POMzites: A Family of Zeolitic Polyoxometalate Frameworks from a Minimal Building Block Library

Small Copper Nanoclusters Synthesized through Solid-State Reduction inside a Ring-Shaped Polyoxometalate Nanoreactor

Cu nanoclusters exhibit distinctive physicochemical properties and hold significant potential for multifaceted applications. Although Cu nanoclusters are synthesized by reacting Cu ions and reducing agents by covering their surfaces using organic protecting ligands or supporting them inside porous materials, the synthesis of surface-exposed Cu nanoclusters with a controlled number of Cu atoms remains challenging. This study presents a solid-state reduction method for the synthesis of Cu nanoclusters employing a ring-shaped polyoxometalate (POM) as a structurally defined and rigid molecular nanoreactor. Through the reduction of Cu2+ incorporated within the cavity of a ring-shaped POM using H2 at 140 °C, spectroscopic studies and single-crystal X-ray diffraction analysis revealed the formation of surface-exposed Cu nanoclusters with a defined number of Cu atoms within the cavities of POMs. Furthermore, the Cu nanoclusters underwent a reversible redox transformation within the cavity upon alternating the gas atmosphere (i.e., H2 or O2). These Cu nanoclusters produced active hydrogen species that can efficiently hydrogenate various functional groups such as alkenes, alkynes, carbonyls, and nitro groups using H2 as a reductant. We expect that this synthesis approach will facilitate the development of a wide variety of metal nanoclusters with high reactivity and unexplored properties.

Computational Study into the Effects of Countercations on the [P8W48O184]40- Polyoxometalate Wheel

Porous metal oxide materials have been obtained from a ring-shaped macrocyclic polyoxometalate (POM) structural building unit, [P8W48O184]40-. This is a tungsten oxide building block with an integrated "pore" of 1 nm in diameter, which, when connected with transition metal linkers, can assemble frameworks across a range of dimensions and which are generally referred to as POMzites. Our investigation proposes to gain a better understanding into the basic chemistry of this POM, specifically local electron densities and locations of countercations within and without the aforementioned pore. Through a rigorous benchmarking process, we discovered that 8 potassium cations, located within the pore, provided us with the most accurate model in terms of mimicking empirical properties to a sufficient degree of accuracy while also requiring a relatively small number of computer cores and hours to successfully complete a calculation. Additionally, we analyzed two other similar POMs from the literature, [As8W48O184]40- and [Se8W48O176]32-, in the hopes of determining whether they could be similarly incorporated into a POMzite network; given their close semblance in terms of local electron densities and interaction with potassium cations, we judge these POMs to be theoretically suitable as POMzite building blocks. Finally, we experimented with substituting different cations into the [P8W48O184]40- pore to observe the effect on pore dimensions and overall reactivity; we observed that the monocationic structures, particularly the Li8[P8W48O184]32- framework, yielded the least polarized structures. This correlates with the literature, validating our methodology for determining general POM characteristics and properties moving forward.

Ce-mediated molecular tailoring on gigantic polyoxometalate {Mo132} into half-closed {Ce11Mo96} for high proton conduction

Precise synthesis of polyoxometalates (POMs) is important for the fundamental understanding of the relationship between the structure and function of each building motif. However, it is a great challenge to realize the atomic-level tailoring of specific sites in POMs without altering the major framework. Herein, we report the case of Ce-mediated molecular tailoring on gigantic {Mo132}, which has a closed structural motif involving a never seen {Mo110} decamer. Such capped wheel {Mo132} undergoes a quasi-isomerism with known {Mo132} ball displaying different optical behaviors. Experiencing an 'Inner-On-Outer' binding process with the substituent of {Mo2} reactive sites in {Mo132}, the site-specific Ce ions drive the dissociation of {Mo2*} clipping sites and finally give rise to a predictable half-closed product {Ce11Mo96}. By virtue of the tailor-made open cavity, the {Ce11Mo96} achieves high proton conduction, nearly two orders of magnitude than that of {Mo132}. This work offers a significant step toward the controllable assembly of POM clusters through a Ce-mediated molecular tailoring process for desirable properties.

Surface-exposed silver nanoclusters inside molecular metal oxide cavities

The surfaces of metal nanoclusters, including their interface with metal oxides, exhibit a high reactivity that is attractive for practical purposes. This high reactivity, however, has also hindered the synthesis of structurally well-defined hybrids of metal nanoclusters and metal oxides with exposed surfaces and/or interfaces. Here we report the sequential synthesis of structurally well-defined {Ag₃₀} nanoclusters in the cavity of ring-shaped molecular metal oxides known as polyoxometalates. The {Ag₃₀} nanoclusters possess exposed silver surfaces yet are stabilized both in solution and the solid state by the surrounding ring-shaped polyoxometalate species. The clusters underwent a redox-induced structural transformation without undesirable agglomeration or decomposition. Furthermore, {Ag₃₀} nanoclusters showed high catalytic activity for the selective reduction of several organic functional groups using H₂ under mild reaction conditions. We believe that these findings will serve for the discrete synthesis of surface-exposed metal nanoclusters stabilized by molecular metal oxides, which may in turn find applications in, for example, the fields of catalysis and energy conversion.

Polyoxometalate-cyclodextrin supramolecular entities for real-time in situ monitoring of dopamine released from neuroblastoma cells

Optimized and sensitive biomarker detection has recently been shown to have a critical impact on quality of diagnosis and medical care options. In this research study, polyoxometalate-γ-cyclodextrin metal-organic framework (POM-γCD MOF) was utilized as an electrocatalyst to fabricate highly selective sensors to detect in-situ released dopamine. The POM-γCD MOF produced multiple modes of signals for dopamine including electrochemical, colorimetric, and smartphone read-outs. Real-time quantitative monitoring of SH-SY5Y neuroblastoma cellular dopamine production was successfully demonstrated under various stimuli at different time intervals. The POM-CD MOF sensor and linear regression model were used to develop a smartphone read-out platform, which converts dopamine visual signals to digital signals within a few seconds. Ultimately, POM-γCD MOFs can play a significant role in the diagnosis and treatment of various diseases that involve dopamine as a significant biomarker.

Heteroatom-Modulated Assembly of Hexalanthanoid-Containing Polyoxometalate-Based Coordination Networks

Two series of lanthanoid (Ln)-containing polyoxometalates (POMs) {[Ln₆(ampH)₄(H₂O)_24-n(ampH₂)_n(PW₁₁O₃₉)₂]·21H₂O (Ln = Tb, n = 0 (1), Ln = Er, n = 1 (2)) and K₂[Ln₆(ampH)₄(H₂O)₂₂(SiW₁₁O₃₉)₂]·23H₂O (Ln = Tb (3), Er (4)) (ampH₂ = (aminomethyl) phosphonic acid)} have been synthesized with tri-lacunary Keggin-type POMs containing different types of heteroatoms. Compounds 1 and 2 display neutral organic-inorganic hybrid POM molecules containing {Ln₆(ampH)₄} ({Ln₆}) cores sandwiched by two {PW₁₁O₃₉} units. By changing the heteroatoms from P^V to Si^IV, the extended 2D networks of 3 and 4 were successfully isolated where the adjacent {Ln₆} clusters were connected by {SiW₁₁O₃₉} moieties. Luminescence performances and magnetic properties of 1-4 have been systematically surveyed. The solid-state fluorescence spectra of 1-4 display characteristic emissions of Ln components resulting from the 4f-4f transitions, and energy transfer from the POM segments to Ln³⁺ centers in 1 and 3 has been observed based on the lifetime decay behaviors. Furthermore, all compounds can be utilized as electrocatalysts toward reduction of nitrite with high stability.

Space-Confined Nucleation of Semimetal-Oxo Clusters within a [H7P8W48O184]33- Macrocycle: Synthesis, Structure, and Enhanced Proton Conductivity

Spatially confined assembly of semimetallic oxyanions (AsO₃^3- and SbO₃^3-) within a [H₇P₈W₄₈O₁₈₄]^33- (P₈W₄₈) macrocycle has afforded three nanoscale polyanions, [{As^III₅O₄(OH)₃}₂(P₈W₄₈O₁₈₄)]^32- (As₁₀), [(Sb^IIIOH)₄(P₈W₄₈O₁₈₄)]^32- (Sb₄), and [(Sb^IIIOH)₈(P₈W₄₈O₁₈₄)]^24- (Sb₈), which were crystallized as the hydrated mixed-cation salts (Me₂NH₂)₁₃K₇Na₂Li₁₀[{As^III₅O₄(OH)₃}₂(P₈W₄₈O₁₈₄)]·32H₂O (DMA-KNaLi-As₁₀), K₂₀Li₁₂[(Sb^IIIOH)₄(P₈W₄₈O₁₈₄)]·52H₂O (KLi-Sb₄), and (Me₂NH₂)₈K₆Na₅Li₅[(Sb^IIIOH)₈(P₈W₄₈O₁₈₄)]·65H₂O (DMA-KNaLi-Sb₈), respectively. A multitude of solid- and solution-state physicochemical techniques were employed to systematically characterize the structure and composition of the as-made compounds. The polyanion of As₁₀ represents the first example of a semimetal-oxo cluster-substituted P₈W₄₈ and accommodates the largest As^III-oxo cluster in polyoxometalates (POMs) reported to date. The number of incorporated SbO₃^3- groups in Sb₄ and Sb₈ could be customized by a simple variation of Sb^III-containing precursors. Encapsulation of semimetallic oxyanions inside P₈W₄₈ sets out a valid strategy not only for the development of host-guest assemblies in POM chemistry but also for their function expansion in emerging applications such as proton-conducting materials, for which DMA-KNaLi-As₁₀ showcases an outstanding conductivity of 1.2 × 10^-2 S cm^-1 at 85 °C and 70% RH.

The missing link between zeolites and polyoxometalates

Open framework materials such as zeolites and metalorganic frameworks are garnering tremendous interest because of their intriguing architecture and attractive functionalities. Thus, new types of open framework materials are highly sought after. Here, we present the discovery of completely new inorganic framework materials, where, in contrast to conventional inorganic open frameworks, the scaffold is not based on tetrahedral EO₄ (E = main group element) but octahedral MO₆ (M = transition metal) building blocks. These structural features place them closer to polyoxometalates than zeolites. The first representatives of this class of materials are [(R)₂₄(NH₄)₁₄(PO(OH)₂)₆]·[M₁₃₄(PO₃(OH,F))₉₆F₁₂₀] (M = Co, R = C₂Py = 1-ethylpyridinium and M = Ni, R = C₄C₁Py = 1-butyl-3-methylpyridinium) featuring interlinked fullerene-like nanosphere cavities. Having a transition metal building up the framework brings about interesting properties, for example, spin-glass behavior, and, with this particular topology, a hedgehog-like spin orientation.

PEG-Infiltrated Polyoxometalate Frameworks with Flexible Form-Factors

We present the synthesis of metal oxide frameworks composed of the Preyssler anion, [NaP₅W₃₀O₁₁₀]^14-, bridged with transition-metal cations and infiltrated with polyethylene glycol. The frameworks can be dissolved in water to form freestanding rigid or flexible films or gels. Powder X-ray diffraction shows that all form-factors maintain the short-range order of the original crystals. Raman spectroscopy reveals that, similar to hydrogels, the macroscopic mechanical properties of these composites are dependent on the water content and the extent of hydrogen-bonding within the water network. The understanding gained from these studies facilitates solution-phase processing of polyoxometalate frameworks into flexible form factors.

A Heniconuclear {Mn21} Cluster-Based Coordination Polymer with Manganese(II) Linkers Showing High Proton Conductivity

An unprecedented metal-linked cluster-based coordination polymer, composed of heniconuclear {Mn₂₁} clusters and Mn(II) ions as the nodes and linkers, respectively, was self-assembled from a facile aqueous synthesis. The structural analysis reveals that the compound possesses a rare 3D 8-connected hex framework topology. Significantly, the compound demonstrates a high proton conductivity of 1.06 mS cm^-1 at 373 K and 98% RH and exhibits a magnetocaloric effect with a magnetic entropy change of -9.94 J kg^-1 K^-1 at H = 80 kOe and T = 6.0 K.

Coordination of Pt(IV) by {P8W48} Macrocyclic Inorganic Cavitand: Structural, Solution, and Electrochemical Studies

Hydrothermal reaction of a macrocyclic inorganic POM cavitand Li₁₇(NH₄)₂₁H₂[P₈W₄₈O₁₈₄] with [Pt(H₂O)₂(OH)₄] results in coordination of up to six {Pt(H₂O)_x(OH)_4-x} fragments to the internal surface of the polyoxoanion. The product was isolated as K₂₂(NH₄)₉H₃[{Pt(OH)₃(H₂O)}₆P₈W₄₈O₁₈₄]·79H₂O (1) and characterized by multiple techniques in the solid state (SCXRD, XRPD, XPS, FTIR, and TGA) and in solution (NMR, ESI-MS, and HPLC-ICP-AES). Electrochemical properties were studied both in solution and as components of the paste electrode. The complex shows electrocatalytic activity in water oxidation.

Assembly of ϵ-Keggin Polyoxometalate from Molecular Crystal to Zeolitic Octahedral Metal Oxide

Zeolitic octahedral metal oxides are inorganic crystalline microporous materials with adsorption and redox properties. New ϵ-Keggin nickel molybdate-based zeolitic octahedral metal oxides have been synthesized. ³¹ P NMR spectroscopy shows that reduction of Mo^VI -based molybdates forms an ϵ-Keggin polyoxometalate that immediately transfers to the solid phase. Investigation of the formation process indicates that a low Ni concentration, insoluble reducing agent, and long synthesis time are the critical factors for obtaining the zeolite octahedral metal oxides rather than the ϵ-Keggin polyoxometalate molecule. The synthesized zeolitic nickel molybdate with Na⁺ is used as the adsorbent, which effectively separates C₂ hydrocarbon mixtures.

A Keggin-type polyoxometalate-based metal-organic complex as a highly efficient heterogeneous catalyst for the selective oxidation of alkylbenzenes

The direct oxidation of C-H bonds in organic materials into necessary oxygen-containing compounds under mild conditions has attracted increasing attention. A Keggin-type polyoxometalate-based metal-organic complex (MOC), [CuII4Cu^I(H₂trz)₄(C₂O₄)(H₂O)₄(H₃PW_11.18CuII0.82O₄₀)]·8H₂O (1) (H₃trz = 1,2,4-triazole), was designed and synthesized under hydrothermal conditions, and was structurally characterized by single crystal X-ray diffraction, PXRD, IR spectroscopy, TGA, and XPS. Complex 1 is a 3D 4,6,8-connected architecture derived from [Cu₅(H₂trz)₄(C₂O₄)₂(H₂O)₂]_n units and [PW_11.18Cu_0.82O₄₀]^6- anions, which can catalytically oxidize various types of alkylbenzenes. Gas chromatographic analysis showed that complex 1 as a heterogeneous catalyst could effectively catalyze the oxidation of diphenylmethane with 93% conversion and 99% selectivity within 6 h. In addition, the conversion for the catalytic oxidation of ethylbenzene was 96% with 99% selectivity. Compared with some reported catalysts, complex 1 exhibited a better catalytic effect and lower reaction time. Meanwhile, the catalytic oxidation of other benzyl derivatives with complex 1 was also investigated, which indicated that complex 1 possessed excellent catalytic performance.

Nanostructured Manganese Oxides within a Ring-Shaped Polyoxometalate Exhibiting Unusual Oxidation Catalysis

Nanosized manganese oxides have recently received considerable attention for their synthesis, structures, and potential applications. Although various synthetic methods have been developed, precise synthesis of novel nanostructured manganese oxides are still challenging. In this study, using a structurally defined nanosized cavity inside a ring-shaped polyoxometalate, we succeeded in synthesizing two types of discrete 18 and 20 nuclear nanostructured manganese oxides, Mn18 and Mn20, respectively. In particular, Mn18 showed much higher catalytic activity than other manganese oxides for the oxygenation of alkylarenes including electron-deficient ones, and the reaction proceeded through a unique reaction mechanism due to its unusual manganese oxide structure.

Discovery of a Neutral 40-PdII-Oxo Molecular Disk, [Pd40O24(OH)16{(CH3)2AsO2}16]: Synthesis, Structural Characterization, and Catalytic Studies

We report on the synthesis and structural characterization of a giant, discrete, and neutral molecular disk, [Pd₄₀O₂₄(OH)₁₆{(CH₃)₂AsO₂}₁₆] (Pd₄₀), comprising a 40-palladium-oxo core that is capped by 16 dimethylarsinate moieties, resulting in a palladium-oxo cluster (POC) with a diameter of ∼2 nm. Pd₄₀, which is the largest known neutral Pd-based oxo cluster, can be isolated either as a discrete species or constituting a 3D H-bonded organic-inorganic framework (HOIF) with a 12-tungstate Keggin ion, [SiW₁₂O₄₀]^4- or [GeW₁₂O₄₀]^4-. ¹H and ¹³C NMR as well as ¹H-DOSY NMR studies indicate that Pd₄₀ is stable in aqueous solution, which is also confirmed by ESI-MS studies. Pd₄₀ was also immobilized on a mesoporous support (SBA15) followed by the generation of size-controlled Pd nanoparticles (diameter ∼2-6 nm, as based on HR-TEM), leading to an effective heterogeneous hydrogenation catalyst for the transformation of various arenes to saturated carbocycles.

Deliberate Construction of Polyoxoniobates Exploiting the Carbonate Ligand

Polyoxometalates (POMs, metals=V^V , Nb^V , Ta^V , Mo^VI , W^VI ) are molecular metal oxides that can be isolated without capping ligands. The high negative charge of polyoxoniobates (PONb) provides strong interactions with heterocations, advantageous for electrostatic assembly of modular materials. In four single-crystal X-ray structures, we demonstrate that carbonate combined with the very reactive decaniobate [Nb₁₀ O₂₈ ]^6- reassembles into a new decaniobate, [Nb₁₀ O₂₅ (CO₃ )₆ ]^12- , featuring three carbonate-ligated Nb-polyhedra. These Nb-sites can be replaced by heterometals (lanthanides), and the tridentate carbonate can serve as an anchor point to build niobate-frameworks. Small-angle X-ray scattering and two additional X-ray structures reveal that the reaction pathway proceeds through a Nb₂₄ -PONb intermediate, and the obtained PONb (with or without carbonate) is counterion, temperature, and solvent-dependent (water or mixed water-methanol). This provides an uncommon level of control for PONb chemistry.

Discovery and Supramolecular Interactions of Neutral Palladium-Oxo Clusters Pd16 and Pd24

We report on the synthesis, structure, and physicochemical characterization of the first three examples of neutral palladium-oxo clusters (POCs). The 16-palladium(II)-oxo cluster [Pd16 O24 (OH)8 ((CH3 )2 As)8 ] (Pd16 ) comprises a cyclic palladium-oxo unit capped by eight dimethylarsinate groups. The chloro-derivative [Pd16 Na2 O26 (OH)3  Cl3  ((CH3 )2  As)8 ] (Pd16 Cl) was also prepared, which forms a highly stable 3D supramolecular lattice via strong intermolecular interactions. The 24-palladium(II)-oxo cluster [Pd24 O44 (OH)8 ((CH3 )2 As)16 ] (Pd24 ) can be considered as a bicapped derivative of Pd16 with a tetra-palladium-oxo unit grafted on either side. The three compounds were fully characterized 1) in the solid state by single-crystal and powder XRD, IR, TGA, and solid-state 1 H and 13 C NMR spectroscopy, 2) in solution by 1 H, 13 C NMR and 1 H DOSY spectroscopic methods, and 3) in the gas phase by electrospray ionization mass spectrometry (ESI-MS).

Peroxouranyl-Containing W48 Wheel: Synthesis, Structure, and Detailed Infrared and Raman Spectroscopy Study

We report on the first example of a peroxouranium-containing {P₈W₄₈} wheel, [{(UO₂)₄(O₂)₄}₂(P₈W₄₈O₁₈₄)]^40- (1), which was synthesized by a one-pot reaction of UO₂(NO₃)₂·6H₂O with the 48-tungsto-8-phosphate wheel [H₇P₈W₄₈O₁₈₄]^33- and aqueous hydrogen peroxide in a pH 6 lithium acetate solution at 50 °C. Polyanion 1 comprises two tetrauranyl squares with side-on peroxo bridging ligands in the cavity of the {P₈W₄₈} wheel, and was isolated as the hydrated potassium-lithium salt K₁₈Li₂₂[{(UO₂)₄(O₂)₄}₂(P₈W₄₈O₁₈₄)]·133H₂O (KLi-1), which was characterized in the solid state by single-crystal X-ray diffraction, as well as thermogravimetric and elemental analyses. A detailed Fourier transform infrared and Raman spectroscopy study was also performed.

36-Nuclearity Organophosphonate-Functionalized Polyoxomolybdates: Synthesis, Characterization and Selective Catalytic Oxidation of Sulfides

The crown-shaped 36-molybdate cluster organophosphonate-functionalized polyoxomolybdates with the highest nuclearity in organophosphonate-based polyoxometalate chemistry, (NH₄ )₁₉ Na₇ H₁₀ [Cu(H₂ O)TeMo₆ O₂₁ {N(CH₂ PO₃ )₃ }]₆ ⋅31 H₂ O, has been reported for the first time. The synthesized 36-molybdate cluster was characterized by routine techniques and tested as a heterogeneous catalyst for selective oxidation of sulfides with impressive catalytic and selective performances after heat treatment. High efficiency (TON=15333) was achieved in the selective oxidation of sulfides to sulfoxides, caused by the synergic effect between copper and polyoxomolybdates and the generation of the cuprous species during the heat treatment.

A Toroidal Zr70 Oxysulfate Cluster and Its Diverse Packing Structures

Herein, we report the discovery of a toroidal inorganic cluster of zirconium(IV) oxysulfate of unprecedented size with the formula Zr₇₀(SO₄)₅₈(O/OH)₁₄₆⋅x(H₂O) (Zr₇₀), which displays different packing of ring units and thus several polymorphic crystal structures. The ring measures over 3 nm across, has an inner cavity of 1 nm and displays a pseudo‐10‐fold rotational symmetry of Zr₆ octahedra bridged by an additional Zr in the outer rim of the ring. Depending on the co‐crystallizing species, the rings form various crystalline phases in which the torus units are connected in extended chain and network structures. One phase, in which the ring units are arranged in layers and form one‐dimensional channels, displays high permanent porosity (BET surface area: 241 m² g⁻¹), and thus demonstrates a functional property for potential use in, for example, adsorption or heterogeneous catalysis.

The Surface Chemistry of Metal Oxide Clusters: From Metal-Organic Frameworks to Minerals

Many metal-organic frameworks (MOFs) incorporate nodes that are small metal oxide clusters. Some of these MOFs are stable at high temperatures, offering good prospects as catalysts-prospects that focus attention on their defect sites and reactivities-all part of a broader subject: the surface chemistry of metal oxide clusters, illustrated here for MOF nodes and for polyoxocations and polyoxoanions. Ligands on MOF defect sites form during synthesis and are central to the understanding and control of MOF reactivity. Reactions of alcohols are illustrative probes of Zr₆O₈ node defects in UiO-66, characterized by the interconversions of formate, methoxy, hydroxy, and linker carboxylate ligands and by catalysis of alcohol dehydration reactions. We posit that new reactivities of MOF nodes will emerge from incorporation of a wide range of groups on their surfaces and from targeted substitutions of metals within them.

Cation-Controlled Assembly of Polyoxotungstate-Based Coordination Networks

The Preyssler polyoxoanion, [NaP₅ W₃₀ O₁₁₀ ]^14- ({P₅ W₃₀ }), is used as a platform for evaluating the role of nonbridging cations in the formation of transition-metal-bridged polyoxometalate (POM) coordination frameworks. Specifically, the assembly architecture of Co²⁺ -bridged frameworks is shown to be dependent on the identity and amount of alkali or alkaline-earth cations present during crystallization. The inclusion of Li⁺ , Na⁺ , K⁺ , Mg²⁺ , or Ca²⁺ in the framework synthesis is used to selectively synthesize five different Co²⁺ -bridged {P₅ W₃₀ } structures. The influence of the competition between K⁺ and Co²⁺ for binding to {P₅ W₃₀ } in dictating framework assembly is evaluated. The role of ion pairing on framework assembly structure and available void volume is discussed. Overall, these results provide insight into factors governing the ability to achieve controlled assembly of POM-based coordination networks.

Switchable Catalytic Polyoxometalate-Based Systems for Biomass Conversion to Carboxylic Acids

We present the Keggin-type polyoxometalate H6[PV3Mo9O40] as a switchable catalyst being able to catalyze the transformation of both glucose and glyceraldehyde to formic acid (42%) and lactic acid (40%), respectively, within 1 h reaction time by simply changing the reaction atmosphere at 160 °C from oxygen to nitrogen in one reactor setup. In detail, we report the influence of different gas atmospheres and reaction temperatures on various vanadium-containing catalysts in the selective transformation of several biogenic substrates to carboxylic acids with a special emphasis on reaction pathways and switchability of the catalyst systems. All investigations were carried out in parallel using either an oxygen or a nitrogen atmosphere of 20 bar performing time-resolved experiments between 0.25 and 5 h and a temperature variation from 160 to 200 °C. Furthermore, a catalyst and a substrate variation led to the reaction system consisting of glyceraldehyde and the Keggin-type polyoxometalates (POM) H6[PV3Mo9O40] as the best switchable reaction system under the applied conditions. This study shows interesting potential for using both Keggin-type and Lindqvist-type POMs as switchable catalysts for selective biomass conversion to platform chemicals.

Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry

Polyoxometalates (POMs) are molecular metal-oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self-assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM-cation interactions in solution, the resulting solid-state compounds, and behavior and properties that emerge from these POM-cation interactions. We will explore how application-inspired research has exploited cation-controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM-cation interactions.

Tunable Metal Oxide Frameworks via Coordination Assembly of Preyssler-Type Molecular Clusters

We present the synthesis of metal oxide frameworks composed of [NaP₅W₃₀O₁₁₀]^14- assembled with Mn, Fe, Co, Ni, Cu, or Zn bridging metal ions. X-ray diffraction shows that the frameworks adopt the same assembly regardless of bridging metal ion. Furthermore, our synthesis allows for the assembly of isostructural frameworks with mixed-metal ion bridges, or with clusters that have been doped with Mo, providing a high degree of compositional diversity. This consistent assembly enables investigation into the role of the building blocks in the properties of the metal oxide frameworks. The presence of bridging metal ions leads to increased conductivity compared to unbridged frameworks, and frameworks bridged with Fe have the highest conductivity. Additionally, Mo-doping can be used to enhance the conductivities of the frameworks. Similar structures can be obtained from clusters in which the central Na⁺ has been replaced with Bi³⁺ or Sm³⁺. Overall, the optical and electronic properties are tunable via choice of bridging metal ion and cluster building block and reveal emergent properties in these cluster-based frameworks. These results demonstrate the promise of using polyoxometalate clusters as building blocks for tunable complex metal oxide materials with emergent properties.

3D designed and printed chemical generators for on demand reagent synthesis

Modern science has developed well-defined and versatile sets of chemicals to perform many specific tasks, yet the diversity of these reagents is so large that it can be impractical for any one lab to stock everything they might need. At the same time, isssues of stability or limited supply mean these chemicals can be very expensive to purchase from specialist retailers. Here, we address this problem by developing a cartridge -oriented approach to reactionware-based chemical generators which can easily and reliably produce specific reagents from low-cost precursors, requiring minimal expertise and time to operate, potentially in low infrastructure environments. We developed these chemical generators for four specific targets; transition metal catalyst precursor tris(dibenzylideneacetone)dipalladium(0) [Pd₂(dba)₃], oxidising agent Dess-Martin periodinane (DMP), protein photolinking reagent succinimidyl 4,4’-azipentanoate (NHS-diazirine), and the polyoxometalate cluster {P₈W₄₈}. The cartridge synthesis of these materials provides high-quality target compounds in good yields which are suitable for subsequent utilization.

Synthetic labs rely on a vast number of chemicals, which are often unstable with time and affected by price fluctuations. Here, the authors report ad hoc developed cartridge reactionware for the synthesis of four different targets in a time- and cost-saving manner.

Controlling the Reactivity of the [P8 W48 O184 ]40- Inorganic Ring and Its Assembly into POMZite Inorganic Frameworks with Silver Ions

The construction of pure-inorganic framework materials with well-defined design rules and building blocks is challenging. In this work, we show how a polyoxometalate cluster with an integrated pore, based on [P8 W48 O184 ]40- (abbreviated as {P8 W48 }), can be self-assembled into inorganic frameworks using silver ions, which both enable reactions on the cluster as well as link them together. The {P8 W48 } was found to be highly reactive with silver ions resulting in the in situ generation of fragments, forming {P9 W63 O235 } and {P10 W66 O251 } in compound (1) where these two clusters co-crystallize and are connected into a POMZite framework with 11 Ag+ ions as linkers located inside clusters and 10 Ag+ linking ions situated between clusters. Decreasing both the concentration of Ag+ ions, and the reaction temperature compared to the synthesis of compound (1), leads to {P8 W51 O196 } in compound 2 where the {P8 W48 } clusters are linked to form a new POMZite framework with 9 Ag+ ions per formula unit. Further tuning of the reaction conditions yields a cubic porous network compound (3) where {P8 W48 } clusters as cubic sides are joined by 4 Ag+ ions to give a cubic array and no Ag+ ions were found inside the clusters.

Self-Assembled Supramolecular Polyoxometalate Hybrid Architecture as a Multifunctional Oxidation Catalyst

Polyoxometalates (POMs) are widely applied as tuneable and versatile catalysts for a variety of oxidation reactions in an aqueous/organic two-phase system. However, the practical applications of POMs-based biphasic catalysis are hampered by low space-time yields and mass-transport limitation between two layers due to extremely low solubility of the organic reactants in the aqueous phase. Here, we first introduced β-cyclodextrin (β-CD) as an inverse phase transfer agent and a supramolecular nanoreactor to construct a supramolecular POM inorganic-organic hybrid framework (KCl₄)Na₇[(β-CD)₃(SiW₁₂O₄₀)]·9H₂O {3CD@SiW₁₂} for various oxidation catalyses. In contrast to free CD, Keggin [SiW₁₂O₄₀]^4- catalysts, and their mixture, the {3CD@SiW₁₂} catalyst, efficiently catalyze oxidation reactions of alcohol, alkene, and thiophene. A comprehensive strategy of experimental, crystallographic, and density functional theory (DFT) calculations elucidates that the catalytic pathway involved three combined aspects of supramolecular recognition, phase transfer property, and POM catalysis. The strategic combination of supramolecular characteristic and POM-based catalysts to fabricate supramolecular POM hybrid materials opens up new economic and green tuning options, thus paving the way to informed catalyst design.

Sequential Isomerization of a Macrocyclic Polyoxometalate Archetype

Controlled isomerization of individual {α-P₂W₁₂O₄₈} polyoxotungstate building blocks under the constricted conditions of the macrocyclic [P₈W₄₈O₁₈₄]^40- archetype ({P₈W₄₈}) is linked to site-specific Cu^II coordination. The derivatives [αγαγ-P₈W₄₈O₁₈₄{Cu(H₂O)}₂]^36- (1), [γγγγ-P₈W₄₈O₁₈₄{Cu(H₂O)_0.5}₄]^32- (2), and [αγγγ-P₈W₄₈O₁₈₄{Cu(H₂O)}₃]^34- (3) feature the {αγαγ-P₈W₄₈} and the hitherto unknown {γγγγ-P₈W₄₈} and {αγγγ-P₈W₄₈} isomers based on {α-P₂W₁₂} and/or Cu^II-stabilized {γ-P₂W₁₂} units and form from the reactions of the classical {P₈W₄₈} (={αααα-P₈W₄₈}) and CuCl₂ in sodium acetate medium (pH 5.2). All products were thoroughly characterized in both the solid state and aqueous solutions, including electrocatalysis assessments.

Inorganic–organic hybrid high-dimensional polyoxotantalates and their structural transformations triggered by water

In this work, novel dimeric polyoxotantalate (POTa) clusters {Cu(en)(Ta₆O₁₉)}₂/{Cu(enMe)(Ta₆O₁₉)}₂ were introduced as SBUs to construct a new family of extended POTa materials, including the first two 3D POTa frameworks and two 2D POTa layers.

Synthesis, structure and electrochemical characterization of an isopolytungstate (W4O16) held by MnII anchors within a superlacunary crown heteropolyanion {P8W48}

The first example of an isopolyanion (W₄O₁₆) held by redox-active Mn^II anchors within an archetypal superlacunary heteropolyanion {P₈W₄₈}.

Selective Hydrolysis of Ovalbumin Promoted by Hf(IV)-Substituted Wells-Dawson-Type Polyoxometalate

The reactivity and selectivity of Wells-Dawson type polyoxometalate (POM), K₁₆[Hf(α₂-P₂W₁₇O₆₁)₂]·19H₂O (Hf1-WD2), have been examined with respect to the hydrolysis of ovalbumin (OVA), a storage protein consisting of 385 amino acids. The exact cleavage sites have been determined by Edman degradation experiments, which indicated that Hf1-WD2 POM selectively cleaved OVA at eight peptide bonds: Phe13-Asp14, Arg85-Asp86, Asn95-Asp96, Ala139-Asp140, Ser148-Trp149, Ala361-Asp362, Asp362-His363, and Pro364-Phe365. A combination of spectroscopic methods including ³¹P NMR, Circular Dichroism (CD), and Tryptophan (Trp) fluorescence spectroscopy were employed to gain better understanding of the observed selective cleavage and the underlying hydrolytic mechanism. ³¹P NMR spectra have shown that signals corresponding to Hf1-WD2 gradually broaden upon addition of OVA and completely disappear when the POM-protein molar ratio becomes 1:1, indicating formation of a large POM/protein complex. CD demonstrated that interactions of Hf1-WD2 with OVA in the solution do not result in protein unfolding or denaturation even upon adding an excess of POM. Trp fluorescence spectroscopy measurements revealed that the interaction of Hf1-WD2 with OVA (K_q = 1.1 × 10⁵ M⁻¹) is both quantitatively and qualitatively slightly weaker than the interaction of isostructural Zr-containing Wells-Dawson POM (Zr1-WD2) with human serum albumin (HAS) (K_q = 5.1 × 10⁵ M⁻¹).

pH-induced phase transition and crystallization of soft-oxometalates (SOMs) into polyoxometalates (POMs): a study on crystallization from colloids

In this work, we demonstrate a simple approach for growing 1D (one-dimensional) inorganic chains of K(C₆H₁₆N)₃Mo₈O₂₆·H₂O polyoxometalates (POMs) from its colloidal soft-oxometalate (SOM) phase through the variation of pH. The structure is composed mainly of a 1D inorganic chain with a β-Mo₈O₂₆^4- binding node linked using K⁺ via Mo-O-K linkages, which results in a cuboctahedral geometry for the K⁺ ions. Crystal structure and Hirshfeld surface studies reveal the role of triethylammonium cations in restricting the growth of the 1D chain into 2D/3D (two-/three-dimensional) structures. During the nucleation process from the heterogeneous SOM phase, some of the intermolecular interactions in the dispersion phase are retained in the crystal structure, which was evidenced from residual O...O interactions. The crystallization of the species from its colloidal form as a function of pH was studied by the use of Raman spectroscopy and it was found that the increase in volume fraction of the β-Mo₈O₂₆^4- species in the crystallizing colloidal mixture with the decrease in pH is responsible for the nucleation. This was monitored by time-dependent DLS (dynamic light scattering) measurement and zeta-potential studies, revealing the co-existence of both the crystal and the colloidal forms at pH 3-2. This brings us to the conclusion that in the crystallization of POMs, the colloidal SOM phase precedes the crystalline POM phase which occurs via a phase transition. This work could open up avenues for the study of POM formation from the stand-point of colloidal chemistry and SOMs.

Organometallic functionalized non-classical polyoxometalates: synthesis, characterization and electrochemical properties

Two monomeric non-classical tellurium-containing heteropolymolybdate-supported metal carbonyl derivatives (NH4)5H3{[Te2Mo12(OH)O44][Mn(CO)3]}·18H2O (1) and (NH4)8{[Te2Mo12(OH)O44][Re(CO)3]}·13H2O (2) have been successfully isolated in good yields. Compounds 1 and 2 are the first two examples of metal carbonyl derivatives of heteromolybdate with tellurium as the central atom. These two compounds have been thoroughly characterized by single-crystal X-ray diffraction, elemental analyses, X-ray powder diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and UV-Vis spectroscopy. The electrochemical catalytic activities of 1 and 2 including cyclic voltammetry and catalytic oxidation of nitrite have also been investigated.

Fabrication and Electrochemical Performance of Polyoxometalate-Based Three-Dimensional Metal Organic Frameworks Containing Carbene Nanocages

Two new polyoxometalate (POM)-based three-dimensional metal organic carbene frameworks, [Ag₁₀(trz)₄(H₂O)₂][HPW₁₂O₄₀] (POMs@MCNCs-1) and [Ag₁₀(trz)₄(H₂O)₆][H₂SiW₁₂O₄₀] (POMs@MCNCs-2), were hydrothermally synthesized, in which Keggin-type polyoxoanions as templates induce the formation of two different kinds of metal-carbene nanocages (MCNCs) for the first time. Combination of the reversible multielectron redox behavior and electron storage functions of POMs with the good electrical conductivity of the single-walled carbon nanotubes (SWNTs) renders the POMs@MCNCs-1/SWNT composite excellent electrochemical performance and good stability as anode materials of lithium-ion batteries, with up to 2000 mA h g^-1 for the first discharge capacity and ca. 859 mA h g^-1 for the second cycle at a current density of 100 mA g^-1. The successful fabrication of unprecedented MCNCs into the POM-based three-dimensional metal-organic frameworks in the present work must initiate extensive research interests in diverse fields.

Polyoxometalate-Based Metal-Organic Frameworks for Selective Oxidation of Aryl Alkenes to Aldehydes

Polyoxometalates (POMs) show considerable catalytic performance toward the selective oxidation of alkenes to aldehydes, which is commercially valuable for the production of pharmaceuticals, dyes, perfumes, and fine chemicals. However, the low specific surface area of POMs as heterogeneous catalysts and poor recyclability as homogeneous catalysts have hindered their wide application. Dispersing POMs into metal-organic frameworks (MOFs) for the construction of POM-based MOFs (POMOFs) suggests a promising strategy to realize the homogeneity of heterogeneous catalysis. Herein, we report two new POMOFs with chemical formulas of [Co(BBTZ)₂][H₃BW₁₂O₄₀]·10H₂O (1) and [Co₃(H₂O)₆(BBTZ)₄][BW₁₂O₄₀]·NO₃·4H₂O (2) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) for the selective oxidation of alkenes to aldehydes. Compound 1 possesses a non-interpenetrated three-dimensional (3D) cds-type open framework with a 3D channel system. Compound 2 displays a 3D polyrotaxane framework with one-dimensional channels along the [100] direction. In the selective oxidation of styrene into benzaldehyde, compound 1 can achieve a 100% conversion in 4 h with 96% selectivity toward benzaldehyde, which is superior to that of compound 2. A series of control experiments reveal that the co-role of [BW₁₂O₄₀]^5- and Co²⁺ active center as well as a more open framework feature co-promote the catalytic property of the POMOFs in this case. This work may suggest a new option for the development of POMOF catalysts in the selective oxidation of alkenes.