Polyoxometalate Anions Research Articles

Polyoxometalate-based room-temperature phosphorescent materials induced by anion-π interactions.

A series of host-guest materials containing polyoxometalate anions and lanthanide-organic layers have been synthesized and structurally characterized. By anion-π interactions between the anions and the π-acidic naphthalenediimide moieties, the materials emit strong red room-temperature phosphorescence and exhibit reversible photochromism.

Photoluminescent polymer hydrogels with stimuli-responsiveness constructed from Eu-containing polyoxometalate and imidazolium zwitterions.

Inorganic-organic co-assembly of anionic polyoxometalates (POMs) with zwitterions provides a facile way to fabricate functional soft materials. In this paper, a translucent, photoluminescent polymer hydrogel was fabricated from Weakley-type POM Na9EuW10O36 (EuW10) and polymerizable imidazole-type zwitterion 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) via a one-step synthesis method. Detailed characterization indicated that the polymerization of double bonds in VIPS and electrostatic interactions between EuW10 and VIPS play important roles in the formation of the hydrogels. Additionally, the introduction of non-polymerizable zwitterions 3-(1-methyl-3-imidazolio)propanesulfonate (MIPS) or 3-(1-decyl-3-imidazolio)propanesulfonate (C10IPS) can improve the mechanical and luminous performances of the hydrogels. Especially, C10IPS with a long alkyl chain would more significantly alter the coordination environment of EuW10, and consequently resulted in a more efficient energy transfer process. Further investigations revealed that the chemical environment around the Eu3+ can be highly influenced by organic solvents with stronger coordination abilities than water molecules, such as acetone. The translucency and luminescence intensity of the hydrogels can be reversibly transformed after alternately immersing in acetone or H2O for several minutes. Our results provided a useful strategy for the fabrication of luminescent hydrogels by regulating the noncovalent interactions between POMs and zwitterions.

Light-powered and transient peptide two-dimensional assembly driven by trans-to-cis isomerization of azobenzene side chains.

Transient nanosheets were fabricated based on the ionic self-assembly of anionic polyoxometalates and cationic peptides appended with azobenzene side chains. The two-dimensional (2D) nanostructures are initiated through ultraviolet (UV) light-actuated trans-to-cis isomerization of azobenzene moieties.

Organic–Inorganic Hybrid Clusters Assembled from Phosphomolybdate Anions and Thioarylated Tetrathiafulvalenes with Diversiform Electron States and Configurations

A series of organic–inorganic hybrid clusters comprising thioarylated tetrathiafulvalene derivatives (A‐S‐TTFs) and Keggin‐type polyoxometalates have been prepared via proton‐induced intermolecular charge transfer or electrochemical oxidation methods, and their structural features have been intensively studied by X‐ray diffraction. The generation of co‐crystals occurs due to the flexible structure of the A‐S‐TTF and its electrostatic interaction with the polyoxometalate anions. The complexes can be divided into three types, i.e., type I, type II, and type III, which have organic/inorganic ratios of 3:1, 2:1, and 1:1, respectively. Multiple intermolecular interactions are observed between the organic and inorganic moieties, which stabilize the structures of the resulting hybrid clusters. In the clusters, Ar‐S‐TTFs are oxidized into radical cations [except in complex (TTF4)·(PMA)·(TBA), where TTF4 is presented as a +2 cation]. There are two factors affecting the packing styles and compositions of the hybrid clusters: 1) valence state of the Ar‐S‐TTF and 2) volume of the aryl groups thereon.

Gıda Endüstrisinde Kullanılmak Üzere Keggin Tipi Polioksometalat/İletken Polimer Kompozitine Dayalı Glukoz Sensörü Hazırlanması

Otomasyonun gerektirdiği sürekli izleme ve kontrol sistemlerinde biyosensörler hassasiyetleri, kararlılıkları, kullanım kolaylıkları ve cevap hızları ile büyük ölçüde avantaj sağlamaktadır. Bu çalışmada, sektörün ihtiyaçlarını karşılamak üzere Keggin tip Polioksometalat (POM), (nBu4N)3[PW9O34(tBuSiOH)3], varlığında amino grubuna sahip karbazol türevinin (3-amino-9-etilkarbazol (AAC)) elektrokimyasal polimerizasyonu tarafından hazırlanan kompozit filme dayalı yeni amperometrik glukoz sensörü geliştirilmiştir. POM/PAAC kompozit ile modifiye edilen yüzeye Glukoz Oksidaz (GOx) enzimi immobilize edilerek enzimatik biyosensör hazırlanmıştır. Biyosensörün hazırlama ve çalışma koşulları optimize edilerek analitik karakterizasyonları gerçekleştirilmiştir. POM/PAAC-GOx modifiyeli yüzeylerin farklı derişimlerdeki glukoz miktarına karşı -0.7 V(Ag/AgCl) potansiyelde elektrokimyasal yanıtları incelenmiştir. Kısa zamanda hazırlanabilen bu yüzeylerden elde edilen veriler incelendiğinde kola ve meyve suyu gibi gıda örneklerindeki glukoz varlığına karşı hassas olduğu belirlenmiştir. Bu çalışmanın literatüre ve endüstriye kattığı yenilik hem düşürülen cevap süresi hem de biyosensörün hazırlanması için geliştirilen yöntemdir. Gıdalardaki glukoz içeriğinin belirlenmesinde yüksek doğrusallık, kısa cevap süresi ve uzun raf ömrü avantajlarının yanı sıra kolay hazırlana bilirliği, uygulamada kolaylık sunması, ölçümlerde zaman ve kimyasallardan kazanım sağlamasıyla ekonomik bir alternatif olacağı düşünülmektedir.

Polyoxometalate intercalated NiFe layered double hydroxides for advanced water oxidation

Exploring high efficient and cost-effective electrocatalysts for oxygen evolution reaction is a determination step towards sustainable green energy applications. Polyoxometalates and layered double hydroxide materials are suggested to be potential catalyst materials; however, those electrocatalytic behaviors are greatly inferior to the state-of-the-art OER electrocatalysts (RuO2 and IrO2). In this work, we employ a self-assembly approach in which polyoxometalate anions are successfully intercalated into NiFe layered double hydroxide. Upon analysis of the composition, the elemental valence state and the infrared spectrum, it confirms that the intercalated polyoxometalate anions in the layers of NiFe layered double hydroxide are in the format of PW12O403−. Even though the partial anions in the as-prepared intercalation NiFe layered double hydroxide are only polyoxometalate anions, its electrochemical performance surpasses the counterpart precursor NiFe layered double hydroxide (NO3−) in many aspects, such as turnover frequencies, overpotentials, kinetics, and stability. The introduction of polyoxometalate anions can optimize the local electronic structure which enhances the charge transport capacity of NiFe layered double hydroxide as well as alter the adsorption/desorption nature of the intermediates during the catalysis process. This work provides a new strategy to boost the OER activity of layered double hydroxide-based electrocatalysts.

Mesoporous Silica vs. Organosilica Composites to Desulfurize Diesel.

The monolacunary Keggin-type [PW11O39]7− (PW11) heteropolyanion was immobilized on porous framework of mesoporous silicas, namely SBA-15 and an ethylene-bridged periodic mesoporous organosilica (PMOE). The supports were functionalized with a cationic group (N-trimethoxysilypropyl-N, N, N-trimethylammonium, TMA) for the successful anchoring of the anionic polyoxometalate. The PW11@TMA-SBA-15 and PW11@TMA-PMOE composites were evaluated as heterogeneous catalysts in the oxidative desulfurization of a model diesel. The PW11@TMA-SBA-15 catalyst showed a remarkable desulfurization performance by reaching ultra-low sulfur levels (<10 ppm) after only 60 min using either a biphasic extractive and catalytic oxidative desulfurization (ECODS) system (1:1 MeCN/diesel) or a solvent-free catalytic oxidative desulfurization (CODS) system. Furthermore, the mesoporous silica composite was able to be recycled for six consecutive cycles without any apparent loss of activity. The promising results have led to the application of the catalyst in the desulfurization of an untreated real diesel supplied by CEPSA (1,335 ppm S) using the biphasic system. The system has proved to be a highly efficient process by reaching desulfurization values higher than 90% for real diesel during three consecutive cycles.

Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry.

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

Discovery of a New Family of Polyoxometalate-Based Hybrids with Improved Catalytic Performances for Selective Sulfoxidation: The Synergy between Classic Heptamolybdate Anions and Complex Cations.

A series of functional cation-regulated isopolymolybdate-based organic-inorganic hybrid compounds, Na2H2[Mo4O12(C8H17O5N)2]·10H2O (1), Na2[M(Bis-tris)(H2O)]2[Mo7O24]·10H2O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris = 2,2-Bis(hydroxymethyl)-2,2',2″-nitrilotriethanol], and (NH4)2[M(Bis-tris)(H2O)]2[Mo7O24]·6H2O (M = Zn, 6; Cu, 7), were synthesized and characterized toward advanced molecular catalyst design. Compound 1 is a covalently bonded adduct, and its self-assembly process can be probed by electrospray ionization mass spectrometry (ESI-MS). Compounds 2-7 are polyoxometalate (POM)-based hybrids containing classic heptamolybdate anions and complex cations with Bis-tris ligands. All of these compounds showed remarkable catalytic effects for selective sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other transition-metal-containing compounds and the compound constructed by Na+ has higher catalytic activity than that constructed by NH4+. The mechanism studies show that the improvements of the catalytic performance are caused by the synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV-vis spectra revealed that the POM anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate thioanisole with complex cations was characterized by NMR experiments and UV-vis spectra. Thus, a new synergistic mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.

Switching on room-temperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions

Room-temperature phosphorescence (RTP) has attracted much attention in the past decades due to its potential applications in optoelectronic devices and bioimaging. So far, most available RTP materials are pure inorganic or organic compounds, few are involved in the organic-inorganic hybrid complexes. The donor-acceptor (D-A) hybrid heterostructures are an emerging class of organic-inorganic hybrid complexes composed of semiconductive organic and inorganic tectons at the molecular level. For these unique hybrids, the primary properties are photochromism and photoinduced electron transfers. Herein, we have demonstrated that the combination of naphthalene diimide (NDI) tectons bearing two divergently oriented pyridyl protons with three polyoxometalate (POM) anions (SiW12O404−, PW12O403− or PMo12O403−) resulted in three isostructural D-A hybrid heterostructures with an alternate arrangement of segregated POM anions and 1-D H-bonded NDI networks as electron donors and acceptors, respectively. Due to the compact contacts between POM anions and NDI tectons induced by charge-assisted anion-π interactions, not only the photochromic speeds of three D-A hybrid heterostructures have been enhanced, but more interestingly their RTP emissions of have also been switched on, and their photoluminescence quantum yields are dependent on anion-π interaction strengths.

The Effect of Nanosized Silicon Molybdate Anions on the Plasma Membrane of Human Fetal Fibroblasts

We used time-of-flight secondary ion mass spectrometry (ToF-SIMS) to study changes in the composition of the plasma membranes of human fetal fibroblasts under the action of nanosized anions of silicon molybdic acid. The dependences of the mass spectra of the main lipids of the plasma membranes on the silicon molybdate concentration were measured and interpreted; the dependences correlate with the layer-by-layer distributions and with the affinity of cholesterol for phospholipids. A new effect for cell biochemistry was discovered, that is, a significant decrease in the relative concentrations of cholesterol and sphingomyelin in plasma membranes under the effect of multiply charged heteropoly anions (HPAs). In aqueous silicon molybdate solutions with a concentration of c ≈ 10 µM/L and an exposure time of 48 h, the amount of cholesterol in plasma membranes decreased by 2–2.5 times, while the amount of sphingomyelin decreased by 20–25%. A new mechanism is proposed for the initial effect of HPA on plasma membranes, which consists of selective etching by multiply charged anions. According to the proposed mechanism, cholesterol and sphingomyelin, the main regulators of permeability and microviscosity of plasma membranes, are extracted from the plasma membrane at the first stage of the interaction of the polyoxometallate anion with the cell. As a consequence of the increased permeability of the plasma membranes in cells, acceleration of vital transmembrane and lateral processes may occur.

Understanding Electrochemical Interface Using Atom-By-Atom Metal Substitution of Redox Species

Molecular-level understanding of electrochemical processes at nanostructured electrode-electrolyte interfaces (EEI) is key to the development of clean and sustainable electrochemical technologies in energy generation and storage as well as water and chemical separation and purification. Herein, we introduced a novel in situ electrochemical cell coupled with ion soft landing to study the precisely defined operating EEI. Ion soft landing is a mass spectrometry based deposition technique which enables to land uniform layers of redox-active ions onto electrode surfaces with precise control over the charge state, composition, and kinetic energy. In this work, we used ion soft landing to study (i) the multi-electron redox activity of Keggin polyoxometalate anions (POMs) to design high-performance EEIs for energy storage (ii) examined how “atom-by-atom” metal substitution affects the redox activity of well-defined mixed addenda POM anions, PMoxW12-xO40 3- (x = 0,1,2,3,6,9,12) at ionic liquid based in situ electrochemical cell. We made a striking observation in in situ experiment and well-supported by theoretical calculation, which is that substantial increase in the first reduction potential obtained by substitution of only few tungsten by molybdenum atoms in the PW12O40 3- anions. More specifically, PMo3W9O40 3- showed the highest redox activity compared to all other mixed-addenda POM anions, which making it as a “super active redox anion” for this class of redox species. This presentation will also focus how these fundamental understanding on the activity of mixed addenda POMs was translated well into achieving device-level supercapacitor performance with superior capacity and capacity retention.

Synergic Catalysts of Polyoxometalate@Cationic Porous Aromatic Frameworks: Reciprocal Modulation of Both Capture and Conversion Materials.

Compositional catalysts based on porous supports and incorporated catalytic nanoparticles have achieved great successes during the past decades. However, rational design of synergic catalysts and modulating the interactions between functional supports and catalytic sites are still far from being well developed. In this work, aiming at overcoming the difficulties of comprehensive screening of porous supports and correspondingly matched catalytic sites, a cationic porous aromatic framework as a capturing platform and polyoxometalate anions as conversion materials are separately designed, and their combination is modularly controlled. The resulting composites show higher catalytic activities than the corresponding conversion sites themselves. Notably, the resulting composites uncommonly exhibit increased surface area and enlarged pore openings after the incorporation of nanoparticles, and lead to the promotion of mass transfer within the porous supports. The emergence of a hierarchical structure with increased surface area induced by guest loading is desired in heterogeneous catalysis. The reciprocal modulation of both capture and conversion materials results in enhanced conversion and increased reaction rate, indicating the successful preparation of synergic catalysts by this separate design approach.

Engineering polyoxometalate anions on porous ionic network towards highly catalytic active noble metal clusters

High surface energy of noble metal clusters (NMCs) with sub-nanometer size (<2 nm) endows their remarkable catalytic activity but naturally increases the risk of deactivation due to aggregation or oxidation. As a result, it is challengeable to construct efficient and stable naked NMCs. Herein, we reported the solvothermal synthesis of polyhedral oligomeric silsesquioxane (POSS) based porous ionic polymers with large surface areas, tunable pore structures and abundant ionic sites that resemble the structure of traditional ionic liquids. A series of highly dispersive and stable NMCs (Pd, Pt, Ru and Rh) were generated on these polymers by modulating their polyoxometalate (POM) anions, attributable to their robust framework and ion-exchange property. These organic cations paired POM anions are found to provide versatile strong interaction towards noble metal species, responsive for the formation and stabilization of NMCs. The obtained Pd clusters effectively catalyzed the solvent-free oxidation of benzyl alcohol with atmospheric dioxygen (O2), affording a high yield of 97% and a huge turnover number (TON) that exceeded 639 times of commercial Pd/C catalyst. The catalyst also exhibited pleasurable reusability and broad substrate compatibility.

A thermodynamic model of non-ionic surfactants' micellization in the presence of polyoxometalates

Polyoxometalates (POMs) are nanometric metal−oxide anions with an unmatched range of chemical and physical properties. During the past decade, significant efforts have been made to study POM surface activity and self-assembly properties that are essential for catalysis applications and for producing organic−inorganic hybrid materials. A previous work highlighted the tungstosilicate (SiW12O404−) and tungstophosphate (PW12O403−) Keggin POM anions' spontaneous and noncovalent adsorption at the micellar surface of non-ionic surfactants. In this study, the critical micelle concentrations (CMC) of two non-ionic surfactants, the n-octyl-β-glucoside (C8G1) and the tetraethylene glycol monooctyl ether (C8E4), were measured in the presence of POMs, and we propose herein thermodynamic models to explain an increase or a decrease of the CMC depending on the choice of the POM/surfactant couple.

Dimensional Control in Polyoxometalate Crystals Hybridized with Amphiphilic Polymerizable Ionic Liquids.

Ionic liquids are an important component for constructing functional materials, and polyxometalate cluster anion is a promising partner for building inorganic–organic hybrid materials comprising ionic liquids. In such hybrid materials, the precise control of the molecular arrangement in the bulk structures is crucial for the emergence of characteristic functions, which can be realized by introducing an amphiphilic moiety into the ionic liquids. Here, an amphiphilic polymerizable imidazolium ionic liquid with a methacryloyl group was firstly hybridized with polyoxometalate anions of octamolybdate ([Mo8O26]4−, Mo8) and silicotungstate ([SiW12O40]4−, SiW12) to obtain inorganic–organic hybrid crystals. The polymerizable ionic liquid with a octyl chain (denoted as MAImC8) resulted in the formation of anisotropic molecular arrangements in the bulk crystal structure, which was compared with the hybrid crystals composed from the polymerizable ionic liquid without a long alkyl chain (denoted as MAIm). Rather densely packed isotropic molecular arrangements were observed in the hybrid crystals of MAIm–Mo8 and MAIm–SiW12 due to the lack of the amphiphilic moiety. On the other hand, using the amphiphilic MAImC8 cation gave rise to a honeycomb-like structure with the Mo8 anion and a layered structure with the SiW12 anion, respectively.

Surface-grafted Europium and Erbium Complexes of the 12-Tungstosilicate Heteropolyoxometalate: A Synthetic and Structural Investigations

The hydrothermal synthesis of two new inorganic-organic hybrid materials in which PDA (PDA = 1,10-phenanthroline-2,9-dicarboxylate) is connected to a well-defined Keggin-type polyoxometalate anion (POM) and a Ln(III) cation (POM = [SiW12O40]4−; Ln = Eu3+ (1), Er3+ (2)), are reported. These hybrids are characterized by elemental analyses, IR spectra and single-crystal X-ray diffraction. Single-crystal X-ray analysis reveals that both compounds have been assembled by hydrogen bonding and anion-π interactions among POM, PDA and lattice water units into a three-dimensional (3D) supramolecular structure. Compound 1 is composed of two different cationic layers of Eu(III) clusters with the POMs located in cavities that are created by the Eu-PDA groups. Compound 2 consists of [SiW12O40]4– Keggin-type POMs, where a single {W3O13} triad is decorated by a trinuclear Er(III)-complex.

Regioselective synthesis and characterization of monovanadium-substituted β-octamolybdate [VMo7O26]5.

The monovanadium-substituted polyoxometalate anion [VMo7O26]5-, exhibiting a β-octamolybdate archetype structure, was selectively prepared as pentapotassium [hexaikosaoxido(heptamolybdenumvanadium)]ate hexahydrate, K5[VMo7O26]·6H2O (VMo7), by oxidation of a reduced vanadomolybdate solution with hydrogen peroxide in a fast one-pot approach. X-ray structure analysis revealed that the V atom occupies a single position in the cluster that differs from the other positions by the presence of one doubly-bonded O atom instead of two terminal oxide ligands in all other positions. The composition and structure of VMo7 was also confirmed by elemental analyses and IR spectroscopy. The selectivity of the synthesis was inspected by a 51V NMR investigation which showed that this species bound about 95% of VV in the crystallization solution. Upon dissolution of VMo7 in aqueous solution, the [VMo7O26]5- anion is substantially decomposed, mostly into [VMo5O19]3-, α-[VMo7O26]4- and [V2Mo4O19]4-, depending on the pH.

Multiscale Self-Assembly of Mobile-Ligand Molecular Nanoparticles for Hierarchical Nanocomposites.

Multiscale hierarchical morphologies are greatly desired for fabricating nanocomposites with tunable macroscopic properties, but challenges remain in precisely manipulating the spatial arrangement of nanoparticles in polymer matrices across multiple length scales. Here, we demonstrate a class of mobile-ligand nanoparticle system built upon 1 nm anionic polyoxometalate molecular nanoparticles and cationic terminated polymer chains by electrostatic interaction. The highly rearrangeable polymer chains can serve as mobile ligands to direct the polyoxometalates to align into sub-10 nm anisotropic superlattice-like nanoarrays in the bulk state. Moreover, these nanoarrays can further serve as structural units to assemble into hierarchically ordered morphologies in polymer matrices, e.g., percolated networks over hundreds of micrometers which are comprised of cylindrically packed polyoxometalate superlattices down to sub-10 nm scale. These hierarchical morphologies enable the nanocomposites with reinforced mechanical performance. The presented mobile-ligand approach can provide a paradigm to design functional polymer nanocomposites with improved properties such as mechanical reinforcement and collective optical and electronic functions.

When Gold Cations Meet Polyoxometalates.

Merging gold(I) cations with polyoxometalate anions results in various interclusters and complexes. Herein, the synthesis of these newly emerging gold(I)/polyoxometalate materials is reviewed. The applications of these promising hybrids in organic catalysis are also summarized and evaluated in terms of the advantages and limitations of the catalysts including efficiency, synergistic effects and recyclability.