Metallopolymers from organically modified polyoxometalates (MOMPs): A review

Self-Healing of Polymers and Polymer Composites

This review is devoted to the description of methods for the self-healing of polymers, polymer composites, and coatings. The self-healing of damages that occur during the operation of the corresponding structures makes it possible to extend the service life of the latter, and in this case, the problem of saving non-renewable resources is simultaneously solved. Two strategies are considered: (a) creating reversible crosslinks in the thermoplastic and (b) introducing a healing agent into cracks. Bond exchange reactions in network polymers (a) proceed as a dissociative process, in which crosslinks are split into their constituent reactive fragments with subsequent regeneration, or as an associative process, the limiting stage of which is the interaction of the reactive end group and the crosslink. The latter process is implemented in vitrimers. Strategy (b) is associated with the use of containers (hollow glass fibers, capsules, microvessels) that burst under the action of a crack. Particular attention is paid to self-healing processes in metallopolymer systems.

Polyoxovanadate-Based Cyclomatrix Polyphosphazene Microspheres as Efficient Heterogeneous Catalysts for the Selective Oxidation and Desulfurization of Sulfides

The [V₆O₁₃]^2- cluster is successfully immobilized to the polymeric framework of cyclomatrix polyphosphazene via the facile precipitation polymerization between the phenol group symmetrically modified [V₆O₁₃]^2- and hexachlorocyclotriphosphazene. The structure of the as-prepared polyoxometalate-containing polyphosphazene (HCCP-V) was characterized by FT-IR, XPS, TGA, BET, as well as SEM and zeta potential. The presence of a rigid polyoxometalate cluster not only supports the porous structure of the polymeric framework but also provides an improved catalytic oxidation property. By using H₂O₂ as an oxidant, the as-prepared HCCP-V exhibited improved catalytic oxidation activity toward MPS, DBT, and CEES, which can achieve as high as 99% conversion. More importantly, the immobilization of POMs in the network of cyclomatrix polyphosphazene also provides better recyclability and stability of the heterogeneous catalyst.

PEG-Infiltrated Polyoxometalate Frameworks with Flexible Form-Factors

We present the synthesis of metal oxide frameworks composed of the Preyssler anion, [NaP₅W₃₀O₁₁₀]^14-, bridged with transition-metal cations and infiltrated with polyethylene glycol. The frameworks can be dissolved in water to form freestanding rigid or flexible films or gels. Powder X-ray diffraction shows that all form-factors maintain the short-range order of the original crystals. Raman spectroscopy reveals that, similar to hydrogels, the macroscopic mechanical properties of these composites are dependent on the water content and the extent of hydrogen-bonding within the water network. The understanding gained from these studies facilitates solution-phase processing of polyoxometalate frameworks into flexible form factors.

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

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

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)...

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.

Solution Dynamics of Hybrid Anderson-Evans Polyoxometalates

Understanding the stability and speciation of metal-oxo clusters in solution is essential for many of their applications in different areas. In particular, hybrid organic-inorganic polyoxometalates (HPOMs) have been attracting increasing attention as they combine the complementary properties of organic ligands and metal-oxygen nanoclusters. Nevertheless, the speciation and solution behavior of HPOMs have been scarcely investigated. Hence, in this work, a series of HPOMs based on the archetypical Anderson-Evans structure, δ-[MnMo₆O₁₈{(OCH₂)₃C-R}₂]^3-, with different functional groups (R = -NH₂, -CH₃, -NHCOCH₂Cl, -N═CH(2-C₅H₄N) {pyridine; -Pyr}, and -NHCOC₉H₁₅N₂OS {biotin; -Biot}) and countercations (tetrabutylammonium {TBA}, Li, Na, and K) were synthesized, and their solution behavior was studied in detail. In aqueous solutions, decomposition of HPOMs into the free organic ligand, [MoO₄]^2-, and free Mn³⁺ was observed over time and was shown to be highly dependent on the pH, temperature, and nature of the ligand functional group but largely independent of ionic strength or the nature of the countercation. Furthermore, hydrolysis of the amide and imine bonds often present in postfunctionalized HPOMs was also observed. Hence, HPOMs were shown to exhibit highly dynamic behavior in solution, which needs to be carefully considered when designing HPOMs, particularly for biological applications.

Introducing borane clusters into polymeric frameworks: architecture, synthesis, and applications

This feature article summarizes the preparation and applications of borane cluster-containing polymers and covers research progress and future trends of borane cluster-containing linear, dendritic, macrocyclic polymers and metal–organic frameworks.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

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.

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.