Self-Sorting of Heteroanions in the Assembly of Cross-Shaped Polyoxometalate Clusters

Cylindrical HPIII-SbIII-Templated Five-Layer Polyoxotungstate with Conspicuous Photochromism Triggered by UV or Visible Light

The heteroanion (HA) component plays an important role in the templating of heteropolyoxometalate (HPA) structures, but polyoxometalates (POMs) formed from two different templates are rarely reported. Herein, we present a five-layer POM [H2N(CH3)2]14{[(HPO3)4W6O10][HPSbW15O54]2}·16H2O (1) prepared by two kinds of different HA templates. The multilayer heteropolyanion {[(HPO3)4W6O10][HPSbW15O54]2}14- in 1 consists of two trivacant diheteroatom-templated ([HPO3]2- and [SbO3]3-) [HPSbW15O54]11- subunits linked by one unusual [(HPO3)4W6O10]8+ subcluster via twelve corner-sharing oxygen atoms. Compound 1 was systematically characterized by IR, UV, PXRD, TGA, XPS, and Raman spectra. Compound 1 exhibits good photochromism under UV irradiation with a half-life (t1/2) of 42.5 s, and it also exhibits noteworthy photochromism under visible light irradiation with a half-life (t1/2) of 157.2 s. The possible photochromic mechanism is proposed and verified by the experimental results.

A Lanthanide-Incorporated Phospho(III)tungstate Aggregate Constructed from [HPIIIW8O31]10- and [W11O39]12- Building Blocks and Its Nanocomposite with CdS for Ultrasensitive Photoelectrochemical Detection of Oxytetracycline

A novel tartronic acid decorated hexa-CeIII-incorporated phospho(III)tungstate aggregate (C4H12NO)6Na18H2[(HPW8O31)2[W11O39]2(H2TAD)4(H2O)4W4Ce6H2P2O14]·84H2O (1, H3TAD = tartronic acid) was synthesized by a one-step assembly strategy. Its main skeleton is constructed from two [W11O39]12- fragments, two [HPIIIW8O31]10- segments and one H2TAD--ornamented dodecanuclear heterometallic [W4Ce6H2PIII2O14(H2TAD)4(H2O)4]18+ cluster. In the structure, the [HPIIIO3]2- groups not only work as the heteroatom template to induce the formation of lacunary [HPIIIW8O31]10- segments but also function as the connector to bridge Ce3+ cations. With the help of a reaction strategy of combining ultrasonication treatment with the continuous ion layer adsorption method, the 1/CdS composite was constructed and exhibits prominent photoelectrochemical activity. The 1/CdS composite was used as a photoelectrochemical sensor for oxytetracycline detection at 0 V (vs Ag/AgCl), which displays excellent properties with quick response and low limit of detection (0.042 nM). This work can provide some helpful references in the construction of novel PIII-induced polyoxometalates consisting of different building blocks and can extend the applications of polyoxometalate-based nanocomposites into photoelectrochemical detection for antibiotics as well as biomolecules.

An Innovative SbIII-WVI-Cotemplated Antimonotungstate with Potential in Sensing Paroxetine Electrochemically

Advances in polyoxometalate (POM) self-assembly chemistry are always accompanied by new developments in molecular blocks. The exploration and discovery of uncommon building blocks offer great possibilities for generating unprecedented POM clusters. An intriguing SbIII-WVI-cotemplated antimonotungstate [H2N(CH3)2]11Na[SbW9O33]Er2(H2O)2Sb2[SbWVIW15O57]·22H2O (1) was synthesized, which comprises a classical trivacant Keggin [SbW9O33]9- ({SbW9}) fragment and an unclassical lacunary Dawson-like [SbWVIW15O57]15- ({SbWVIW15}) subunit. Notably, the Dawson-like {SbWVIW15} subunit is the first example of a [SbO3]3- and [WVIO6]6- mixed-heteroatom-directing POM segment. Hexacoordinated [WVIO6]6- can not only serve as the heteroatom function but its additional oxygen sites can also link to lanthanide, main-group metal, and transition-metal centers to form the innovative structure. {SbWVIW15} and {SbW9} subunits are joined by the heterometallic [Er2(H2O)2Sb2O17]22- cluster to give rise to an asymmetric sandwich-type architecture. To further realize its potential application in electrochemical sensing, a conductive 1@rGO composite was obtained by the electrochemical deposition of 1 with graphene oxide (GO). Using a 1@rGO-modified glassy carbon electrode as the working electrode, an electrochemical biosensor for detecting the antidepressant drug paroxetine (PRX) was successfully constructed. This work can provide a viable strategy for synthesizing mixed-heteroatom-directing POMs and demonstrates the application of POM-based materials for the electrochemical detection of drug molecules.

Single-Molecule Magnet Behavior of Confined Dy(III) in a Mixed Heteroatom-Substituted Polyoxotungstate

Two heteroatom-templated Dy(III)-confined polyoxotungstates [H2N(CH3)2]7Na7[Dy2(H2O)7(W4O9)(HPSeW15O54)(α-SeW9O33)2]·31H2O (1) and [H2N(CH3)2]14K2Na18{[Dy2(H2O)13W14O40]2[α-SeW9O33]4[HPSeW15O54]2}·44H2O (2) were synthesized by a one-pot aqueous reaction and structurally characterized. The most distinctive structural feature of complexes 1 & 2 is the simultaneous presence of both trivacant Keggin [α-SeW9O33]8- and Dawson [HPSeW15O54]10- building blocks containing P(III)-Se(IV) heteroatoms. The trimeric polyanion of 1 can be represented as a fusion of two trivacant Keggin [α-SeW9O33]8- and Dawson [HPSeW15O54]10- building units encapsulating the [Dy2(H2O)7(W4O9)]12+ cluster. On the other hand, hexameric polyoxoanions of 2 are described as four trivacant Keggin [α-SeW9O33]8- and two Dawson [HPSeW15O54]10-, building units anchoring a [Dy4(H2O)26W28O80]20+ cluster. The magnetic investigation revealed the presence of significant magnetic anisotropy and slow relaxation of magnetization behavior for complex 1 with a phenomenological energy barrier, Ueff = 13.58 K in the absence of an external magnetic field, and Ueff = 24.57 K in the presence of a 500 Oe external dc magnetic field. On the other hand, complex 2 favors the QTM relaxation process in the absence of an external magnetic field and shows field-induced slow relaxation of magnetization with Ueff = 11.11 K at 1500 Oe applied dc field. The in-depth analysis of magnetic relaxation dynamics shows that the relaxation process follows the Orbach as well as Raman relaxation pathways. Further, the ab initio calculation of the studied complexes confirms that the highly axial ground and first excited energy states (containing pure highest mJ states) are responsible for the observed single-molecule magnet (SMM) behavior. Remarkably, this is the first example of a mixed heteroatom-based Dy(III)-substituted polyoxotungstate with both trimeric Keggin [α-SeW9O33]8- and Dawson [HPSeW15O54]10- building units showing SMM behavior.

An Unusual Bismuth-Antimony-Europium Cluster-Imbedded Polyoxotungstate and Its Bidirectional Luminescence Detection

An unprecedented aggregate formed by two bismuth-antimony-europium cluster-imbedded tungsten-oxo clusters and one Krebs-type polyoxotungstate linker [H2N(CH3)2]14Na30H6[W4O10][B-β-BiW9O33]2{[Bi5.35Sb0.65Eu3O9(H2O)9][B-α-SbW9O33]3}2·124H2O (1) was prepared. The polyoxoanion skeleton of 1 contains a Krebs-type polyoxotungstate [W4O10][B-β-BiW9O33]2}14- ({Bi2W22}) (1a) as a linker that offers six active coordinate O atoms (two μ3-O and four μ2-O atoms) to grasp two Bi-Sb-Eu cluster-imbedded tungsten-oxo clusters {[Bi5.35Sb0.65Eu3O9(H2O)9][B-α-SbW9O33]3}18- (1b) through Bi-O-W and Sb-O-W bonds. 1b comprises an unprecedented nona-nuclearity Bi-Sb-Eu [Bi5.35Sb0.65Eu3O9(H2O)9]9+ cluster encircled by three trivacant [B-α-SbW9O33]9-segments in a triangular motif through Eu-O-W, Sb-O-W, and Bi-O-W linkages into a trilobal trimer. Moreover, a bidirectional detection method by using 1 as an effective luminescence probe was proposed to recognize both Mn2+ and CO32- through an "on-off-on" mode. 1 can be used as an "on-off" luminescent sensor to detect Mn2+ ions in aqueous solution. The limit of detection was 0.05 μM (9 × 10-6 mg L-1), which is much lower than the World Health Organization (WHO) guideline for Mn2+ concentration in drinking water (0.05 mg L-1). Then the Mn2+-quenching system can be used as an "off-on" sensor to detect CO32- in water system. This work provides a new research idea for the application of rare-earth-imbedded polyoxotungstate-based materials in the field of optical smart detection.

Binary Heterogroup-Templated Scaffolds of Polyoxopalladates as Precatalysts for Plasma-Assisted Ammonia Synthesis

In addition to improving the synthetic efficiency, the template method can do a lot more in the chemistry of polyoxopalladates (POPs), such as the establishment of novel metal-oxo scaffolds. In this endeavor, a binary system comprising heterogroups of nonmetallic {As/SiO4} and metallic {VO4/5} successfully fulfills the templated growth of two POPs with unprecedented seesaw- and spindle-like prototypes. Of these, self-aggregation of heterogroups beacons an effective route to break the highly symmetrical PdII-oxo matrix and to force the arrangement of addenda in a nonconventional manner. Aside from the interest in their structural features, the as-made POPs are available for immobilization on the mesoporous SBA-15 as precatalysts for ammonia synthesis. The outer cover of heterogroups in the POP precursors contributes to the ultrafine size and uniform distribution of derived Pd0 nanoparticles (PdNPs). With the help of plasma activation on H2 and N2, such PdNPs-SBA15 catalysts significantly improve the production performance of NH3, showcasing the maximum synthesis rate of 64.42 μmol/(min·gcat) with the corresponding energy yield as high as 4.38 g-NH3/kWh.

Cerium-Encapsulated SbIII-SeIV-Templating Polyoxotungstate for Electrochemically Sensing Human Multidrug Resistance Gene Segment

A tower-like SbIII-SeIV-templating polyoxotungstate [H2N(CH3)2]12Na7H3[Ce0.5/Na0.5(H2O)5]2[SbSe2W21O75]2·50H2O (1) was synthesized, whose skeleton is assembled from two prolonged lacunary Dawson [SbSe2W21O75]13- units and two [Ce0.5/Na0.5(H2O)5]2+ linkers. The uncommon [SbSe2W21O75]13- unit can be viewed as a combination of one [SeW6O21]2- group grafted onto a trivacant Dawson [SbSeW15O54]11- subunit. The conductive composite 1-Au@rGO containing 1, gold nanoparticles, and reduced graphene oxide (rGO) was conveniently prepared, using which the 1-Au@rGO-based electrochemical genosensor was constructed for detecting human multidrug resistance gene segment. This work enriches structural types of dual-heteroatom-inserted polyoxometalates and promotes the application of polyoxometalates in genosensors.

Hourglass shaped polyoxometalate-based materials as electrochemical sensors for the detection of trace Cr(VI) in a wide pH range

Due to Chromium hexavalent Cr(VI) is one of the most carcinogenic toxic ions, it is essential for finding a low-cost, efficient and highly selective detection method. Considering the wide range of pH detection in water, a major issue is exploring high sensitive electrocatalyst. Thus, two crystalline materials with hourglass {P₄Mo₆} clusters in different metal centers were synthesized and had fabulous Cr(VI) detection performance in a wide pH range. At pH = 0, the sensitivities of CUST-572 and CUST-573 were 133.89 μA μM^-1 and 30.05 μA μM^-1, and the detection limits (LODs) of Cr(VI) were 26.81 nM and 50.63 nM which met World Health Organization (WHO) standard for drinking water. CUST-572 and CUST-573 also had good detection performance at pH = 1-4. In actual water samples, CUST-572 and CUST-573 also possessed sensitivities of 94.79 μA μM^-1 and 20.09 μA μM^-1 and LODs were 28.25 nM and 52.24 nM, showing high selectivity and chemical stability. The difference of the detection performance of CUST-572 and CUST-573 were mainly attributed to the interaction between {P₄Mo₆} and different metal centers of crystalline materials. In this work, electrochemical sensors for Cr(VI) detection in a wide pH range were explored, providing important guidance for the design of efficient electrochemical sensors for ultra-trace detection of heavy metal ions in practical environments.

Lamellar Nanocomposite Based on a 1D Crayfish-like CeIII-Substituted Phospho(III)tungstate Semiconductor and Polyaniline Used as a High-Performance Humidity Sensing Device

In order to meet people's demand for intelligent management of daily life and health, manufacturing and developing humidity monitoring equipment with convenience, high sensitivity, easy miniaturization, and low cost is particularly important in the era of rapid development of artificial intelligence and the Internet of Things. Polyaniline (PANI) is an attractive humidity sensing material due to its designable functional properties. However, PANI modified polyoxometalates (POMs) for humidity sensing are still rare. As a proof of concept, a novel moisture sensing composite material was obtained based on PANI and a novel 1D rare-earth-substituted phospho(III)tungstate [H₂N(CH₃)₂]₉Na₃H₆[Ce₂(H₂O)₃W₅O₁₃(C₂O₄)][HP^IIIW₉O₃₃]₂[(HP^III)₂W₁₅O₅₄]·42H₂O (1). Notably, the anion structure of 1 contains trivacant Keggin-type [B-α-HP^IIIW₉O₃₃]^8- and Dawson-like [(HP^III)₂W₁₅O₅₄]^10- subunits linked by a heterometallic [Ce₂(H₂O)₃W₅O₃₂(C₂O₄)]^30- cluster. Furthermore, the 1/PANI composite shows a typical semiconductive characteristic with a "band-like" conductive mechanism. The fabricated 1/PANI-based humidity sensing device exhibits a broad sensing range (11∼97% relative humidity), fast response/recovery time (3.45 s/3.24 s), good repeatability, and long-term stability (over 3 months). Additionally, the possible sensing mechanism is proposed. This work offers an enormous possibility for the design of high-performance humidity sensing materials through POM material chemistry.

Two Types of Subgroup-Valence Heteroatoms (PIII, TeIV) Synergistically Controlling Octa-CeIII-Encapsulated Heteropolyoxotungstate and Its Electrochemical Recognition Properties

Rapid development of the synthetic chemistry of polyoxometalates (POMs) has greatly driven the generation of structurally variable innovative POM-based materials. Herein, we synthesized a novel P^III and Te^IV synergistically controlling octa-Ce^III-encapsulated heteropolyoxotungstate [H₂N(CH₃)₂]₁₁K₂Na₆H₁₁[Ce₈(CH₃COO)₂(HP^IIIO₃)₂W₈O₂₀(H₂O)₁₂(B-β-TeW₈O₃₀)₂(B-α-TeW₈O₃₁)₄]·64H₂O (1). Its distinctive anion skeleton [Ce₈(CH₃COO)₂(HP^IIIO₃)₂W₈O₂₀(H₂O)₁₂(B-β-TeW₈O₃₀)₂(B-α-TeW₈O₃₁)₄]^30- is built by two tetra-vacancy [B-β-TeW₈O₃₀]^8- and four tetra-vacancy [B-α-TeW₈O₃₁]^10- moieties linked through an inorganic-organic hybrid [Ce₈(CH₃COO)₂(HP^IIIO₃)₂W₈O₂₀(H₂O)₁₂]²⁶⁺ {Ce₈P₂W₈} cluster core. Interestingly, {Ce₈P₂W₈} is assembled from four [W₂O₁₁]^10- groups and two [HP^IIIO₃]^2- anions and eight Ce³⁺ ions. Besides, 1 was further composited with carboxylated multiwalled carbon nanotube (CMCN), resulting in a bi-component 1/CMCN nanocomposite. An electrochemical recognition platform (named as 1/CMCN/GCE) was built by modifying 1/CMCN on a glassy carbon electrode (GCE) for electrochemical detection of dopamine (DPA) at physiological pH (pH = 7.0). The findings have shown that 1/CMCN/GCE exhibits a good detection limit of 4.95 nM for DPA. This work provides considerable inspiration to promote innovative and rational structure designs of POM-based materials and expand their applications to electrochemical and biological detection fields.

Organophosphonic Acid-Regulating Assembly of PV-SbIII Polyoxotungstate and Its Potential in Building a Dual-Signal Readout Electrochemical Aptasensor for Carcinogen Detection

Template-directed assembly of giant cluster-based nanomaterials is an everlasting theme in cluster science. In this work, ethylenediamine tetramethylphosphonic acid [H₈EDTPA = (POCH₂(OH)₂)₄C₂H₄N₂] and [B-α-SbW₉O₃₃]^9- were, respectively, used as an organic template and an inorganic template to prepare an organophosphonic acid-regulating P^V-Sb^III-heteroatom-inserted polyoxotungstate aggregate [H₂N(CH₃)₂]₅Na₁₁H₉[CeW₄O₁₀(HEDTPA)SbW₁₅O₅₀][B-α-SbW₉O₃₃]₂·36H₂O (1). Noteworthily, organophosphonic acid ligand not only works as an organic template leading to the assembly of a [HEDTPASbW₁₅O₅₀]^14- building block but also further bridges the sandwich-type [CeW₄O₁₀(B-α-SbW₉O₃₃)₂]^11- entity. To extend its potential application in electrochemical sensing properties, we prepared a three-dimensional 1@EGO composite (EGO = reduced graphene oxide functionalized by ethylenediamine) with porous architecture and a prominent conducting ability. Furthermore, the 1@EGO composite was explored as a modification material for glassy carbon electrodes to build a dual-signal readout electrochemical aptasensor for carcinogens, which shows much better detection performance for aflatoxin B1 compared with traditional single-signal biosensors.

Nuclearity enlargement from [PW9O34@Ag51] to [(PW9O34)2@Ag72] and 2D and 3D network formation driven by bipyridines

The structural transformations of metal nanoclusters are typically quite complex processes involving the formation and breakage of several bonds, and thus are challenging to study. Herein, we report a case where two lacunary Keggin polyoxometallate templated silver single-pods [PW9O34@Ag51] (SD/Ag51b) fuse to a double-pod [(PW9O34)2@Ag72] by reacting with 4,4’-bipyridine (bipy) or 1,4-bis(4-pyridinylmethyl)piperazine (pi-bipy). Their crystal structures reveal the formation of a 2D 44-sql layer (SD/Ag72a) with bipy and a 3D pcu framework (SD/Ag72c) with pi-bipy. The PW9O349− retains its structure during the cluster fusion and cluster-based network formation. Although the two processes, stripping of an Ag-ligands interface followed by fusion, and polymerization, are difficult to envisage, electrospray ionization mass spectrometry provides enough evidences for such a proposal to be made. Through this example, we expect the structural transformation to become a powerful method for synthesizing silver nanoclusters and their infinite networks, and to evolve from trial-and-error to rational.

The Intrinsic Charge Carrier Behaviors and Applications of Polyoxometalate Clusters Based Materials

Polyoxometalates (POMs) are a series of molecular metal oxide clusters, which span the two domains of solutes and solid metal oxides. The unique characters of POMs in structure, geometry, and adjustable redox properties have attracted widespread attention in functional material synthesis, catalysis, electronic devices, and electrochemical energy storage and conversion. This review is focused on the links between the intrinsic charge carrier behaviors of POMs from a chemistry-oriented view and their recent ground-breaking developments in related areas. First, the advantageous charge transfer behaviors of POMs in molecular-level electronic devices are summarized. Solar-driven, thermal-driven, and electrochemical-driven charge carrier behaviors of POMs in energy generation, conversion and storage systems are also discussed. Finally, present challenges and fundamental insights are discussed as to the advanced design of functional systems based upon POM building blocks for their possible emerging application areas.

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.

Looking for Options to Sustainably Fixate Nitrogen. Are Molecular Metal Oxides Catalysts a Viable Avenue?

Fast and reliable industrial production of ammonia (NH₃) is fundamentally sustaining modern society. Since the early 20^th Century, NH₃ has been synthesized via the Haber-Bosch process, running at conditions of around 350-500°C and 100-200 times atmospheric pressure (15-20 MPa). Industrial ammonia production is currently the most energy-demanding chemical process worldwide and contributes up to 3% to the global carbon dioxide emissions. Therefore, the development of more energy-efficient pathways for ammonia production is an attractive proposition. Over the past 20 years, scientists have imagined the possibility of developing a milder synthesis of ammonia by mimicking the nitrogenase enzyme, which fixes nitrogen from the air at ambient temperatures and pressures to feed leguminous plants. To do this, we propose the use of highly reconfigurable molecular metal oxides or polyoxometalates (POMs). Our proposal is an informed design of the polyoxometalate after exploring the catabolic pathways that cyanobacteria use to fix N₂ in nature, which are a different route than the one followed by the Haber-Bosch process. Meanwhile, the industrial process is a "brute force" system towards breaking the triple bond N-N, needing high pressure and high temperature to increase the rate of reaction, nature first links the protons to the N₂ to later easier breaking of the triple bond at environmental temperature and pressure. Computational chemistry data on the stability of different polyoxometalates will guide us to decide the best design for a catalyst. Testing different functionalized molecular metal oxides as ammonia catalysts laboratory conditions will allow for a sustainable reactor design of small-scale production.

Multinuclear Lanthanide-Implanted Tetrameric Dawson-Type Phosphotungstates with Switchable Luminescence Behaviors Induced by Fast Photochromism

A series of benzoate-decorated lanthanide (Ln)-containing tetrameric Dawson-type phosphotungstates [N(CH₃)₄]₆H₂₀[{(P₂W₁₇O₆₁)Ln(H₂O)₃Ln(C₆H₅COO)(H₂O)₆]}{[(P₂W₁₇O₆₁)Ln(H₂O)₃}]₂Cl₂·98H₂O [Ln = Sm (1), Eu (2), and Gd (3)] were made using a facile one-step assembly strategy and characterized by several techniques. Notably, the Ln-containing tetrameric Dawson-type polyoxoanions [{(P₂W₁₇O₆₁)Ln(H₂O)₃Ln(C₆H₅COO)(H₂O)₆]}{[(P₂W₁₇O₆₁)Ln(H₂O)₃}]₂^24- are all established by four monolacunary Dawson-type [P₂W₁₇O₆₁]^10- segments, encapsulating a Ln³⁺ ion with two benzoates coordinating to the Ln³⁺ ions. 1-3 exhibit reversible photochromism, which can change from intrinsic white to blue for 6 min upon UV irradiation, and their colors gradually recover for 30 h in the dark. The solid-state photoluminescence spectra of 1 and 2 display characteristic emissions of Ln components based on 4f-4f transitions. Time-resolved emission spectra of 1 and 2 were also measured to authenticate the energy transfer from the phosphotungstate and organic chromophores to Eu³⁺. In particular, 1 shows an effectively switchable luminescence behavior induced by its fast photochromism.

Tricarboxylic-Ligand-Decorated Lanthanoid-Inserted Heteropolyoxometalates Built by Mixed-Heteroatom-Directing Polyoxotungstate Units: Syntheses, Structures, and Electrochemical Sensing for 17β-Estradiol

Organic-inorganic hybrid metal-oxide clusters have been pursued for many years, benefiting from their abundant structures and prominent performances. Upon our exploration, a family of unusual mixed-heteroatom (Sb^III, P^III)-directing lanthanoid (Ln)-inserted heteropolyoxotungstates (Ln-HPOTs), [(CH₃)₂NH₂]₂Na₇H₃[Ln₄(HP^III)W₈(H₂O)₁₂(H₂ptca)₂O₂₈][Sb^IIIW₉O₃₃]₂·27H₂O [Ln = Ce³⁺ (1), La³⁺ (2), Pr³⁺ (3)], functionalized by 1,2,3-propanetricarboxylic acid (H₃ptca) was achieved. The intriguing trimeric [Ln₄(HP^III)W₈(H₂O)₁₂(H₂ptca)₂O₂₈][Sb^IIIW₉O₃₃]₂^12- polyanion was established by two trivacant [B-α-Sb^IIIW₉O₃₃]^9- segments mounted on both sides and one rare [HP^IIIW₄O₁₈]^8- segment at the bottom, which are bridged via an organic-inorganic hybrid [W₄Ln₄(H₂O)₁₂O₁₀(H₂ptca)₂]¹⁴⁺ central moiety. Such Ln-HPOTs involving dual-heteroatom-directing mixed building blocks, and even simultaneously modified by tricarboxylic ligands, are rather unseen in polyoxometalate chemistry. Moreover, the detection of 17β-estradiol through a 1-based electrochemical biosensor has been explored, demonstrating a low detection limit (7.08 × 10^-14 M) and considerable stability.

{HPO3} and {WO4} Simultaneously Induce the Assembly of Tri-Ln(III)-Incorporated Antimonotungstates and Their Photoluminescence Behaviors

A double template agent strategy has been adopted to prepare a group of triacetate-adorned tri-Ln(III)-incorporated trimeric antimonotungstates (AMTs) K₃Na₂₁H[Ln₃(CH₃COO)₃(HPO₃)(WO₄)][B-α-SbW₉O₃₃]₃·36H₂O [Ln = Eu³⁺ (1), Dy³⁺ (2), Ho³⁺ (3), or Er³⁺ (4)] in a CH₃COOH-CH₃COONa buffer system. Therein, H₃PO₃ and Na₂WO₄·2H₂O have been, respectively, transformed into the capped {HPO₃} and {WO₄} tetrahedra during the assembly process, which are situated at the center of polyoxoanions of 1-4 and simultaneously perform as structure-directing templates to induce the assembly of 1-4. The hexahedral configuration supramolecular stacking is the same as the shape of a crystal of 1, which illustrates that the supramolecular stacking mode plays a significant role in forming the crystal shape of 1-4. Under the Ln³⁺ f-f excitation, the photoluminescence behaviors involving the emission spectrograms and fluorescence decay curves of 1-4 were systematically researched. The modulation of the excitation wavelength has realized the emission color tuning from blue to red, blue to green, blue to yellow, and green to yellow for 1-4. On the basis of the excitation of O → W charge transfer (OWCT), the energy-transfer procedure from AMT units to Eu³⁺ centers in 1 is mainly accomplished in the form of energy reabsorption. This work proposes a typical case for the construction of a new type of AMT clusters by using the double template agent strategy and confirms the great potential of Ln-containing AMTs in optic applications.

Mo[O8Mo4]3 Lewis acid-base cluster pairs: highly efficient and stable Lewis catalysis fields frustrated in crystalline nanoclusters

By using metal compounds or oxide/organic acid and enhanced reaction temperatures in the controlled solvothermal oxidation of [Mo3O2(MeCO2)6(H2O)3]2+, more interstitial metal atoms were introduced to produce the largest nanoscale MoIV-polyoxomolybdates, [M2@(MoIV3py3)4Mo18Ox]q- (M = Al, V, Mo). Each [H4V2@(MoIV3py3)4Mo18O84]12- (2a) nanocluster is surrounded by 12 [V3Mo12O42] to build a Lewis catalysis field (LCF) composed of MoIV3[O8Mo4]3 Lewis acid-base cluster pairs in the crystalline 2, accounting for the excellent and stable catalysis performance in the hydrazine reduction of nitroarenes to arylamines in varied solvents. The proposed new concept LCF provides a new way of thinking for designed synthesis and real applications of highly efficient LCF catalysts.

A trimeric tri-Tb3+ including antimonotungstate and its Eu3+/Tb3+/Dy3+/Gd3+-codoped species with luminescence properties

A trimeric tri-Tb3+-including antimonotungstate (AMT) hybrid Na17{(WO4)[Tb(H2O)(Ac)(B-α-SbW9O31(OH)2)]3}·50H2O (Tb3W28) was successfully synthesized, in which the capped tetrahedral {WO4} group plays a significant template role in directing the aggregation of three [B-α-SbW9O33]9- fragments and three Tb3+ ions. Eu3+/Tb3+/Dy3+/Gd3+-codoped AMT materials based on Tb3W28 were firstly prepared and their luminescence properties were investigated. The red emitter Eu3+, yellow emitter Dy3+, and nonluminous Gd3+ ions were codoped into Tb3W28 to substitute Tb3+ ions for investigating the energy transfer (ET) mechanism among Eu3+, Tb3+, and Dy3+ ions. Upon the 6H15/2 → 4I13/2 excitation at 389 nm of the Dy3+ ion, the ET1 mechanism (Dy3+ → Tb3+) was confirmed as a non-radiative dipole-dipole interaction. Under the 7F6 → 5L10 excitation at 370 nm of the Tb3+ ion, the ET2 mechanism (Tb3+ → Eu3+) was identified as a non-radiative quadrupole-quadrupole interaction. Under excitation at 389 nm, the two-step successive Dy3+ → Tb3+ → Eu3+ ET3 process was proved in Dy1.2Tb3zEu0.03Gd1.77-3zW28. Through changing the excitation wavelengths, the emission color of Dy1.2Tb1.2Eu0.03Gd0.57W28 can vary from blue to yellow, in which a near-white-light emission case was observed upon excitation at 378 nm. This work not only provides a systematic ET mechanism study of hetero-Ln-codoped AMTs, but also offers some useful guidance for designing novel performance-oriented Ln-codoped polyoxometalate-based materials.

Recent Advances in Electrochemical Monitoring of Chromium

The extensive use of chromium by several industries conducts to the discharge of an immense quantity of its various forms in the environment which affects drastically the ecological and biological lives especially in the case of hexavalent chromium. Electrochemical sensors and biosensors are useful devices for chromium determination. In the last five years, several sensors based on the modification of electrode surface by different nanomaterials (fluorine tin oxide, titanium dioxide, carbon nanomaterials, metallic nanoparticles and nanocomposite) and biosensors with different biorecognition elements (microbial fuel cell, bacteria, enzyme, DNA) were employed for chromium monitoring. Herein, recent advances related to the use of electrochemical approaches for measurement of trivalent and hexavalent chromium from 2015 to 2020 are reported. A discussion of both chromium species detections and speciation studies is provided.

An unprecedented polyhydroxycarboxylic acid ligand bridged multi-EuIII incorporated tellurotungstate and its luminescence properties

The first polyhydroxycarboxylic acid ligand bridged multi-EuIII-incorporated tellurotungstate K14H10[Eu4(H2O)4W6(H2glu)4O12(B-α-TeW9O33)4]·60H2O (H6glu = d-gluconic acid) (1) was synthesized via an organic ligand-driven self-assembly strategy. The polyhydroxycarboxylic acid ligand bridged tetrameric polyoxoanion [Eu4(H2O)4W6(H2glu)4O12(B-α-TeW9O33)4]24- in 1 can be viewed as an aggregation of four trivacant Keggin [B-α-TeW9O33]8- fragments and an innovative heterometallic [Eu4(H2O)4W6(H2glu)4O12]8+ cluster, in which four high-coordinate polyhydroxy flexible H2glu4- ligands chelate W and Eu centers through carboxyl and hydroxyl groups, giving rise to a heterometallic cluster. The hexagonal packing of the tetrameric polyoxoanions in 1 along the c axis provides excellent porous channels, which greatly increases the specific surface area of the whole framework and may be of benefit for fluorescence sensing in aqueous solution. 1 can function as a "turn-off" luminescence sensor to detect Cu2+ ions in aqueous solution. The limit of detection (LOD) of the 1-sensor is 8.82 × 10-6 mM, which is the lowest among the reported polyoxometalate-based fluorescence sensors. As for the Cu2+-quenching system, it can function as an "off-on" sensor to detect cysteine in an aqueous system, affording a LOD of 1.75 × 10-4 mM. This work opens up an avenue to broaden the applications of polyoxometalate-based materials in the optical intelligence detection field.

Multi-praseodymium-and-tungsten bridging octameric tellurotungstate and its 2D honeycomb composite film for detecting estrogen

Under coordination driving force of tungsten and rare-earth (RE) bridges, we synthesized a novel giant multi-tungsten-and-RE-bridging octameric tellurotungstate (TT) [H₂N(CH₃)₂]₁₆K₈Na₆H₁₀[Pr₈(H₂O)₂₀W₁₆O₄₈][B-α-TeW₉O₃₃]₈·70H₂O (1) in CH₃CN-H₂O mixed solvent. The cluster anion {[Pr₈(H₂O)₂₀W₁₆O₄₈][B-α-TeW₉O₃₃]₈}^40- features sixteen W^VI bridges, eight Pr^III bridges and eight trivacant Keggin [B-α-TeW₉O₃₃]^8- fragments, which the square {W₄O₁₂} cluster can be imagined as a seed to induce the aggregation of eight [B-α-TeW₉O₃₃]^8- fragments by coordination driving force of additional twelve W^VI bridges and eight Pr^III ions. Furthermore, the 2D 1@DODA (dimethyldioctadecyl ammonium bromide = DODA·Br) honeycomb composite material was prepared. The honeycomb morphology of the 1@DODA composite material provides rich binding sites for electrodepositing Au nanoparticles to make Au/1@DODA electrodes. The aptamer of 17β-estradiol (E2) hormone can be grafted to the Au/1@DODA electrodes via Au-S bonding interaction to construct the Au/1@DODA aptamer biosensors. By virtue of the specific recognition interaction of aptamer and the electrochemical signal amplification function of methylene blue and cDNA, the Au/1@DODA aptamer biosensors can realize the electrochemical detection of E2. This finding not only offers an electrochemical biosensing platform for detecting E2, but also expands POM-based composite materials in the applications of clinical detection and biological analysis.

Polyoxometalate-based crystalline materials as a highly sensitive electrochemical sensor for detecting trace Cr(vi)

It is crucial to find a convenient and sensitive method for quantitative determination of heavy metal chromium(vi) ions. Developing crystalline materials coupled with polyoxometalates as an electrochemical sensor is a promising approach to address the above issues. Here we reported two reductive polyoxometalate-based crystalline compounds with the formula of (H2bpp)2[Na4Fe(H2O)7][Fe(P4Mo6O31H6)2]·2H2O (1) and (H2bpp)6(bpp)2[Fe(P4Mo6O31H8)2]2·13H2O (2) (bpp = 1,3-bi(4-pyridyl)propane). Structural analysis indicated that both two compounds were composed of inorganic polyanionic clusters and organic protonated bpp cations. The difference lies in the arrangement mode of the inorganic moiety: crystal 1 shows a unique three-dimensional (3-D) inorganic porous skeleton, while crystal 2 consists of isolated 0-D polyanionic clusters. When used as electrochemical sensors in the determination of trace Cr(vi), crystal 1 shows a broad linearity range (2-2610 μM) with a low limit of detection (LOD) of 0.174 μM (9 ppb), which is superior to that of compound 2 (a LOD of 0.33 μM) and meets the standard of Cr(vi) in drinking water set by the WHO (less than 0.962 μM or 50 ppb). Importantly, crystal 1 showed benign selectivity to Cr(vi) in the presence of various heavy metal ions and good reproducibility in a real water sample, which prove its strong anti-interference ability. In addition, experimental results showed that the spatial arrangement of polyanionic clusters could affect the final electrochemical behavior of crystalline materials. This work provides some insights into the design of cost-effective POM-based electrochemical sensors at the molecular level.

Polyoxometalate-based composite materials in electrochemistry: state-of-the-art progress and future outlook

Polyoxometalates (POMs) have been developed as a class of promising smart material candidates not only due to their multitudinous architectures but also their good redox activities and outstanding electron and proton transport capacities. Recently, abundant studies on POMs composited with metal nanoparticles (NPs), carbon materials (e.g., carbon nanotubes (CNTs), carbon quantum dots (CQDs), graphene), and conducting polymers or highly-porous framework materials (e.g., MOFs, ZIFs) have been performed and POM-based composite materials (PCMs) undoubtedly show enhanced stability and improved electrochemical performances. Therefore, POMs and PCMs are of increasing interest in electrocatalysis, electrochemical detection and energy-related fields (such as fuel cells, redox flow batteries and so on), thus, developing novel PCMs has long been the key research topic in POM chemistry. This review mainly summarizes some representative advances in PCMs with electrochemical applications in the past ten years, expecting to provide some useful guidance for future research.

Trinuclear ruthenium core-containing polyoxometalate-based hybrids: preparation, characterization and catalytic behavior

Two ruthenium-containing polyoxometalate-based hybrids (Ru-POMs), KH8[Ru3O(Trz)6Cl3]2[(NaO6)W6(H2O)6(AsW9O33)4]·28H2O (1) and H3[Ru3O(Trz)6Cl3]2[(WO)2W(OH)(AsW9O33)2]·6H2O (2), (Trz = 1,2,4-triazole), were successfully isolated by a one-step hydrothermal method under different acidic conditions and further characterized by single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), IR spectroscopy, electrospray ionization and mass spectrometry (ESI-MS), thermogravimetric (TG) analyses and elemental analyses. Single-crystal X-ray diffraction analyses reveal two polyanions comprised of four/two identical trivacant Keggin-type polyanion building blocks, leading to the formation of tetrameric/dimeric assembly stabilized by additional metal-oxo cores, respectively. ESI-MS shows that the polyanion units [(NaO6)W6(H2O)6(AsW9O33)4]11- and [(WO)2W(OH)(AsW9O33)2]5- are intact in mixed solvent. Moreover, the two heterogeneous catalysts with trinuclear ruthenium cations were further investigated, and 1 was found to exhibit much higher yield (94.1%)/conversion (95.1%) and better selectivity (>99%) towards the oxidation of thioanisole to sulfoxide than 2.

First series of mixed (PIII, SeIV)-heteroatomoriented rare-earth-embedded polyoxotungstates containing distinct building blocks

A series of unprecedented mixed heteroatoms-oriented rare-earth-embedded heteropolyoxotungstates 1–8 were prepared and the 1@CMWCNT-GCE electrochemical sensing performances toward DA and UA were studied.

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.

A novel peroxopolyoxoniobate incorporating mixed heteroatoms: [P2Se2Nb6(O2)6O22]8-

A novel hetero selenato-phosphato-peroxopolyoxoniobate with the formula Cs₄H₄[P₂Se₂Nb₆(O₂)₆O₂₂]·10H₂O has been successfully isolated in an acidified aqueous hydrogen peroxide solution. The synthesized cluster represents the first example of a selenium-containing polyoxoniobate. Furthermore, the ESI-MS spectra show that the polyoxoanion structural unit [P₂Se₂Nb₆(O₂)₆O₂₂]^8- remains intact in aqueous solution.

Blurring the line between addenda and heteroatoms in a giant polyoxotungstotellurite

The 3.5 nm-sized polyanion [H₄TeWO₃₇₂]^80-, exhibiting an exceptionally high Te : W ratio, expands the structural role of Te^IV ions from classical heterogroups within polyoxotungstate subunits to addenda ions. Investigations on the self-assembly conditions, proceeding in a simple one-pot reaction, point to the pH as one critical factor for circumventing this limitation.

Anisotropic Polyoxometalate Cages Assembled via Layers of Heteroanion Templates

The synthesis of anisotropic redox-active polyoxometalates (POMs) that can switch between multiple states is critical for understanding the mechanism of assembly of structures with a high aspect ratio, as well as for their application in electronic devices. However, a synthetic methodology for the controlled growth of such clusters is lacking. Here we describe a strategy, using the heteroanion-directed assembly, to produce a family of 10 multi-layered, anisotropic POM cages templated by redox-active pyramidal heteroanions with the composition [W₁₆Mo₂O₅₄(XO₃)]^n-, [W₂₁Mo₃O_75/76(XO₃)₂]^m-, and [W₂₆Mo₄O₉₃(XO₃)₃]^o- for the double, triple, and quadruple layered clusters, respectively. It was found that the introduction of reduced molybdate is essential for self-assembly and results in mixed-metal (W/Mo) and mixed-valence (W^VI/Mo^V) POM cages, as confirmed by an array of analytical techniques. To probe the archetype in detail, a tetrabutyl ammonium (TBA) salt derivative of a fully oxidized two-layered cage is produced as a model structure to confirm that all the cages are a statistical mixture of isostructures with variable ratios of W/Mo. Finally, it was found that multilayered POM cages exhibit dipolar relaxations due to the presence of the mixed valence W^VI/Mo^V metal centers, demonstrating their potential use for electronic materials.

Ligand-Directed Template Assembly for the Construction of Gigantic Molybdenum Blue Wheels

Template-mediated synthesis is a powerful approach to build a variety of functional materials and complex supramolecular systems. However, the systematic study of how templates structurally evolve from basic building blocks, and then affect the templated self-assembly, is critical to understanding and utilizing the underlying mechanism, to work towards designed assembly. Here we describe the templated self-assembly of a series of gigantic Mo Blue (MB) clusters 1-4 using l-ornithine as a structure-directing ligand. We show that by using l-ornithine as a structure director, we can form new template⊂host assemblies. Based on the structural relationship between encapsulated templates of {Mo8 } (1), {Mo17 } (2) and {Mo36 } (4), a pathway of the structural evolution of templates is proposed. This provides insight into how gigantic Mo Blue cluster rings form and could lead to full control over the designed assembly of gigantic Mo-blue rings.

Temperature-controlled repeatable scrambling and induced-sorting of building blocks between cubic assemblies

Separation of a homogeneous mixture of different components to reach an ordered out-of-equilibrium state in solution has attracted continuous attention. While this can be achieved using external chemical fuels or photo energy, an alternative energy source is heat. Here we realize a temperature-controlled cycle of transitions between ordered and disordered states based on a mixture of two kinds of building blocks that self-assemble into cubic structures (nanocubes). An almost statistical mixture of nanocubes (disordered state) is thermodynamically most stable at lower temperature (25 °C), while homoleptic assemblies composed of single components are preferentially produced at higher temperature (100 °C) followed by rapid cooling. The scrambling of the building blocks between the nanocubes takes place through the exchange of free building blocks dissociated from the nanocubes. Based on this mechanism, it is possible to accelerate, retard, and perfectly block the scrambling by the guest molecules encapsulated in the nanocubes.

In this paper, the authors study the temperature-controlled dynamic behavior of a system of nanocubes self-assembled from two different building blocks. Non-intuitively, the disordered, equilibrium state (a mixture of heteroleptic cubes) and the ordered, out-of-equilibrium state (a mixture of homoleptic cubes) are cycled by heating and subsequent rapid cooling.

Polyoxometalates as components of supramolecular assemblies

The non-covalent interaction of polyoxometalates (POMs) with inorganic- or organic-based moieties affords hybrid assemblies with specific physicochemical properties that are of high interest for both fundamental and applied studies, including the discovery of conceptually new compounds and unveiling the impact of their intra-supramolecular relationships on the fields of catalysis, molecular electronics, energy storage and medicine. This minireview summarises the recent advances in the synthetic strategies towards the formation of such non-covalent POM-loaded assemblies, shedding light on their key properties and the currently investigated applications. Four main emerging categories according to the nature of the conjugate are described: (i) POMs in metal-organic frameworks, (ii) POMs merged with cationic metal complexes, (iii) architectures generated with solely POM units and (iv) POMs assembled with organic molecular networks.

Preparation, characterization and electrocatalysis performance of a trimeric ruthenium-substituted isopolytungstate

The ruthenium-containing isopolytungstate Rb₁₀K₃H₆[SeO₃(H₉Ru_5.5W_30.5O₁₁₄)]Cl₃·48H₂O was isolated and then served as a catalyst, showing electrochemical catalytic activity towards the oxidation reaction of nitrite.