Polyoxometalate-supported transition metal complexes for the oxidative cross-coupling of amines and alcohols

Three new hybrid polyoxometalates (POMs) based on transition metals, namely, copper metal with γ-[Mo8O26]4- polyoxometalates [Cu(DMF)4(Mo8O26)·2C3H8NO]·1.8 DMF (1), cobalt metal with hexamolybdate [Co(DMSO)6][Mo6O19] (2), and nickel metal with hexamolybdate [Ni(DMSO)6][Mo6O19] (3), were developed. These materials were characterised by applying both analytical and spectroscopic techniques (IR, TGA, and CHNO elemental analysis), and structural elucidation was performed using single-crystal X-ray diffraction studies. Among these hybrid POMs, octamolybdate with copper was used for the catalytic production of benzimidazoles. The incorporation of [Cu(DMF)4] between two octamolybdates results in the transformation of POMs into a useful catalyst, which effectively catalyses the oxidative cross-coupling of anilines, benzyl alcohol, and sodium azide to produce benzimidazole. This catalytic reaction occurs in the presence of tert-butyl hydroperoxide (TBHP) at a moderate temperature. Benzimidazole and its derivatives are highly preferred owing to their wide-ranging biological activities and clinical applications. These compounds are exceptionally effective in terms of their selectivity ratio and inhibitory activity. The catalytic reaction presented herein is a one-pot procedure that includes a series of reactions, such as C-H functionalization, condensation, ortho selective amination, and cyclization.

Cluster-Cation Pairs Mediated Assembly of Subnanometer Polyoxometalates Superstructures

Superstructures assembled by subnanometer polyoxometalate (POM) clusters are interesting for their attractive structures and excellent properties. However, the complex interactions between clusters and cations make it challenging to control the assembly of POM clusters at the subnanometer scale. Here, 20 cluster-assembled superstructures built by two types of MP2W17O61 (M = La-Lu) clusters are successfully synthesized. The precise structures and configurations of the subnanostructures, including nanowires, tetragonal nanosheets, and rectangular nanosheets, are characterized and presented. Molecular dynamics (MD) simulations reveal that the difference in interactions of POM clusters and cations leads to the formation of distinct superstructures. Two mechanisms of superstructure formation are proposed. Furthermore, the EuP2W17 nanosheet behaves with a high Faradaic efficiency of 90.2% and selectivity of 87.3% for glycolic acid in the electrocatalytic ethylene glycol oxidation reaction, which is much higher than that of isolated cluster components. This work connects the cluster topologies and cluster-cation pairs to the superstructures of cluster assemblies, providing general guidelines for the supramolecular self-assembly of POM clusters.

Designing diverse coordination modes for the covalent attachment of Wells-Dawson type polyoxometalate onto porphyrins

Five covalently bonded polyoxometalate (POM)-porphyrin hybrids were synthesized by reacting the Wells-Dawson type polyoxometalate [N(C4H9)4]5H4P2W15V3O62 with five tris-functionalized porphyrins containing different numbers of tris groups at different peripheral positions. These hybrids were thoroughly characterized using elemental analysis, NMR (1H, 31P, and 51V), mass spectrometry (ESI-MS, MALID-TOF-MS), FT-IR, UV-Vis, and fluorescence spectroscopies. The results proved that different quantities (one, two, and three) of the vanadium-capped Wells-Dawson type metal-oxide cluster P2W15V3O629- can be grafted onto a porphyrin moiety via covalent bonding with different orientations, depending on the number and position of peripheral functional groups on the porphyrin. Interestingly, remarkable fluorescence quenching (60% in 3Py-P@1POM, 75% in trans-2PyP@2POM, 80% in cis-2PyP@2POM, 85% in cis-2PhP@2POM, and 55% in 1Py-P@3POM, as compared to the fluorescence intensity of their corresponding porphyrin precursor) was observed under excitation (λexc = 328 nm), indicating electron transfer from the porphyrin moiety to the POM moiety through covalent linkage.

An Inorganic-Organic Hybrid Framework Composed of Polyoxotungstate and Long-Chained Bolaamphiphile

Surfactants are functional molecules utilized in various situations. The self-assembling property of surfactants enables several molecular arrangements that can be employed to build up nanometer-sized architectures. This is beneficial in the construction of functional inorganic-organic hybrids holding the merits of both inorganic and organic components. Among several surfactants, bolaamphiphile surfactants with two hydrophilic heads are effective, as they have multiple connecting or coordinating sites in one molecule. Here, a functional polyoxotungstate inorganic anion was successfully hybridized with a bolaamphiphile to form single crystals with anisotropic one-dimensional alignment of polyoxotungstate. Keggin-type metatungstate ([H₂W₁₂O₄₀]^6-, H₂W₁₂) was employed as an inorganic anion, and 1,12-dodecamethylenediammonium (C₁₂N₂) derived from 1,12-dodecanediamine was combined as an organic counterpart. A simple and general ion-exchange reaction provided a hybrid crystal consisting of H₂W₁₂ and C₁₂N₂ (C₁₂N₂-H₂W₁₂). Single crystal X-ray structure analyses revealed a characteristic honeycomb structure in the C₁₂N₂-H₂W₁₂ hybrid crystal, which is possibly effective for the emergence of conductivity due to the dissociative protons of C₁₂N₂.

Polyoxomolybdate Layered Crystals Constructed from a Heterocyclic Surfactant: Syntheses, Pseudopolymorphism and Introduction of Metal Cations

Crystals with layered structures are crucial for the construction of functional materials exhibiting intercalation, ionic conductivity, or emission properties. Polyoxometalate crystals hybridized with surfactant cations have distinct layered packings due to the surfactants which can form lamellar structures. Introducing metal cations into such polyoxometalate-surfactant hybrid crystals is significant for the addition of specific functions. Here, polyoxomolybdate–surfactant hybrid crystals were synthesized as single crystals, and unambiguously characterized by X-ray structure analyses. Octamolybdate ([Mo₈O₂₆]^4–, Mo₈) and heterocyclic surfactant of 1-dodecylpyridinium (C₁₂py) were employed. The hybrid crystals were composed of α-type and β-type Mo₈ isomers. Two crystalline phases containing α-type Mo₈ were obtained as pseudopolymorphs depending on the crystallization conditions. Crystallization with the presence of rubidium and cesium cations caused the formation of metal cation-introduced hybrid crystals comprising β-Mo₈ (C₁₂py-Rb-Mo₈ and C₁₂py-Cs-Mo₈). The yield of the C₁₂py-Rb-Mo₈ hybrid crystal was almost constant within crystallization temperatures of 279–303 K, while that of C₁₂py-Cs-Mo₈ decreased over 288 K. This means that the C₁₂py-Mo₈ hybrid crystal can capture Rb⁺ and Cs⁺ from the solution phase into the solids as the C₁₂py-Rb-Mo₈ and C₁₂py-Cs-Mo₈ hybrid crystals. The C₁₂py-Mo₈ hybrid crystals could be applied to ion-capturing materials for heavy metal cation removal.

Novel fluorescent biosensor for carcinoembryonic antigen determination via atom transfer radical polymerization with a macroinitiator

A novel fluorescence method for CEA via β-CD and BIBB-initiated atom transfer radical polymerization (ATRP) was reported.

Ruthenium(ii)-supported phosphovanadomolybdates [Ru(dmso)3PMo6V3O32]6− and [Ru(PMo6V3O32)2]14−, and their use as heterogeneous catalysts for oxidation of alcohols

Two stable ruthenium(ii)-substituted phosphovanadomolybdates [Ru^II(dmso)₃PMo^VI₆V^V₃O₃₂]⁶⁻ and [Ru^II(PMo^VI₆V^V₃O₃₂)₂]¹⁴⁻ are synthesized. They could catalyze oxidation of alcohols with good conversion and selectivity.

COF-inspired fabrication of two-dimensional polyoxometalate based open frameworks for biomimetic catalysis

The development of highly efficient and robust biomimetic catalysts is an essential and feasible strategy to overcome the intrinsic drawbacks of natural enzymes. Inspired by the synthetic strategy of covalent organic frameworks, we adopted a covalent-bond-driven strategy to prepare polyoxometalate (POM) based open frameworks (NKPOM-OFs = Nankai University POM-OFs) with abundant Mo[double bond, length as m-dash]O groups that can mimic the active center of sulfite oxidase. Four 2-dimensional (2D) NKPOM-OFs were designed and synthesized via the condensation reaction of linear amino-containing POMs with planar tetra-aldehyde monomers. Benefitting from the high crystallinity, the structures of 2D POM-OFs can be successfully determined from structural simulations. The results unveiled that NKPOM-OFs possessed 2D staggered stacking layered structures with the sql topology. All these NKPOM-OFs exhibited high crystallinity and stability and demonstrated outstanding performance to serve as biomimetic catalysts of sulfite oxidase with good recyclability. Notably, exfoliation of NKPOM-OFs under ultrasonic treatment can significantly boost the catalytic activity with almost two times faster reaction rates. This study not only enriches the facile and versatile synthesis strategy for POM-OFs but also provides new biomimetic platforms for biocatalysis.

The regulatory effects of the number of VP(N-vinylpyrrolidone) function groups on macrostructure and photochromic properties of polyoxometalates/copolymer hybrid films

Nanocomposite films were synthesized by introducing phosphotungstic acid (PWA) into the poly (vinylpyrrolidone and methyl acrylate) (P (VP(N-vinylpyrrolidone)coMA(methyl acrylate))). FT-IR curves illustrated that non-covalent bond interaction was built between PWA particles and organic matrixes. In combination the results of SEM, TG-DTA and DSC, it was conducted that the ratio of functional groups of VP had a critical effect on the surface morphology, thermal stability of the hybrid films. Irradiated with ultraviolet light, the transparent thin hybrid films change from colorless to blue. Stored in air, the hybrid films could recover colorless gradually. The analysis of kinetics told us that higher VP content within the different systems always accompanied more superior photochromic performance, faster coloring and fading speed. Appearance of W5+ in ESR spectra indicated that the photo-reduction process between PWA and copolymer matrix occurred.

Twining Poly(polyoxometalate) Chains into Nanoropes

Organic polymers and inorganic clusters belong to two different disciplines and have completely different properties and structures. When a cluster is attached to the backbone of a polymer as a pendant, the resultant hybrid polymers (polyclusters) exhibit unique behaviours totally different from those of conventional polymers owing to the nanoscale size of the cluster and its particular interactions. Herein, the aggregation of a poly(polyoxometalate)-a polynorbornene backbone with inorganic polyoxometalate cluster pendants-upon addition of a non-solvent to its dilute solution is reported. A three-dimensional network of tangled and snake-like nanothreads was observed. Direct visualisation of individual nanoscale clusters enabled identification of single chains within the nanothreads. These observations suggest that during the process of aggregation, the hybrid polymer forms curved or extended chains as a consequence of an armouring effect in which the collapsed cluster pendants wrap around the backbone. The collapse occurs because they become less soluble in the solvent/non-solvent mixture. The extended chains then become entwined and form nanoropes consisting of multiple chains wound around each other. This study provides a deeper understanding of the nature of polyclusters and should also prove useful for their future development and application.

Preparation of polyoxometalate-doped aminosilane-modified silicate hybrid as a new barrier of chem-bio toxicant

Nanohybrid membranes based on the Keggin-type polyoxometalate (POM) H5PV2Mo10O40 and aminosilane-modified silicate (Ormosil and Ormosil(NR4+Cl-)) hybrids were synthesized as a new barrier to protect against simulants of chemical and biological toxicant. The 31P NMR and XPS results indicated that POM was bound to the Ormosil and Ormosil(NR4+Cl-) hybrids after impregnation. The antibacterial effects of the hybrids and hybrid-impregnated fabrics against Gram-negative and Gram-positive bacteria were investigated with zone of inhibition, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and plate-counting method. The MIC/MBC values of Ormosil(NR4+Cl-)/POM and Ormosil/POM against bacteria were 0.267/2.67 and 2.67/26.7, respectively, and the percentage reduction of bacteria was approximately 100% after 20 laundry cycles of their fabrics. The reaction products and mechanisms of the adsorptive degradation of 2-chloroethylethylsulfide (CEES) by hybrids were investigated with 13C NMR. The results of this study showed that POM-doped Ormosil systems are capable of destroying bacteria and CEES.

Polyoxometalate-Polymer Hybrid Materials as Proton Exchange Membranes for Fuel Cell Applications

As one of the most efficient pathways to provide clean energy, fuel cells have attracted great attention in both academic and industrial communities. Proton exchange membranes (PEMs) or proton-conducting electrolytes are the key components in fuel cell devices, which require the characteristics of high proton conductivity as well as high mechanical, chemical and thermal stabilities. Organic-inorganic hybrid PEMs can provide a fantastic platform to combine both advantages of two components to meet these demands. Due to their extremely high proton conductivity, good thermal stability and chemical adjustability, polyoxometalates (POMs) are regarded as promising building blocks for hybrid PEMs. In this review, we summarize a number of research works on the progress of POM-polymer hybrid materials and related applications in PEMs. Firstly, a brief background of POMs and their proton-conducting properties are introduced; then, the hybridization strategies of POMs with polymer moieties are discussed from the aspects of both noncovalent and covalent concepts; and finally, we focus on the performance of these hybrid materials in PEMs, especially the advances in the last five years. This review will provide a better understanding of the challenges and perspectives of POM-polymer hybrid PEMs for future fuel cell applications.

Toward Advanced Functional Systems: Honeycomb-Like Polymeric Surfaces Incorporating Polyoxovanadates with Surface-Appended Copper-Cyclam Complexes

In this work the immobilization of hybrid polyoxometalates (POMs) onto functional polymeric surfaces is exposed and discussed. Thus, various hybrid polymer‒inorganic films were prepared by anchoring selected hybrid POMs onto tailored polymeric surfaces that consisted of breath figures (BFs) made of polystyrene-b-poly(acrylic acid)/polystyrene (PS-b-PAA/PS) blends. Functionalization of the BF films was performed by selective arrangement of acrylic acid groups of the amphiphilic block copolymer on the surface pores because of their affinition for the water condensed during breath figure formation. These carboxylic acid functional groups contained within the PAA blocks were then employed to anchor [Cu(cyclam)][{Cu(cyclam)}₂(V₁₀O₂₈)]·10H2O (1-CuV10) and [{Cu(cyclam)}(VO₃)₂]·5H₂O (1-CuV1), hybrid POMs by immersing the films into aqueous solutions of the in situ formed hybrid clusters, resulting in the hybrid films BF1 and BF2, respectively. Superficial analysis of these hybrid polymeric films was carried out by the sophisticated ion beam-based technique time-of-flight secondary ion mass spectrometry (ToF-SIMS) that was revealed to be an excellent method for the superficial compositional mapping of patterned surfaces.

Bottlebrush-Colloid Janus Nanoparticles

A bottlebrush-colloid Janus nanoparticle (JNP) with a ball-and-stick structure is reported. A single poly(4-vinyl benzyl chloride) (PVBC) polymer chain was grafted onto the amine-capped Fe₃O₄@NH₂ nanoparticle. pH-responsive 2-diethylaminoethyl methacrylate (DEAEMA) and water-soluble oligo(ethylene glycol) methacrylate (OEGMA) were sequentially grown from the PVBC backbone by ATRP, forming a core-shell bottlebrush. The synthesized PVBC₂₀₈-g-(PDEAEMA₁₃-b-POEGMA₈)-Fe₃O₄@NH₂ JNPs are dispersible in water and can be manipulated by a magnet. The Fe₃O₄ NPs with exposed -NH₂ groups facilitate accumulation at acidic sites. Hydrophobic dyes can be loaded within the PDEAEMA at pH ≥ 7.5, while they are released at pH values below 6.8. The composite JNPs are promising as a guided pH-responsive delivery vector toward acidic solid tumors.

A supramolecular approach of modified polyoxometalate polymerization and visualization of a single polymer chain

Photo-polymerization based on pre-assembly of anthracene-grafted polyoxometalate through host–guest inclusion of cyclodextrins is realized and a single polymer chain is visualized.

Multi-Tasking POM Systems

Polyoxometalate (POM)-based materials of current interest are summarized, and specific types of POM-containing systems are described in which material facilitates multiple complex interactions or catalytic processes. We specifically highlight POM-containing multi-hydrogen-bonding polymers that form gels upon exposure to select organic liquids and simultaneously catalyze hydrolytic or oxidative decontamination, as well as water oxidation catalysts (WOCs) that can be interfaced with light-absorbing photoelectrode materials for photoelectrocatalytic water splitting.

Heterogeneous catalysis by tungsten-based heteropoly compounds

In this review, the recent works on heterogeneous catalytic applications of polyoxotungstates in liquid-phase organic reactions are reviewed.

Conjugated hybrid films based on a new polyoxotitanate monomer

Electropolymerisation of the novel polyoxotitanate (POT) hexamer [Ti(μ₃-O)(OⁱPr)(TA)]₆ (TA = thiophene-3-acetate) with 3,4-ethylenedioxythiophene (EDOT) gives films of hybrid conjugated copolymer, Poly-(EDOT-POT)s, the morphologies of which are, uniquely, influenced by the electropolymerisation potential.

A simple synthetic route to polyoxovanadate-based organic–inorganic hybrids using EEDQ as an ester coupling agent

A simple synthetic strategy for the post-functionalization of organically derivatized hexavanadates is presented.

Dual Role of Zirconium Oxoclusters in Hybrid Nanoparticles: Cross-Linkers and Catalytic Sites

Organic-inorganic hybrid nanoparticles are prepared by free-radical copolymerization of methyl methacrylate (MMA) with the structurally well-defined methacrylate-functionalized zirconium oxocluster Zr₄O₂(methacrylate)₁₂. The polymerization process occurs in the confined space of miniemulsion droplets. The formation of covalent chemical bonds between the organic and the inorganic counterparts improves the distribution of the guest species (oxoclusters) in the polymer particles, overcoming problems related to migration, leaching, and stability. Because of the presence of a high number of double bonds (12 per oxocluster), the oxoclusters act as efficient cross-linking units for the resulting polymer matrix, thus ruling its swelling behavior in organic solvents. The synthesized hybrid nanostructures are applied as heterogeneous systems in the catalytic oxidation of an organic sulfide to the corresponding sulfoxide and sulfone by hydrogen peroxide, displaying quantitative sulfide conversion in 4-24 h, with overall turnover numbers (TON) up to 8000 after 4 cycles.

Tube-graft-Sheet Nano-Objects Created by A Stepwise Self-Assembly of Polymer-Polyoxometalate Hybrids

In this work, we report the preparation of complex nano-objects by means of a stepwise self-assembly of two polymer-polyoxometalate hybrids (PPHs) in solution. The PPHs are designed and synthesized by tethering two linear poly(ε-caprolactone)s (PCL) of different molecular weights (MW) on a complex of a Wells-Dawson-type polyoxometalate (POM) cluster and its countraions. The higher MW PCL-POM self-assembled into nanosheets, while the lower MW PCL-POM assembled into nanotubes just by altering the ratio of water in the DMF-water mixed solvent system. The two nano-objects have a similar membrane structure in which a PCL layer is sandwiched by the two POM-based complex layers. The PCL layer in the nanosheets is semicrystalline, while the PCL layer in the nanotubes is amorphous. We further exploited this MW-dependence to self-assemble the nanotubes on the nanosheet edges to create complex tube-graft-sheet nano-objects. We found that the nanotubes nucleate on the four {110} faces of the PCL crystal and then further grow along the crystallographic b-axis of the PCL crystal. Our findings offer hope for the further development of nano-objects with increasing complexity.

Main-chain polyoxometalate-containing donor–acceptor conjugated copolymers: synthesis, characterization, morphological studies and applications in single-component photovoltaic cells

Single-component solar cells of main-chain polyoxometalate-containing donor–acceptor conjugated copolymers show efficiencies comparable to those conjugated polymers with covalently bonded fullerenes.

A poly(polyoxometalate)-b-poly(hexanoic acid) block copolymer: synthesis, self-assembled micelles and catalytic activity

A poly(polyoxometalate)-polymer hybrid block copolymer (H-BCP) was prepared via ring-opening metathesis polymerization (ROMP). The H-BCP self-assembles into hybrid micelles with a poly(polyoxometalate) shell and a polymer core in acetonitrile.