Hybrid polyoxometalate materials for photo(electro-) chemical applications

Efficient reduction of CO2 and inhibition of hydrogen precipitation by polyoxometalate photocatalyst modified with the metal Mn

Photocatalytic reduction of CO2 to chemical fuels is attractive for solving both the greenhouse effect and the energy crisis, but the key challenge is to design and synthesize photocatalysts with remarkable performance under visible light irradiation. Efficient catalytic carbon dioxide reduction (CO2RR) with light is considered a promising sustainable and clean approach to solve environmental problems. Herein, we found a new photocatalyst ([Mn(en)2]6[V12B18O54(OH)6]) (abbreviated as Mn6V12) based on the modifiability of polyoxometalates, in which Mn acts as a modifying unit to efficiently reduce CO2 to CO and effectively inhibit the hydrogen precipitation reaction. This Mn modified polyoxometalate catalyst has a maximum CO generation rate of 4625.0 μmol g-1 h-1 and a maximum H2 generation rate of 499.6 μmol g-1 h-1, with a selectivity of 90.3% for CO generation and 9.7% for H2 generation. This polyoxometalate photocatalyst can effectively reduce CO and inhibit the hydrogen precipitation reaction. It provides a new idea for the efficient photocatalytic carbon dioxide reduction (CO2RR) with polyoxometalate catalysts.

Impact of POM's coordination mode and Mo-hybrid constituents on the binding, stability, and catalytic properties of hybrid (pre)catalysts

The assembly of MoVIO2 2+ and methoxy-substituted salicylaldehyde nicotinoyl hydrazone ligands afforded two classes of hybrid polyoxometalates (POMs). In the Class I architectures, [MoO2(HL1-3)(D)]2[Mo6O19]·xCH3COCH3 (D = CH3COCH3 or H2O, x = 0 or 2, and L1-3 = ligands bearing the OMe group at position 3, 4 and 5, respectively), the main driving force for self-assembly is the electrostatic interaction between the components. Class II architectures are composed of a POM anion covalently linked to two Mo-complex units through the terminal Ot or bridging μ2-OPOM oxygen atoms, as found in Lindqvist-based hybrids [{MoO2(HL1-3)}2Mo6O19]·xCH3CN (x = 0 or 2) and the asymmetrical β-octamolybdate-based hybrid [{Mo2O4(HL2)(H2L)}{MoO2(HL2)}2Mo8O26]·CH3CN·H2O. Quantum chemical calculations were applied to evaluate the impact of the POM hybrid constituents on the hybrid-type stability, showing that it strongly depends on the ligand substituent position and ancillary ligand nature. Hybrids were tested as catalysts for cyclooctene epoxidation using tert-butyl hydroperoxide (TBHP in water or decane) and with or without the addition of acetonitrile (CH3CN) as an organic solvent. The catalytic results provided by the use of TBHP in decane are the best ones and classify all the prepared catalysts as very active, with the conversion of cyclooctene >90%, and high selectivity towards epoxide, >80%. We also examined the influence of the ligand structure, POM's hybrid type, and coordination mode on the Mo-hybrid activity and selectivity.

A New Molybdenum Blue Structure Type: How Uranium Expands this Family of Polyoxometalates

The assembly of molybdenum polyoxometalates (POMs) has afforded large discrete nanoclusters with varied degrees of reduction such as the ~20 % reduced molybdenum blues. While many heterometals have been incorporated into these clusters to afford new properties, uranium has yet to be reported. Here we report the first uranium containing molybdenum blue clusters and the unique properties exhibited by this incorporation. The uranyl ion (UO2 2+) directs formation of Mo72U8, a square POM comprised of two faces connected by eight edge-sharing molybdenum dimers. Mo72U8, a chiral cluster, crystallizes as a racemic mixture and, in the solid state, has a 'negative' charge localized on one face of the cluster opposite the 'positively' charged face of another cluster. Using U(IV) as both heterometal and molybdenum reductant afforded crystals of Mo97U10, a wheel cluster with a heptamolybdate cap on one face. Mo97U10 dissociates in solution, losing the heptamolybdate, to form Mo90U10. Using more solvent during synthesis afforded crystals of Mo90U10S4 which, instead of heptamolybdate, contains four sulfate ions. Crystals of Mo90U10S4 undergo a dehydration induced phase change where clusters form a sheet through oxide bridges. Half of the bridges are cation-cation interactions between the uranyl oxygen atom and molybdenum, the first reported of this kind.

A reduced polyoxometalate-encapsulated organo cobalt modified phosphate framework for improving photocatalytic reduction of CO2

A reduced polyoxometalate-based organo-metallophosphate (MOPO) framework formulated as [Co4(PO4)(C7H8N4)6](PWVI10WV2O40) (Co-PO4-PW12) with an ultra-high CO production rate of 13 676 μmol g-1 h-1 has been presented through photocatalytic CO2 reduction investigations.

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

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

Tungsten-based Lindqvist and Keggin type polyoxometalates as efficient photocatalysts for degradation of toxic chemical dyes

Photocatalytic dye degradation employing polyoxometalates (POMs) has been a research focus for several years. We report the facile synthesis of tungsten-based Lindqvist and Keggin-type POMs that degrade toxic chemical dyes, methyl orange (MO) and methylene blue (MB), respectively. The Lindqvist POM, sodium hexatungstate, Na2W6O19, degrades MO under 100 W UV light irradiation within 15 min, whereas the Keggin POM, Ag4PW11VO40, degrades MB under 20 W visible light source within 180 min. The effect of various operating parameters, such as photocatalyst concentration, pH, time, and initial dye concentration, were assessed in the degradation of both dyes. The photoelectrochemical performance of the as-synthesized polyoxometalates shows that the Ag4PW11VO40 shows 2.4 times higher photocurrent density than Na2W6O19 at a potential of 0.9 V vs. Ag/AgCl. Electrochemical impedance analysis reveals that Ag4PW11VO40 exhibits much lower charge transfer resistance as compared to Na2W6O19, which indicates facile charge transfer at the electrode-electrolyte interface. Further Mott-Schottky measurements reveal that both the catalysts possess n-type semiconductivity and the charge carrier concentration of Ag4PW11VO40 (5.89 × 1019 cm-3) is 1.4 times higher as compared to Na2W6O19 (4.25 × 1019 cm-3). This work offers a new paradigm for designing polyoxometalates suitable for efficient photocatalytic degradation of organic dyes.

Crystal structure of (μ-hydrogen di-sulfato)-μ-oxido-bis-[(4,4'-di-tert-butyl-2,2'-bi-pyridine)-oxidovanadium(IV/V)] aceto-nitrile monosolvate

The dinuclear oxidovanadium(IV/V) complex, [V2(HS2O8)O3(C18H24N2)2]·CH3CN or [V2O2(μ-O)(μ-H(SO4)2)(4,4'-tBubpy)2]·CH3CN (4,4'-tBubpy = 4,4'-di-tert-butyl-2,2'-bi-pyridine), has crystallographic C 2 symmetry and exhibits a distorted octa-hedral geometry around the vanadium center, where the two 4,4'-tBubpy ligands are nearly orthogonal to each other. The two vanadium ions are linked by an oxo anion and a unique protonated sulfate anion [H(SO4)2 3-]. In the crystal, inter-molecular C-H⋯π and π-π inter-actions between the 4,4'-tBubpy ligands are present, leading to a three-dimensional network.

Photoinduced electron transfer between a noble-metal-free [Mo6I8Cl6]2- cluster and polyoxometalates

Evidence for photoinduced intermolecular electron transfer from the excited state of the [Mo6I8Cl6]2- electron-rich cluster to polyoxometalates (POMs) is reported. We demonstrate that the global charge density of POMs affects the efficiency of electron transfer. This work paves the way for the rational design of photocatalytic systems using cluster-based complexes as robust noble-metal-free photosensitizers.

Diphosphoryl-functionalized Polyoxometalates: Structurally and Electronically Tunable Hybrid Molecular Materials

Herein, we report the synthesis and characterization of a new class of hybrid Wells-Dawson polyoxometalate (POM) containing a diphosphoryl group (P2 O6 X) of the general formula [P2 W17 O57 (P2 O6 X)]6- (X=O, NH, or CR1 R2 ). Modifying the bridging unit X was found to impact the redox potentials of the POM. The ease with which a range of α-functionalized diphosphonic acids (X=CR1 R2 ) can be prepared provides possibilities to access diverse functionalized hybrid POMs. Compared to existing phosphonate hybrid Wells-Dawson POMs, diphosphoryl-substituted POMs offer a wider tunable redox window and enhanced hydrolytic stability. This study provides a basis for the rational design and synthesis of next-generation hybrid Wells-Dawson POMs.

A dual-mode system based on molybdophosphoric heteropoly acid and fluorescent microspheres for the reliable and ultrasensitive detection of alkaline phosphatase

A novel colorimetric and fluorescent dual-mode sensing system based on molybdophosphoric heteropoly acid (PMA) and fluorescent microspheres (FMs) was established for monitoring the activity of alkaline phosphatase (ALP). In the presence of ALP, L-ascorbic acid-2-phosphate (AAP) could be hydrolyzed catalytically to ascorbic acid (AA), which could reduce PMA to phosphorus molybdenum blue (PMB), accompanied by the generation of colorimetric signals depending on the level of ALP. Meanwhile, the fluorescence of FMs was quenched markedly by the PMB produced due to the inner-filter effect, which constituted the response mechanism for the dual-mode sensing systems of ALP. On this basis, a PMA-FMs based dual-mode sensing system was used for the detection of ALP, which not only possessed remarkable sensitivity, with a limit of detection of 0.27 U L^-1 and 0.11 U L^-1, but also exhibited good analytical performance in biological samples with satisfactory results. Moreover, a simple and portable test kit for the visual detection of ALP in real serum samples was fabricated utilizing a smartphone with image-recognition and data-processing capabilities as a visual-detection platform.

Organic-Inorganic Hybrid Films of the Sulfate Dawson Polyoxometalate, [S2W18O62]4-, and Polypyrrole for Iodate Electrocatalysis

The Dawson-type sulfate polyoxometalate (POM) [S₂W₁₈O₆₂]^4- has successfully been entrapped in polypyrrole (PPy) films on glassy carbon electrode (GCE) surfaces through pyrrole electropolymerization. Films of varying POM loadings (i.e., thickness) were grown by chronocoulometry. Film-coated electrodes were then characterized using voltammetry, revealing POM surface coverages ranging from 1.9 to 11.7 × 10^-9 mol·cm^-2, and were stable over 100 redox cycles. Typical film morphology and composition were revealed to be porous using atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, and the effects of this porosity on POM redox activity were probed using AC impedance. The hybrid organic-inorganic films exhibited a good electrocatalytic response toward the reduction of iodate with a sensitivity of 0.769 μA·cm^-2·μM^-1.

Facile synthesis of polyoxometalate supported on magnetic graphene oxide as a hybrid catalyst for efficient oxidation of aldehydes

In the present study, Anderson-type polyoxometalate [N(C₄H₉)₄] [FeMo₆O₁₈(OH)₆] (FeMo₆) was immobilized on amino-modified magnetic graphene oxide and employed as a new hybrid catalyst in oxidation of aldehydes to carboxylic acids. The synthesized hybrid catalyst Fe₃O₄/GO/[FeMo₆] was characterized using thermogravimetric analysis (TGA), scanning electron microscopies (SEM), Fourier transform infrared (FT-IR), vibrating sample magnetometry (VSM), energy-dispersive X-ray analysis (EDX), Raman spectroscopy and inductively coupled plasma atomic emission spectroscopy (ICP-OES). The results indicated that our catalyst was quite active in oxidizing the aldehydes to their corresponding carboxylic acids in the presence of hydrogen peroxide. The synthesized catalyst can be easily separated from the reaction medium and reused for six consecutive runs without a significant reduction in reaction efficiency.

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.

Colloidal nanomaterials for water quality improvement and monitoring

Water is the most important resource for all kind forms of live. It is a vital resource distributed unequally across different regions of the globe, with populations already living with water scarcity, a situation that is spreading due to the impact of climate change. The reversal of this tendency and the mitigation of its disastrous consequences is a global challenge posed to Humanity, with the scientific community assuming a major obligation for providing solutions based on scientific knowledge. This article reviews literature concerning the development of nanomaterials for water purification technologies, including collaborative scientific research carried out in our laboratory (nanoLAB@UA) framed by the general activities carried out at the CICECO-Aveiro Institute of Materials. Our research carried out in this specific context has been mainly focused on the synthesis and surface chemical modification of nanomaterials, typically of a colloidal nature, as well as on the evaluation of the relevant properties that arise from the envisaged applications of the materials. As such, the research reviewed here has been guided along three thematic lines: 1) magnetic nanosorbents for water treatment technologies, namely by using biocomposites and graphite-like nanoplatelets; 2) nanocomposites for photocatalysis (e.g., TiO2/Fe3O4 and POM supported graphene oxide photocatalysts; photoactive membranes) and 3) nanostructured substrates for contaminant detection using surface enhanced Raman scattering (SERS), namely polymers loaded with Ag/Au colloids and magneto-plasmonic nanostructures. This research is motivated by the firm believe that these nanomaterials have potential for contributing to the solution of environmental problems and, conversely, will not be part of the problem. Therefore, assessment of the impact of nanoengineered materials on eco-systems is important and research in this area has also been developed by collaborative projects involving experts in nanotoxicity. The above topics are reviewed here by presenting a brief conceptual framework together with illustrative case studies, in some cases with original research results, mainly focusing on the chemistry of the nanomaterials investigated for target applications. Finally, near-future developments in this research area are put in perspective, forecasting realistic solutions for the application of colloidal nanoparticles in water cleaning technologies.

Lanthanoid-containing polyoxometalate nanocatalysts in the synthesis of bioactive isatin-based compounds

Lanthanoid-containing polyoxometalates (Ln-POMs) have been developed as effective and robust catalysts due to their Lewis acid–base active sites including the oxygen-enriched surfaces of POM and the unique 4f. electron configuration of Ln. As an extension of our interest in Ln-POMs, a series of as-synthesized nanocatalysts K₁₅[Ln(BW₁₁O₃₉)₂] (Ln-B₂W₂₂, Ln = La, Ce, Nd, Sm, Gd, and Er) synthesized and fully characterized using different techniques. The Ln³⁺ ion with a big ionic radius was chosen as the Lewis acid center which is sandwiched by two mono-lacunary Keggin [BW₁₁O₃₉]⁹⁻ units to form Ln-containing sandwiched type cluster. Consequently, the catalytic activity of nanocatalysts with different Ln was examined in the synthesis of bioactive isatin derivatives and compared under the same optimized reaction conditions in terms of yields of obtained products, indicating the superiority of the nano-Gd-B₂W₂₂ in the aforementioned simple one-pot reaction. The effects of different dosages of nanocatalyst, type of solvent, reaction time, and reaction temperature in this catalytic system were investigated and the best results were obtained in the presence of 10 mol% of nano-Gd-B₂W₂₂ in water for 12 min at the reflux condition.

Bipolar Hemicyanine-Based Photodynamic Modulation of Type I Pathway for Efficient Sterilization and Real-Time Monitoring

The development of photosensitizers with low oxygen dependence for generating type I ROS is in high demand to be able to treat pathogenic infections in hypoxic conditions. Here, we report a series of cationic bipolar hemicyanines (C3, C6, and C10) with alkyl linkers of varying lengths that are found to exclusively produce hydroxyl radicals and superoxide radicals with the aid of white light and that have different antibacterial abilities toward a variety of pathogens. Furthermore, hemicyanines could differentiate live from dead bacteria to track the status of pathogens in real time. It is expected that hemicyanines could be applied for combatting various microbial infections in hypoxia and real-time tracking.

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

Encapsulation of Polymetallic Oxygen Clusters in a Mesoporous/Microporous Thorium-Based Porphyrin Metal-Organic Framework for Enhanced Photocatalytic CO2 Reduction

Solar-initiated CO₂ reduction is significant for green energy development. Herein, we have prepared a new mesoporous/microporous porphyrin metal-organic framework (MOF), IHEP-20, loaded with polymetallic oxygen clusters (POMs) to form a composite material POMs@IHEP-20 for visible-light-driven photocatalytic CO₂ reduction. The as-made composite material exhibits good stability in water from pH 0 to 11. After POMs were introduced to IHEP-20, they showed superior activity toward photocatalytic CO₂ reduction with a CO production rate of 970 μmol·g^-1·h^-1, which is 3.27 times higher than that of pristine IHEP-20. This study opens a new door for the design and synthesis of high-performance catalysts for the photocatalytic reduction of CO₂.

Self-assembly of polyoxometalate clusters into two-dimensional clusterphene structures featuring hexagonal pores

Two-dimensional (2D) structures have been shown to possess interesting and potentially useful properties. Because of their isotropic structure, however, clusters tend to assemble into 3D architectures. Here we report the assembly of polyoxometalate clusters into layered structures that feature uniform hexagonal pores and in-plane electron delocalization properties. Because these structures are 2D and visually reminiscent of graphene, they are referred to as 'clusterphenes'. A series of multilayer and monolayer clusterphenes have been constructed with 13 types of polyoxometalate cluster. The resulting clusterphenes were shown to exhibit substantially improved stability and catalytic efficiency towards olefin epoxidation reactions, with a turnover frequency of 4.16 h^-1, which is 76.5 times that of the unassembled clusters. The catalytic activity of the clusterphenes derives from the electron delocalization between identical clusters within the 2D layer, which efficiently reduces the activation energy of the catalytic reaction.

Niobium polyoxometalate–folic acid conjugate as a hybrid drug for cancer therapeutics

In this work, covalently bonded folic acid to niobium substituted Wells-Dawson polyoxometalate, (Bu4N)5H4[P2W15Nb3O62]-folic acid, has been synthesized and characterized. Afterward, the bioactivity behavior of this hybrid compound against cervical (HeLa)...

Incorporating highly basic polyoxometalate anions comprising Nb or Ta into nanoscale reaction fields of porous ionic crystals

Polyoxometalates (POMs) are oxide cluster anions composed of high-valence early transition metals and are widely used as catalysts. Yet base catalysis of POMs remains an ongoing challenge; group V (V, Nb, and Ta) elements form more negatively charged POMs than group VI (Mo and W) elements, and in particular, polyoxoniobates and polyoxotantalates are known to show strong basicity in solution due to the highly negative surface oxygen atoms. Herein, we report for the first time porous ionic crystals (PICs) comprising Nb or Ta. The PICs are composed of Dawson-type Nb/W or Ta/W mixed-addenda POMs with oxo-centered trinuclear Cr^III carboxylates and potassium ions as counter cations to control the crystal structure. Among the PICs, those with Nb or Ta tri-substituted POMs exhibit the highest yield (78-82%) and selectivity (99%) towards the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate (353 K, 6 h), which is a typical base-catalyzed reaction, as reusable solid catalysts, and they can also catalyze the reaction of other active methylene compounds. A detailed investigation into the crystal structures together with DFT calculations and in situ IR spectroscopy with methanol as a basic probe molecule shows that the exposure of [Nb₃O₁₃] or [Ta₃O₁₃] units with highly negative surface oxygen atoms to the pore surface of PICs is crucial to the catalytic performance. These findings based on the composition-structure-function relationships show that Nb- and Ta-containing PICs can serve as platforms for rational designing of heterogeneous base catalysts.

Multifunctional Polyoxometalate Platforms for Supramolecular Light-Driven Hydrogen Evolution*

Multifunctional supramolecular systems are a central research topic in light‐driven solar energy conversion. Here, we report a polyoxometalate (POM)‐based supramolecular dyad, where two platinum‐complex hydrogen evolution catalysts are covalently anchored to an Anderson polyoxomolybdate anion. Supramolecular electrostatic coupling of the system to an iridium photosensitizer enables visible light‐driven hydrogen evolution. Combined theory and experiment demonstrate the multifunctionality of the POM, which acts as photosensitizer/catalyst‐binding‐site^[1] and facilitates light‐induced charge‐transfer and catalytic turnover. Chemical modification of the Pt‐catalyst site leads to increased hydrogen evolution reactivity. Mechanistic studies shed light on the role of the individual components and provide a molecular understanding of the interactions which govern stability and reactivity. The system could serve as a blueprint for multifunctional polyoxometalates in energy conversion and storage.

Application of polyoxometalates in photocatalytic degradation of organic pollutants

Organic pollutants are highly toxic, accumulative, and difficult to degrade or eliminate. As a low-cost, high-efficiency and energy-saving environmental purification technology, photocatalytic technology has shown great advantages in solving increasingly serious environmental pollution problems. The development of efficient and durable photocatalysts for the degradation of organic pollutants is the key to the extensive application of photocatalysis technology. Polyoxometalates (POMs) are a kind of discrete metal-oxide clusters with unique photo/electric properties which have shown promising applications in photocatalytic degradation. This review summarizes the recent advances in the design and synthesis of POM-based photocatalysts, as well as their application in the degradation of organic dyes, pesticides and other pollutants. In-depth perspective views are also proposed in this review.

Chiral Phosphotungstate Functionalized with (S)-1-Phenylethylamine: Synthesis, Characterization, and Asymmetric Epoxidation of Styrene

In the present work, an attempt has been made to induce chirality in copper-substituted phosphotungstate (PW₁₁Cu) by functionalization with (S)-(+)-1-phenylethylamine (S-PEA) via a ligand substitution approach. The formation of a N→Cu dative bond was confirmed by ¹³C NMR, while ¹H NMR, circular dichroism spectroscopy and optical rotation studies confirmed the introduction of chirality to the Keggin structure. The synthesized material was used as the heterogeneous catalyst for the asymmetric epoxidation of styrene using various green oxidants to obtain high enantiomeric excess (ee), and the reaction with molecular oxygen was found to give the best ee. Regeneration studies were carried out, and the catalyst was found to be suitable for the same. A probable mechanism is also proposed. A comparison with other copper-based polyoxometalate catalysts clearly demonstrate the superiority and novelty of the present catalyst in terms of the reaction conditions as well as the obtained ee.

Bimetal zeolite imidazolate framework derived Mo0.84Ni0.16-Mo2C@NC nanosphere for overall water splitting in alkaline solution

The bifunctional efficient electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are in urgent need for the advanced overall water splitting (OWS) device. Restricted by the thermodynamic limitations of the catalytic active center for OER and the reaction kinetics limitations induced by the structure of the electrocatalysts, the development of OWS catalysts requires more effort. Herein, a porous carbon-based bimetal electrocatalyst of Mo_0.84Ni_0.16-Mo₂C@NC nanosphere is prepared by hydrothermal treatment of PMo₁₂@PVP@Zn/Ni-ZIF which is synthesized via one-pot self-assembled hydrothermal method. Our study confirms that the Mo-Ni alloy and Mo₂C nanoparticles homogeneously distribute in nitrogen-rich carbon-based materials. Furthermore, the porous structure exposes rich active sites and increases the effective specific area for redox reactions. The obtained Mo_0.84Ni_0.16-Mo₂C@NC catalyst requires low overpotentials of 151 and 285 mV to reach a current density of 10 mA cm^-2 towards the water reduction and oxidation in 1 M KOH solution, respectively, and possesses good catalytic stability for one day. This work introduces an advanced method for the synthesis of the bimetal electrocatalyst.

Optical, third order non-linear optical and electrochemical properties of dipolar, centrosymmetric and C2v organoimido polyoxometalate derivatives

A family comprising seven arylimido-polyoxometalate (POM) hybrid chromophores (three of which are new), with linear dipolar, C_2v and linear centrosymmetric geometries have been synthesised and studied by electronic absorption spectroscopy, electrochemistry, Z-scans (two photon absorption, TPA) and computation (DFT/TD-DFT). These reveal that POM acceptor units are an effective basis for TPA materials: the centrosymmetric bis-POM chromophores produce significant cross sections (δ up to 82 GM) from a single aryl bridge, a similar performance to larger dipolar π-systems combining carbazole or diphenylamino donors with the imido-POM acceptor. DFT/TD-DFT calculations indicate strong communication between POM and organic components is responsible for the linear and non-linear optical behaviour of these compounds, while electrochemical measurements reveal class II mixed valence behaviour resulting from an interplay of through-bond and through-space effects.

Efficient Electron Transfer from Electron-Sponge Polyoxometalate to Single-Metal Site Metal-Organic Frameworks for Highly Selective Electroreduction of Carbon Dioxide

In this work, by combining the superiority of polyoxometalates (POMs) and catalytic single-metal site Co of metalloporphyrin, a series of mixed-valence POM-based metal-organic frameworks (MOFs) composites is synthesized by a post-modification method. The electron-transfer property of POM@PCN-222(Co) composite is significantly enhanced owing to the directional electron-transfer from POM to single-metal site Co in PCN-222(Co). In particular, H-POM@PCN-222(Co) gives a high Faradaic efficiency of 96.2% for electroreduction of CO₂ into CO and good stability over 10 h. DFT calculations confirm that the directional electron transfer, which accelerates the multi-electron transfer from the electrode to active single-metal site Co, enriches the electron density of the Co center, and ultimately reduces the energy of the rate-determining step, thus increasing the catalytic activity of CO₂ reduction reaction (CO₂ RR). This work therefore suggests some new insight for the design of efficient electrocatalysts for CO₂ RR.

Constructing chiral polyoxometalate assemblies via supramolecular approaches

Polyoxometalates (POMs), as a typical class of discrete metal oxide clusters that are known in inorganic and structural chemistry since long, have displayed more and more interesting applications over recent years. However, in comparison to the chemical synthesis, the photochemical, electrochemical, and magnetic properties, the structural asymmetry, and relative characteristic investigations arising therefrom are far behind even if they are very important for functional materials, especially in solution systems. One of the main reasons is that it is hard to control and maintain a stable chiral state of POMs to carry out further corresponding performances. Aiming to overcome these disadvantages, the main concerns of this review are to discuss the generation of the chirality for discrete metal oxide clusters, chirality transfer via a supramolecular approach, chirality amplification in self-assemblies, and the related functional properties such as photochromism, catalysis, and bioactivities in solutions. Considering that some previous reviews dealt with chiral structures and packing architectures in the crystalline solids of POMs, this article only concentrates on the induced chirality and material properties in solution systems, which have been more active recently but no review article has been involved in this interesting area.

Photoactive Organic/Inorganic Hybrid Materials with Nanosegregated Donor-Acceptor Arrays

The synthesis of the first mesogenic donor-acceptor polyoxometalate (POM)-based hybrid is herein described. The structural and electronic properties of the hybrid compound were evaluated through combination of small- and wide-angle X-ray scattering, optical microscopy, electrochemistry and photoluminescence. In the solid state, the compound behaves as a birefringent solid, displaying a lamellar organization in which double-layers of POMs and bis(thiophene)thienothiophene organic donors alternate regularly. Noticeably, the sub-unit organizations in the composite are similar to that observed for the individual POM and organic donor precursors. Photophysical studies show that in the hybrid, the fluorescence of the organic donor unit is considerably quenched both in solution and in the solid state, which is attributed to occurrence of intramolecular charge-separated state.