₄₂]⁴⁻. According to cyclic voltammograms, the above-described compounds exhibit quasi-reversible redox behaviour.

The extended solid-state structures of the hydrothermally prepared compounds [Zn(2,2′-bipy)₂]₂[V₁₄As₈O₄₂(H₂O)]·H₂O (Fig. 17a), [Cd(2,2′-bipy)₃][Cd(dien)V₁₄As₈O₄₂(H₂O)], and [Ni(en)₂]₃[V₁₄As₈O₄₂(HPO₃)]·4H₂O (Table 3) display the “fully-reduced” α-[V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanions interconnected through the corresponding TMC complexes [Zn(2,2′-bipy)₂]²⁺, [Cd(dien)]²⁺, and [Ni(en)₂]²⁺, respectively.¹⁴⁴ The Zn(II)- and Cd(II)-containing compounds organise into 1D chain structures and are characterised by overall antiferromagnetic coupling. In contrast, the Ni(II)-containing compound exhibits a 2D structure and produces no signal for V^IV ions in the EPR spectrum at room temperature.

Fig. 17 Polyhedral representations of segments of the polymeric solid-state structures of [Zn(2,2′-bipy)₂]₂[V₁₄As₈O₄₂(H₂O)]·H₂O (a), [Co(2,2′-bipy)₂]₂[V₁₄As₈O₄₂(H₂O)]·H₂O (b), and [Cu(en)₂]₃[V₁₄As₈O₄₂(CO₃)]·10H₂O (c; only four [Cu(en)₂]²⁺ bridging groups are shown). Hydrogen atoms are omitted for clarity. Colour code: C, grey; N, blue; As, rose; O, red; V^IV, sky blue; V^IVO_x, sky-blue polyhedra; Co, plum; Cu, black; Zn, dark green.

The antiferromagnetic compound [Ni(en)₂]₃[V₁₄As₈O₄₂(SO₄)]·4.5H₂O with a 2D layer structure was prepared under hydrothermal conditions (Table 3) and reveals strong covalent attachment between the adjacent α-[SO₄@V^IV₁₄As₈O₄₂]⁶⁻ polyoxoanions through V=O–Ni–O=V connectivities.¹⁴⁵ A 2D network is formed as POV is bound to four neighbouring polyoxoanions via an octahedrally coordinated Ni(II) centre.

The compounds [{Ni(en)₂}₄(4,4′-bipy)₄{Ni(H₂O)₂}]₂[V₁₄As₈O₄₂(NO₃)]₄·16H₂O and [Ni(en)₂(H₂O)₂]₂[{Ni(en)₂(H₂O)}₂V₁₄As₈O₄₂(NO₃)][{Ni(en)₂}V₁₄As₈O₄₂(NO₃)]·6H₂O (Table 3) were hydrothermally prepared and showed high-dimensional organic–inorganic hybrid nanostructures.¹⁴⁶ The compound bearing 4,4′-bipy ligands consists of two bridging [{Ni(en)₂}₄(4,4′-bipy)₄{Ni(H₂O)₂}]¹⁰⁺ fragments and four α-[NO₃@V^IV₁₄As₈O₄₂]⁵⁻ polyoxoanions which are joined through Ni–O bonds to form a pillared box-like structure with a cavity of ca. 600 ų (Fig. 18). In contrast to the previous compound, the compound [Ni(en)₂(H₂O)₂]₂[{Ni(en)₂(H₂O)}₂V₁₄As₈O₄₂(NO₃)][{Ni(en)₂}V₁₄As₈O₄₂(NO₃)]·6H₂O is based on the “fully-reduced” β-[NO₃@V^IV₁₄As₈O₄₂]⁵⁻ polyoxoanions. Its crystal structure shows three crystallographically independent motifs: (i) bis-{Ni(en)₂(H₂O)}-decorated AsPOV anion [{Ni(en)₂(H₂O)}₂V₁₄As₈O₄₂(NO₃)]⁻; (ii) 1D chain polymer consisting of [{Ni(en)₂}₂V₁₄As₈O₄₂(NO₃)]⁻ monoanions; (iii) [Ni(en)₂(H₂O)₂]²⁺ complexes.

Fig. 18 Two different views of the box-like motif found in the solid-state structure of [{Ni(en)₂}₄(4,4′-bipy)₄{Ni(H₂O)₂}]₂[V₁₄As₈O₄₂(NO₃)]₄·16H₂O. Hydrogen atoms and lattice water molecules are omitted for clarity. Colour code: C, grey; N, blue; As, rose; O, red; V^IVO_x, sky-blue polyhedra; Ni, bright green.

The hydrothermal synthesis of ‘metal-controlled’ inorganic–organic self-assemblies with compositions [Ni(enMe)₃]₄[Ni(enMe)₂][V₁₄As₈O₄₂(NO₃)]₂·8H₂O and [Co(en)₃][Co(en)₂V₁₄As₈O₄₂(H₂O)]·16H₂O (Table 3) was reported. These antiferromagnetic compounds are based on the “fully-reduced” α-[V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanions accommodating NO₃⁻ and H₂O as guests.¹²⁵ The crystal structure of the former AsPOV consists of [Ni(enMe)₂{V₁₄As₈O₄₂(NO₃)}₂]⁸⁻ dimers, four [Ni(enMe)₃]²⁺ countercations and lattice H₂O molecules. The [Ni(enMe)₂{V₁₄As₈O₄₂(NO₃)}₂]⁸⁻ dimer is constructed from two “fully-reduced” [NO₃@V^IV₁₄As₈O₄₂]⁵⁻ moieties linked through the bridging [Ni(enMe)₂]²⁺ complex. The crystal structure of the latter compound is characterised by 1D chains consisting of the [Co(en)₂V₁₄As₈O₄₂(H₂O)]²⁻ hybrids, [Co(en)₃]²⁺ countercations and lattice H₂O molecules. The [Co(en)₂]²⁺ complexes act here as bridging ligands for the neighbouring AsPOVs.

Another Co(II)-containing compound with the formula [Co(2,2′-bipy)₂]₂[V₁₄As₈O₄₂(H₂O)]·H₂O (Fig. 17b) and a tubular structure was prepared hydrothermally in acidified solution (Table 3).¹⁴⁷ The crystal structure of this solid being stable up to ca. 370 °C shows closed helical chains of {[Co(2,2′-bipy)₂][V^IV₁₄As₈O₄₂(H₂O)]}_∞ hybrid building blocks where the [Co(2,2′-bipy)₂]²⁺ complexes covalently link the α-isomeric AsPOVs through Co–O bonds.

A number of high-dimensional hybrid materials based on the α-[V^IV₁₄As₈O₄₂]⁴⁻ building block, namely [M(bbi)₂]₂[V₁₄As₈O₄₂(H₂O)] (M = Co, Ni, Zn) and [Cu(bbi)]₄[V₁₄As₈O₄₂(H₂O)] were synthesised under hydrothermal conditions (pH of ca. 5.0) where oxalic acid can act as a reducing agent for the V^V source (Table 3).¹⁴⁸ These compounds containing the soft N-donor 1,1′-(1,4-butanediyl)bis(imidazole) (= bbi, C₁₀H₁₄N₄) extend a series of POV-templated structures including, e.g., that of the [H₄V₁₈O₄₆(SiO)₈(dab)₄(H₂O)]·4H₂O compound bearing the bidentate dab ligand.⁹⁷ The isostructural [M(bbi)₂]₂[V₁₄As₈O₄₂(H₂O)] (M = Co, Ni, Zn) compounds with antiferromagnetic properties are characterised by a binodal (4,6)-connected 2D network structure, Schläfli symbol (3⁴·4²)(3⁴·4⁴·5⁴·6³)₂, where each of the neighbouring AsPOVs is covalently coordinated to four bridging [M(bbi)₂]²⁺ complexes through M–O bonds. The coordination modes of the bbi ligands allow them to connect these four M²⁺ ions in such a manner that each polyoxoanion is located within a closed {M(bbi)}₄ ring. In the structure of the compound [Cu(bbi)]₄[V₁₄As₈O₄₂(H₂O)] a 3D network is observed made up of the AsPOVs interacting covalently with the bbi-bridged Cu⁺ cations from the adjacent wave-like chains of the [Cu(bbi)]_n moieties. The latter surrounding each polyoxoanion up and down results in the formation of 1D ladder-like [Cu(bbi)]⁺ double chains.

The compound [Cu(en)₂]₃[V₁₄As₈O₄₂(CO₃)]·10H₂O with a 2D network structure was prepared hydrothermally (Table 3) and contains α-[CO₃@V^IV₁₄As₈O₄₂]⁶⁻ with a carbonate ion in the inner void of the polyoxoanion.¹⁴⁹ In the solid state, each AsPOV is surrounded by six [Cu(en)₂]²⁺ bridging groups with Cu–O_term–V linkages (Fig. 17c).

Lanthanoid–AsPOV hybrids. The first representatives of AsPOVs with covalently attached aqua-lanthanoid(III) complexes were reported in 2009.¹⁵⁰ Arumuganathan and Das isolated three isostructural [{Ln(H₂O)₆}₂V₁₄As₈O₄₂(SO₃)]·8H₂O compounds (Ln = La^III, Sm^III, and Ce^III) from aqueous reaction solutions at room temperature using (NH₄)₆[V₁₄As₈O₄₂(SO₃)] and Ln(NO₃)₃·6H₂O as starting materials (Table 3). The main structural motif of the all-inorganic {V₁₄As₈}-type compounds represents the “fully-reduced” β-[V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanion with approximate D_2d symmetry, which encapsulate a sulfite anion. As was demonstrated by thermal analysis and mass spectrometry experiments, the aqua-Ln(III)-capped AsPOVs release gaseous SO₂ at temperatures 520–580 °C, whereas the (NH₄)₆[V₁₄As₈O₄₂(SO₃)] precursor releases SO₂ already at 480–520 °C. The crystal structures of these compounds display 2D layered coordination polymers where each of six [Ln(H₂O)₆]³⁺ complexes surrounding the [SO₃@V^IV₁₄As₈O₄₂]⁶⁻ polyoxoanion through one covalent Ln–O_term–V bond coordinates to two other adjacent AsPOVs. Thus, the aqua-Ln(III) complexes can indeed link to AsPOVs, which resulted in the formation of extended structures where La^III ions are nine-fold coordinated and reside in a monocapped square-antiprismatic coordination environment. Magnetic susceptibility studies performed for the samples with the 4f⁰-La^III, 4f⁵-Sm^III and 4f¹-Ce^III ions indicated that [{Ln(H₂O)₆}₂V₁₄As₈O₄₂(SO₃)]·8H₂O are antiferromagnetically coupled materials, with no significant coupling between the lanthanide(III) ions and the POV.

3.2.5 {V₁₅As₆}-type polyoxoanions.
Discrete AsPOV with potassium countercations. The first synthesised AsPOV dates back to 1988, when Müller and Döring published the compound K₆[V₁₅As₆O₄₂(H₂O)]·8H₂O which was prepared by reduction of vanadate with hydrazinium sulfate in the presence of As₂O₃ in aqueous solution at 85 °C (Table 3).¹⁵¹ This compound is composed of the “fully-reduced” [H₂O@V^IV₁₅As₆O₄₂]⁶⁻ polyoxoanion with crystallographic D₃ symmetry and six K⁺ countercations. The “hydrated” potassium ions provide the networking of the AsPOVs in the crystal structure (rhombohedral cell; for hexagonal cell, see K₆[V₁₅As₆O₄₂(H₂O)]·6H₂O).¹⁵² The [H₂O@V^IV₁₅As₆O₄₂]⁶⁻ polyoxoanion with an encapsulated H₂O molecule (Fig. 19a) is the first member of the series of compounds with general formula [V_18–zAs_2zO₄₂] (z = 3). The structural motif of [H₂O@V^IV₁₅As₆O₄₂]⁶⁻ is derived from the [V₁₈O₄₂]¹²⁻ structure by replacing three {VO₅} units by three handle-like {As₂O₅} groups (for comparison, see [V^IV₁₅Si₆O₄₂(OH)₆]⁶⁻ in Fig. 6c). This polyoxoanion thus consists of fifteen distorted {VO₅} square pyramids that are interlinked through the basal O vertices and edges and of six {AsO₃} trigonal pyramids. Interestingly, Müller and Döring noticed that this prominent AsPOV “may be regarded as a model for species that are formed during poisoning of the V₂O₅catalyst (which contains V^IVcenters) by arsenic”.¹⁵¹

Fig. 19 (a) The “fully-reduced” [H₂O@V^IV₁₅As₆O₄₂]⁶⁻ polyoxoanion. (b) The “fully-reduced” [{Zn(en)(enMe)}V^IV₁₅As₆O₄₂]⁴⁻ hybrid polyoxoanion. Hydrogen atoms and encapsulated water molecule are not shown. (c) A segment of the polymeric structure of [Zn₂(dien)₃(H₂O)₂]_0.5[{Zn₂(dien)₃}V₁₅As₆O₄₂(H₂O)]·2H₂O. (d) A segment of the extended solid-state structure of [Co(enMe)₂]₃[V₁₅As₆O₄₂(H₂O)]·2H₂O. (e) A segment of the extended solid-state structure of [Cu(en)₂]_1.5[H₃V₁₅As₆O₄₂(H₂O)]·3H₂O. Colour code: C, grey; N, blue; As, rose; O, red; V^IV, sky blue; V^IVO_x, sky-blue polyhedra; Co, plum; Cu, black; Zn, dark green.

TMC–AsPOV hybrids without specific cation–anion interactions. The compound [Zn(H₂O)₄]₂[H₂V₁₅As₆O₄₂(H₂O)]·2H₂O was obtained under hydrothermal conditions (Table 3) and is composed of the doubly protonated [H₂O@H₂V^IV₁₅As₆O₄₂]⁴⁻ polyoxoanion and two [Zn(H₂O)₄]²⁺ countercations.¹⁵³ In the structure, these two constituents are held together by hydrogen O–H⋯O_term bonds, which results in the formation of a 3D network structure. In studies of third-order nonlinear optical (NLO) properties of [Zn(H₂O)₄]₂[H₂V₁₅As₆O₄₂(H₂O)]·2H₂O the compound exhibited strong nonlinear absorption.
TMC–AsPOV hybrids with specific cation–anion interactions. The structures of two hydrothermally synthesised compounds [Zn(en)₂][Zn(en)₂(H₂O)₂][{Zn(en)(enMe)}V₁₅As₆O₄₂(H₂O)]·4H₂O and [Zn₂(enMe)₂(en)₃][{Zn(enMe)₂}V₁₅As₆O₄₂(H₂O)]·4H₂O (Table 3) consist of the [{Zn(en)(enMe)}V^IV₁₅As₆O₄₂(H₂O)]⁴⁻ (Fig. 19b) and [{Zn(enMe)₂V^IV₁₅As₆O₄₂(H₂O)}]⁴⁻hybrid polyoxoanions whose AsPOV shells are coordinatively functionalised with Zn²⁺ complexes through Zn–O_term–V bonds.¹⁵⁴ Charge neutrality of the compounds is achieved by cationic {[Zn(en)₂][Zn(en)₂(H₂O)₂]}⁴⁺ and [Zn₂(enMe)₂(en)₃]⁴⁺ complexes. By contrast, a [{{Zn(en)₂}₂V₁₅As₆O₄₂(H₂O)}₂{Zn(en)₂}]²⁻ dimer in another hydrothermally prepared compound (Hen)₂[{{Zn(en)₂}₂V₁₅As₆O₄₂(H₂O)}₂{Zn(en)₂}]·3H₂O (Table 3) is charge-balanced by two singly protonated Hen⁺ cations.¹⁵⁵ The crystal structure of the latter compound shows two [H₂O@V^IV₁₅As₆O₄₂{Zn(en)₂}₂]²⁻ building blocks which are joined by a bridging [Zn(en)₂]²⁺ unit through Zn–O bonds.

The “fully-reduced” [H₂O@V^IV₁₅As₆O₄₂]⁶⁻ polyoxoanions in the crystal structures of the antiferromagnetic compounds [Zn₂(dien)₃(H₂O)₂]_0.5[{Zn₂(dien)₃}V₁₅As₆O₄₂(H₂O)]·2H₂O (Fig. 19c, Table 3) and [Ni(2,2′-bipy)₃]₂[{Ni(en)₂}V₁₅As₆O₄₂(H₂O)]·9.5H₂O are interlinked by [Zn₂(dien)₃]²⁺ and [Ni(en)₂]²⁺ moieties to form 1D helical chains and 1D infinite straight chains, respectively.¹⁵⁶ The 1D helical chains are involved in extensive hydrogen bonding interactions with discrete [Zn₂(dien)₃(H₂O)₂]⁴⁺ complexes, thus generating a 2D network structure; a further expansion of the hydrogen bond pattern involving [Zn₂(dien)₃]⁴⁺ bridging units results in a 3D supramolecular array. The 1D chains based on the [H₂O@V^IV₁₅As₆O₄₂{Ni(en)₂}]⁴⁻ polyoxoanions have been reported as possessing molecular recognition ability for the chiral [Ni(2,2′-bipy)₃]²⁺ guests.

The hydrothermally prepared compound [Co(enMe)₂]₃[V₁₅As₆O₄₂(H₂O)]·2H₂O (Fig. 19d and Table 3) is characterised by a 2D structure where each [H₂O@V^IV₁₅As₆O₄₂]⁶⁻ polyoxoanion is coordinated by six [Co(enMe)₂]²⁺ moieties which in turn link the neighbouring AsPOVs to each other through Co–O_term–V bonds.¹⁵⁶ The crystal structure of the hydrothermally synthesised compound [Co(en)₃][{Co(en)₂}₂V₁₅As₆O₄₂]·4H₂O (Table 3) shows an 1D infinite chain structure generated by Co–O_apical–V linkages.¹⁵⁷ The neighbouring [V^IV₁₅As₆O₄₂]⁶⁻ polyoxoanions are covalently linked through two six-coordinate cobalt(II) ions of μ₂-{Co^II(en)₂}²⁺ moieties; isolated [Co^II(en)₃]²⁺ cations reside in interchain regions. This “fully-reduced” AsPOV exhibits weak antiferromagnetic exchange interactions between the vanadyl moieties.

A {V₁₅As₆}-type polyoxoanion with a higher degree of protonation was found in the hydrothermally synthesised compound [Cu(en)₂]_1.5[H₃V₁₅As₆O₄₂(H₂O)]·3H₂O (Table 3).¹⁵⁸ In the crystal structure, the “fully-reduced” [H₂O@H₃V^IV₁₅As₆O₄₂]³⁻ polyoxoanions are bridged by [Cu(en)₂]²⁺ moieties leading to formation of a 1D sinusoidal chain (Fig. 19e). The series of protonated [H₃V^IV₁₅As₆O₄₂]³⁻ and [H₂V^IV₁₅As₆O₄₂]^4− 153 polyoxoanions was further expanded by a [HV^IV₁₅As₆O₄₂]⁵⁻ polyoxoanion as observed in the compound [Cu(enMe)₂]_2.5[HV₁₅As₆O₄₂(H₂O)]·2H₂O exhibiting antiferromagnetic properties.¹³⁶ This compound was prepared hydrothermally (Table 3) and shows a brick-wall-like 2D layer structure where the “fully-reduced” [H₂O@HV^IV₁₅As₆O₄₂]⁵⁻ polyoxoanions are doubly-bridged by the [Cu(enMe)₂]²⁺ moieties.

3.2.6 {V₁₆As₄}-type polyoxoanion. The {V₁₆As₄O₄₂} building block of D_2h symmetry, which is derived from the general formula [V_18−zAs_2zO₄₂] with z = 2, was isolated hydrothermally as the antiferromagnetic compound [Zn₂(dien)₃][{Zn(dien)}₂V₁₆As₄O₄₂(H₂O)]·3H₂O (Table 3).¹⁵⁹ Its crystal structure is characterised by 1D chains where the neighbouring, “fully-reduced” [H₂O@V^IV₁₆As₄O₄₂]⁸⁻ polyoxoanions are bridged by dual mononuclear {Zn(dien)}²⁺ complexes through Zn–O bonds (Fig. 20). The dinuclear [Zn₂(dien)₃]⁴⁺ countercations reside in the interchain regions. To point out some structural relationships, this AsPOV can formally be converted, on the one hand, into the {V₁₈O₄₂} archetype by replacing two diagonal handle-like {As₂O₅} groups in the former by two square-pyramidal {VO₅} units or, on the other hand, the formal substitution of two diagonal {VO₅} square pyramids – that interlock two eight-membered rings in the {V₁₆As₄O₄₂} building block – by two additional {As₂O₅} groups will result in the polyoxoanion with the composition β-[V^IV₁₄As₈O₄₂]⁴⁻ (z = 4).

Fig. 20 Polyhedral representation of a segment of the polymeric structure of [Zn₂(dien)₃][{Zn(dien)}₂V₁₆As₄O₄₂(H₂O)]·3H₂O. Colour code: C, grey; N, blue; As, rose; O, red; V^IVO_x, sky-blue polyhedra; Zn, dark green.

3.2.7 POVs with organoarsonate ligands.
{O₃AsPh}/{O₄AsPh}-functionalised AsPOVs. The vanadium oxide compounds containing phenylarsonate (O₃AsPh)²⁻ moieties were first described in the early 1990s. The antiferromagnetic compound [V₂O₄(HO₃AsPh)]·H₂O with a layered mixed-valent (V^V/V^IV) structure was hydrothermally synthesised and characterised by Huan et al.,¹⁶⁰ while the compound displaying a phenylarsonate POV structure of higher nuclearity was described by Zubieta and colleagues^161,162 The mixed-valent [H₂{V^IV₂V^V₄O₁₀(O₃AsPh)₆}]²⁻ polyoxoanion was isolated as the compound (N_nBu₄)₂[H₂{V₆O₁₀(O₃AsPh)₆}]·2H₂O (Table 3).¹⁶¹

Later on, the solvothermally prepared compounds (H₇O₃)₂(N_nBu₄)₂[(MeOH)₂V₁₂O₁₄(OH)₄(O₃AsPh)₁₀]·H₂O and (NEt₄)₂[V₁₂O₁₂(OH)₂(H₂O)₄(O₃AsPh)₁₀(HO₃AsPh)₄]·6H₂O (Table 3) exhibiting, respectively, the nanoscopic organoarsonate POV cages with compositions [(MeOH)₂@V₁₂O₁₄(OH)₄(O₃AsPh)₁₀]⁴⁻ (with approx. D_2h symmetry) and [(H₂O)₂@V₁₂O₁₂(OH)₂(H₂O)₂(O₃AsPh)₁₀(HO₃AsPh)₄]²⁻ that cannot be derived from the {V₁₈O₄₂} archetype.¹⁶² The “fully-reduced” [(MeOH)₂@V^IV₁₂O₁₄(OH)₄(O₃AsPh)₁₀]⁴⁻ polyoxoanion is built up of edge- and corner-sharing {VO₅} square pyramids, phenylarsonate tetrahedra and square pyramids and can be regarded as being composed of two {V^IV₄O₅(O₃AsPh)} fragments bridged by two {V^IV₂O₂(OH)₂(O₃AsPh)₄}⁶⁻ moieties. The dianionic, “fully-reduced” AsPOV [(H₂O)₂@V^IV₁₂O₁₂(OH)₂(H₂O)₂(O₃AsPh)₁₀(HO₃AsPh)₄]²⁻ consists of {VO₅} square pyramids, {VO₆} octahedra and phenylarsonate tetrahedra and square pyramids. The structure of this AsPOV is characterised by two {V^IV₅O₆(O₃AsPh)₃}²⁺ fragments bridged by two {V^IV(OH)(H₂O)(O₃AsPh)₂(HO₃AsPh)₂}³⁻ moieties. The geometric equivalence of the {AsPh}⁴⁺ groups to {VO}³⁺ by virtue of the nearly identical As^V–O and V^V–O bond lengths was stressed by Khan and Zubieta.

In 2011, Zhang and Schmitt reported a unique series of symmetrical nanoscopic vanadium oxide supramolecular coordination cages functionalised with phenylarsonate ligands (Fig. 21) and exploited the topologies of their hollow structures together with the template effects of the octahedral {X_z(H₂O)_6−z} guest assemblies (X = Br, Cl and z = 2, 4, 6).¹⁶³ Such “fully-reduced” V^IV, mixed-valent V^V/V^IV and fully-oxidised V^V AsPOVs characterised by the high-nuclearity {V₁₆As₈}, {V₁₆As₁₀}, {V₂₀As₈} and {V₂₄As₈} cages were found in the blue compounds (HNEt₃)₂[{Br₂(H₂O)₄}(V^IVO)₁₆(OH)₈(O₄AsPh)₂(O₃AsPh)₈]·6MeCN and (HNEt₃)₂[{Cl₂(H₂O)₄}(V^IVO)₁₆(OH)₈(O₄AsPh)₂(O₃AsPh)₈]·2H₂O as well as the green compounds H₅[{Cl₄(H₂O)₂}(V^VO)₁₆O₁₆(O₃AsPh)₈]Cl·4H₂O·3MeCN, [{Cl₄(H₂O)₂}(V^VO)₁₆(V^IVO)₄O₁₆(OH)₄(O₃AsPh)₈]·7H₂O·3MeCN and H₁₀[{Cl₆}(V^VO)₁₆(V^IVO)₈O₂₄(O₃AsPh)₈]Cl₄·10H₂O·2MeCN (Table 3). Their structures contain four- ({O₃AsPh}) or five-fold ({O₄AsPh}) coordinated As^V centres. The isomeric, “fully-reduced” {V₁₆As₁₀} cages in the blue compounds consist of sixteen {VO₅} square-pyramids and ten fully deprotonated phenylarsonate ligands. Their AsPOV building block represents a modified [V₁₈O₄₂]¹²⁻ structure in which two {VO₅} caps on one diagonal are replaced with two {O₄AsPh} moieties and two groups by four {O₃AsPh} moieties slice the structure horizontally to isolate an eight-membered {VO₅}-belt (Fig. 21, top). The key structural difference between the {V₁₆As₁₀} cages accommodating octahedral {Br₂(H₂O)₄} and {Cl₂(H₂O)₄} guest templates in their inner voids is that the two convex {V^IV₄O₅(O₃AsPh)} moieties in the Cl-containing AsPOV are rotated by 45° (isomer 2). This structural change is the result of the considerably different ionic radii of Br⁻ and Cl⁻ ions and the energies of V⋯Br and V⋯Cl interactions. The encapsulated symmetrical {Br₂(H₂O)₄} and {Cl₂(H₂O)₄} octahedra with the Br⁻ and Cl⁻ ions situated in the apical positions show Br⋯H₂O and Cl⋯H₂O distances ranging from 3.13 to 3.36 Å and from 3.03 to 3.18 Å, respectively. In addition, Br⋯Br distance of 5.28 Å is slightly shorter than the Cl⋯Cl distance of 5.31 Å. The intramolecular V⋯Br and V⋯Cl separations are between 3.47 and 3.51 Å. It was concluded that the nature and geometry of these octahedral guest assemblies determines the nuclearity, size, and topology of the produced organoarsonate POV cages.

Fig. 21 Polyhedral representations of the “fully-reduced” {V₁₆As₁₀}-nuclearity AsPOVs (top) as well as the fully-oxidised {V₁₆As₈}-nuclearity AsPOV, and the mixed-valent {V₂₀As₈}- and {V₂₄As₈}-nuclearity AsPOVs (bottom). Encapsulated octahedral guest assemblies are omitted for clarity. Colour code: C, grey; As, rose; O, red; V^IVO_x, sky-blue polyhedra; V^VO_x, light-orange polyhedra; V^V/V^IVO_x, dark-grey polyhedra.

The fully-oxidised {V₁₆As₈}- and mixed-valent {V₂₀As₈}- and {V₂₄As₈}-type building blocks (Fig. 21, bottom) of the above-mentioned green compounds were synthesised using Dy(NO₃)₃·nH₂O (n ≈ 6), where presumably nitrate acts as an oxidant, different quantities of which was shown to influence the nuclearity of these AsPOV cages and the formal oxidation states of their V atoms. Thus, a gradual increase in the amount of the oxidant led ultimately to the fully-oxidised {V₁₆As₈}-nuclearity cage. The molecular structures of these three AsPOVs are constructed of sixteen, twenty and twenty-four edge- and corner-sharing {VO₅} square-pyramids, respectively. Each POV shell is ligated by eight phenylarsonate groups. The {V₁₆As₈}-toroid cage encapsulates an octahedral {Cl₄(H₂O)₂} guest template, in which the closest Cl⋯Cl/Cl⋯H₂O distances amount to 3.97 Å/3.41 Å. The intramolecular V⋯Cl separations range from 2.82 Å to 3.02 Å. The {V₂₀As₈} cage can be seen as the toroidal structure of {V₁₆As₈} decorated with two {(V^IVO)₂(μ₂-OH)₂} units and is stabilised by the {Cl₄(H₂O)₂} assembly as well. The Cl⋯Cl and Cl⋯H₂O distances in this {Cl₄(H₂O)₂} octahedron are 3.98 Å and 3.24–3.80 Å, respectively. The closest intramolecular V⋯Cl distances lie in the range 2.81–3.10 Å. The two encapsulated H₂O molecules and the V ions of the {(V^IVO)₂(μ₂-OH)₂} entities are involved in V⋯O_water interactions at a distances of 2.36 Å. The {V₂₄As₈} cage encapsulates a {Cl₆} aggregate with d_Cl⋯Cl = 3.91–4.01 Å the vertices of which are capped by six square {V₄O₈} fragments with electrophilic inner and nucleophilic outer environments. The closest V⋯Cl separations are in the range 2.94–3.033. The Archimedean body {V₂₄} polyhedron (14 faces, 36 edges, 24 vertices) deduced from the mixed-valent {(V^VO)₁₆(V^IVO)₈O₂₄(O₃AsPh)₈} fragment that can be regarded as a Keplerate displays a truncated octahedral topology. The {V₂₄} polyhedron in the {V₂₄As₈} cage can thus be classified as one of the thirteen Archimedean solids.¹⁶⁴ The vanadium skeletons of some of the discussed organoarsonate POV cages are illustrated in Fig. 22.

Fig. 22 Topological representations of the vanadium skeletons deduced from some high-nuclearity organoarsonate POV cages represented in Fig. 21 and 23. Colour code: V^IV, sky blue; V^V, light orange.

{O₃AsC₆H₄-4-NH₂}-functionalised AsPOVs. The group of Schmitt also reported two nanoscopic cages with compositions [V₁₄O₁₆(OH)₈(O₃AsC₆H₄-4-NH₂)₁₀]⁴⁻ (Fig. 23), which shows a defect AsPOV structure from the {V₁₆As₁₀}-nuclearity cage (isomer 1, Fig. 21), and [V₁₀O₁₈(O₃AsC₆H₄-4-NH₂)₇(DMF)₂]⁵⁻.¹⁶⁵ The compounds based on these AsPOVs were isolated from the condensation reactions under reducing conditions in aqueous solution (Table 3). The different nature of acids (HCl and HNO₃) used in the synthesis of these compounds strongly influenced the nuclearity of the cages, resulting in the {V₁₄As₁₀}- and {V₁₀As₇}-type polyoxoanions, respectively. In contrast to the {O₃AsPh}/{O₄AsPh}-functionalised AsPOVs, the POV shells of these polyoxoanions are decorated with terminal (4-aminophenyl)arsonate ligands {O₃AsC₆H₄-4-NH₂}. Their skeleton structures are composed exclusively of {VO₅} square-pyramids. Whereas the electrophilic void of the “fully-reduced” {V₁₄As₁₀} cage (dimensions: 5.8 × 5.9 × 5.9 Å) with an almost ideal cubic arrangement of the As atoms contains two Cl⁻ ions and four H₂O molecules forming a stabilising octahedral assembly, the mixed-valent asymmetric {V₁₀As₇} cage is characterised by a hexagonal packing due to the amine functionalities engaged in hydrogen bonds in the structure.

Fig. 23 The “fully-reduced” [V^IV₁₄O₁₆(OH)₈(O₃AsC₆H₄-4-NH₂)₁₀]⁴⁻ (left) and [V^IV₁₂O₁₄(OH)₄(O₃AsC₆H₄-4-NH₂)₁₀]⁴⁻ (right) AsPOVs. H omitted for clarity. Colour code: C, grey; N, blue; As, rose; O, red; V^IVO_x, sky-blue polyhedra.

The [V₁₂O₁₄(OH)₄(O₃AsC₆H₄-4-NH₂)₁₀]⁴⁻ polyoxoanion with the V^IV/arsonate nuclearity [12(VO₅):10(O₃AsR)], lower than that [14(VO₅): 10(O₃AsR)] of the above-mentioned [V₁₄O₁₆(OH)₈(O₃AsC₆H₄-4-NH₂)₁₀]⁴⁻ polyoxoanion, was isolated as the compound Na₄(H₂O)₁₀[{V₁₂O₁₄(OH)₄(H₂O)₃(O₃AsC₆H₄-4-NH₂)₁₀}]·1.5DMF·1.25H₂O (Table 3) from a condensation reaction involving p-arsanilic acid in a H₂O/dimethylformamide (DMF) mixture.¹⁶⁶ The main structural difference between these two AsPOVs (Fig. 22) is the number of the {VO₅} square pyramids sandwiched between two tetranuclear {LV^IV₄O₁₂} subunits where L is the (4-aminophenyl)arsonate ligand. The {V₁₄As₁₀} cage comprises two separated hydroxy-bridged {O₄V^IV(OH)₂V^IV(OH)₂V^IVO₄} trimers, whilst the {V₁₂As₁₀} cage has only two separated, partially hydrated {O₄V^IV(OH)₂V^IVO₄} dimers. The {VO₅} square pyramids in the belt of each of these molecular cages are connected to two tetranuclear {LV^IV₄O₁₂} subunits via additional eight (4-aminophenyl)arsonate ligands (Fig. 23). The synthesis and structural characterisation of the compound Na₅[V₅O₉(O₃AsC₆H₄-4-NH₂)₄]·20.5H₂O·3DMF with the low-nuclearity inorganic–organic calix-type building block [V₅O₉(O₃AsC₆H₄-4-NH₂)₄]⁵⁻ comprising mixed-valent V^V/V^IV atoms were described as well (Table 3).¹⁶⁶ The preliminary magnetic studies performed for these two compounds showed the presence of antiferromagnetic exchange interactions and S = 0 ground states.

An example of the {O₃AsC₆H₄-4-NH₂}-functionalised AsPOV with a quasi-planar, polycyclic-type structure was also reported. The fully-oxidised [V^V₁₀O₂₄(O₃AsC₆H₄-4-NH₂)₃]⁴⁻ (Fig. 24a) polyoxoanion was isolated as the compound (N_nBu₄)₂(NH₄)₂[V₁₀O₂₄(O₃AsC₆H₄-4-NH₂)₃] (Table 3).¹⁶¹ The structure of this AsPOV can be regarded as a [V₉O₂₁(O₃AsC₆H₄-4-NH₂)₃]³⁻ toroid that encloses a {VO₃}⁻ unit in the centre of the wheel. The most unique feature of [V^V₁₀O₂₄(O₃AsC₆H₄-4-NH₂)₃]⁴⁻ is that it contains a wheel-type [V₇O₂₄]¹³⁻ substructure (Fig. 24b). Remarkably, the structure of this heptanuclear [V₇O₂₄]¹³⁻ polyoxoanion displays striking similarities to the Anderson-type structures, e.g. of [TeM₆O₂₄]⁶⁻,¹⁶⁷ [M₇O₂₄]⁶⁻ (M = Mo, W),¹⁶⁸ and [Bi₇I₂₄]^3− 169 (Fig. 24c). The overall composition of [V^V₁₀O₂₄(O₃AsC₆H₄-4-NH₂)₃]⁴⁻ formally consists of a double layer of polyhedra (bicapped Anderson type), with the one being composed of [V₇O₂₄]¹³⁻ and the second, of a [V₃(RAsO₃)₃]⁹⁺ ring. Another interesting feature is that it can be reversibly reduced by one electron to give the green-brown compound with composition [V₁₀O₂₄(O₃AsC₆H₄-4-NH₂)₃]⁵⁻. According to electrochemical and magnetic measurements, this compound as well as the aforementioned (N_nBu₄)₂[H₂{V₆O₁₀(O₃AsPh)₆}]·2H₂O¹⁶¹ are characterised by extensive electron storage and coupled electron–proton transfer processes.

Fig. 24 The fully-oxidised [V^V₁₀O₂₄(O₃AsC₆H₄-4-NH₂)₃]⁴⁻ polyoxoanion (a), its [V₇O₂₄]¹³⁻ core (b), and the structurally related [Bi₇I₂₄]³⁻ polyiodoanion (c). Colour code: C, grey; N, blue; As, rose; O, red; V^V, light orange; Bi, brown; I, violet.

3.2.8 Corollary for AsPOVs. The AsPOVs discussed so far exhibit various symmetries as exemplified by [V^IV₈V^V₄As₈O₄₀]⁵⁻ and β-[V^IV₁₄As₈O₄₂]⁴⁻ with D_4h, α-[V^IV₁₄As₈O₄₂]⁴⁻ with D_2d, [H₃KV^IV₄V^V₈As₃O₃₉(AsO₄)]⁶⁻ with C₃, [V^IV₁₄As₈O₄₂]⁴⁻ with S₄, [V^IV₁₅As₆O₄₂]⁶⁻ with D₃, and [V^IV₁₆As₄O₄₂]⁸⁻ with D_2h. Fully-oxidised, mixed-valent and “fully-reduced” AsPOVs with As atoms in their formal oxidation states of +3 and, more rarely, +5, were reported to date. AsPOVs a variety of structural motifs, ranging from spherical-shaped to wheel-type structures. Dimensionalities from 0D (isolated AsPOVs) to 3D were observed in the solid state. The AsPOVs easily undergo transition metal functionalisation, resulting in novel inorganic–organic frameworks where multidentate organoamines are very commonly captured by transition metal ions introduced into the backbones of the AsPOVs. The hydrogen bonds characteristically expand the dimensionality of the AsPOV structures into 3D networks.

The discovery of the seminal [H₂O@V^IV₁₅As₆O₄₂]⁶⁻ polyoxoanion¹⁵¹ that was shown to act as a textbook example for quantum spin frustration and as a qubit with relatively long coherence lifetimes paved the way for studies focussed on the optical, electronic and magnetic properties of other iso- and heteroPOVs. The nanoscale magnetism of this {V₁₅}-type AsPOV with a symmetrical layer structure of spin centres and spin frustration effects in the central V^IV triangle is now well-documented. The [H₂O@V^IV₁₅As₆O₄₂]⁶⁻ polyoxoanion is a low-spin (spin-1/2) molecular nanomagnet and its characteristics were explored extensively: non-adiabatic Landau–Zener transitions,¹⁷⁰ low-energy spin excitations by elastic neutron scattering study of K₆[V₁₅As₆O₄₂]·9D₂O,¹⁷¹ low-energy excitations from proton NMR and μSR,¹⁷² adiabatic Landau–Zener–Stückelberg transitions with or without dissipation,¹⁷³ mechanism of ground-state selection,¹⁷⁴ local spin moment configuration determined by NMR,¹⁷⁵ static magnetisation at ultra-low temperatures,¹⁷⁶ quantum oscillations,¹⁷⁷ and direct spin-phonon transitions.¹⁷⁸ In general, such AsPOVs show extraordinary magnetic properties and can be used as models to study different fundamental phenomena as e.g. spin frustration¹⁷⁹ and frustrated spin coupling, spin-phonon bottlenecks and butterfly hysteresis,¹⁸⁰ Dzyaloshinsky–Moria interactions and other splitting effects,¹⁸¹ molecular-scale switching,¹⁸² quantum-spin tunnelling, spin coherence and low-temperature spin relaxation processes.¹⁸³

3.3 Polyoxovanadatoantimonates (SbPOVs)

3.3.1 Discrete SbPOVs.
{V₁₄Sb₈}-type polyoxoanions. The “fully-reduced” α-[H₂O@V^IV₁₄Sb₈O₄₂]⁴⁻ polyoxoanion, with rhombicuboctahedral topology and isostructural to α-[V^IV₁₄As₈O₄₂]⁴⁻ (Fig. 15a), was isolated as the compound [(H₂en)₂{V₁₄Sb₈O₄₂(H₂O)}]·(en)·4H₂O under hydrothermal conditions¹⁸⁴ (Table 4). Its crystal structure shows a 1D double-chain in which the sphere-like α-isomeric SbPOVs are interlinked through non-bonding Sb⋯O contacts of 2.74 and 2.79 Å. The intermolecular N–H⋯O hydrogen bonds between the terminal oxygen atoms of the SbPOVs and the hydrogen atoms of ethylenediamine molecules were observed.

The β-isomeric [V^IV₁₄Sb₈O₄₂]⁴⁻ polyoxoanion is a component of the solvothermally prepared ammonium salt (NH₄)₄[V₁₄Sb₈O₄₂]·2H₂O (Table 4).¹⁸⁵ The structure is further characterised by comparably short intercluster Sb⋯O contacts of 2.83–2.98 Å.

{V₁₅Sb₆}-type polyoxoanion. The compound (H₃tren)₂[V₁₅Sb₆O₄₂]·0.33(tren)·nH₂O (n = 3–5) was obtained under solvothermal conditions (Table 4),¹⁸⁶ comprising an isostructural antimony analogue [V^IV₁₅Sb₆O₄₂]⁶⁻ of the molecular magnet [H₂O@V^IV₁₅As₆O₄₂]⁶⁻. The nanosized structure of this “fully-reduced” polyoxoanion (Fig. 25a, left) is viewed as a derivative of the {V₁₈O₄₂} archetype where three {VO₅} square-pyramids are substituted by three handle-like {Sb₂O₅} groups. The structure features {V₁₅}-nuclearity building bocks being arranged in hexagonal layers via weak, intercluster Δ(Sb–μ-O⋯Sb–μ-O⋯Sb–μ-O) interactions within the distances of ca. 2.9 Å, thus resulting in the formation of trimeric super-structures ([V^IV₁₅Sb₆O₄₂]⁶⁻)₃. The spin-1/2 ground structure of the [V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanion “represents an interesting reference system for the concise characterisation of the microscopic magnetism of these spin 1/2 triangle systems”¹⁸⁶ (Fig. 25a, right).

Fig. 25 (a) Left: Polyhedral representation of the “fully-reduced” [V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanion isolated as the compound (H₃tren)₂[V₁₅Sb₆O₄₂]·0.33(tren)·nH₂O with n = 3–5. Right: Ball-and-stick representation of [V^IV₁₅Sb₆O₄₂]⁶⁻ emphasising the equilateral S = 1/2 V^IV₃ triangle that defines the low-temperature magnetic properties. (b) A segment of the extended solid-state structure of (H₂aep)₄[V₁₆Sb₄O₄₂]·2H₂O, highlighting the weak Sb⋯O interactions that interlink the neighbouring SbPOVs. Colour code: Sb, dark yellow; O, red; V^IV, sky blue; V^IVO_x, sky-blue polyhedra.

{V₁₆Sb₄}-type polyoxoanion. The above class of SbPOV-based compounds with the general formula [(Hamine)_mV_18−zSb_2zO₄₂]·nH₂O (z = 2–4) was extended by a new member of this series, namely (H₂aep)₄[V₁₆Sb₄O₄₂]·2H₂O, prepared under solvothermal conditions (Table 4).¹⁸⁵ This compound is composed of the “fully-reduced” [V^IV₁₆Sb₄O₄₂]⁸⁻ polyoxoanion (D_2h symmetry) charge-balanced by four doubly protonated (C₆H₁₇N₃)²⁺ groups (=H₂aep). The structure of the SbPOV showing the common rhombicuboctahedral topology consists of sixteen {VO₅} square pyramids and four {SbO₃} trigonal pyramids and can be interpreted as being derived from the {V₁₈O₄₂} archetype when replacing two {VO₅} units in the latter with two {Sb₂O₅} groups. In the structure of (H₂aep)₄[V₁₆Sb₄O₄₂]·2H₂O, the SbPOVs are connected into infinite chains via intercluster Sb⋯O interactions of ca. 2.85 Å (Fig. 25b).

3.3.2 Vanadyl(IV)-extended SbPOV. The solvothermal synthesis of the compound [V₁₆Sb₄O₄₂(H₂O){VO(dach)₂}₄]·(dach)·10H₂O containing a neutral SbPOV building block¹⁸⁷ (Table 4) can be derived from a general composition {V_18−zSb_2zO₄₂} showing a modified [V₁₈O₄₂]¹²⁻ structure where two [VO₅]⁶⁻ square pyramids are replaced by two diagonal-lying, handle-like [Sb₂O₅]⁴⁻ groups (Fig. 26). The terminal O atoms of two diagonally oriented, non-substituted {VO₅} square pyramids and those from the two {VO₅} square-pyramids interlocking two orthogonal eight-membered rings are involved in the bonding to the four square-pyramidal [V^IVO(dach)₂]²⁺ complexes. These {N₄VO}-type constituents cause charge neutrality. Their coordination results in the discrete [V₁₆Sb₄O₄₂{VO(dach)₂}₄] nanostructure with a diameter of ca. 17.5 Å. The compound is soluble in polar organic solvents (methanol, ethanol, and dimethylformamide) and may find application in homogeneous redox catalysis. According to the magnetic susceptibility data, the coupling between the O-bridged V^IV ions of the four {N₄VO} groups and those of the {V₁₆Sb₄O₄₂} building block is very weak. The “fully-reduced” [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ polyoxoanion (D_2h symmetry) itself is characterised by strong antiferromagnetic coupling between the spin-1/2 vanadyl {VO}²⁺ moieties.

Fig. 26 The “fully-reduced” [H₂O@V^IV₁₆Sb₄O₄₂{VO(dach)₂}₄] self-assembly (right) and its constituents, [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ (left) and [V^IVO(dach)₂]²⁺ (middle). Hydrogen atoms are not shown. Colour code: C, grey; N, blue; Sb, dark yellow; O, red; V^IVO_x, sky-blue polyhedra.

3.3.3 SbPOVs with covalent Sb–N bonds. First organo-SbPOV structures formed due to the covalent attachment of the primary and secondary amines to the SbPOVs were reported in 2011.¹⁸⁸ The compounds [V₁₄Sb₈(Haep)₄O₄₂(H₂O)]·4H₂O and (H₂aep)₂[V₁₅Sb₆(Haep)₂O₄₂(H₂O)]·2.5H₂O were hydrothermally synthesised upon adjustment of pH to the alkaline media (Table 4). The crystal structures of these compounds show hexagonal layers and rows of SbPOVs with intercluster Sb⋯O distances of ca. 2.73 Å in the (001) and of >3 Å in the (100) planes. The Sb atoms of the β-[H₂O@V^IV₁₄Sb₈O₄₂]⁴⁻ and [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanions are covalently bound to ammonium groups (C₆H₁₆N₃)⁺ (=Haep), thus resulting in Sb–N bonds lengths ranging from 2.50 to 2.54 Å. Interestingly, the molecular charge of the {V₁₄}-nuclearity polyoxoanion is completely neutralised by the four monoprotonated 1-(2-aminoethyl)piperazine groups, while the charge of the {V₁₅}-nuclearity polyoxoanion in the respective compound is only partly compensated by two Haep groups covalently attached to the Sb atoms of the {Sb₂O₅} handle-like groups (Fig. 27). Hence, [V₁₄Sb₈(Haep)₄O₄₂(H₂O)]·4H₂O is a neutral compound and (H₂aep)₂[V₁₅Sb₆(Haep)₂O₄₂(H₂O)]·2.5H₂O is a zwitterionic complex composed of two doubly protonated (C₆H₁₇N₃)²⁺ amine countercations (=H₂aep) and the “fully-reduced” [H₂O@V^IV₁₅Sb₆(Haep)₂O₄₂]⁴⁻ polyoxoanion. The (C₆H₁₆N₃)⁺ groups in the compound [V₁₄Sb₈(Haep)₄O₄₂(H₂O)]·4H₂O coordinate to the Sb sites of the handle-like {Sb₂O₅} groups through terminal NH₂ groups of 1-(2-aminoethyl)piperazinium as well as the N atoms of the piperazinium rings. The amine ligands in [H₂O@V^IV₁₅Sb₆(Haep)₂O₄₂]⁴⁻ favour only the second coordination type to the Sb^III atoms. The magnetic properties of two compounds are characterised by strong antiferromagnetic interactions between the spin-1/2 vanadyl {VO}²⁺ moieties. In line with the low or neutral charge of the clusters, both compounds are soluble in methanol and ethanol. The UV/Vis spectra of their solutions are characterised by a strong absorption band at ca. 230 nm and a weaker shoulder at ca. 280 nm.

Fig. 27 The “fully-reduced” organo-SbPOVs [H₂O@V^IV₁₄Sb₈(Haep)₄O₄₂] (a) and [H₂O@V^IV₁₅Sb₆(Haep)₂O₄₂]⁴⁻ (b). Colour code: C, grey; N, blue; Sb, dark yellow; O, red; V^IVO_x, sky-blue polyhedra.

3.3.4 TMC-supported SbPOVs.
Zinc–SbPOV hybrids. Two organic–inorganic hybrid solids [Zn₂(dien)₃][{Zn(dien)}₂V₁₆Sb₄O₄₂(H₂O)]·4H₂O and [Zn(dien)₂]₂[{Zn(dien)}₂(V₁₄Sb₈O₄₂)₂(H₂O)]·4H₂O were obtained by pH-controlled hydrothermal syntheses (Table 4).¹⁸⁹ These compounds contain the [V^IV₁₆Sb₄O₄₂]⁸⁻ and β-[V^IV₁₄Sb₈O₄₂]⁴⁻ building blocks and in the solid state are arranged as 1D infinite linear and zig-zag chains, respectively. In the structure of the former compound, neighbouring [H₂O@V₁₆Sb₄O₄₂]⁸⁻ polyoxoanions with D_2h symmetry are linked by the [Zn(dien)]²⁺ bridging groups via covalent Zn–O bonds (Fig. 28a), while the [Zn₂(dien)₃]⁴⁺ complexes resemble structurally discrete countercations. Similarly to the previous compound, the crystal structure of [Zn(dien)₂]₂[{Zn(dien)}₂(V₁₄Sb₈O₄₂)₂(H₂O)]·4H₂O contains neighbouring β-[V^IV₁₄Sb₈O₄₂]⁴⁻ polyoxoanions that are bridged by single [Zn(dien)]²⁺ moieties through covalent Zn–O bonds (Fig. 28b). The two [Zn(dien)₂]²⁺ cations compensate the negative charge of the [{Zn(dien)}₂(V₁₄Sb₈O₄₂)₂(H₂O)]⁴⁻ assembly.

Fig. 28 (a and b) Segments of the 1D infinite linear (a) and zig-zag (b) chains in the crystal structures of [Zn₂(dien)₃][{Zn(dien)}₂V₁₆Sb₄O₄₂(H₂O)]·4H₂O and [Zn(dien)₂]₂[{Zn(dien)}₂(V₁₄Sb₈O₄₂)₂(H₂O)]·4H₂O. (c) A segment of the extended solid-state structure of [{Co(en)₂}₂V₁₄Sb₈O₄₂(H₂O)]·6H₂O, illustrating the “fully-reduced” β-[H₂O@V^IV₁₄Sb₈O₄₂]⁴⁻ polyoxoanion decorated with four bridging [Co(en)₂]²⁺ groups. (d and e) Pairs of [H₂O@V₁₅Sb₆O₄₂{M(aepda)₂}]⁴⁻ hybrid polyoxoanions (M = Co, left; M = Ni, right) formed by weak Sb⋯O (d) and Sb⋯N (e) interactions in the solid-state structures of [Co(aepda)₂]₂[{Co(aepda)₂}V₁₅Sb₆O₄₂(H₂O)]·5H₂O and [Ni(aepda)₂]₂[{Ni(aepda)₂}V₁₅Sb₆O₄₂(H₂O)]·8H₂O, respectively. Colour code: C, grey; N, blue; Sb, dark yellow; O, red; V^IV, sky blue; V^IVO_x, sky-blue polyhedra; Co, plum; Ni, bright green; Zn, dark green.

A [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ polyoxoanion in which the geometric position of the handle-like {Sb₂O₅} groups reduce the symmetry of the {V₁₆} skeleton to C₂ (vs. D_2h symmetry reported for all hitherto characterised {V₁₆}-nuclearity AsPOVs and SbPOVs) was also identified (Fig. 29).¹⁹⁰ For comparison, the [V^IV₁₆Si₄O₄₆]¹²⁻ polyoxoanion illustrated in Fig. 6a has D_2h symmetry, whereas the [V^IV₁₆Ge₄O₄₂(OH)₄]⁸⁻ in Fig. 8a also displays C₂ symmetry. Such changes in the SbPOV structure influenced the geometrical parameters, specifically the V⋯V distances of this polyoxoanion. While the shortest non-bonding V⋯V distances in the C₂-symmetrical [H₂O@V₁₆Sb₄O₄₂]⁸⁻ structure are between 2.73 and 3.10 Å, those in the isonuclear, but D_2h-symmetrical analogues fall in the range 2.85–3.06 Å in (H₂aep)₄[V₁₆Sb₄O₄₂]·2H₂O,¹⁸⁵ 2.92–3.03 Å in [V₁₆Sb₄O₄₂(H₂O){VO(dach)₂}₄]·(dach)·10H₂O,¹⁸⁷ and 2.92–3.05 Å in [Zn₂(dien)₃][{Zn(dien)}₂V₁₆Sb₄O₄₂(H₂O)]·4H₂O.¹⁸⁹ The “fully-reduced” [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ polyoxoanion with its new pseudorhombicuboctahedral topology is a component of the [Zn(tren)(H₂tren)]₂[V₁₆Sb₄O₄₂(H₂O)]·nH₂O compound (n = 6–10) obtained under solvothermal conditions (Table 4). Two trigonal bipyramidal {Zn(tren)(H₂tren)}⁴⁺ complexes assume the role of countercations. The crystal structure of this compound that is soluble in methanol and ethanol contains channels between the SbPOVs arranged in the pairs by weak intercluster Sb⋯O interactions because of the specific orientation of the handle-like [Sb₂O₅]⁴⁻ groups. The intermolecular N–H⋯O hydrogen bonds between the cations, polyoxoanions and H₂O molecules expand the structure of [Zn(tren)(H₂tren)]₂[V₁₆Sb₄O₄₂(H₂O)]·nH₂O into a 3D network. The low-field magnetic susceptibility studies revealed that the C₂-symmetrical [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ polyoxoanion exhibits strong antiferromagnetic coupling between the spin-1/2 vanadyl {VO}²⁺ moieties. Remarkably, a significant difference in the susceptibility data was found between the compounds [Zn(tren)(H₂tren)]₂[V₁₆Sb₄O₄₂(H₂O)]·nH₂O (with C₂-symmetrical {V₁₆} spin polytope) and [Zn₂(dien)₃][{Zn(dien)}₂V₁₆As₄O₄₂(H₂O)]·3H₂O¹⁵⁹ (with D_2h-symmetrical {V₁₆} spin polytope), thus showing the effect of the pnictogen oxide positions on the magnetism of the isonuclear, but not isostructural AsPOV and SbPOV structures.

Fig. 29 The “fully-reduced” [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ polyoxoanions featuring {V₁₆} skeletons with different symmetries (D_2h and C₂). Colour code: Sb, dark yellow; O, red; V^IV, sky blue; V^IVO_x, sky-blue polyhedra.

Cobalt– and nickel–SbPOV hybrids. The compound [{Co(en)₂}₂V₁₄Sb₈O₄₂(H₂O)]·6H₂O prepared under hydrothermal conditions (Table 4) was the first example of functionalisation of a POV building block with antimony.¹⁹¹ The crystal structure features 2D arrays of the spherical β-[H₂O@V^IV₁₄Sb₈O₄₂]⁴⁻ polyoxoanions that are joined through the covalent bonds between the apical oxygen atoms of the β-SbPOVs and the bridging Co²⁺ ions of the [Co(en)₂]²⁺ complexes (Fig. 28c). Similar to the β-[V^IV₁₄As₈O₄₂]⁴⁻ polyoxoanion (Fig. 15b), this “fully-reduced” SbPOV represents a modified [V₁₈O₄₂]¹²⁻ structure whose four {VO₅} square pyramids are replaced by four handle-like {Sb₂O₅} groups.

The compounds [Co(tren)(H₂O)]₃[V₁₅Sb₆O₄₂(H₂O)]·H₂O, [Co₂(tren)₃]₂[Co(tren)(en)][{V₁₅Sb₆O₄₂(H₂O)(Co(tren)₂)}V₁₅Sb₆O₄₂(H₂O)]·nH₂O (n ≈ 11), and [Co(tren)(H₂tren)]₂[V₁₆Sb₄O₄₂(H₂O)]·6H₂O containing distinct Co²⁺ complexes, namely [Co(tren)(H₂O)]²⁺, [Co₂(tren)₃]⁴⁺, [Co(tren)(en)]²⁺, [Co(tren)₂]²⁺, and [Co(tren)(H₂tren)]⁴⁺ were synthesised under solvothermal conditions using different concentrations of amine molecules (Table 4).¹⁹² Notably, the solvothermal reaction yielding [Co₂(tren)₃]₂[Co(tren)(en)][{V₁₅Sb₆O₄₂(H₂O)(Co(tren)₂)}V₁₅Sb₆O₄₂(H₂O)]·nH₂O included a partial decomposition of tren molecules yielding bidentate en ligands. The inorganic–organic hybrid solids presented above are based on the [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ or [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ polyoxoanions with diameters of ca. 11 Å. The “fully-reduced” SbPOV shells with the shortest V⋯V distances of 2.81–3.07 Å are derived from the {V₁₈O₄₂} shell showing D_4d symmetry, i.e. 3 {VO₅} ↔ 3 {Sb₂O₅}. The [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanion in the crystal structure of [Co(tren)(H₂O)]₃[V₁₅Sb₆O₄₂(H₂O)]·H₂O are involved in weak Sb⋯O intercluster interactions, with the shortest one being ca. 2.90 Å. These Sb⋯O interactions are comparable with those identified e.g., in the 2D layered structures of the compounds (H₂en)₂[V^IV₁₄Sb₈O₄₂(H₂O)]·3H₂O (d_Sb⋯O = 2.72–2.92 Å) and (H₂ppz)₂[V^IV₁₄Sb₈O₄₂(H₂O)] (d_Sb⋯O = 2.83–3.01 Å) with the discrete β-isomeric SbPOV motifs (d_V⋯V = 2.90– 3.10 Å).¹⁹³ The solvothermal formation of these compounds is accompanied by a decomposition of N,N,N′,N′-tetramethylethylenediamine to ethylenediamine and in situ fragmentation of 1-methylpiperazine to piperazine, respectively. Like for the compounds (H₂en)₂[V₁₄Sb₈O₄₂(H₂O)]·3H₂O and (H₂ppz)₂[V₁₄Sb₈O₄₂(H₂O)], O_SbPOV⋯H_amine hydrogen bonds are observed in the crystal structure of [Co(tren)(H₂O)]₃[V₁₅Sb₆O₄₂(H₂O)]·H₂O. The crystal structure of [Co₂(tren)₃]₂[Co(tren)(en)][{V₁₅Sb₆O₄₂(H₂O)(Co(tren)₂)}V₁₅Sb₆O₄₂(H₂O)]·nH₂O (n ≈ 11) is characterised by short Sb⋯N interactions (2.60 Å, 2.65 Å, and 2.74 Å), which engage the N atoms of primary amine molecules (tren) and the Sb atoms of the SbPOVs. Relatively short Sb⋯O intercluster contacts (2.88–2.93 Å) were also observed. The observed N–H⋯O hydrogen bonds generate a 3D network. Interestingly, the central [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ polyanion of the compound [Co(tren)(H₂tren)]₂[V₁₆Sb₄O₄₂(H₂O)]·6H₂O is isostructural to that of [Zn(tren)(H₂tren)]₂[V₁₆Sb₄O₄₂(H₂O)]·nH₂O (n = 6–10)¹⁹⁰ (see Fig. 29) and can be viewed as being derived from the T_d-symmetric {V₁₈O₄₂} shell, i.e. by formal metathesis 2{VO₅} ↔ 2{Sb₂O₅}. The shortest V⋯V distances in this C₂-symmetrical SbPOV polyoxoanion are in the range 2.73–3.09 Å.

The compounds [Co(aepda)₂]₂[{Co(aepda)₂}V₁₅Sb₆O₄₂(H₂O)]·5H₂O and [Ni(aepda)₂]₂[{Ni(aepda)₂}V₁₅Sb₆O₄₂(H₂O)]·8H₂O (aepda = N-(2-aminoethyl)-1,3-propanediamine = C₅H₁₅N₃) were also obtained under solvothermal conditions¹⁹⁴ (Table 4) and their crystal structures feature the [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanion being structurally related to the well-known {V₁₈O₄₂} archetype, as is the case for the AsPOV analogue [H₂O@V^IV₁₅As₆O₄₂]⁶⁻. Each of these [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanions comprises a terminal oxygen position covalently bonded to the transition metal complexes [M(aepda)₂]²⁺ (M = Co, Ni). As a result, the M²⁺ centre has an octahedral M(N_aepda)₅O_SbPOV coordination environment. The two remaining [M(aepda)₂]²⁺ moieties in each compound serve as charge-balancing cations. As in the case of several other TMC-functionalised heteroPOVs, the paramagnetic M²⁺ ions (M = Co, Ni) from the terminally coordinated [M(aepda)₂]²⁺ complexes couple only very weakly with the spin-1/2 vanadyl {VO}²⁺ moieties of the [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanions. In the crystal structure of the compound comprising Co²⁺ ions, the polyoxoanions are connected through weak intercluster Sb⋯O (3.13 Å) interaction, thus forming pairs of SbPOVs (Fig. 28d). In contrast, in the crystal structure of the compound containing Ni²⁺ ions pairs of SbPOVs linked by weak Sb⋯N contacts (2.65 Å) are observed. These Sb⋯N interactions involve the Sb atoms of the “fully-reduced” [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanion and the N atoms of the propylamine chains from the adjoined [Ni(aepda)₂]²⁺ fragments (Fig. 28e). In addition, intermolecular N–H⋯O hydrogen bonds generating higher dimensional networks were observed in the structures of both these compounds.

A dimeric {[Ni₂(tren)₃(V₁₅Sb₆O₄₂(H₂O)_0.5)]₂}⁴⁻ fragment where two “fully-reduced” [V^IV₁₅Sb₆O₄₂(H₂O)_0.5]⁶⁻ polyoxoanions (shortest d_V⋯V = 2.84–3.08 Å) are bridged by an in situ-produced [Ni₂(tren)₃]⁴⁺ complex via a multidentate μ-1,7 tren ligand was identified in the solvothermally synthesised, highly insoluble compound [Ni(Htren)₂][Ni₂(tren)₃(V₁₅Sb₆O₄₂(H₂O)_0.5)]₂·H₂O (Table 4).¹⁹⁵ Its crystal structure is characterised by double chains along the crystallographic b axis, formed by short Sb⋯N contacts (d_Sb⋯N = ca. 2.69 Å) between the neighbouring heterometallic dimers. Again, an additional [Ni(Htren)₂]⁴⁺ complex with protonated tren ligands acts as countercation. For comparison, the covalent Sb–N distances in the structures of the aforementioned compounds [V₁₄Sb₈(Haep)₄O₄₂(H₂O)]·4H₂O and (H₂aep)₂[V₁₅Sb₆(Haep)₂O₄₂(H₂O)]·2.5H₂O (Fig. 27) are d_Sb–N = 2.53 and 2.54 Å and d_Sb–N = 2.50 and 2.54 Å, respectively.¹⁸⁸ A 3D network in the crystal structure of [Ni₂(tren)₃(V₁₅Sb₆O₄₂(H₂O)_0.5)]₂[Ni(Htren)₂]·H₂O is generated by a complex hydrogen bonding pattern between H atoms of water molecules and the amine moieties and oxygen atoms of the SbPOVs.

Two pseudopolymorphic compounds with compositions [Ni(dien)₂]₃[V₁₅Sb₆O₄₂(H₂O)]·nH₂O (n = 12 and 8) and the compound [Ni(dien)₂]₄[V₁₆Sb₄O₄₂(H₂O)] were prepared solvothermally by adjusting the reaction temperature (Table 4).¹⁹⁶ The pseudopolymorphs display the spherical [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanions which are virtually isostructural to the molecular magnet [H₂O@V^IV₁₅As₆O₄₂]⁶⁻. The structural motif of the “fully-reduced” [V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanion is shown in Fig. 25a and that of [V^IV₁₆Sb₄O₄₂]⁸⁻, a component of the compound [Ni(dien)₂]₄[V₁₆Sb₄O₄₂(H₂O)], is illustrated in Fig. 26. Interestingly, the crystal structures of the pseudopolymorphs possess different composition of the asymmetric units that involve four crystallographically independent [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanions and twelve [Ni(dien)₂]²⁺ complexes adopting the s-fac, mer-, and u-fac-configurations (tetragonal space group) or a third of the polyoxoanion and a mer-{Ni(dien)₂}²⁺ complex (rhombohedral space group). In the crystal structure of the compound crystallising in the tetragonal space group, the SbPOVs are arranged as tetrameric superstructures with composition ([H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻)₄ (layer-like arrangement). The individual polyoxoanions are held together by weak Sb⋯O interactions (2.68 to 2.97 Å), which are significantly shorter than the sum of van der Waals radii (3.52 Å) for Sb and O atoms. For the structure crystallising in the rhombohedral space group, the shortest intercluster Sb⋯O distances exceed 5 Å, because of a triangular-like arrangement of dicationic mer-[Ni(dien)₂]²⁺ complexes around the [V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanions. The 3D network structure of this compound is realised by intermolecular N–H⋯O hydrogen bonding interactions. The compound [Ni(dien)₂]₄[V₁₆Sb₄O₄₂(H₂O)] comprises octahedrally coordinated Ni²⁺ ions in the [Ni(dien)₂]²⁺ cations adopting the s-fac- and u-fac-configurations. In the crystal structure of [Ni(dien)₂]₄[V₁₆Sb₄O₄₂(H₂O)], the [H₂O@V^IV₁₆Sb₄O₄₂]⁸⁻ polyoxoanions are arranged in layers in the (100) plane.

Iron–SbPOV hybrids. The solvothermally synthesised compound [{Fe(dach)₂}₃{V₁₅Sb₆O₄₂(H₂O)}]·8H₂O with the unique porous 3D network topology in its extended solid-state structure (Fig. 30) is the first example of an iron-augmented SbPOV architecture also exhibiting a unique magnetic structure.¹⁹⁷ The amine molecules used in the synthesis of this compound (Table 4) were shown to carry out versatile functions, acting as ligands, solvents, and reducing agents. In the structure, the [H₂O@V^IV₁₅Sb₆O₄₂]⁶⁻ polyoxoanion is expanded by six in situ-generated [Fe(dach)₂]²⁺ complexes, which coordinate terminally to the polyoxoanion via Fe–O bonds to form octahedral {FeN₄O₂} units that further interlink the adjoined SbPOVs. Relatively strong N–H⋯O hydrogen bonds were observed. The compound is further characterised by an optical energy gap of 2.47 eV. The low-field magnetic susceptibility revealed ferromagnetic exchange interactions between the spin-1/2 vanadyl {VO}²⁺ moieties of the polyoxoanion and the high-spin (S = 2) Fe²⁺ ions of the adjoined [Fe(dach)₂]²⁺ complexes. We note that this heterometal-vanadyl coupling differs significantly from that established for the aforementioned TMC-supported AsPOVs and SbPOVs, which showed only weak or negligible, and mostly antiferromagnetic interactions between the spin centres of the central building blocks and the attached TMC fragments.

Fig. 30 3D network in the crystal structure of [{Fe(dach)₂}₃{V₁₅Sb₆O₄₂(H₂O)}]·8H₂O. The channels along [−110] are shown. Crystal solvent (water) molecules are omitted for clarity.

3.3.5 Corollary for SbPOVs. All hitherto reported SbPOVs are based on the {V₁₄}-, {V₁₅}-, and {V₁₆}-type skeletons composed exclusively of V^IV ions. The different symmetries of these “fully-reduced” SbPOVs were determined as D_2d for α-[V^IV₁₄Sb₈O₄₂]⁴⁻, D_2h for β-[V^IV₁₄Sb₈O₄₂]⁴⁻, D₃ for [V^IV₁₅Sb₆O₄₂]⁶⁻, and D_2h and C₂ for [V^IV₁₆Sb₄O₄₂]⁸⁻. No mixed-valent SbPOVs have yet been reported. The SbPOV-based compounds display 1D, 2D and 3D solid-state structures where the polyoxoanions were found to form weak intercluster Sb⋯O and/or Sb⋯N interactions. On one hand, the SbPOVs exhibit interesting reactivity towards the covalent attachment of organic amines, which can reduce or fully balance the high negative molecular charge of the polyoxoanion shell. On the other hand, the SbPOVs also allow for functionalization with transition metal complexes to generate TMC-supported SbPOV structures, which are of considerable interest due to the addition of multiple spin centres with different magnetic moments. The possible influence of these adjoined TMC fragments on the spin (de)coherence in SbPOVs with an odd number of V^IV atoms (and spin-1/2 ground states) remains as an interesting problem. S = 0 ground states were established for the {V₁₄}- and {V₁₆}-nuclearity building blocks with the even number of V^IV atoms.

The study of the chemical and physical properties and reactivity of the SbPOVs has been motivated, in particular, by the following points closely related to catalysis:

(i) The Sb^III ions exhibited stabilising effects on the Keggin-type POM structures at high temperatures (>450 °C) and this played an important role in employing the Sb-modified POMs as catalysts, e.g. for the oxidative dehydrogenation of ethane to ethylene.¹⁹⁸

(ii) Since Sb-modified vanadium catalysts were shown to selectively oxidise o-xylene to phthalic anhydride,¹⁹⁹ SbPOVs might find potential applications as pre- or catalysts in heterogeneous selective oxidation reactions.

4. Summary and outlook

We reviewed the structural aspects and key properties of heavier group 14 and 15 element-functionalised POVs and their inorganic–organic hybrid compounds involving transition metal and lanthanoid complexes as charge compensating units and structure modifiers. We provided an overview of the synthetic routes to SiPOVs, GePOVs, AsPOVs and SbPOVs whose central structural motifs typically are derived from the {V₁₈O₄₂} archetype and display rhombicuboctahedral topologies. The V⁴⁺ cations in these robust polyoxoanions incorporating handle-like {E₂O₇}/{E₂O₅S₂} (E = Si or Ge) and {E₂O₅} (E = As or Sb) groups adopt the very common square-pyramidal {VO₅} coordination geometry, although a number of compounds involving {VO₆} octahedra and {VO₄} tetrahedra were also reported. Note that the crystal chemical aspects of vanadium oxide polyhedra were comprehensively analysed by Schindler et al., in 2000²⁰⁰ and were, therefore, not discussed herein. The topological aspects of POV macropolyhedra can be found in the work of King dating back to 1995.²⁰¹

It was shown that the {Ge₂O₅S₂} and {As₂O₅}/{Sb₂O₅} substituents in the {V₁₅E₆}-type building blocks play crucial roles in superexchange interactions within the central spin-1/2 {V₃} triangle that define the low-temperature magnetism of these heteroPOVs. Even though the [H₂O@V₁₅Ge₆O₄₂S₆]^12− 102 and [V₁₅Sb₆O₄₂]^6− 186 polyoxoanions feature spin topologies very similar to that of the prominent molecular magnet [H₂O@V₁₅As₆O₄₂]⁶⁻,¹⁵¹ the magnetic exchange properties of the {Ge₂O₅S₂}-modified POVs differed significantly from those of the corresponding As- and Sb-functionalised POVs.

As was demonstrated by a large number of studies, the discrete heteroPOVs can be extended further towards the hybrid inorganic–organic frameworks by introducing TMCs or lanthanoid salts to the reactions mixtures. These TMCs and lanthanoid complex cations commonly act as bridging ligands between the neighbouring heteroPOVs and often exhibit unusual coordination geometries and connectivities. The transition metal ions (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺) can be introduced into the backbone structures of the vanadium oxide shells to facilitate the linking of these heteroPOVs into 1D, 2D and 3D networks and to enrich/modify their magnetic properties. We did not highlight the structural chemistry of TMC fragments in these structures; discussion on their coordination geometries and bonding patterns can be found in the original literature. The use of adjoined inorganic moieties (e.g. lanthanoid complex cations¹⁵⁰) was shown to result in high coordination numbers (up to 6) for the POV shells and, thus, to remarkably increase the overall thermal stabilities of these heteroPOV-based materials (even exceeding 500 °C). The transition metal functionalisation of the polynuclear molecular vanadium oxides open up new possibilities for making new heteroPOV spin architectures with e.g. spin-glass behaviour by virtue of the fact that the vanadyl {VO}²⁺ moieties in these polyoxoanions can be replaced by divalent transition metal ions.

Another way to improve the thermal stability of heteroPOVs may be through the preparation of heteroPOV-based ionic liquids, which can be achieved by pairing negatively charged heteroPOV building blocks with appropriate cations such as imidazolium, pyridinium, phosphonium species. For instance, a number of Keggin and Lindqvist polyoxomolybdate and polyoxotungstate-based ionic liquids²⁰² and tetraalkylphosphonium decavanadates²⁰³ have been reported.

The reactivity of heteroPOVs towards organic groups was also illustrated. The organic amines usually introduced in the reaction mixtures are commonly not only acting as reducing agents, but also as structure-directing and charge-compensating groups and, furthermore, display interesting transformations under hydro-/solvothermal conditions in the presence of V^V sources. The organic functionalisation of magnetic polyoxoanions, as exemplified by [H₂O@V^IV₁₄Sb₈(Haep)₄O₄₂] and [H₂O@V^IV₁₅Sb₆(Haep)₂O₄₂]⁴⁻,¹⁸⁸ enabled their dissolution in polar protic solvents (methanol, ethanol, etc.). It also can partly or fully compensate the high negative charges of the heteroPOVs, an important issue for molecular deposition and subsequent (micro)spectroscopic studies with this class of compounds. Such a direct covalent attachment of organic ligands allows for the expansion of heteroPOVs into classical coordination clusters and may significantly influence their redox and acid–base properties.²⁰⁴

The relevance of the presented polyoxoanions to different fields of chemical and physical sciences was briefly discussed. The heteroPOVs are envisaged to find potential applications in molecular electronics and spintronics, optics, sorption, catalysis and artificial photosynthesis.²⁰⁵ However, the search for heteroPOV compounds with sufficient solubility in organic solvents and a high structural and thermal stability is crucial in this context. The synthesis of new discrete and multidimensional SiPOVs, GePOVs, AsPOVs and SbPOVs is highly desirable in order to discover strategies for altering their chemical and physical properties.

Since the ratio of V^V/V^IV and V^IV/V^III redox couples and molecular connectivities are strongly influenced by the nature of the heteroelements, and since the overall E/V^V/V^IV/V^III ratio impacts the overall properties of the heteroPOV to a large extent, the chemical modification of the conventional POV shells by heavier metal ions from groups 14 such as Sn and Pb is appealing and worthwhile. For example, functionalisation of POVs with bismuth, the heaviest group-15 element, resulted in interesting photocatalytic activity of the designed molecular bismuth vanadium oxide clusters.²⁰⁶ The reactivities of SiPOVs, GePOVs, AsPOVs and SbPOVs towards transition metals e.g. from the platinum group and actinoids are yet to be discovered. Integration of lanthanoid ions into heteroPOV shells may lead to interesting magnetic phenomena as well as visible-light photocatalytic properties. The controlled interlinking of heteroPOV spin structures is of high importance for the development of smart materials with electron storage functions and inter-site communications. The coexistence of (partly) delocalised 3d-vanadium electron density with localised spins of transition metals and lanthanoids in the mixed-valent heteroPOV-based inorganic–organic hybrid materials may result in unusual spintronic effects, “in particular, if the corresponding charge transport can be confined to one or two dimensions”.¹¹⁴

The molecular magnetism of heteroPOVs described so far is characterised by the presence of strong antiferromagnetic exchange interactions between the vanadium spin centres, with the exception of several compounds showing ferrimagnetic ordering characteristics. Because there are some indications of ferromagnetic coupling interactions in heteroPOVs (see the mixed-valent [HCO₂@V^IV₆V^V₆As₈O₄₀]³⁻ polyoxoanion^119,120) as well as alkoxoPOVs (see the mixed-valent [V^IV₄V^V₂O₇(OR)₁₂] compound⁷⁸ and the mixed-valent [V^IIIV^IV₅O₆(OMe)₈(calix)(MeOH)]⁻ polyoxoanion²⁰⁷ where calix = p-tert-butylcalix[4]arene), the magnetic properties of these compounds should be explored in more detail. Building a deeper understanding of the magnetism of the mixed-valent, “fully-reduced” and “highly-reduced” SiPOVs, GePOVs, AsPOVs and SbPOVs, which can potentially act as “nanoscale quantum magnets” and spin qubits remains challenging, because of the absence of magnetochemical models suitable for the fitting of their magnetic susceptibility data.

Only a few studies have employed quantum chemical models to study the electronic structures of fully-oxidised POVs²⁰⁸ and of some of the herein-described “fully-reduced”, semimetal-functionalised POVs with the V₁₄ and V₁₅ nuclearities.²⁰⁹ Reliable computational methods for assessing the electronic and magnetic properties of the mixed-valent heteroPOVs are still being developed.

The biological and medicinal relevance of POVs²¹⁰ deserves mention, as the archetypal decavanadate [V₁₀O₂₈]⁶⁻ has demonstrated non-trivial chemical behaviour and reactivity towards biomolecules,²¹¹e.g.: (i) inhibition of various enzymes,²¹² (ii) hydrolytic DNA cleavage,²¹³ (iii) inhibition of oxygen consumption in membranes.²¹⁴ The interactions of other, lower nuclearity, but more labile oxovanadates ([VO₄]³⁻ as a structural and electronic analogue of phosphate, [V₂O₇]⁴⁻, and [V₄O₁₂]⁴⁻) with enzymes²¹⁵ have also been recognised and well-documented.^1,216 Over the past two decades, much attention has been paid to biochemical, cell biological, antidiabetic and antitumor studies involving vanadium^217–219 and organic biomolecules for the reason that “understanding the nature of protein interactions with oxovanadates and other oxometalates is important for further development of drugs based on vanadium complexes and polyoxoanions”.¹ In view of the biological innocuity of bismuth and its extensive uses in medicine, pharmaceuticals, and cosmetics, the class of polyoxovanadatobismuthates is relevant to biochemical studies.

Although the number of reports on magnetic heteroPOVs has grown substantially in the last few decades, the field is hampered by the lack of systematic research on this class of molecular compounds. Many questions concerning their inorganic synthesis, reactivity in aqueous and non-aqueous solutions, molecular magnetism, spectroscopy (in particular, NMR studies in solution and in the solid state), excited states and computational application remain entirely open. The benefits of effective adjustment of the heteroPOV building block's properties from the standpoint of chemical reactivity, electrochemistry, molecular magnetism, photophysics and surface physics will be remarkable.

Finally, we note that the self-assembly formation mechanisms of heteroPOVs remain poorly understood and for the development of rational syntheses these mechanisms should be unveiled. There are first promising attempts in this direction: in situ energy-dispersive X-ray diffraction experiments were performed under hydrothermal conditions in order to gain insight into the synthetic parameters influencing the formation of SbPOVs.²²⁰ These studies were able to demonstrate directly that e.g. the amine concentration plays a crucial role for the crystallisation of a distinct SbPOV, where the formation of [V₁₄Sb₈(Haep)₄O₄₂(H₂O)]·4H₂O¹⁸⁸ is observed at the lowest amine concentration, whereas (H₂aep)₂[V₁₅Sb₆(Haep)₂O₄₂(H₂O)]·2.5H₂O¹⁸⁸ crystallised at slightly larger amine concentration and finally the SbPOV with the highest number of V^IV centres, (H₂aep)₄[V₁₆Sb₄O₄₂]·2H₂O,¹⁸⁵ was obtained at the highest amine concentration. In addition, solvothermal syntheses performed under static conditions yield mixtures of these three SbPOVs while stirring afforded formation of phase pure materials at a distinct amount of amine in the reaction slurries. These empirical in situ studies pave the way for a more comprehensive understanding of the complex self-assembly processes in vanadate reaction solutions, in line with similarly revealing in situ spectroscopy studies in the chemistry of other polyoxometalate families.²²¹

Abbreviations

Methods

CV	Cyclic voltammetry
DFT	Density functional theory
EA	Elemental analysis
EDXS	Energy-dispersive X-ray spectroscopy
EMP	Electron microprobe
EPR	Electron paramagnetic resonance
ESI-MS	Electrospray ionisation mass spectrometry
INS	Inelastic neutron scattering
IR	Infrared spectroscopy
NMR	Nuclear magnetic resonance
powder XRD	Powder X-ray diffraction
SEM	Scanning electron micrograph
single-crystal XRD	Single-crystal X-ray diffraction
TGA	Thermogravimetric analysis
UV-vis	Ultraviolet-visible spectroscopy
XPS	X-ray photoelectron spectroscopy

Inorganic compounds

POV	Polyoxovanadate
heteroPOV	Heteropolyoxovanadate
alkoxoGePOV	Polyoxoalkoxovanadatogermanate
AsPOV	Polyoxovanadatoarsenate
GePOV	Polyoxovanadatogermanate
SiPOV	Polyoxovanadatosilicate
SbPOV	Polyoxovanadatoantimonate
TMC	Transition metal complex

Organic compounds

2,2′-bipy	2,2′-Bipyridine
aep	1-(2-Aminoethyl)piperazine
aepda	N-(2-Aminoethyl)-1,3-propanediamine
bbi	1,1′-(1,4-Butanediyl)bis(imidazole)
bpe	1,2-Bis(4-pyridyl)ethylene
dab	1,4-Diaminobutane
dach	(±)-trans-1,2-Cyclohexanediamine
dien	Diethylenetriamine
en	Ethylenediamine
enMe	1,2-Diaminopropane
pdn	1,3-Propanediamine
phen	1,10-Phenanthroline
ppz	Piperazine
salen	N,N′-(Ethylene)bis(salicylideneiminate)
teos	Tetraethyl orthosilicate
tepa	Tetraethylenepentamine
theed	N,N,N′,N′-Tetrakis(2-hydroxyethyl)ethylenediamine
tren	Tris(2-aminoethyl)amine

Acknowledgements

K.Y.M. is grateful to the Deutsche Forschungsgemeinschaft (DFG) for a postdoctoral reintegration fellowship and the Jülich-Aachen Research Alliance – Future Information Technology (JARA-FIT) for a Seed Fund grant. We thank Dr Claire Besson, Dr Jeff Rawson (RWTH Aachen University) and Dr Natalya Izarova (Forschungszentrum Jülich) for helpful suggestions.

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