Recent advances in transition-metal-containing Keggin-type polyoxometalate-based coordination polymers

Polyoxovanadate-Based Metal-Organic Frameworks with Dual Active Sites for the Synthesis of p-Benzoquinones

p-Benzoquinones are important organic intermediates in the synthesis of biopharmaceuticals and fine chemicals. In this study, two crystalline 3D polyoxovanadate-based metal-organic frameworks, H[Cu(tpi)2]{Cu2V7O21}·H2O (1, tpi = C18N5H13) and [Co(Htpi)2]{V4O12} (2, Htpi = C18N5H14), were synthesized, which as heterogeneous catalysts showed excellent catalytic activities for the synthesis of p-benzoquinones. Both compounds were characterized by IR, UV-vis diffuse reflectance spectroscopy, TG, XPS, X-ray diffraction, etc. In 1, {Cu2V7} clusters are connected together by copper cations and 1D Cu-organic coordination chains to yield a 3D polyoxometalate-based metal-organic framework (POMOF); in 2, adjacent 2D bimetallic oxide layers, constructed from 1D polyoxovanadate chains and cobalt ions, are further connected by 1D Co-organic coordination chains to form a 3D POMOF. Noteworthily, in the synthesis of trimethyl-p-benzoquinone, the key intermediate of vitamin E, using 2,3,6-trimethylphenol as the model substrate, the turnover frequency values for compounds 1 and 2 can, respectively, reach 607 and 380 h-1 in 8 min. Furthermore, both compounds demonstrated excellent recyclability and structural stability, characterized by PXRD and IR. The catalytic mechanism reveals that both the homolytic radical mechanism and heterolytic oxygen atom transfer mechanism are involved.

Design of a new nanocomposite based on Keggin-type [ZnW12O40]6- anionic cluster anchored on NiZn2O4 ceramics as a promising material towards the electrocatalytic hydrogen storage

Extensive research efforts have been dedicated to developing electrode materials with high capacity to address the increasing complexities arising from the energy crisis. Herein, a new nanocomposite was synthesized via the sol-gel method by immobilizing K6ZnW12O40 within the surface of NiZn2O4. ZnW12O40@NiZn2O4 was characterized by FT-IR, UV-Vis, XRD, SEM, EDX, BET, and TGA-DTG methods. The electrochemical characteristics of the materials were examined using cyclic voltammogram (CV) and charge-discharge chronopotentiometry (CHP) techniques. Multiple factors affecting the hydrogen storage capacity, including current density (j), surface area of the copper foam, and the consequences of repeated cycles of hydrogen adsorption-desorption were evaluated. The initial cycle led to an impressive hydrogen discharge capability of 340 mAh/g, which subsequently increased to 900 mAh/g after 20 cycles with a current density of 2 mA in 6.0 M KOH medium. The surface area and the electrocatalytic characteristics of the nanoparticles contribute to facilitate the formation of electrons and provide good diffusion channels for the movement of electrolyte ions throughout the charge-discharge procedure.

2,5-Thiophenedicarboxylic Acid Bridging Hexameric CeIII-Substituted Selenotungstate and Its Application for Detecting Mucin 1

The development of polyoxometalate chemistry not only is derived from the continuous discovery of novel polyoxometalates (POMs) but also stems from the exploitation of their new functionalities. In this work, we obtained a rigid sulfur-containing heterocyclic ligand-linking aggregate [N(CH3)4]10Na6H6[Ce8(H2O)26W8(HTDA)2(TDA)2O20][SeW4O18]2[SeW9O33]4·112H2O (1) (H2TDA = 2,5-thiophenedicarboxylic acid). Its polyanionic unit consists of one [Ce4(H2O)13W4O10(HTDA)(TDA)O10]18+ cluster and two kinds of Keggin-type [SeW4O18] and [SeW9O33] segments. It is noteworthy that H2TDA ligands not only work as connectors to link two symmetrical {[Ce4(H2O)13W4(HTDA)(TDA)O10][SeW4O18][SeW9O33]2}11- units but also function as ornaments to graft to the polyanionic backbone. Furthermore, 1 and 3,4-ethylenedioxythiophene (EDOT) were deposited on the glassy carbon electrode (GCE) by the electropolymerization (EPM) method, resulting in a 1-poly(3,4-ethylenedioxythiophene) (1-PEDOT) composite film, which can provide sufficient binding sites to immobilize Au nanoparticles (Au NPs). Hereafter, the Au NPs-immobilized 1-PEDOT modified electrode (Au/1-PEDOT/GCE) was used to construct an electrochemical aptasensor to detect mucin 1, showing a low detection limit of 29.5 fM in the Tris solution. This work not only demonstrates that rigid heterocyclic ligands are beneficial for the creation of novel rare-earth-substituted selenotungstate hybrids but also provides more enlightenment for POM-based materials used for electrochemical detection of cancer markers.

Effective oxygen activation on polyoxometalate-based hybrids for epoxidation of alkenes

Two polyoxometalate-based hybrids, [M(btap)3(H2O)3(HPW12O40)]·xH2O (M-PW, M = Co/Mn, btap = 3,5-bis(1',2',4'-triazol-1'-yl)pyridine) were synthesized. Co-PW exhibited higher activity and selectivity towards olefin epoxidation than Mn-PW due to the synergistic effect between CoII and PW, in which the Co centers activate O2 to ˙O2- and further binding of free H+ from PW affords the active peroxyacid.

Intensifying Photocatalytic Baeyer-Villiger Oxidation of Ketones with the Introduction of Ru Metalloligands and Bimetallic Units in POM@MOF

The synthesis of visible light-responsive and efficient photocatalysts toward green Baeyer-Villiger oxidation organic synthesis is of extraordinary significance. In this work, we have synthesized two examples of visible light responsive crystalline polyoxometalate@metal-organic framework materials Ru-NiMo and Ru-CoMo by introducing Ru metalloligands and {CdM3O12} bimetallic units (M = Ni or Co). This is the first report of metalloligand-modified polyoxometalate@metal-organic framework materials with bimetallic nodes, and the materials form a three-dimensional framework directly through coordination bonds between {CdM3O12} bimetallic units and metalloligands. In particular, Ru-NiMo can achieve efficient photocatalytic conversion of cyclohexanone to ε-caprolactone in yields as high as 95.5% under visible light excitation in the range of λ > 400 nm, achieving a turnover number and turnover frequency of 955 and 440 h-1, respectively, which are the best known photocatalysts for Baeyer-Villiger oxidation, while apparent quantum yield measured at 485 nm is 4.4%. Moreover, Ru-NiMo exhibited excellent structural stability and recyclability, producing a 90.8% yield after five cycles of recycling.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

Polyoxometalate nanocluster-infused triple IPN hydrogels for excellent microplastic removal from contaminated water: detection, photodegradation, and upcycling

Microplastic (MP) pollution pervades global ecosystems, originating from improper plastic disposal and fragmentation due to factors like hydrolysis and biodegradation. These minute particles, less than 5 mm in size, have become omnipresent, impacting terrestrial, freshwater, and marine environments worldwide. Their ubiquity poses severe threats to marine life by causing physical harm and potentially transferring toxins through the food chain. Addressing this environmental crisis necessitates a sustainable strategy. Our proposed solution involves a highly efficient copper substitute polyoxometalate (Cu-POM) nanocluster infused triple interpenetrating polymer network (IPN) hydrogel, comprising chitosan (CS), polyvinyl alcohol (PVA), and polyaniline (PANI) (referred to as pGel@IPN) for mitigating MP contamination from water. This 3D IPN architecture, incorporating nanoclusters, also enhances the hydrogel's photodegradation capabilities. Our scalable approach offers a sustainable strategy to combat MPs in water bodies, as demostrated by the adsorption behaviors on the hydrogel matrix under varying conditions, simulating real-world scenarios. Evaluations of physicochemical properties, mechanical strength, and thermal behavior underscore the hydrogel's robustness and stability. Detecting minute MP particles remains challenging, prompting us to label MPs with Nile red for fluorescence microscopic analysis of their concentration and adsorption on the hydrogel. The catalytic properties of POM within the hydrogel facilitate UV-induced MP degradation, highlighting a sustainable solution. Our detailed kinetics and isotherm studies revealed pseudo-first-order and Langmuir models as fitting descriptors for MP adsorption, exhibiting a high maximum adsorption capacity (Qm). Notably, pGel@IPN achieved ∼95% and ∼93% removal efficiencies for polyvinyl chloride (PVC) and polypropylene (PP) MPs at pH ∼ 6.5, respectively, also demonstrating reusability for up to 5 cycles. Post-end-of-life, the spent adsorbent was efficiently upcycled into carbon nanomaterials, effectively removing the heavy metal Cr(VI), exemplifying circular economy principles. Our prepared hydrogel emerges as a potent solution for MP removal from water, promising effective mitigation of the emerging pollutants of MPs while ensuring sustainable environmental practices.

Recent advances in lanthanide-based POMs for photoluminescent applications

Since the first formation of the famous "Peacock-Weakley" anions [Ln(W5O18)2]8/9-, a steady stream of breakthroughs have been made in the chemistry of multitalented lanthanide (Ln)-based polyoxometalates (POMs) for their potentially desirable properties. In particular, LnIII ions are generally recognised as the "vitamins of the modern industry" owing to their ability to cover a wide emission range, endowing Ln-based POMs with great potential for versatile and diverse luminescence-related applications. In this frontier, we discuss the synthesis strategies and intramolecular energy transfer in Ln-based POM derivatives. Then, the progressive improvements achieved with Ln-based POMs in photoluminescence applications are highlighted, focusing mainly on luminescent and fluorescent probes. Finally, the challenges for Ln-based POM materials for photoluminescence applications are discussed.

Visible-Light-Responsive Polyoxometalate@Metal-Organic Frameworks Involving Ir Metalloligands for Highly Selective Photocatalytic Oxidation of Sulfides to Sulfoxide

The synthesis of highly efficient visible-light-responsive photocatalysts is fundamental to solving the problems of low efficiency and poor selectivity in photocatalytic organic synthesis reactions. We synthesized a crystalline polyoxometalate @metal-organic framework material {Zn4 (H2 O)8 [Ir(ppy)2 (dcbpy)]4 [SiW12 O40 ]} ⋅ 4H2 O (Ir-SiW) by self-assembly of Ir metalloligands with POMs. The introduction of Ir metalloligands extends the light absorbing range to visible light, improving the efficient utilization of solar energy. The transfer of photogenerated electrons from Ir metalloligands to SiW12 was observed under visible light irradiation, which boosted the carrier separation efficiency. The synergistic effect of the two components increased the photocatalytic thioether oxidation activity, and the product methyl phenyl sulfoxide for 2.5 h under visible light irradiation (λ >400 nm) reached 99.5 %, which was higher than those of other POM-based photocatalysts. Meanwhile, the yield of methyl phenyl sulfoxide was still higher than 97 % after three cycles, demonstrating the high stability and reusability of Ir-SiW.

An electrochemical sensor based on carbon composites derived from bisbenzimidazole biphenyl coordination polymers for dihydroxybenzene isomers detection

Co-based coordination polymers (CoCP) based on 4,4'-bis(1H-benzo[d]imidazol-1-yl)-1,1'-biphenyl (BMB) ligand have been synthesized for the first time by the solvothermal method. The CoCP was carbonized at 700 °C under a nitrogen atmosphere to obtain carbide coordination polymer (C-CoCP) with a unique two-dimensional layered network structure. C-CoCP@GO was obtained by binding with GO and C-CoCP, its morphology and structure were investigated by XRD, SEM, EDS, FTIR, and TGA, which confirmed its two-dimensional stacked layered structure with high catalytic activity and large specific surface area. A highly sensitive electrochemical sensor was constructed for the simultaneous detection of hydroquinone and catechol based on the prepared carbon-based composite. Under optimized conditions, the working potentials (vs. Ag/AgCl) of HQ and CC are at 0.097 V and 0.213 V, respectively. The sensor exhibited an extremely wide linear range of 3-600 μM and 3-1750 μM for hydroquinone (HQ) and catechol (CC), respectively, with limits of detection (LOD) of 0.46 μM and 0.27 μM. The electrode material demonstrated stability over 14 days without significant attenuation of the response signal. Impressively, the sensor shows high stability, reproducibility, and selectivity due to the stable carbon skeleton structure of the C-CoCP material. In addition, it can be applied to the detection of hydroquinone in real samples with high interference immunity and high recovery. Hence, the C-CoCP@GO composite proved to be a great prospect and highly sensitive sensing platform for the detection of phenolic isomers.

N-N-Bridged Polynuclear POM-Based Coordination Polymers Based on a V-Type Ligand: Proton Conduction and Magnetism

The construction of polyoxometalate (POM)-based coordination polymers, in the presence of a nitrogen heterocyclic ligand, is intriguing due to the potential for obtaining diverse structures. These structures exhibit extensive application possibilities in the fields of proton conductivity and magnetism. Herein, four new POM-based polynuclear coordination polymers with the formulas of {[Fe2(btb)3(H2O)2(SiW12O40)]·3H2O}n (1), {[Cd2(btb)2(H2O)6(HPMoVI10MoV2O40)]·2H2O}n (2), {[Co3(OH)2(btb)2(H2O)5(HPMoVI10MoV2O40)]·7H2O}n (3), and {[Cu3(OH)(btb)2(H2O)(HP2Mo5O23)]·6H2O}n (4) have been prepared using the V-type 1,3-bis(4H-1,2,4-triazole-4-yl)benzene (btb) ligand. Compounds 1 and 2 feature similar two-dimensional (2D) structures, derived from the binuclear Fe2N6 and Cd2N4 subunits connected by tridentate btb ligands. Meanwhile, in compound 3, hexanuclear Co6(OH)4 units are bound by quadridentate btb ligands forming a 2D layer with the same 4-c sql topology simplification as compounds 1 and 2. In compound 1, Keggin-type polyoxoanions are monodentate-coordinated to metal ions and suspended on the 2D structure, while, in compounds 2 and 3, they act as discrete counterions residing in the interstitial spaces between two adjacent layers, thereby extending the 2D structures into 3D structures through hydrogen bonding interactions. In compound 4, trinuclear Cu3(OH) subunits are further constructed into a 3D framework through cooperation with four tridentate and quadridentate btb ligands as well as Strandberg-type anions. Furthermore, the proton conduction of the four compounds has been investigated. They display high proton conductivities at 358 K and 98% RH with powdered samples, which are 1.26 × 10-3, 1.24 × 10-3, 3.24 × 10-4, and 2.57 × 10-4 S cm-1, respectively. Interestingly, by mixing with Nafion, the composite membranes of compounds 2 and 4 exhibit enhanced proton conductivities, measuring at 4.87 × 10-2 and 1.28 × 10-2 S cm-1, respectively, at 358 K and 98% RH, which suggests excellent potential for applications. In addition, compounds 1, 3, and 4 display antiferromagnetic behaviors due to similar magnetic interactions. This work can provide research insights into the assembly of 2D POM-based coordination polymers with nitrogen heterocyclic ligands and Keggin-type POMs and further promote their research progress in proton conduction.

Advances in Polyoxometalates as Electron Mediators for Photocatalytic Dye Degradation

The increasing concerns over the environment and the growing demand for sustainable water treatment technologies have sparked substantial interest in the field of photocatalytic dye removal. Polyoxometalates (POMs), known for their intricate metal-oxygen anion clusters, have received considerable attention due to their versatile structures, compositions, and efficient facilitation of photo-induced electron transfers. This paper provides an overview of the ongoing research progress in the realm of photocatalytic dye degradation utilizing POMs and their derivatives. The details encompass the compositions of catalysts, catalytic efficacy, and light absorption propensities, and the photocatalytic mechanisms inherent to POM-based materials for dye degradation are exhaustively expounded upon. This review not only contributes to a better understanding of the potential of POM-based materials in photocatalytic dye degradation, but also presents the advancements and future prospects in this domain of environmental remediation.

Organic Hybrid Antimoniotungstate Layered Ionic Crystal: Synthesis, Structure, and Interlayer-Confined Proton Conduction

A synchronous crystal- and microstructure-dependent strategy was implemented to synthesize the organic hybrid antimoniotungstate layered ionic crystal Na_5.5H_6.5[(SbW₉O₃₃)₂{WO₂(OH)}₂{WO₂}RuC₇H₃NO₄]·36H₂O, and the layered structure was constructed through the Na⁺ bridged sheet and the hydrogen-bonded layers. It displayed an effective proton conductivity of 2.97 × 10^-2 S cm^-1 at 348 K and 75% RH, owing to the complete interlayer confined hydrogen-bond network formed by the hydrogens of interlayer crystal waters, organic ligands ({RuC₇H₃NO₄}²⁺, {C₇H₃NO₄} is formed by the hydrolysis of pyridine 2,5-dicarboxylic acid (C₇H₅NO₄)), and acidic protons (H⁺), along with the interlayer domain as a transport channel. Furthermore, the hydrogen-bond network originating from interlayer organic ligands and acidic protons was more stable at a higher temperature of 423 K, preserving a high conductivity of 1.99 × 10^-2 S cm^-1.

Discrete Arsonate-Grafted Inverted-Keggin 12-Molybdate Ion [Mo12O32(OH)2(4-N3C2H2-C6H4AsO3)4]2- and Formation of a Copper(II)-Mediated Metal-Organic Framework

The discrete inverted-Keggin ion [Mo₁₂O₃₂(OH)₂(4-N₃C₂H₂-C₆H₄AsO₃)₄]^2- (1) has been prepared in an aqueous acidic (pH 0.8) medium by the reaction of MoO₃ with the (4-triazolylphenyl)arsonic acid 4-N₃C₂H₂-C₆H₄AsO₃H₂ under hydrothermal conditions and was isolated as a sodium salt in 21% yield. The exact same reaction in the presence of Cu²⁺ ions resulted in the neutral metal-organic framework (MOF) Cu₂[Mo₁₂O₃₄(4-N₃C₂H₂-C₆H₄AsO₃)₄] (Cu-1) in 68% yield. The inverted-Keggin ion 1 comprises a metal-oxo core, which is capped by four organoarsonate groups, and in Cu-1, individual polyanions are linked in the solid state by coordination of the Cu²⁺ ions with the triazolyl groups. The discrete ion 1 was characterized by single-crystal X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and atomic absorption (AA) spectroscopy, as well as thermogravimetric analysis (TGA), and the POM-MOF Cu-1 was characterized by single-crystal and powder XRD, FT-IR, TGA, and gas sorption. Cu-1 has channels with a diameter of around ∼0.9 nm and exhibits a water-vapor adsorption capacity of 89.7 cm³ g^-1 (p/p₀ = 0.95).

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.

Proton-Conductive Polyoxometalate Architectures Constructed from Lanthanide-Incorporated Polyoxoniobate Cages

Two new extended polyoxometalate (POM) architectures based on lanthanide-incorporated polyoxoniobate (Ln-incorporated PONb) cages, namely, H₄[Cu^II(en)₂]₄{K₄(H₂O)₂[Cu^II(en)₂]₅[Cu^II₅(trz)₂(en)₄(OH)₂][Dy₂Cu^II₂(en)₂(CO₃)₃(H₂O)₂(OH)₃][Dy(H₂O)₄][DyNb₂₃O₆₈(H₂O)₄]₂}·60H₂O (1, en = ethylenediamine) and H₂₀[Cu^II(en)₂]₄{[Cu^II(en)₂]₄[Dy₂(C₂O₄)(H₂O)₄]₂[(Nb₃₂(OH)₄(H₂O)₃O₈₉]₂}·54H₂O (2), have been successfully synthesized and structurally characterized, demonstrating a feasible strategy to develop functional POM materials. In addition, the proton conductivity and magnetic behaviors of both 1 and 2 were studied.

A novel nanocomposite (g-C3N4/Fe3O4@P2W15V3) with dual function in organic dyes degradation and cysteine sensing

Among the important needs of human societies is the elimination of environmental pollution and also the construction of high-performance and inexpensive biosensors. In this regard, the construction of multi-functional composites has been considered. A novel magnetic graphite carbon nitride decorated by tri-vanadium substituted Dawson-type heteropolytungstate nanocomposite (C₃N₄/Fe₃O₄@P₂W₁₅V₃) effectively synthesized and characterized by prevalent functional analysis. The prepared nano-catalyst presents bi-functional usage involving photocatalytic removal of dyes (methylene blue, congo red and phenyl red) (around 98%) under visible light radiation and greatly sensitive colorimetric sensing of cysteine in an aqueous media. Moreover, synthesized nano-catalyst successfully recovered five times without any considerable deficiency on its photocatalytic ability. Further, Moreover, we propose a novel method for cysteine detection based on the C₃N₄/Fe₃O₄@P₂W₁₅V₃ nanocomposite. This nanocomposite displayed a privileged catalytic feature for cysteine oxidation to extend a clock reaction of methylene blue as an indicator in the presence of NaBH₄ in acidic solution. More importantly, this colorimetric sensing method of cysteine presents an easy, low-cost, selective, and rapid colorimetric assay with a detection limit value of 7.2 μM in the acceptable linear range of 5-600 μM.

Synthesis, Structure, and Catalytic Activities of Two Multi-Rh-Decorated Polyoxometalates

Two multi-Rh-incorporated polyoxometalates [NH₂(CH₃)₂]₁₀[Na₄(H₂O)₈]H₃[As₄W₄₂O₁₄₂(OH)₄(CH₃COO)₂Rh₃(H₂O)₄]·13H₂O·4[NH(CH₃)₂] (1) and [K₄Na(H₂O)₆]KH₁₀[As₄W₄₀O₁₄₀Rh₄(H₂O)₄]·34H₂O (2) have been synthesized in acetate buffer solution. Polyanion 1a is built up atop of an acetate-modified rectangular framework [As₄W₄₂O₁₄₂(OH)₄(CH₃COO)₂]^26-, while polyanion 2a contains a pure inorganic cryptand [As₄W₄₀O₁₄₀]^28-. All Rh atoms of these two compounds share the same hexa-coordinate distorted-octahedral geometry and are embedded into their cavities through As-Rh bonding with a bond length around 2.304(4)-2.436(5) Å. Besides, they not only represent structural novelty but also demonstrate controllable proton conduction properties. Catalysts 1 and 2 can catalyze cycloaddition of epoxides with CO₂ in a solvent-free system in conjunction with 1-ethyl-1-methylpyrrolidinium bromide.

The Construct and Interpretation of Chelated Coordination Polymers and Their Use in Nanomaterials Research

Presently, an important step from basic research to practical applications is synthesizing nanostructured materials. Metal-organic structures, as well as coordination polymers, are a diverse group of materials with a wide range of potential and properties applications. It has been difficult to get these materials into commercial use because of many drawbacks. Polymers containing chelated units are described and assessed for their advancements and problems in preparation, properties, and structure. Here, a proposed approach based on designing coordination polymeric materials with chelated units using the metal-ligand approach (CPM-CU-MA) has been introduced for a columnar-layered plan, supramolecular components, and building levels. Nanocomposite materials can be formed through the thermal transformation of coordination polymers based on donor atoms. The polymeric metal chelates (PMCs) are categorized according to luminescent coordination polymer (LCoP) development. It is classified as macrocyclic intracomplex, polynuclear, and molecular according to its macrostructure. Supramolecular networks (SMNs) can be transformed into a coordination polymer by introducing cyclo-dehydrogenation of natural building blocks on a surface. The structure-property connections of LCPs can influence a framework of liquid crystal display (LCP) that has been given based on LC phase modulators with a large modulation depth and has useful applications in LC lens. In the spatial organization of PMCs, the main focus is on the commonalities and contrasts between higher- and lower-molecular-weight chelates based on molecularly imprinted sensors (MISs) and nanomaterial sensors for a wide range of uses. New functional nanoparticles based on the molecular components have exciting potential, as demonstrated by these findings based on parameters risk factors for human health, hazards reduction in the environment, lack of cost-effectiveness, environmental sustainability, and bioavailability of polymers with an overall performance of 95.3%.

Three Types of Lanthanide-Containing Polyoxoniobates and Their Luminescence Properties

Fourteen lanthanide (Ln)-containing hetero selenato-phosphato-peroxopolyoxoniobates in three series: (1) Cs₂{[Ln(H₂O)₄][H₄P₂Se₂Nb₆(O₂)₆O₂₂]}₂·nH₂O [Ln = Ce (I-Ce), Pr (I-Pr), Sm (I-Sm), Eu (I-Eu), Gd (I-Gd); I-Ce, I-Pr, I-Eu, I-Gd n = 12; I-Sm n = 16]; (2) Cs₂K₃{[Ln(H₂O)₆][H_2.5P₂Se₂Nb₆(O₂)₆O₂₂]}₂·nH₂O [Ln = Dy (II-Dy), Tm (II-Tm), Yb (II-Yb), Lu (II-Lu); II-Dy n = 14; II-Tm n = 24; II-Yb, II-Lu n = 13]; (3) Cs₅K₂H_m{[Ln(H₂O)₆][P₂Se₂Nb₆(O₂)₆O₂₂]}₂{[Ln_x(H₂O)_y]₂[P₂Se₂Nb₆(O₂)₆O₂₂]}·nH₂O [Ln = Ho (III-Ho), Er (III-Er), Tm (III-Tm), Yb (III-Yb), Lu (III-Lu), III-Ho-III-Yb m = 5, x = 1; y = 5; III-Lu m = 6.5; x = 0.75; y = 4; III-Ho-III-Tm n = 19, III-Yb n = 34; III-Lu n = 31], have been isolated by a one-pot reaction and structurally characterized by various physical and chemical analyses. These compounds represent the first group of Ln-functionalized polyoxoniobates with mixed heteroatoms. In addition, the photoluminescence properties of compounds I-Sm, I-Eu, and II-Dy were measured, and the time-resolved emission spectra showed that the {P₂Se₂Nb₆} segment contributed to the energy transfer from the polyoxoniobate units to the Ln³⁺ center, which effectively promoted the emission of the Ln³⁺ center.

Porous {P6Mo18O73}-type Poly(oxometalate) Metal-Organic Frameworks for Improved Pseudocapacitance and Electrochemical Sensing Performance

{P₆Mo₁₈} poly(oxometalate) (POM) clusters have huge steric hindrance and limited active oxygen atoms, which make them difficult to combine with metal-organic units to form three-dimensional (3D) porous structures. Therefore, functionalization of such POMs has always been a bottleneck that is difficult to break through. In this study, {P₆Mo₁₈} POM was successfully grafted on a lock-like metal-organic chain to generate a multiporous coordination polymer, [{Na(H₂O)(H₂btb)}{Cu₄^I(H₂O)(pz)₅Cl}{H₂Sr⊂P₆Mo₂^VMo₁₆^VIO₇₃}]·3H₂O (1) (pz = pyrazine; btb = 1,4-bis(1,2,4-triazole) butane). Meanwhile, a zero-dimensional (0-D) control compound with only btb ligands as counterions, (H₄btb)[H₄Sr⊂P₆Mo₂^VMo₁₆^VIO₇₃]·3H₂O (2), was also obtained via a hydrothermal reaction. Compound 1 represents the first basket-type 3D poly(oxometalate) metal-organic framework (POMOF) assembly, which possesses interpenetrating pores and complex topology. 1-GO-CPE displays improved supercapacitor (SC) performance (the specific capacitance of 929.4 F g^-1 at a current density of 3 A g^-1 with 94.1% of cycle efficiency after 5000 cycles) compared with 2-GO-CPE and most reported POMOF electrode materials, which may be due to the outstanding redox capability of basket-POM, introduction of metal-organic chains, intersecting pores, and excellent conductivity of graphene. An asymmetric SC device with 1-GO-CPE as the negative electrode exhibits an energy density of 29.7 Wh kg^-1 with a power density of 3148.2 W kg^-1 and long-lasting cycling life. In addition, 1-GO-GCE as an electrochemical sensor responds to dopamine (DA) at a voltage of 0.40 V and shows lower detection limits (0.19 μM (signal-to-noise ratio (SNR) = 3)), higher selectivity, and good reproducibility in the linear range of 0.56 μM to 0.24 mM. The ability to accurately detect the content of DA in biological samples further proves the feasibility of the sensor in practical applications.

Enhanced Carrier Separation in Visible-Light-Responsive Polyoxometalate-Based Metal-Organic Frameworks for Highly Efficient Oxidative Coupling of Amines

Photocatalytic technology is widely studied, while it comes with drawbacks such as low sunlight utilization efficiency and high carrier recombination rates. Herein, for the first time, we present two crystalline polyoxometalate (POM)-based metal-organic frameworks (POMOFs), {[Cd(DMF)₂Ru(bpy)₂(dcbpy)]₂(POMs)(DMF)₂} xDMF (PMo-1, POMs = [PMo^VI₁₁Mo^VO₄₀]^4-, x = 5; SiW-2, POMs = [SiW₁₂O₄₀]^4-, x = 4) through assembling the photosensitizer [Ru(bpy)₂(H₂dcbpy)]Cl₂ and POMs into a single framework. The assembly not only enhances light absorption in the visible light regime but also improves carrier separation efficiency; atop of that, both POMOFs demonstrate activities in the photocatalytic oxidative coupling of amines. Particularly, PMo-1 enables the quantitative completion of oxidative coupling of benzylamine reaction within 30 min (yield = 99.6%) with a high turnover frequency (TOF = 6631.6 h^-1). To our knowledge, the PMo-1 catalyst outperforms any other photocatalysts previously reported in similar use cases where TOF values were usually obtained <2000 h^-1.

Role of Polyoxometalate Contents in Polypyrrole: Linear Actuation and Energy Storage

A combination of polyoxometalates with polypyrrole is introduced in this work. Our goal was to include phosphotungstic acid (PTA) in different molar concentrations (0.005, 0.01, and 0.05 M) in the electropolymerization of pyrrole doped with dodecylbenzene sulfonate (DBS) and phosphotungstinates (PT), forming PPy/DBS-PT films. Scanning electron microscopy (SEM) revealed that the PPy/DBS-PT films became denser and more compact with increasing PTA concentrations. The incorporation of PT in PPy/DBS was analyzed using Fourier-transform infrared (FTIR) and energy dispersive X-ray (EDX) spectroscopy. The linear actuation in cyclic voltammetry and potential square wave steps in an organic electrolyte revealed increasing mixed actuation, with major expansion upon oxidation found for PPy/DBS-PT films with a PTA concentration of 0.005 M. Best results of a strain of 12.8% and stress at 0.68 MPa were obtained for PPy/DBS-PT (0.01 M). The PPy/DBS-PT films polymerized in the presence of 0.05 M of PTA and showed main expansion upon reduction, changing the actuation direction. Chronopotentiometric measurements of PPy/DBS-PT samples were conducted to determine the specific capacitance optimal for a 0.01 M PTA concentration in the range of 80 F g⁻¹ (±0.22 A g⁻¹).

Recent Advances in Polyoxometalates with Enzyme-like Characteristics for Analytical Applications

Artificial enzymes based on inorganic solids with both enzyme-mimetic activities and the special material features has been a promising candidate to overcome many deleterious effects of native enzymes in analytical applications. Polyoxometalates (POMs) are an importance class of molecular metal-oxygen anionic clusters. Their outstanding physicochemical properties, versatility and potential applications in energy conversion, magnetism, catalysis, molecular electronics and biomedicine have long been studied. However, the analytical applications of them is limited. Recently, the intrinsic enzymatic activities of POMs have also been found and become an area of growing interest. In this review, along with other reports, we aimed to classify the enzymatic activity of POMs, summarize the construction of POMs-based enzymes, and survey their recent advances in analytical fields. Finally, the current challenges and trends of the polyoxometalates with enzymatic activity in future chemo-/bio-sensing applications are briefly discussed.

Copper-Containing Polyoxometalate-Based Metal-Organic Frameworks as Heterogeneous Catalysts for the Synthesis of N-Heterocycles

Three new polyoxometalate-based metal-organic frameworks (POMOFs) [Cu₄(μ₃-OH)₂(tba)₃(H₂O)₅(SiW₁₂O₄₀)_0.5](H₂SiW₁₂O₄₀)_0.5·2.5H₂O (CuSiW), [Cu₃(μ₃-OH)(tba)₃(Htba)(H₂O)₂(HPMo₁₂O₄₀)]·7H₂O (CuPMo), and [Cu₄(μ₃-OH)₂(tba)₃(H₂O)₃(PW₁₂O₄₀)_0.5]₂(PW₁₂O₄₀)·0.5H₂O (CuPW) were constructed using multinuclear copper clusters, 3-(4H-1,2,4-triazol-4-yl)benzoic acid (Htba), and Keggin polyoxometalates (POMs). Different POMs regulate the formation of different multinuclear copper clusters ("boat" tetranuclear clusters in CuSiW, trinuclear clusters in CuPMo, and "chair" tetranuclear clusters in CuPW) and different topological structures of CuSiW, CuPMo, and CuPW (3-connected two-dimensional (2D) network for CuSiW, 4-connected 2D network for CuPMo, and (4,6)-connected three-dimensional network for CuPW). CuSiW, CuPMo, and CuPW as heterogeneous catalysts combine the high stability of MOFs in polar solvents and excellent catalytic activity of POMs and could be used for the synthesis of nitrogen-heterocycle compounds. The condensation cyclization reactions of 2-aminophenols/benzenesulfonyl hydrazines with 1,3-diketones produce benzoazoles and pyrazoles in good to excellent yields under the catalysis of CuPMo. Moreover, the catalyst could be reused at least for 7 runs, and this protocol was suitable for gram-scale reactions.

Investigation on the Mechanism of Radical Intermediate Formation and Moderate Oxidation of Spiro-OMeTAD by the Synergistic Effect of Multisubstituted Polyoxometalates in Perovskite Solar Cells

Conventional oxidation of 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) by air would bring various drawbacks for perovskite solar cells (PSCs), such as low power conversion efficiency (PCE) and poor stability. Here, a series of heteroatom-substituted Keggin-type polyoxometalates (POMs), H₄PMo₁₁VO₄₀ (PMo₁₁V), H₅PMo₁₀V₂O₄₀ (PMo₁₀V₂), and H₆PMo₉V₃O₄₀ (PMo₉V₃) are prepared and applied as p-type dopants to realize quantitative and controllable oxidation of Spiro-OMeTAD under an inert condition. The possible mechanism and electron donor regions in the oxidation of Spiro-OMeTAD are investigated using two-dimensional nuclear magnetic resonance (NMR) spectra and the relationship between POM structures and the oxidation degree of Spiro-OMeTAD is proposed. In addition, the synergistic effect of heteroatoms makes V₂-substituted PMo₁₀V₂ exhibit appropriate oxidation of Spiro-OMeTAD and promoted the highest efficient hole extraction as well as the decreased charge recombination. Therefore, the champion device doped with PMo₁₀V₂ shows a PCE of 20.41% and a superior open circuit voltage (V_oc) of 1.133 V, surpassing that of the pristine device (18.61%). This work presents a fresh perspective to the controllable oxidation of Spiro-OMeTAD employing economical inorganic POM dopants, which would promote the commercialization of PSCs.

Insights into organic-inorganic hybrid molecular materials: organoimido functionalized polyoxomolybdates

Polyoxometalates (POMs) are polyatomic anions that comprise transition metal group 5 (V, Nb, Ta) or group 6 (Mo, W) oxyanions connected together by shared oxygen atoms. POMs are fascinating because of their exclusive and remarkable characteristics. One of the most interesting features of POMs is their capability to function as an electron relay by performing stepwise multi-electron redox reactions while maintaining their structural integrity. Functionalization of POMs with amino organic compounds results in organoimido derivatives of polyoxometalates, which have aroused interest due to augmentation of their properties. Comprehensive study has shown that the synthesis methodologies to obtain desired organoimido derivatives of POMs by employing various imido-releasing reagents have progressed drastically in recent decades, particularly the innovative DCC-dehydrating technique. These organoimido functionalized POMs have been used as major building blocks to develop unique nanostructured organic-inorganic hybrid molecular materials. Many conventional organic synthesis processes such as Pd-catalyzed carbon-carbon coupling and esterification reactions have been performed with organoimido functionalized POMs where the presence of POM triggered the reaction process. Thus, investigation of the reactivity of organoimido derivatives of POMs foreshadows the intriguing future of POMs chemistry.

Copper-Containing Polyoxometalate-Based Metal-Organic Framework as a Catalyst for the Oxidation of Silanes: Effective Cooperative Catalysis by Metal Sites and POM Precursor

The oxidation of silanes into silanols is a very necessary transformation, and yet the rational fabrication of efficient catalysts for this reaction remains a challenging task. Here, a 3D polyoxometalate-based metal-organic framework (POMOF), [Cu^Ι₃(pz)₃{PMo₁₂O₄₀}]·H₂O (HENU-8, HENU = Henan University; pz = pyrazine) was consciously prepared and first employed in the oxidation of dimethylphenylsilane with tert-butyl hydroperoxide (TBHP) as an oxidant, achieving 89% yield at a production rate of 132 mmol·g^-1·h^-1. Control experiments indicated that polyoxometalates and Cu atoms together affected the ultimate outcome in this catalytic system, and the designed catalyst followed a free radical mechanism.

Simple preparation of Ag-BTC-modified Co3Mo7O24 mesoporous material for capacitance and H2O2-sensing performances

{Co3Mo7O24}@Ag-BTC-2 was synthesized by a grinding method, and it showed excellent performance in a supercapacitor and H2O2 sensing.

Various amide derivatives induced Keggin-type SiW12O404−-based cobalt complexes: assembly, structure, electrochemical sensing and dye adsorption properties

Three new [SiW12O40]4−-based complexes assembled from three amide derivative ligands displayed excellent electrochemical sensing activities for BrO3−, NO2− and Cr(vi) ions and showed good adsorption capacities for methylene blue and neutral red dyes.

Construction of cucurbit[n]uril-based supramolecular frameworks via host-guest inclusion and functional properties thereof

Frameworks utilizing cucurbit[n]uril-based chemistry build on the rapid developments in the fields of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and supramolecular organic frameworks (SOFs), and as porous materials have found...

pH-Controlled assembly of [ZnW12O40]6--based hybrids from a 0D dimer to a 2D network: synthesis, crystal structure, and photocatalytic performance in transformation of toluene into benzaldehyde

Polyoxometalate-based organic-inorganic hybrids have attracted considerable attention due to their fascinating structures and wide application prospects. In this work, using the same building blocks, ligands and metal ions (ZnW₁₂O₄₀^6-(ZnW₁₂), 2,2'-bipyridine (2,2'-bipy), and Cu²⁺), we synthesized three new POM-based hybrids by controlling the pH values of the reaction systems. These three compounds {(Zn_0.6(H₂)_0.4W₁₂O₄₀)[Cu(2,2'-bipy)(H₂O)][Cu(2,2'-bipy)(H₂O)₂][Cu(2,2'-bipy)(H₂O)₃]}₂·6H₂O (1), (Me₄N)₂{ZnW₁₂O₄₀[Cu(2,2'-bipy)(H₂O)][Cu(2,2'-bipy)(H₂O)₃]}·5H₂O (2), and {(Zn_0.5(H₂)_0.5W₁₂O₄₀)[Cu(2,2'-bipy)][Cu(2,2'-bipy)(H₂O)][Cu(2,2'-bipy)(H₂O)₂]}·5H₂O (3) have been structurally characterized by single-crystal X-ray diffraction. Compound 1 appears as a dimeric cluster structure, while compounds 2 and 3 appear as a 1D chain structure and a 2D network, respectively. The semiconducting properties of compounds 1-3 are different, which was demonstrated by band gap (E_g) and photocurrent response measurements. Compound 3 can efficiently catalyze the photooxidation of toluene to benzaldehyde with high selectivity using molecular oxygen as the oxidant component. Moreover, compound 3 was recycled and reused three times without significant degradation in conversion and selectivity. In addition, the mechanism of the photocatalytic reaction was also investigated.

An unprecedented polyoxometalate-based 1D double chain compound with opposite charges enables conductivity improvement

A POM-based one-dimensional (1D) chain compound, {BW₁₂O₄₀[Cu(2,2'-bipy)₂]₂[Cu(2,2'-bipy)(H₂O)]}{BW₁₂O₄₀[Cu(2,2'-bipy)₂][Cu(2,2'-bipy)(H₂O)₂]}·7H₂O, has been synthesized and structurally characterized, which represents an unprecedented 1D double chain structure with opposite charges. In contrast to common POMs, this compound exhibits a relatively high electrical conductivity of 1.17 × 10^-9 S cm^-1 at 25 °C. In addition, its semiconducting properties have also been investigated by application of photoelectrochemical sensing of H₂O₂.

Assembly of a Hexameric Cluster of Polyoxomolybdotriphosphonate Builts from [Zn(H2O){TeMo6O21}{N(CH2PO3)3}]6- Subunits and Its Optical and Catalytic Properties

The crown-shaped organotriphosphonate-modified 36-molybdenum cluster (NH₄)₁₈Na₇H₁₁[Zn(H₂O)TeMo₆O₂₁{N(CH₂PO₃)₃}]₆·23H₂O (1) has been synthesized, which is the largest zinc-containing organophosphonate-based polyoxometalate to date. Compound 1 was prepared in buffer solution (pH 5.5) with heptamolybdate and amino trimethylene phosphonic acid (ATMP) as the organic ligand. The polyanion constructed from a hexmeric assembly of [Zn(H₂O){TeMo₆O₂₁}{N(CH₂PO₃)₃}]^6- subunits has been fully investigated by a few characterization methods. In this work, we discovered that 1 exhibited reversible photochromism and it changed from white to reddish brown upon UV irradiation. In addition, compound 1, as a catalyst, can oxidize sulfides to sulfoxides, showing a high yield/conversion and a good selectivity.

A Rh-substituted polyoxometalate with an acetate-modified building block {As2W22O76(CH3COO)2}

The first example of Rh-substituted organic-inorganic hybrid arsenotungstate [H₂N(CH₃)₂]₈{[Na(H₂O)₄]NaAs₂W₂₂(CH₃COO)₂O₇₆ Rh₂(N(CH₃)₂)₂}·H₂O (1) synthesised in an aqueous solution by the conventional method is reported. Two [Rh(N(CH₃)₂)]³⁺ fragments are imbedded into the vacancy of the [NaAs₂W₂₂(CH₃COO)₂O₇₆]^15- unit by Rh-As bonds [2.439(1) Å], which are observed in POM chemistry for the first time.

Organophosphonate-Functionalized Telluromolybdate Containing a [TeMo10O37]10- Building Block and Its Catalytic Efficiency for Knoevenagel Condensation

A novel organodiphosphonate-based telluromolybdate cluster, (NH₄)₆Na₃H₁₃[TeMo₁₀O₃₇(CoMo₂O₆L)₄]·11H₂O [1; L = (O₃P)₂C(O)(CH₂)₃NH₂], has been successfully synthesized by a simple one-pot aqueous reaction. Intriguingly, the [TeMo₁₀O₃₇]^10- subunit with tetrahedral geometry of TeO₄ is observed in the organophosphonate-functionalized polyoxometalates for the first time. Compound 1 was prepared in a buffer solution (pH = 5.5) with alendronic acid (Ale) and (NH₄)₆Mo₇O₂₄·4H₂O as raw materials. The polyanion [TeMo₁₀O₃₇(CoMo₂O₆L)₄]^22- was constructed from four {Mo₂O₆L} subunits encapsulating an interesting Te-Mo heterometal subunit [TeMo₁₀O₃₇]^10- through four CoO₆ octahedra and has been fully characterized by routine techniques. In addition, compound 1, as a heterogeneous catalyst, shows good conversion (92%) and high selectivity (99%) for Knoevenagel condensation reaction.