Polyoxometalate Derivatives Research Articles

Epoxidation at Isolated Titanium Site Modeled by Ti‐Siloxy‐Polyoxometalates Built on [α–A–XW9O34]n− and [α–B–YW9O33]9−: Comparative Study of Their Hydrolytic Stability

AbstractThis report investigates the structural differences in a series of titanium complexes constructed from silanol functionalized polyoxometalate (SiloxPOMs) derivatives, designed to create a constrained coordination site for titanium (IV) cations, namely (THA)3[PW9O34(tBuSiO)3Ti(OiPr)] and (THA)3[SbW9O33(tBuSiO)3Ti(OiPr)]. The complexes serve as structural and functional models for titanium silicates, facilitating the epoxidation of allylic alcohols and alkenes by aqueous hydrogen peroxide solutions. The different activity and selectivity observed between the two derivatives are attributed to variations in the polyoxotungstic platform used, A‐type–[XW9O34]n− versus B‐type –[YW9O33]3−. A combined experimental and theoretical investigation highlights the influence of these structural differences on water interaction and hydrolytic stability, with A‐type structures proving more susceptible to hydrolysis. In addition, the study also delves into the nuclearity of the active sites, a monomeric titanium (IV)‐hydroperoxide [Ti]–(OOH) active species evidenced by diffusion NMR spectroscopy, and the influence of the presence of water on catalytic performance in epoxidation reaction, thus shedding light on the relationship between catalyst stability, intermediates formed and reaction pathway. The study finally demonstrates the suitability of B‐type SiloxPOM derivatives as models for titanium silicates, offering insights into their stability and catalytic activity for epoxidation reactions.

Polyoxometalate inhibition of SOX2-mediated tamoxifen resistance in breast cancer

BackgroundIncreased cancer stem cell (CSC) content and SOX2 overexpression are common features in the development of resistance to therapy in hormone-dependent breast cancer, which remains an important clinical challenge. SOX2 has potential as biomarker of resistance to treatment and as therapeutic target, but targeting transcription factors is also challenging. Here, we examine the potential inhibitory effect of different polyoxometalate (POM) derivatives on SOX2 transcription factor in tamoxifen-resistant breast cancer cells.MethodsVarious POM derivatives were synthesised and characterised by infrared spectra, powder X-ray diffraction pattern and nuclear magnetic resonance spectroscopy. Estrogen receptor (ER) positive breast cancer cells, and their counterparts, which have developed resistance to the hormone therapy tamoxifen, were treated with POMs and their consequences assessed by gel retardation and chromatin immunoprecipitation to determine SOX2 binding to DNA. Effects on proliferation, migration, invasion and tumorigenicity were monitored and quantified using microscopy, clone formation, transwell, wound healing assays, flow cytometry and in vivo chick chorioallantoic membrane (CAM) models. Generation of lentiviral stable gene silencing and gene knock-out using CRISPR-Cas9 genome editing were applied to validate the inhibitory effects of the selected POM. Cancer stem cell subpopulations were quantified by mammosphere formation assays, ALDEFLUOR activity and CD44/CD24 stainings. Flow cytometry and western blotting were used to measure reactive oxygen species (ROS) and apoptosis.ResultsPOMs blocked in vitro binding activity of endogenous SOX2. [P2W18O62]6− (PW) Wells-Dawson-type anion was the most effective at inhibiting proliferation in various cell line models of tamoxifen resistance. 10 µM PW also reduced cancer cell migration and invasion, as well as SNAI2 expression levels. Treatment of tamoxifen-resistant cells with PW impaired tumour formation by reducing CSC content, in a SOX2-dependent manner, which led to stem cell depletion in vivo. Mechanistically, PW induced formation of reactive oxygen species (ROS) and inhibited Bcl-2, leading to the death of tamoxifen-resistant cells. PW-treated tamoxifen-resistant cells showed restored sensitivity to tamoxifen.ConclusionsTogether, these observations highlight the potential use of PW as a SOX2 inhibitor and the therapeutic relevance of targeting SOX2 to treat tamoxifen-resistant breast cancer.

Facilitated hydrogen storage properties of MgH2 by Ni nanoparticles anchored on Mo2C@C nanosheets

Magnesium hydride (MgH2) holds promise as a hydrogen storage material, but its practical application is impeded by high desorption temperatures and slow kinetics, presenting substantial challenges. Herein, an efficient approach is proposed to synthesize Mo2C@C nanosheets-supported Ni nanoparticles (Ni/Mo2C@C-2) materials, which can operate as a highly efficient catalyst to enhance hydrogen desorption of MgH2. The ball-milled MgH2–7.5 wt% Ni/Mo2C@C-2 demonstrates prominent hydrogen desorption kinetics with an activation energy (Ea) of 97.22 kJ mol−1, alongside remarkable cycling stability, maintaining a capacity retention of 97.8% after 20 cycles. Consequently, the results reveal that the improvement of hydrogen storage properties for MgH2 can be attributed to the synergistic effect of the in-situ generated Mg2NiH4 with highly dispersion, the sustained Mo2C nanosheets, and highly graphitized carbon. This work provides a reliable strategy for constructing polyoxometalates derivatives to facilitate the hydrogen desorption and cycling of MgH2.

Rigid-flexible-ligand-ornamented lanthanide-incorporated selenotungstates and photoluminescence properties

Currently, organic-inorganic hybrid lanthanide-incorporated polyoxometalates (POMs) have emerged as a prominent research area. Herein, we employ a simple raw material assembly method to synthesize two neoteric mixed-organic-ligand-ornamented lanthanide (Ln) incorporated selenotungstates [H2N(CH3)2]16Na2[Ln4(H2O)6(HPZDA)2(HFMA)2W8O21][B-α-SeW9O33]4·29H2O (Ln = Sm3+ (1), La3+ (2); H2PZDA = 2,3-pyrazine dicarboxylic acid, H2FMA = fumaric acid). 1 and 2 are isomorphic with the polyanions constructed from four trivacant Keggin [B-α-SeW9O33]8– ({SeW9}) segments and a rigid-flexible-ligand-ornamented dodeca-nuclear W–Ln heterometallic [Ln4(H2O)6(HPZDA)2(HFMA)2W8O21]14+ cluster. Moreover, the solid-state fluorescence spectrum of 1 at room temperature mainly exhibits the characteristic emission peak of Sm3+ cations. Additionally, energy transfer from {SeW9} to Sm3+ ions in 1 has been demonstrated by time-resolved spectroscopy. This work presents a feasible dual-ligand synergistic strategy for constructing novel POM derivatives and POM-based fluorescent materials.

Bridge improvement work: maximising non-linear optical performance in polyoxometalate derivatives.

New phenyl and stilbene-bridged polyoxometalate (POM) charge-transfer chromophores with diphenylamino donor groups produce, respectively, the highest intrinsic and absolute quadratic hyperpolarisabilities measured for such species. The β0,zzz obtained for the phenyl bridge - at 180 × 10-30 esu - is remarkable for a short conjugated system while changing to the stilbene (260 × 10-30 esu) produces a substantial increase in non-linearity for a minimal red-shift in the absorption profile. Together with TD-DFT calculations, the results show that maximising conjugation in the π-bridge is vital to high performance in such "POMophores".

Low-frequency noise in nanoparticle-molecule networks and implications for in materio reservoir computing.

We studied the low-frequency noise, i.e. flicker noise, also referred to as 1/f noise, in 2D networks of molecularly functionalized gold nanoparticles (NMN: nanoparticle-molecule network). We examined the noise behaviors of the NMN hosting alkyl chains (octanethiol), fatty acid oleic acids (oleylamine), redox molecule switches (polyoxometalate derivatives) or photo-isomerizable molecules (azobenzene derivatives) and we compared their 1/f noise behaviors. These noise metrics are used to evaluate which molecules are the best candidates to build in materio reservoir computing molecular devices based on the NMNs.

Halide-free CO2 cycloaddition onto styrene oxide catalysed by first row transition-metal derivatives of polyoxotungstates.

Quaternary ammonium salts of metal derivatives of polyoxometalates [XW11O39M(H2O)]n- (X = P, Si; M = Cr, Mn, Co, Ni, Zn) were successfully tested instead of quaternary ammonium halides as catalysts in the cycloaddition of CO2 to styrene oxide. Remarkably, they gave very satisfactory yields of styrene carbonate at moderate temperature (80 °C).

Single-atom Co dispersed on polyoxometalate derivatives confined in bamboo-like carbon nanotubes enabling efficient dual-site lattice oxygen mediated oxygen evolution electrocatalysis for acidic water electrolyzers

In situ generated confined single-atom Co3+ sites with a low coordination number and abundant oxygen vacancies demonstrate enhanced stability and activity for acidic oxygen evolution with broken scaling relations.

Carbon nanofibers confined polyoxometalate derivatives as flexible self-supporting electrodes for robust sodium storage

Flexible self-supporting film electrodes, which eliminate the need for additional adhesive, conductive agents, or current collectors, offer significant advantages in terms of mechanical properties, specific capacity, and energy density for energy storage applications. In this study, we successfully developed a flexible film electrode by incorporating derivatives of Mo and Fe-based polyoxometalates (POMs-D) into carbon nanofibers (CNFs). The integration of CNFs significantly enhanced the structural stability of POMs-D, while the internally formed electrical field facilitated efficient electron transfer, resulting in good performance in sodium storage. The film electrode demonstrated a high capacitive contribution of 90.0 % for sodium uptake/release at a scan rate of 1.0 mV s−1. It maintained a capacity of approximately 170 mA h g−1 even after 8000 cycles at a current density of 3.0 A g−1. Moreover, the film electrode exhibited a decent capacity with a 40.0-fold increase in current density, along with high power capability and energy density in sodium-ion hybrid supercapacitors, showcasing the versatility. These findings unveil the structure-functionality relationship and offer an advanced approach for developing high-performance film electrode materials, opening new possibilities in the fields of material science and energy storage.

Functionalized PCN-777-supported Lindqvist-type polyoxometalate derivatives for oxidative desulfurization: Role of electron-donating effect and hydrophobicity

The electronic structure and hydrophobicity of heterogeneous catalysts are of great importance to catalytic performance. A novel catalyst of functionalized PCN-777-supported Lindqvist-type polyoxometalate derivatives was designed and prepared. The hydrophobicity of the catalyst was improved by simple surface modification. The catalyst was applied in the ODS of dibenzothiophene (DBT) in this study, and the support and catalyst were characterized by Fourier transform infrared (FT-IR), contact angle test, and other methods. Systematic characterization and ODS experiments further confirmed the electron-donating effect of functionalized PCN-777 (tBu-PCN-777) with high hydrophobicity on [C12Py]3(NH4)3Mo7O24. The electron-donating effect leads to a narrow bandgap and high electron density of Mo active sites, ensuring that the catalyst [C12Py]3(NH4)3Mo7O24/tBu-PCN-777-35% exhibits excellent ODS activity. In particular, [C12Py]3(NH4)3Mo7O24/tBu-PCN-777-35% completely removed DBT at 60 °C in 50 min, and it continued to be stable after 30 cycles. The sulfur removal efficiency of real diesel oil reached 92.26%. This study demonstrates that functionalized PCN-777 has desulfurization potential and provides new ideas for increasing ODS catalytic activity from the design of electron-donating effect and hydrophobicity.

“Perfect match” of the carbazole-based conducting polymer and polyoxometalate nanocomposite components for enhanced optical and electrical properties

Conducting polymers (CPs) have been recognized as ideal matrices for electroactive species because of their ability to host them at the molecular level and can interact synergistically. One of the promising approaches is to make the optical and electrical properties of CPs easily tunable with nano-sized reactive inorganic species such as electroactive molecules, metal oxides, and polyoxometalates. In this study, organic–inorganic nanocomposites that include a triazine-centered carbazole and different types of polyoxometalate (Keggin and Dawson) derivatives have been electrochemically synthesized to obtain enhanced electrical and optical properties by determining the perfect match of the components. In situ incorporating POM clusters into cross-linked conducting polymers lead to organic–inorganic hybrid materials with not only value-adding properties but also synergistic effects. It has been observed that charge density and optical contrast values increased with the introduction of POM derivatives into the conductive polymer structure, while these values decreased when the feed ratio of POM derivatives increased. Furthermore, the highest optical contrast and charge density values were obtained in the molybdenum (POM4) based POM derivative when compared to all POM derivatives. Within the scope of technological application, dual-type electrochromic devices (ECDs) have been fabricated using poly(3,4-ethylenedioxythiophene) (PEDOT) and PCT3 or POM4/PCT3 and their characterization studies have been carried out.

Construction of Sb-capped Dawson-type POM derivatives for high-performance asymmetric supercapacitors

Modified polyoxometalate (POM) derivatives are promising pseudocapacitive active materials due to their abundant active sites and high redox capacity. Here, we constructed four antimony-capped Dawson-type molybdate clusters with high specific capacitance properties for the first time through the traditional hydrothermal method, formulated as [Cu(im)2]4{[Cu2(im)2]1/2(im)}[H3/2(Sb1Sb1/3Sb2/3)P2Mo7VMo11VIO62]·H2O (1); [Cu(bzim)2][H4Sb1Sb1/2 × 2P2Mo7VMo11VIO62]·(bzim)4·2H3O (2); [(bzim)4(NO3)1/2][H5/2Sb2P2Mo3VMo15VIO62]·H3O (3); [(bzim)4(bzim)1/3][HSb2P2Mo2VMo16VIO62]·H3O (4) (im= imidazole, bzim= benzimidazole). The compounds 1–4 all contain Sb-capped Dawson type {P2Mo18} anions with 3D supramolecular channel structures. Especially, compound 1 expresses a porous 2D metal-organic layer formed by Cu-ims transition metal complexes (TMCs) through the Cu-O interaction, and Sb capped-polyanions are exactly filled in the holes. Importantly, compound 1 exhibits a higher specific capacitance (Cs) value of 1401.75 F g−1 than the other three compounds (1141 F g−1, 1027.3 F g−1, and 978.25 F g−1) at a current density of 1.4 A g−1. In addition, the asymmetric supercapacitor Sb-P2Mo18//AC assembled with compound 1 and active carbon exhibits good cycling stability (89.14 %), and possesses an excellent energy density of 17.78 W h Kg−1, which could be attributed to the high redox capacity and excellent electronic conductivity. The experimental results demonstrate the feasibility of the synthesis strategy and pave the way for exploring novel high-performance polyoxometalate-based pseudocapacitive materials.

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, C2v 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.

Polyoxometalate‐Derived Ir/WOx/rGO Nanocomposites for Enhanced Electrocatalytic Water Splitting

Iridium (Ir)‐based nanomaterials are promising electrocatalysts for water splitting, and to alleviate their costs as well as improve the performances are always important tasks. Polyoxometalates (POMs) composed of abundant metal, oxygen, and heteroatoms are nanoclusters with defined structures. Benefitting from the inherent advantages of POMs, highly dispersive and “unprotected” Ir nanoparticles originating from Ir‐based colloid solution were successfully anchored on POM‐derived WO3/rGO nanocomposites for the first time. Interestingly, the obtained hybrid material Ir/WOx/rGO delivered improved electrocatalytic performance for water splitting, which is attributed to the addition of only quite small amount of POM derivatives. This work is also the first proof that POM can be employed as precursor to construct metal oxides to support Ir catalysts, providing a new vision for the design of advanced multi‐metal electrocatalysts.

Switchable NLO response induced by rotation of the fulvalene diruthenium-linked polyoxometalate derivatives

In this paper, the relationship between reversible reactions and second-order nonlinear optical (NLO) responses forFvRu2(CO)4-linkedpolyoxometalate (POM) derivatives have been systematically investigated by using density functional theory (DFT) method. The order of β values illustrates that the NLO responses can be enhanced by introducing POM and electron donating group(-NH2).According to the TDDFT results, the large βtot values of the studied systems arise mainlydue to the electronic transition from FvRu2(CO)4 group to the POM group.It is noticeable thatthe geometric interconversion resulting for systems 1/1′-4/4′allows the NLO responses to be switched “ON” or “OFF”.The aim of this work is to combine the attractive reversible reactions and NLO response of FvRu2(CO)4-linked POM derivatives to obtain the switchable NLO materials.

Deep desulfurization of fuels using supported ionic liquid-polyoxometalate hybrid as catalyst: A comparison of different types of ionic liquids

In this work, polyoxometalates-based monomer ionic liquid, dimer ionic liquid and polyionic liquid were designed and prepared. Then supported catalysts were synthesized by loading polyoxometalate derivatives on the surface of graphene oxide (GO). The catalysts before and after loading were characterized via many tests such as scanning electron microscope (SEM), infrared spectroscopy (IR), X-ray powder diffraction (XRD) and so on. The influences of ionic liquid type and carrier on desulfurization activity were carried out. The result shows that supported catalyst-based polyionic liquid (P[Vim]POM/GO) performed high activity and excellent recyclability in extraction-oxidation desulfurization (EODS) due to unique state of polyoxometalate and the support of graphene oxide. In addition, the possible mechanism of oxidation dibenzothiophene (DBT) with H2O2 was proposed according to the kinetic study and gas chromatography-mass spectrometer (GC–MS) result.

Novel morphologies including cowry-like crystal of polyoxometalates derivatives via coupled twinning between enantiomers

A single-sided, organically decorated Anderson-type polyoxometalates hybrid [AlMo6O18(OH)3{(OCH2)3CNH2}Cl]4- was successfully synthesized. In the presence of a chiral template, the polyoxometalates-organic derivative was crystallized in the chiral space group P21; this was confirmed via single-crystal X-ray diffraction analysis. Interestingly, Cowry-like crystal morphologies with chiral enantiomeric dimers can be formed by adjusting the ionic strength of the solvent, besides chiral template. Furthermore, the crystallization led to a series of crystals with different morphologies by adjusting the supersaturation, besides chiral template and ionic strength.

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.

Facile and efficient preparation of organoimido derivatives of [Mo6O19]2- using accelerated reactions in Leidenfrost droplets.

Reaction acceleration is a hot topic in recent years since it is very useful for rapid reaction screening and small-scale synthesis on a short timescale. It is known that the rates of chemical reactions are often accelerated in confined volumes (small droplets or thin films) where the unique chemical reactivities of molecules at the air-droplet/thin film interface, usually different from that in the bulk and gas phases, play a dominant role in acceleration. The Leidenfrost effect was employed to create small levitated droplets with no net charge. These droplets can accelerate many kinds of organic reactions. Our first accelerated synthesis of a series of organoimido-functionalized polyoxometalate (POM) clusters using Leidenfrost droplets with product analysis by electrospray ionization mass spectrometry (ESI-MS) demonstrated that this method can be successfully extended to the synthesis of inorganic/organic hybrids, a very promising area for developing POM-based functional materials. Comparable amounts of synthetic products [Mo6O18(NC6H4R)]2- (R = H (6), m/z 477; p-i-C3H7 (7), m/z 498; p-OCH3 (8), m/z 492; p-NO2 (9), m/z 500) were prepared within minutes in Leidenfrost droplets versus in hours in the corresponding bulk reactions under the same reaction conditions in the presence of the DCC catalyst, suggesting that both concentration and interfacial effects are pivotal in causing reaction acceleration in the Leidenfrost droplet. Compared to the conventional bulk reactions, the acceleration factors (AFs) were 92, 136, 126, and 89 for the four model reactions (1)-(4), respectively. We also found out that substitution affects the rate of reactions occurring in droplets, and hence the magnitude of AF. The rates increase in the order of R = NO2 < H < i-C3H7 < OCH3, in which the electron-donating groups (i.e., R = OCH3, i-C3H7) on the benzene ring are more favorable to the reaction than the electron-withdrawing group (i.e., R = NO2). This experimental result is in good agreement with the DFT calculation which indicates that the free-energy barriers for the direct imidoylization of POM with RNH2 are linearly correlated with the basicity constants (pKb) of amines.

Application of polyoxometalate derivatives in rechargeable batteries

Various polyoxometalate-derived transition metal composites can be applied as electrode materials in multiple rechargeable batteries with excellent electrochemical performances.