Synthesis of Polyoxometalate−Polymer Hybrid Polymers and Their Hybrid Vesicular Assembly

Development of a chromatographic method for optimizing the thiol-maleimide coupling of polyoxometalate-polymer hybrids

The covalent attachment of polyoxometalates (POMs) to polymers has been developed as a strategic approach for the advancement of POM-based hybrid materials with versatile applications. In this study, we utilized thiol-maleimide Michael addition to investigate the kinetics and efficacy of the "one-to-one" conjugation between Keggin type POM and polystyrene. We explored the effects of solvent polarity, catalyst, molecular weight of PS and synthetic strategies on the reaction kinetics and efficiency, by means of reverse-phase high-performance liquid chromatography (RP-HPLC). A series of comparative analysis affirmed the superior efficiency of the one-pot method, particularly when facilitated by the addition of a high-polarity solvent and an excess of maleimide. These findings offer valuable insights into the intricate interplay between reaction conditions, kinetics, and selectivity in thiol-maleimide reactions of POMs and polymers. They hold profound implications for advancing the study of POM-based multifunctional materials and the synthesis of complex hybrid molecules.

Self-Assembly of Polyoxometalate-Based Nanoparticle Surfactants in Solutions

Nanoparticle surfactants (NPSs) are an emergent class of amphiphiles attractive for their controllable assembly at the liquid-liquid interface. In this work, intriguing self-assembly behavior and stimuli-responsiveness of NPSs in homogeneous solutions are presented. With β-cyclodextrin-grafted polyoxometalates (POMs) and ferrocene (or azobenzene)-terminated polystyrene in water/tetrahydrofuran, POM-based NPSs are formed via host-guest interactions and self-organize to vesicles driven by solvent-phobic effects. The tunable supramolecular interactions allow these assemblies to be responsive to redox or light stimulus, respectively, affording an on-demand assembly/disassembly capacity that shows promise in delivery and release applications.

Polyoxometalates as chemically and structurally versatile components in self-assembled materials

Polyoxometalates (POMs) are anionic molecular metal oxides with expansive diversity in terms of their composition, structure, nuclearity and charge. Within this vast collection of compounds are dominant structural motifs (POM platforms), that are amenable to significant chemical tuning with minimal perturbation of the inorganic oxide molecular structure. Consequently, this enables the systematic investigation of these compounds as inorganic additives within materials whereby structure and charge can be tuned independently i.e. [PW12O40]3- vs. [SiW12O40]4- while also investigating the impact of varying the charge balancing cations on self-assembly. The rich surface chemistry of POMs also supports their functionalisation by organic components to yield so-called inorganic-organic hybrids which will be the key focus of this perspective. We will introduce the modifications possible for each POM platform, as well as discussing the range of nanoparticles, microparticles and surfaces that have been developed using both surfactant and polymer building blocks. We will also illustrate important examples of POM-hybrids alongside their potential utility in applications such as imaging, therapeutic delivery and energy storage.

Supramolecular assemblies of organo-functionalised hybrid polyoxometalates: from functional building blocks to hierarchical nanomaterials

This review provides a comprehensive overview of recent advances in the supramolecular organisation and hierarchical self-assembly of organo-functionalised hybrid polyoxometalates (hereafter referred to as hybrid POMs), and their emerging role as multi-functional building blocks in the construction of new nanomaterials. Polyoxometalates have long been studied as a fascinating outgrowth of traditional metal-oxide chemistry, where the unusual position they occupy between individual metal oxoanions and solid-state bulk oxides imbues them with a range of attractive properties (e.g. solubility, high structural modularity and tuneable properties/reactivity). Specifically, the capacity for POMs to be covalently coupled to an effectively limitless range of organic moieties has opened exciting new avenues in their rational design, while the combination of distinct organic and inorganic components facilitates the formation of complex molecular architectures and the emergence of new, unique functionalities. Here, we present a detailed discussion of the design opportunities afforded by hybrid POMs, where fine control over their size, topology and their covalent and non-covalent interactions with a range of other species and/or substrates makes them ideal building blocks in the assembly of a broad range of supramolecular hybrid nanomaterials. We review both direct self-assembly approaches (encompassing both solution and solid-state approaches) and the non-covalent interactions of hybrid POMs with a range of suitable substrates (including cavitands, carbon nanotubes and biological systems), while giving key consideration to the underlying driving forces in each case. Ultimately, this review aims to demonstrate the enormous potential that the rational assembly of hybrid POM clusters shows for the development of next-generation nanomaterials with applications in areas as diverse as catalysis, energy-storage and molecular biology, while providing our perspective on where the next major developments in the field may emerge.

Self-assembly of amphiphilic polyoxometalates for the preparation of mesoporous polyoxometalate-titania catalysts

Amphiphilic polyoxometalate (POM) surfactants were prepared by covalently grafting double hydrophobic tails with chain lengths C12H25, C14H29, C16H33 or C18H37 onto the lacunary Wells-Dawson {P2W17O61} headgroup. The critical micelle concentrations (CMCs) of these novel surfactants in aqueous solutions were determined by conductivity, and micelle formation was studied by small angle neutron scattering (SANS). Surprisingly, the amphiphiles with longer hydrophobic tails tend to form less elongated and more globular micelles in water. The self-assembled amphiphilic polyoxometalates were used as templates in the hydrothermal synthesis of mesoporous TiO2 containing dispersed, immobilised {P2W17O61} units, which showed enhanced activity for the photodegradation of rhodamine B (RhB). The catalyst was recycled eight times with no loss of efficiency, demonstrating the stability of the hybrid structure. The amphiphilic polyoxometalates, therefore have excellent potential for the synthesis of various types of catalytically active porous materials.

Antagonistic mixing in micelles of amphiphilic polyoxometalates and hexaethylene glycol monododecyl ether

HYPOTHESIS

Polyoxometalates (POMs) are metal oxygen clusters with a range of interesting magnetic and catalytic properties. POMs with attached hydrocarbon chains show amphiphilic behaviour so we hypothesised that mixtures of a nonionic surfactant and anionic surfactants with a polyoxometalate cluster as headgroup would form mixed micelles, giving control of the POM density in the micelle, and which would differ in size and shape from micelles formed by the individual surfactants. Due to the high charge and large size of the POM, we suggested that these would be nonideal mixtures due to the complex interactions between the two types of surfactants. The nonideality and the micellar composition may be quantified using regular solution theory. With supplementary information provided by small-angle neutron scattering (SANS), an understanding of this unusual binary surfactant system can be established.

EXPERIMENTS

A systematic study was performed on mixed surfactant systems containing polyoxometalate-headed amphiphiles (K₁₀[P₂W₁₇O₆₁OSi₂(C_nH_2n+1)₂], abbreviated as P₂W₁₇-2C_n, where n = 12, 14 or 16) and hexaethylene glycol monododecyl ether (C₁₂EO₆). Critical micelle concentrations (CMCs) of these mixtures were measured and used to calculate the interaction parameters based on regular solution theory, enabling prediction of micellar composition. Predictions were compared to micelle structures obtained from SANS. A phase diagram was also established.

FINDINGS

The CMCs of these mixtures suggest unusual unfavourable interactions between the two species, despite formation of mixed micelles. Micellar compositions obtained from SANS concurred with those calculated using the averaged interaction parameters for P₂W₁₇-2C_n/C₁₂EO₆ (n = 12 and 14). We attribute the unfavourable interactions to a combination of different phenomena: counterion-mediated interactions between P₂W₁₇ units and the unfolding of the ethylene oxide headgroups of the nonionic surfactant, yet micelles still form in these systems due to the hydrophobic interactions between surfactant tails.

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.

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.

Controlled introduction of metal cations into polymerizable ionic liquid-polyoxomolybdate hybrid crystals

The first syntheses of polyoxomolybdate hybrid crystals were achieved by using polymerizable ionic-liquid.

Synthesis and spectroscopic properties of silver-fluorescein co-doped phosphotungstate hollow spheres

Herein, silver-fluorescein co-doped phosphotungstate hollow spheres have been successfully synthesized using a chemical precipitation method by introducing silver ions and fluorescein at room temperature. This hybrid nanomaterial possesses a uniform structure, exhibiting a strong emission spectrum with a peak centered at 517 nm. It is anticipated that silver and fluorescein as functionalized dopants for the phosphotungstate hollow sphere structure would provide this material with multiple properties. The control experiments indicate that fluorescein plays a key role in the formation of the hollow sphere structure. Hence, the co-doping strategy is proposed to be a general method to endow polyoxometalate nanomaterials with new structures, new functions and potential applications in bioimaging, fluorescent chemical sensors, and antibacterial or optoelectronic devices.

Polymerizable Ionic Liquid Crystals Comprising Polyoxometalate Clusters toward Inorganic-Organic Hybrid Solid Electrolytes

Solid electrolytes are crucial materials for lithium-ion or fuel-cell battery technology due to their structural stability and easiness for handling. Emergence of high conductivity in solid electrolytes requires precise control of the composition and structure. A promising strategy toward highly-conductive solid electrolytes is employing a thermally-stable inorganic component and a structurally-flexible organic moiety to construct inorganic-organic hybrid materials. Ionic liquids as the organic component will be advantageous for the emergence of high conductivity, and polyoxometalate, such as heteropolyacids, are well-known as inorganic proton conductors. Here, newly-designed ionic liquid imidazolium cations, having a polymerizable methacryl group (denoted as MAImC₁), were successfully hybridized with heteropolyanions of [PW₁₂O₄₀]^3- (PW₁₂) to form inorganic-organic hybrid monomers of MAImC₁-PW₁₂. The synthetic procedure of MAImC₁-PW₁₂ was a simple ion-exchange reaction, being generally applicable to several polyoxometalates, in principle. MAImC₁-PW₁₂ was obtained as single crystals, and its molecular and crystal structures were clearly revealed. Additionally, the hybrid monomer of MAImC₁-PW₁₂ was polymerized by a radical polymerization using AIBN as an initiator. Some of the resulting inorganic-organic hybrid polymers exhibited conductivity of 10^-4 S·cm^-1 order under humidified conditions at 313 K.

Ionic Complexes of Metal Oxide Clusters for Versatile Self-Assemblies

The combination of rational design of building components and suitable utilization of driving force affords spontaneous molecular assemblies with well-defined nanostructure and morphology over multiple length scales. The serious challenges in constructing assemblies with structural advantages for the realization of functions programmed into the building components usually lie ahead since the process that occurs does not always follow the expected roadmap in the absence of external intervention. Thus, prefabricated intermediates that help in governing the target self-assemblies are developed into a type of unique building blocks. Metal oxide cluster polyanions are considered as a type of molecular nanoclusters with size scale and structural morphology similar to those of many known inorganic particles and clusters but possess distinctive characteristics. Following the understanding of these clusters in self-assembly and the rationalization of their most efficient design strategy and approach, the obtained fundamental principles can also be applied in common nanoparticle- and cluster-based systems. On the other hand, the deliberate synergy offered by organic countercations that support the self-assembly of these clusters greatly expands the opportunity for the functionalization of complex building units via control of multiple interactions. The ionic combination of the inorganic clusters with hydrophilicity and the cationic organic component with hydrophobicity leads to discrete properties of the complexes. Significantly, the core-shell structure with rigid-flexible features and amphiphilicity will pave the way for hierarchical self-assemblies of the obtained complexes, while the intrinsic characteristics of the metal oxide clusters can be modulated through external physicochemical stimuli. Within this context, over the past decade we have extensively explored the ionic combination of inorganic polyanionic clusters with cationic organic amphiphiles and devoted our efforts to establishing the general rules and structure-property relationships of the formed complexes for constructing self-assemblies at the interface, in solution, and in solid matrixes. Specific interest has been focused on the functional synergy deriving from the incompatible components in highly organized self-assemblies. In this Account, we describe the recent progress on the ionic complexation of polyoxometalate clusters with cationic amphiphiles and the construction of diverse self-assembled nanostructures. First, the fundamental structural characteristics and molecular geometries of the prepared complexes are analyzed. The construction principle and diversity of the self-assembly based on the complexes and the smart stimuli response are then discussed, subject to the adjustment of various non-covalent interactions occurring in the assemblies. Subsequently, we enumerate the functional applications of the ionic complexes assembling into organic, inorganic, and even biological matrixes. The inspiration from the construction of ionic complexation and self-assembly in this Account provides vivid profiles for the design of hybrid materials involving nanoclusters and/or nanoparticles with rich potentials in addition to polyoxometalate chemistry.

A mild post-functionalization method for the vanadium substituted P2W15V3 Wells–Dawson polyoxometalate based on a copper catalyzed azide–alkyne cycloaddition

A novel post-functionalization method for the vanadium substituted P₂W₁₅V₃ Wells–Dawson polyoxometalate has been developed using a copper catalyzed azide–alkyne cycloaddition.

Grafting of Secondary Diolamides onto [P2 W15 V3 O62 ](9-) Generates Hybrid Heteropoly Acids

The Dawson tungstovanadate [P2 W15 V3 O62 ](9-) can be grafted to secondary diolamides. The electron-withdrawing character of the polyanion increases the acidity of the amide proton, leading to an organo-polyoxometalate, which can be used as a Brønsted organocatalyst. High-field NMR and DFT modeling indicate that the amide proton stays on the nitrogen and that the exalted acidity derives from the interaction between the organic and inorganic parts of the organo-polyoxometalate. The amide-inserted vanadotungstates thus form a new family of (hybrid) heteropolyacids, offering new perspectives for the application of POM-based catalysis in organic synthesis.

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.

Versatility of the ionic assembling method to design highly luminescent PMMA nanocomposites containing [M6Qi8La6]n− octahedral nano-building blocks

Strongly luminescent hybrid nanocomposites were designed by integrating via an ionic technique high content of different transition metal clusters.

Supermolecular assembly of polyoxoanion and metal–organic cationic units towards a model for core–shell nanostructures

Two novel core–shell-like molecular composites [NiL₃]₄[HV^IV₁₂V^V₆O₄₂(PO₄)] and [NiL₂]₄[HV^IV₁₂V^V₆O₄₂(PO₄)] were designed and synthesized, which exhibited electrocatalytic activity for the reduction of H₂O₂ and oxidation of NO₂⁻.

Ion mediated charge carrier transport in a novel radiation sensitive polyoxometalate–polymer hybrid

We elucidate the carrier transport mechanism in a novel polyoxometalate–polymer (POM–MAPDST) hybrid containing molybdenum transition metal.

Covalently-linked polyoxometalate–polymer hybrids: optimizing synthesis, appealing structures and prospective applications

In this perspective, the field of covalent polyoxometalate–polymer hybrids has been reviewed and some perspectives are provided.

Polyoxometalate-conductive polymer composites for energy conversion, energy storage and nanostructured sensors

The exchange of electric charges between a chemical reaction centre and an external electrical circuit is critical for many real-life technologies. This perspective explores the "wiring" of highly redox-active molecular metal oxide anions, so-called polyoxometalates (POMs) to conductive organic polymers (CPs). The major synthetic approaches to these organic-inorganic hybrid materials are reviewed. Typical applications are highlighted, emphasizing the current bottlenecks in materials development. Utilization of the composites in the fields of energy conversion, electrochemical energy storage, sensors and nanoparticle "wiring" into conductive materials are discussed. The outlook section presents the authors' views on emerging fields of research where the combination of POMs and CPs can be expected to provide novel materials for groundbreaking new technologies. These include light-weight energy storage, high-sensitivity toxin sensors, artificial muscles, photoelectrochemical devices and components for fuel cells.

A covalent polyoxomolybdate-based hybrid with remarkable electron reservoir properties

A new polyoxomolybdate-based hybrid platform TBA4[PMo11O39{Sn(p-C6H4I)}] is reported. The presence of a post-functionalisable iodo-aryl moiety allows the grafting of a ferrocenyl moiety onto the POM. The electrochemical characterisation shows the effect of molybdenum on the electron reservoir properties of POM-based hybrids, which are further enhanced upon the addition of an acid.