Self-Assembly of Anionic Polyoxometalate–Organic Architectures Based on Lacunary Phosphomolybdates and Pyridyl Ligands

High-Selectivity Tandem Photocatalytic Methanation of CO2 by Lacunary Polyoxometalates-Stabilized *CO Intermediate

Stabilizing specific intermediates to produce CH4 remains a main challenge in solar-driven CO2 reduction. Herein, g-C3N4 is modified with saturated and lacunary phosphotungstates (PWx, x=12, 11, 9) to tailor the CO2 reduction pathway to yield CH4 in high selectivity. Increased lacuna of phosphotungstates leads to higher CH4 yield and selectivity, with a superior CH4 selectivity of 80 % and 40.8 μmol ⋅ g-1 ⋅ h-1 evolution rate for PW9/g-C3N4. Conversely, g-C3N4 and PWx alone show negligible CH4 production. The conversion of CO2 to CH4 follows a tandem catalytic process. CO2 is initially activated on g-C3N4 to form *CO intermediates, meanwhile photogenerated electrons derived from g-C3N4 transfer to PWx. Then the reduced PWx captures *CO, which is subsquently hydrogenated to CH4. With the injection of two photogenerated electrons, PW9 is capable of adsorbing and activating *CO. However, the reduced PW12 and PW11 are incapable of adsorbing *CO due to the small energy of occupied molecular orbitals, which is the reason for the poorer activity of PWx/g-C3N4 (x=12, 11) compared with that of PW9/g-C3N4. This work provides new insights to regulate highly selective CO2 photoreduction to CH4 by utilizing lacuna of polyoxometalates to enhance the interaction of metals in polyoxometalates with key intermediates.

Making an inverted Keggin ion lacunary

The century-old inverted Keggin ion has been revisited in an effort to unleash its potential in the structural engineering and functional development of polyoxomolybdates (POMos). Over the past hundred years, attempts to program the metal-oxo scaffold of inverted Keggins have been conducted continually but without any success. In this work, a structurally inert, inverted Keggin-type POMo could finally be altered by means of a binary heterogroup-templated approach, resulting in the successful isolation of two lacunary species. The local structure and charge distribution of these species are adjustable, and hence they serve as available building blocks for the subsequent controlled assembly of a CeIII-incorporated derivative. From the plenary to the lacunary, the enclosed structure of the inverted Keggin has been opened up significantly, resulting in less steric hindrance, along with a transition from an electron neutral species to a negatively charged species. Owing to these beneficial properties, the emerging defect-containing polyanions demonstrated outstanding Lewis acid-base catalytic activity in the high efficiency production of pyrazoles.

Synthesis of polyoxothiometalates through site-selective post-editing sulfurization of polyoxometalates

Polyoxometalates (POMs) function as platforms for synthesizing structurally well-defined inorganic molecules with diverse structures, metals, compositions, and arrangements. Although post-editing of the oxygen sites of POMs has great potential for development of unprecedented structures, electronic states, properties, and applications, facile methods for site-selective substitution of the oxygen sites with other atoms remain limited. Herein, we report a direct site-selective oxygen-sulfur substitution method that enables transforming POMs [XW12O40]4- (X = Si, Ge) to Keggin-type polyoxothiometalates (POTMs) [XW12O28S12]4- using sulfurizing reagents in an organic solvent. The resulting POTMs retain the original Keggin-type structure, with all 12 surface W[double bond, length as m-dash]O groups selectively converted to W[double bond, length as m-dash]S without sulfurization of other oxygen sites. These POTMs show high stability against water and O2 in organic solvents and a drastic change in the electronic states and redox properties. The findings of this study represent a facile method for converting POMs to POTMs, leading to the development of their unique properties and applications in diverse fields, including (photo)catalysis, sensing, optics, electronics, energy conversion, and batteries.

Self-assembled molecular hybrids comprising lacunary polyoxometalates and multidentate imidazole ligands

Self-assembly via coordination bonding facilitates the creation of diverse inorganic-organic molecular hybrids with distinct structures and properties. Recent advances in this field have been driven by the versatility of organic ligands and inorganic units. Lacunary polyoxometalates are a class of well-defined metal-oxide clusters with a customizable number of reactive sites and bond directions, which make them promising inorganic units for self-assembled molecular hybrids. Herein, we report a novel synthesis method for self-assembled molecular hybrids utilizing the reversible coordination of multidentate imidazole ligands to the vacant sites of lacunary polyoxometalates. We synthesized self-assembled molecular hybrids including monomer, dimers, and tetramer, demonstrating the potential of our method for constructing intricate hybrids with tailored properties and functionalities.

Selective Hydrolysis of Ovalbumin by Zr-Based Lacunary Polyoxotungstate in Surfactant Solutions

This study aims to design an artificial metalloprotease based on a Zr-containing polyoxometalate Na8[Zr(W5O18)2] [Zr(W5)2] for the hydrolysis of ovalbumin (OVA) in the presence of different surfactants, which can be used in many areas of the biological and medical sciences, particularly for targeted proteolytic drug design. For this reason, parameters, including the free energy of binding, the chemical nature of amino acid residues, secondary structures, and electrostatic potentials, of Zr(W5)2-OVA and Zr(W5)2-OVA-surfactant were analyzed by molecular docking simulations. The investigations showed that the presence of surfactants decreases the binding affinity of Zr(W5)2 for OVA amino acids, and hydrogen bonds and van der Waals interactions are formed between Zr(W5)2 and OVA amino acids. Additionally, GROMACS further illustrated the significance of SDS and CTAB surfactants in influencing the conformational changes of the OVA that lead to selective protein hydrolysis. In agreement with molecular dynamics simulation results, the experimental analysis showed more protein hydrolysis for the Zr(W5)2-OVA-surfactant systems. For instance, circular dichroism spectroscopy indicated that Zr(W5)2-OVA-CTAB and Zr(W5)2-OVA-TX-100 were more hydrolytically efficient due to the increased level of β-structures rather than α-chains, which showed that surfactants can facilitate the accessibility of Zr(W5)2 to the cleavage sites by inducing partial unfolding of the OVA structure.

Porphyrin-Polyoxotungstate Molecular Hybrid as a Highly Efficient, Durable, Visible-Light-Responsive Photocatalyst for Aerobic Oxidation Reactions

Organic-polyoxometalate (POM) hybrids have recently attracted considerable interest because of their distinctive properties and wide-ranging applications. For the construction of organic-POM hybrids, porphyrins are promising building units owing to their optical properties and reactivity, including strong visible-light absorption and subsequent singlet-oxygen (1O2*) generation. However, the practical utilization of porphyrins as photocatalysts and photosensitizers is often hindered by their own degradation by 1O2*. Therefore, there is a substantial demand for the development of porphyrin-derived photocatalysts with both high efficiency and durability. Herein, we present a porphyrin-polyoxotungstate molecular hybrid featuring a face-to-face stacked porphyrin dimer (I) fastened by four lacunary polyoxotungstates. Hybrid I exhibited remarkable efficiency and durability in photocatalytic aerobic oxidation reactions, and the selective oxidation of various dienes, alkenes, sulfides, and amines proceeded using just 0.003 mol % of the catalyst. Mechanistic investigations suggested that the high activity of I stems from the efficient generation of 1O2*, resulting from the heavy-atom effect of POMs. Furthermore, despite its high efficiency in 1O2* generation compared to free porphyrins, I exhibited superior durability against 1O2*-induced degradation under photoirradiation.

Ultra-stable and highly reactive colloidal gold nanoparticle catalysts protected using multi-dentate metal oxide nanoclusters

Owing to their remarkable properties, gold nanoparticles are applied in diverse fields, including catalysis, electronics, energy conversion and sensors. However, for catalytic applications of colloidal gold nanoparticles, the trade-off between their reactivity and stability is a significant concern. Here we report a universal approach for preparing stable and reactive colloidal small (~3 nm) gold nanoparticles by using multi-dentate polyoxometalates as protecting agents in non-polar solvents. These nanoparticles exhibit exceptional stability even under conditions of high concentration, long-term storage, heating and addition of bases. Moreover, they display excellent catalytic performance in various oxidation reactions of organic substrates using molecular oxygen as the sole oxidant. Our findings highlight the ability of inorganic multi-dentate ligands with structural stability and robust steric and electronic effects to confer stability and reactivity upon gold nanoparticles. This approach can be extended to prepare metal nanoparticles other than gold, enabling the design of novel nanomaterials with promising applications.

Polyoxometalate-Organic Hybrid "Calixarenes" as Supramolecular Hosts

Calixarenes are among the most useful and versatile macrocycles in supramolecular chemistry. The one thing that has not changed in the 80 years since their discovery, despite numerous derivatizations, is their fully organic, covalent scaffolds. Here, we report a new type of organic-inorganic hybrid "calixarenes" constructed by means of coordination-driven assembly. Replacing acetate ligands on the {SiW10 Cr2 (OAc)2 } clusters with 5-hydroxyisophthalates allows these 95° inorganic building blocks to be linked into bowl-shaped, hybrid "calix[n]arenes" (n=3, 4). With a large concave cavity, the metal-organic calix[4]arene can accommodate nanometer-sized polyoxoanions in an entropically driven process. The development of hybrid variants of calixarenes is expected to expand the scope of their physicochemical properties, guest/substrate binding, and applications on multiple fronts.

Organoarsonates Enable Single-Site Condensation of Hexalacunary {P2W12} Polyoxotungstates

A systematic study of the condensation reactions of arylarsonic-functionalized [α-P2W12O48]14- units in acidic aqueous media identified that the specific presence of an amino group in the ortho position of the phenyl rings induces a dimerization process that allowed isolation of discrete dimeric polyanions [(o-H2N-C6H4-AsO3)4P4W24O85]14- (1) with an unprecedented polyoxometalate skeleton characterized by two seminal {P2W12} groups joined via a single W-O-W bridge. At the same time, addition of divalent transition metal ions (MnII, CoII, and NiII) in the reaction mixture directed a condensation process on a completely different pathway resulting in one-dimensional (1D) coordination polymers based on V-shaped [{M(H2O)4}P4W24O92(C6H6AsNO)2]14- polyanions (M = MnII (2), CoII (3), and NiII (4)). All polyanions were isolated as hydrated mixed potassium/dimethylammonium salts and thoroughly characterized in the solid state. 31P NMR studies showed that the discrete cluster 1 is comparatively stable in 1 M LiCl aqueous solution and thus represents a potential precursor for subsequent reactions.

Clustering Six Electrons within "Dawson-Like" Polyoxometalate: An Open Route toward Its Post-functionalization

Super-reduction of polyoxometalates (POMs) in solution is of fundamental interest for designing innovative energy storage systems. In this article, we show that the "Dawson-like" POM can undergo a disproportionation process during its massive electron uptake, leading to species containing three metal-metal bonds as evidenced by X-ray diffraction, multi-nuclear magnetic resonance spectroscopy (1 H and 183 W NMR), extended X-ray absorption fine structure (EXAFS), UV/Vis, and voltammetry techniques. This result indicates that electron storing within metal-metal bonds is not a unique property of Keggin-type POM as postulated since the 70s. Besides, we demonstrate that the presence of an electron-rich triad in the "Dawson-like" POM allows its post-functionalization with additional tungstate ions, generating a chiral molecule that is also the largest WIV -containing POMs known to date.

Molecular hybrids of trivacant lacunary polyoxomolybdate and multidentate organic ligands

Functional molecular inorganic-organic hybrids of lacunary polyoxometalates and organic ligands attract much attention for advanced material applications. However, the inherent instability of lacunary polyoxomolybdates hinders the synthesis of hybrids and their utilization. Herein, we present a viable approach for the synthesis of molecular hybrids of trivacant lacunary Keggin-type polyoxomolybdates and multidentate organic ligands including carboxylates and phosphonates, which is based on the use of a lacunary structure stabilized by removable pyridyl ligands as a starting material.

Multifunctional Aryl Sulfonium Decavanadates: Tuning the Photochromic and Heterogeneous Oxidative Desulfurization Catalytic Properties Using Salicylaldehyde-type Functional Moieties on Counterions

Multifunctional materials based on polyoxovanadates (POVs) have rarely been reported. Herein, we used aryl sulfonium counterions (ASCIs) bearing a salicylaldehyde-type functionality to tune the properties of decavanadate ([V10O28]6-)-based hybrids for their application in photochromism and heterogeneous oxidative desulfurization (ODS) catalysis. The counterions FHPDS ((3-formyl-4-hydroxyphenyl)dimethylsulfonium), DFHPDS ((3,5-diformyl-4-hydroxyphenyl)dimethylsulfonium), and EFPDS ((4-ethoxy-3-formylphenyl)dimethylsulfonium) were clubbed with the decavanadate cluster to generate the hybrids (FHPDS)4[H2V10O28](H2O)4 (HY1), (DFHPDS)4[H2V10O28](H2O)3 (HY2), and (EFPDS)4[H2V10O28](H2O)6 (HY3). The photochromic properties of these hybrids were tested under 365 nm irradiation, which showed a color change from yellow to green. Different hybrids exhibited different photocoloration half-life (t1/2) values in the range of 0.77-28.38 min, suggesting the dependence of the photocoloration properties upon functional groups on the counterions. The hybrid HY2, having a 2,6-diformyl phenol moiety on the ASCI, exhibited an impressive t1/2 of 0.77 min. UP to 70% reversibility of photocoloration was achieved for the best photochromic hybrid HY2 in 48 h at 70 °C under an oxygen atmosphere. Theoretical and experimental data suggested that some of these aryl sulfonium POVs follow a different e--h+ stabilization mechanism than traditional sulfonium POM hybrids. Further, the salicylaldehyde-type ASCIs control the solubility of the decavanadate hybrids, which enables their application as heterogeneous catalysts for the selective oxidation of various sulfides. The nature of the substituents on the ASCIs also affected their catalytic activities; the counterion that facilitates the reversible V4+/V5+ switching enhances the catalytic ODS efficiency of the hybrids. Using HY2 as the catalyst, up to 99% conversion and 96% selectivity toward sulfones were achieved in dibenzothiophene (DBT) oxidation. The present study suggests a new promising approach for controlling POVs' photoresponsive and catalytic properties by using ASCIs bearing salicylaldehyde-type functional moieties.

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.

Two-dimensional metal-phase layered molybdenum disulfide for electrocatalytic hydrogen evolution reaction

The two-dimensional (2D) basal plane of metal-phase molybdenum disulphide (1T-MoS₂) provides a large area of active sites to significantly reduce the overpotential of the hydrogen evolution reaction (HER), but the long preparation period limits its industrial application. Here, 1T-MoS₂ catalysts derived from molybdenum blue solution (MBS) were prepared in one step using a rapid high-pressure microwave (MW-MoS₂) strategy. This method eliminated the thermodynamic process with a long time required for Mo-O trioxide bond breakage and reduction (Mo^VI → Mo^IV) of the conventional hydrothermal method. Additionally, the introduction of heteroatomic oxygen atoms effectively reduced the build-up of MW-MoS₂ and improved the monolayer/few-layer state and stability. Impressively, MW-MoS₂ has outstanding electrocatalytic performance, with a low overpotential (62 mV) at 10 mA cm^-2 and a small Tafel slope (42 mV dec^-1). This provides a simple strategy for the rapid preparation of a 2D sulphide HER catalyst with performance close to that of commercial Pt/C.

Recent Advances of Ti/Zr-Substituted Polyoxometalates: From Structural Diversity to Functional Applications

Polyoxometalates (POMs), a large family of anionic polynuclear metal-oxo clusters, have received considerable research attention due to their structural versatility and diverse physicochemical properties. Lacunary POMs are key building blocks for the syntheses of functional POMs due to their highly active multidentate O-donor sites. In this review, we have addressed the structural diversities of Ti/Zr-substituted POMs based on the polymerization number of POM building blocks and the number of Ti and Zr centers. The synthetic strategies and relevant catalytic applications of some representative Ti/Zr-substituted POMs have been discussed in detail. Finally, the outlook on the future development of this area is also prospected.

A photo-switchable molecular capsule: sequential photoinduced processes

The metastable trilacunary heteropolyoxomolybdate [PMo₉O₃₁(py)₃]^3- - {PMo₉}; py = pyridine) and the ditopic pyridyl bearing diarylethene (DAE) (C₂₅H₁₆N₂F₆S₂) self-assemble via a facile ligand replacement methodology to yield the photo-active molecular capsule [(PMo₉O₃₁)₂(DAE)₃]^6-. The spatial arrangement and conformation of the three DAE ligands are directed by the surface chemistry of the molecular metal oxide precursor with exclusive ligation of the photo-active antiparallel rotamer to the polyoxometalate (POM) while the integrity of the assembly in solution has been verified by a suite of spectroscopic techniques. Electrocyclisation of the three DAEs occurs sequentially and has been investigated using a combination of steady-state and time-resolved spectroscopies with the discovery of a photochemical cascade whereby rapid photoinduced ring closure is followed by electron transfer from the ring-closed DAE to the POM in the latent donor-acceptor system on subsequent excitation. This interpretation is also supported by computational and detailed spectroelectrochemical analysis. Ring-closing quantum yields were also determined using a custom quantum yield determination setup (QYDS), providing insight into the impact of POM coordination on these processes.

Ligand-Protecting Strategy for the Controlled Construction of Multinuclear Copper Cores within a Ring-Shaped Polyoxometalate

The ring-shaped polyoxometalate (POM) [P₈W₄₈O₁₈₄]^40- contains a large cavity and is an attractive inorganic multidentate ligand for accumulating metal cations. Until now, several multinuclear metal cores are constructed within the {P₈W₄₈} framework in aqueous solvents. However, it is still challenging to control the number and arrangement of introduced metal cations because of the numerous coordination sites inside the {P₈W₄₈} framework. In this study, we developed a novel approach for the selective synthesis of several multinuclear copper-containing ring-shaped POMs in organic solvents using methoxy groups as organic protecting ligands for the reactive coordination sites. Reacting a tetra-n-butylammonium (TBA) salt of [P₈W₄₈O₁₈₄]^40- (P8W48) with 4 and 8 equiv of copper(II) acetate in the presence of methanol (MeOH), tetra- and octacopper cores were incorporated into the cavity to form TBA₁₁H₁₃[Cu₄(H₂O)₄P₈W₄₈O₁₇₆(OCH₃)₈]·28H₂O·3CH₃NO₂ (Cu4) and TBA₁₄H₂[Cu₈(H₂O)₁₂P₈W₄₈O₁₇₆(OCH₃)₈]·24H₂O·CH₃CN (Cu8), respectively. For both structures, methoxy groups served as protecting ligands and temporarily inactivated vacant coordination sites. Without MeOH, dodeca- and hexadecacopper cores were constructed inside the cavity to form TBA₁₄H₂[Cu₁₂(H₂O)₁₆P₈W₄₈O₁₈₄]·4H₂O (Cu12) and TBA₁₆H₈[Cu₁₆(OH)₁₆(H₂O)₄P₈W₄₈O₁₈₄]·12H₂O·C₃H₆O (Cu16), respectively. The arrangement of copper ions on the same {P₂W₁₂} units could be controlled by the input number of copper ions. Moreover, all four POMs could be synthesized from P8W48 by the stepwise addition of 4 equiv of copper(II) acetate, clarifying the introduction process.

When Identification of the Reduction Sites in Mixed Molybdenum/Tungsten Keggin-Type Polyoxometalate Hybrids Turns Out Tricky

The mixed molybdenum/tungsten Keggin-type polyoxometalate (POM) hybrid (TBA)4[PW9Mo2O39{Sn(C6H4I)}] (TBA = tert-butylammonium) has been prepared by the reaction between [α-PW9Mo2O39]7- and [Cl3Sn(C6H4I)] in dried acetonitrile, in the presence of tetra-n-butylammonium bromide. A further coupling reaction affords the ferrocenyl derivative (TBA)4[PW9Mo2O39{Sn(C6H4)C≡C(C6H4)Fc}]. The POM hybrids have been thoroughly characterized by NMR and IR spectroscopies. Electrochemical analysis confirms their ease of reduction compared to the all-W analogue, albeit with a second reduction process occurring at a lower potential than in the all-Mo species. It is noteworthy that the second reduction is accompanied by an unusual red shift of the electronic absorption spectrum. Whereas there is no doubt that the first reduction deals with Mo, the location of the second electron in the bireduced species, on the second Mo or on W, has thus been the subject of a cross-investigation by spectroelectrochemistry, electron spin resonance, and theoretical calculations. Finally, it came out that the second reduction is also Mo-centered with two unpaired and antiferromagnetically coupled extra electrons.

Binuclear Ru(III)-Containing Polyoxometalate with Efficient Photocatalytic Activity for Oxidative Coupling of Amines to Imines

A novel binuclear ruthenium-based polyoxometalate, K₆H[{Ru₂Cl(H₂O)(CH₃COO)₂}{WO(H₂O)}₂(PW₉O₃₄)₂]·14H₂O (1), was successfully synthesized by the conventional hydrothermal method. Compound 1 was well-characterized by single-crystal X-ray diffraction, X-ray powder diffraction (PXRD), infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), electrospray ionization-mass spectrometry (ESI-MS), thermogravimetric analyses (TGA), and elemental analysis. The structural unit of compound 1 contains two [A-α-PW₉O₃₄]^9- building blocks at the upper and lower positions connected by two W atoms and two Ru atoms, where the W atoms and Ru atoms are arranged in a trapezoidal arrangement and the Ru atoms are bridged by acetic acid. Furthermore, compound 1 features characteristic absorption bands in the visible region, which allows the investigation of its photocatalytic properties in visible light. Under simulated sunlight radiation (λ > 400 nm), compound 1 exhibits high photocatalytic activity and good circularity toward the oxidative coupling of amines to imines at room temperature with O₂ as the sole oxidant.

Structural features of selenate based {Mo132} keplerate capsules

A new structural feature and significant stabilizing effect were found for keplerate-type nanospherical polyoxometalates.

Polyoxometalate-based metal–organic complexes and their derivatives as electrocatalysts for energy conversion in aqueous systems

The research progress on polyoxometalate-based metal–organic complexes and their derivatives as electrocatalysts in sustainable and clean energy conversion applications in aqueous systems is summarized.

Carbazole-based bis-imidazole ligand-involved synthesis of inorganic–organic hybrid polyoxometalates as electrochemical sensors for detecting bromate and efficient catalysts for selective oxidation of thioether

Considering the potential application on preparing electrode and catalyst materials of inorganic–organic hybrid polyoxometalates, a bis-imidazole ligand with carbazole as a connector, 3,6-di(1H-imidazol-1-yl)-9H-carbazole (L), was used for preparing inorganic–organic hybrid polyoxometalates. As a result, three complexes formulated by [NiL₂(Mo₂O₇)] (1), [Cu(H₂O)₂(HL)₂ (β-Mo₈O₂₆)]·H₂O (2) and [Ni₂(H₂O)₄L₂ (CrMo₆(OH)₅O₁₉)]·6H₂O (3) were obtained successfully. Structural analysis indicated that the different polyoxoanions and metal ions showed important influences on the formation of structures. In the presence of Ni²⁺ ions and heptamolybdate, a 2D network constructed from Ni²⁺ ions and L ligands was formed in complex 1, in which the [Mo₄O₁₄]⁴⁻ polyoxoanions were encapsulated. But the use of Cu²⁺ ions led to a 1D chain of complex 2, which was composed of [β-Mo₈O₂₆]⁴⁻ polyoxoanions and mononuclear {CuL₂} units. By utilizing [CrMo₆(OH)₅O₁₉]⁴⁻ as the inorganic building block, complex 3 showed a 2D (4, 4)-connected layer. Complexes 1–3 could be employed as electrode materials for sensing bromate with the limits of detection of 0.315 μM for 1, 0.098 μM for 2 and 0.551 μM for 3. Moreover, these complexes showed efficient catalytic activity for the selective oxidation of thioethers.

Three inorganic–organic hybrid polyoxometalates were prepared using a bis-imidazole ligand featuring carbazole as a connector, exhibiting not only diverse structures, but also good electrochemical sensing activities for bromate, as well as efficient catalytic performances for oxidation of thioether.

Roles of Organic Fragments in Redirecting Crystal/Molecular Structures of Inorganic-Organic Hybrids Based on Lacunary Keggin-Type Polyoxometalates

Lacunary polyoxometalates (LPOMs) are key precursors for the synthesis of functional POMs. To date, reviews dedicated to behavioral studies of LPOMs often comprise the role of metal ions, including transition metal (TM) and rare earth (RE) ions, in extending and stability of high-nuclearity clusters. In contrast, the role of organic ligands in the structures and properties of lacunary-based hybrids has remained less explored. In this review, we focus on the role of organic fragments in the self-assembling process of POM-based architectures and discuss relationships between the nature and structure of organic ligand and properties such as the topology of hybrid inorganic–organic material in RE and TM-RE heterometallic derivatives of lacunary Keggin-type POMs. The effects of organic fragment in mixed ligand hybrids are also briefly reviewed.

The Smallest Polyoxotungstate Retained by TRIS-Stabilization

A polycondensation reaction of the orthotungstate anion WO₄^2–, buffered at pH 7.5 in a TRIS-HCl (0.15 M) solution, results in the first example of a discrete polyoxotungstate anion, with just two W ions stabilized with TRIS ligands. It was isolated and characterized as Na₂[W^VI₂O₆(C₄O₃NH₁₀)₂]·6H₂O by single-crystal and powder X-ray diffraction, FT-IR spectroscopy, thermogravimetrical analysis (TGA), and elemental analysis in solid state and by electro-spray ionization mass spectrometry (ESI-MS), ¹³C, and ¹⁸³W NMR, as well as Raman spectroscopy in solution. This synthesis demonstrates the crucial and new role of the added tris-alkoxy ligand in the development of a new hybrid TRIS-isopolytungstate with the lowest known nuclearity (so far) and the terminal oxygens substituted with two nitrogen atoms arising from amines of the TRIS ligands.

We report on the synthesis and characterization of a new hybrid isopolytungstate Na₂[W^VI₂O₆(C₄O₃NH₁₀)₂]·6H₂O with the, so far, lowest known nuclearity and the terminal oxygen atoms substituted with nitrogen arising from amine.

Porphyrins and Polyoxometalate Scaffolds

Polyoxometalates (POMs) can act as unique reservoirs for multiple electron transfers. As POMs display only weak absorption in the visible spectrum, they can be associated with chromophores such as porphyrins and porphyrin antennae. In this Minireview, the research dedicated to the combination of porphyrins and polyoxometalates is put in context and the state of the art identifying the challenges addressed in the optimization of hybrid materials for applications is detailed.

Hexameric to Trimeric Lanthanide-Included Selenotungstates and Their 2D Honeycomb Organic-Inorganic Hybrid Films Used for Detecting Ochratoxin A

Two types of organic-inorganic hybrid structure-related lanthanide (Ln)-included selenotungstates (Ln-SeTs) [H₂N(CH₃)₂]₁₁Na₇[Ce₄(H₂PTCA)₂(H₂O)₁₂(HICA)]₂[SeW₄O₁₇]₂[W₂O₅]₄[SeW₉O₃₃]₄·64H₂O (1, H₃PTCA = 1,2,3-propanetricarboxylic acid, H₂ICA = itaconic acid) and [H₂N(CH₃)₂]₆Na₄[Ln₄SeW₈(H₂O)₁₄(H₂PTCA)₂O₂₈] [SeW₉O₃₃]₂·31H₂O [Ln = Pr³⁺ (2), Nd³⁺ (3)] were obtained by Ln nature control. The primary frameworks of 1-3 are composed of trivacant Keggin-type [B-α-SeW₉O₃₃]^8- and [SeW₄O_m]^n- [Ln = Ce³⁺ (1), m = 17, n = 6; Ln = Pr³⁺ (2), Nd³⁺ (3), m = 18, n = 8] fragments bridged by organic ligands and Ln clusters. Intriguingly, Ln nature results in the degradation of hexameric 1 to trimeric 2-3. Besides, 1@DMDSA and 3@DMDSA composites (DMDSA·Cl = dimethyl distearylammonium chloride) were prepared through the cation exchange method, which were then reorganized to form two-dimensional (2D) honeycomb thin films by the breath figure method. Using these honeycomb thin films as electrode materials, the aptasensors were further established by utilizing methylene blue as an indicator and cDNA and Au nanoparticles as signal amplifiers to enhance the response signal so as to realize the purpose of ochratoxin A (OTA) detection. This work provides a new platform for detecting OTA and explores the application potential of POM-based composites in biological and clinical analyses.

Microwave-Assisted Synthesis of Tris-Anderson Polyoxometalates for Facile CO2 Cycloaddition

Four new tris-Anderson polyoxometalates (POMs), (NH₄)₄[ZnMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (1), (NH₄)₄[CuMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (2), (TBA)₃(NH₄)[ZnMo₆O₁₇(C₅H₉O₃)₂(OH)]·10H₂O (3) (TBA = n-C₁₆H₃₆N), and (NH₄)₄[CuMo₆O₁₈(C₅H₉O₃)₂]·16H₂O (4), were synthesized by a microwave-assisted method. Single-crystal X-ray diffraction revealed that 1 and 2 contained a tris (trihydroxyl organic compounds) ligand grafted on one side, while two tris ligands were grafted on two sides to form χ/δ and δ/δ isomers in 3 and 4, respectively. ¹H and ¹³C NMR spectra of the χ/δ isomer 3 were obtained for the first time, with six methylenes showing six peaks in the ¹H NMR spectrum and only four peaks in the ¹³C NMR spectrum. Mass spectrometry monitoring revealed that during the microwave-assistant process the tris ligand can graft onto POMs to form 1, while tris directly coordinates with metallic heteroatoms to form isopolymolybdates during the conventional reflux synthesis process. In addition, 1-4 can catalyze CO₂ with epoxides into cyclic carbonates with high selectivity and yields at an atmospheric pressure of CO₂, which is lower than the pressure of CO₂ in other catalysis using POMs as catalysts. Furthermore, 1-4 showed good catalytic stability and cycling properties. Mechanism studies substantiated POMs cocatalyzed with Br^- to improve the catalytic yields.

Lacunary {Se4V10} Heteropolyoxovanadate Precursor with Monometal, Metal-Richer-Sandwiched Derivatives {Se8V20M} and {Se8V20M3}: Correlations between the Synthesis, Structure, and Catalytic Property

A new family of trinuclear transition-metal (TM)-sandwiched heteropolyoxovanadates (hetero-POVs) [(SeV₁₀O₂₈(SeO₃)₃M(H₂O)₃)₂(M(H₂O)₄)]^10- (Se₈V₂₀M₃, where M = Mn²⁺, Co²⁺, and Zn²⁺) were prepared using two feasible approaches: a stepwise assembly strategy atop the POV precursor Se₄V₁₀ and a one-pot reaction approach of KVO₃, SeO₂, TM²⁺, and a proline ligand. The crystallographic studies reveal that Se₈V₂₀M₃ consist of two asymmetric [SeV₁₀O₂₈(SeO₃)₃M(H₂O)₃)]^6- units, linked by another TM²⁺ ion, thus forming an interesting staggered sandwich-type arrangement. Additionally, Se₈V₂₀M₃ and Se₈V₂₀M present strong correlations between their structures and catalytic properties. In particular, Se₈V₂₀M₃ demonstrate an analogical heterogeneous catalytic performance as Se₈V₂₀M in the sulfoxidation reaction of thioethers owing to their structural similarities.

A protecting group strategy to access stable lacunary polyoxomolybdates for introducing multinuclear metal clusters

Although metal-containing polyoxomolybdates (molybdenum oxide clusters) exhibit outstanding catalytic properties, their precise synthetic method has not yet been developed. This is mainly because the very low stability of the multivacant lacunary polyoxomolybdates limited their use as synthetic precursors. Here, we present a "protecting group strategy" in polyoxometalate synthesis and successfully develop an efficient method for synthesising multinuclear metal-containing polyoxomolybdates using pyridine as a protecting group for unstable trivacant lacunary Keggin-type polyoxomolybdate [PMo₉O₃₄]^9-. Specifically, tetranuclear cubane- and planar-type manganese clusters were selectively synthesised in the polyoxomolybdates using the present method. The importance of this work is that, in addition to being the first practical way of utilizing multivacant lacunary polyoxomolybdates as precursors, this new "protecting group strategy" will make it possible to produce polyoxometalates with unexplored structures and properties.

Synthesis of a phosphomolybdate with a tetranuclear vanadium core by installing vanadium atoms in a lacunary template using the protecting group strategy

Vanadium-containing phosphomolybdates have outstanding catalytic and electrochemical properties. However, their traditional one-pot synthesis in aqueous media generates a mixture of randomly distributed isomers and undesirable impurities, which has hampered the in-depth study of the catalysis and reaction mechanism of these compounds. Here, the selective synthesis of a tetranuclear vanadium-containing phosphomolybdate, [PV3Mo9O40(VO)]3-, was achieved for the first time by using a pyridine-protected [A-α-PMo9O31(py)3]3- (py = pyridine) as the template.

Keggin-type polyoxometalate-containing metal–organic hybrids as friction materials for triboelectric nanogenerators

The POM-based inorganic–organic hybrids with different structures were assembled and used as the friction materials to construct TENGs and the results demonstrated that the output performance was closely related to the dielectric constant.

Coordination-Assembled Molecular Cages with Metal Cluster Nodes

Molecular cages have attracted great attention because of their fascinating topological structures and well-defined functional cavities. These discrete cages were usually fabricated by coordination assembly approach, a process employing directional metal-ligand coordination bonds due to the nature of the divinable coordination geometry and the required lability to encode dynamic equilibrium/error-correction. Compared to these coordination molecular cages with mononulcear metal-nodes, an increasing number of molecular cages featuring dinuclear and then polynuclear metal-cluster nodes have been synthesized. These metal-cluster-based coordination cages (MCCCs) combine the merits of both metal clusters and the cage structure, and exhibit excellent performances in catalysis, separation, host-guest chemistry and so on. In this review, we highlight the syntheses of MCCCs and their potential functions that is donated by the metal-cluster nodes.