Polyoxometalate Hybrid Research Articles

Efficient Removal of Methylene Blue Using an Organic–Inorganic Hybrid Polyoxometalate as a Dual-Action Catalyst for Oxidation and Reduction

Efficient Removal of Methylene Blue Using an Organic–Inorganic Hybrid Polyoxometalate as a Dual-Action Catalyst for Oxidation and Reduction

Redox-active polyoxovanadates as cofactors in the development of functional protein assemblies

The interactions of polyoxovanadates (POVs) with proteins have increasingly attracted interest in recent years due to their potential biomedical applications. This is especially the case because of their redox and catalytic properties, which make them interesting for developing artificial metalloenzymes. Organic-inorganic hybrid hexavanadates in particular offer several advantages over all-inorganic POVs. However, they have been scarcely investigated in biological systems even though, as shown in this work, hybrid hexavanadates are highly stable in aqueous solutions up to relatively high pH. Therefore, a novel bis-biotinylated hexavanadate was synthesized and shown to selectively interact with two biotin-binding proteins, avidin and streptavidin. Bridging interactions between multiple proteins led to their self-assembly into supramolecular bio-inorganic hybrid systems that have potential as artificial enzymes with the hexavanadate core as a redox-active cofactor. Moreover, the structure and charge of the hexavanadate core were determined to enhance the binding affinity and slightly alter the secondary structure of the proteins, which affected the size and speed of formation of the assemblies. Hence, tuning the polyoxometalate (POM) core of hybrid POMs (HPOMs) with protein-binding ligands has been demonstrated to be a potential strategy for controlling the self-assembly process while also enabling the formation of novel POM-based biomaterials that could be of interest in biomedicine.

Supramolecular assembled Keggin based porous material: Synthesis, crystal structure, electrochemical and spectroscopic properties

In this paper, a new supramolecular assembled Keggin polyoxometalate (POM) based organic-inorganic porous material (A; [HSiMo12O40].3[C10H11N4O2S].48H2O) containing the compound sulfadiazine (SZD) has been prepared and its molecular structure was determined. To characterize the hybrid compound (A), X-ray crystallography, MALDI-TOF, UV–Vis, photoluminescence and FTIR methods were used. The electrochemical properties of the Keggin POM, SZD and porous hybrid (A) compounds were investigated in the −1.8 – 1.8 V range and the redox processes at some potentials of the hybrid compound (A) are irreversible at 250 and 500 mV/s scan rates. Thermal stability properties of the hybrid compound (A) were investigated by the TGA/DTA instruments. The loss of the adsorbed water molecules on the hybrid compound (A) starts at 28 °C temperature. Crystal structure of the hybrid compound contains a Keggin type polyoxometalate [HSiMo12O40]3− and three monoprotonated sulfadiazinium (HSZD)+ cations. In the structure of the hybrid material, there are solvent accessible voids along a axis filled with water molecule.

Highly efficient catalytic reduction of organic dyes, Cr (VI) and 4‐nitrophenol, and photocatalytic degradation of MB by an inorganic–organic hybrid polyoxometalate

It is urgent to remove various pollutants from the water environment. Therefore, an inorganic–organic hybrid polyoxometalate (POM) [PMoVI6MoV2VIV8O44{Co(2,2′‐bipy)2(H2O)}4]‐[PMoVI4MoV4VIV8O44{Co(2,2′‐bipy)2(H2O)}2]·4H2O (PMoVCo) was fully characterized by XRD, XPS, FTIR, SEM, BET and zeta potential techniques, and applied for the catalytic reduction of hazardous methylene blue (MB), methyl orange (MO), rhodamine B (RhB), Cr (VI) and 4‐nitrophenol (4‐NP) in aqueous media over NaBH4 under mild condition. The removal ratios were 99.6% (MB, 2.5 min), 97.0% (MO, 3.0 min), 92.5% (RhB, 2.0 min), 89.2% (Cr (VI), 2.0 min) and 89.6% (4‐NP, 2.5 min), respectively. The pseudo‐first‐order rate constants of the reactions were 1.992 min−1 (MB), 0.810 min−1 (MO), 1.768 min−1 (RhB), 1.186 min−1 (Cr (VI)) and 0.934 min−1 (4‐NP), respectively. Additionally, PMoVCo exhibited satisfactory catalytic activity for the photo‐Fenton‐like degradation of MB at neutral pH under visible light irradiation with a degradation rate up to 99.9% in 480 min. The results of the recycling experiment prove that PMoVCo has high catalytic reusability. Moreover, the possible mechanisms of the catalytic reduction and degradation processes were proposed. The catalytic activities of PMoVCo are better than other inorganic–organic hybrid POMs. All the results of this study indicate that PMoVCo has the potential to be a satisfactory catalyst for wastewater remediation via reduction or photocatalytic degradation.

Manipulating the supramolecular assembly of γ-cyclodextrin and Anderson polyoxometalate hybrids through variations in remote functionality

Manipulating the supramolecular assembly of γ-cyclodextrin and Anderson polyoxometalate hybrids through variations in remote functionality

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.

Metalorganic Polyhedra Polyoxometalate Hybrid Salts for Tandem Photooxidative‐Hydrolytic Pesticide Detoxification

AbstractThe extensive use of pesticides to ensure global food production presents significant threats to both human health and the environment. In this study, the photochemical, redox, and acid‐base properties of Zr‐metal‐organic polyhedra (Zr‐MOP) and polyoxometalates (POM) are utilized to fabricate functional hybrid salts, denoted as [Zr‐MOP][POM], designed for pesticide detoxification. Upon light irradiation, [Zr‐MOP][POM] salts generate a charge‐separated state with oxidizing properties. This activated state selectively photooxidizes highly toxic and persistent fenamiphos into fenamiphos sulfoxide and subsequently hydrolytically breaks down it into non‐toxic 3‐methyl‐4‐(methylsulfinyl)phenol and phosphate components. The hydrolytic degradation of fenamiphos is facilitated by the reduced basicity of the 3‐methyl‐4‐(methylsulfinyl)phenolate residue (ΔpKb = ‐2.4) in the photooxidized form of the pesticide molecule. Furthermore, the hybrid salts demonstrate efficacy in the breakdown of multicomponent pesticide mixtures such as fenamiphos and methylparaoxon.

Impact of different spacers on the conjugation between Anderson-Evans polyoxometalates and peptides

The Anderson-Evans polyoxometalates (POM) display a promising anticancer activity. The conjugation with the GRP-receptor antagonist peptide Demobesin (fQWAVGHL-NHEt) was exploited to impart cell targeting capabilities and improve the selectivity of such polyanions. However, the POM interacts with the grafted peptides, inducing chains folding and self-assembly of the resulting hybrids, thus decreasing their recognition ability. Within this context, a tailored spacer, including two domains, i.e., a hydrophilic one (1,13-diamino-4,7,10-trioxatridecan-succinamic acid, Ttds) and a tetra-anionic one (Glu-Glu-Glu-Glu-βAla, EEEE-βA) was previously utilized to mitigate such interaction. In this work, hybrid POMs containing only Ttds or EEEE-βA were prepared and the contribution of the two spacers was separately studied by using 2D NMR, fluorimetry and circular dichroism (CD). Transmission electron microscopy (TEM) was also used to observe the impact of the different spacers on self-assembly. Owing to the relevant effects observed for EEEE-βA, MD calculations were finally performed to elucidate its behavior when incorporated in the hybrid POM. Our results show that, despite the stronger impact of EEEE-βA spacer, only when both spacer are present together it is possible to observe a significant effect on the retention of peptide's secondary structure and recognition capability.

Synthesis and oxidative desulfurization activity of vanadium-substituted Keggin-type polyoxometalate hybrid compounds

Synthesis and oxidative desulfurization activity of vanadium-substituted Keggin-type polyoxometalate hybrid compounds

An organic-inorganic hybrid based on bicapped Keggin polyoxometalate as a heterogeneous catalyst for MB removal via Fenton-like/photo-Fenton-like degradation or reduction

An organic-inorganic hybrid polyoxometalate (POM) [CuI(phen)2]4[PMoVI8VIV6O42{CuI(phen)}2]•H5O2 (CuPMoV) was explored as heterogeneous Fenton-like catalyst for methylene blue (MB) degradation in the absence/presence of visible light at neutral pH. CuPMoV was characterized by XRD, SEM, FT-IR and UV–vis DRS techniques. The effect of initial pH, the concentrations of H2O2 and dye, and CuPMoV dosage on the degradation was evaluated. The possible mechanism for the Fenton-like/photo-Fenton-like was proposed. Under the optimum conditions, the MB degradation rates were 85.3 % (Fenton-like reaction) and 95.2 % (photo-Fenton-like reaction) in 12 min, respectively. CuPMoV also exhibited excellent catalytic performance for MB reduction in the presence of NaBH4, with the reduction rate of 98.6 % in 90 s. The pseudo-first-order rate constant of the reduction was estimated as 0.0246 s−1. The high reusability and stability of CuPMoV enabled it to undergo five cycles of usage without significant loss of catalytic activity. The multi-metal composition of CuPMoV and the bicapped Keggin unit were the root of its excellent catalytic activities. CuPMoV may be a potential candidate for the practical treatment of MB contaminated wastewater via oxidation or reduction.

A New Avenue in Hybrid Polyoxometalate Chemistry: Synthesis and Characterization of Mn (II) Functionalized POM Based Ionic Liquids

The synthesis of novel hybrid materials, combining the properties of organic and inorganic components in one compound, results in materials with unique physical and chemical properties. The organic-functionalization of POMs with covalently or non-covalently linked organic species opens a large spectrum of possibilities and research challenges. The development of such novel systems for metal−organic architectures is an attractive research field. Lacunary polyoxometalates (POMs) offer structurally well-defined coordination sites with various coordination directions; thus, lacunary POMs are ideal building blocks for inorganic−organic architectures. Here, we report the successful self-assembly of transition metal substituted anionic POM along with a new generation POM based ionic liquid. We know that the counter-cations in polyoxometalate chemistry play immense role beyond charge balance. That is why the resultant structure is furthermore utilized as a stable and reactive building block to synthesize POM based ionic liquid (POM-IL), an inorganic−organic architecture comprising 3d-metal substituted mono-lacunary POM and tetrabutyl ammonium (TBA) ligands.

Supramolecular Self-Assembly of Proteins Promoted by Hybrid Polyoxometalates.

Controlling the formation of supramolecular protein assemblies and endowing them with new properties that can lead to novel functional materials is an important but challenging task. In this work, a new hybrid polyoxometalate is designed to induce controlled intermolecular bridging between biotin-binding proteins. Such bridging interactions lead to the formation of supramolecular protein assemblies incorporating metal-oxo clusters that go from several nanometers in diameter up to the micron range. Insights into the self-assembly process and the nature of the resulting biohybrid materials are obtained by a combination of Small Angle X-ray Scattering (SAXS), Transmission Electron Microscopy (TEM), and Dynamic Light Scattering (DLS), along with fluorescence, UV-vis, and Circular Dichroism (CD) spectroscopy. The formation of hybrid supramolecular assemblies is determined to be driven by biotin binding to the protein and electrostatic interactions between the anionic metal-oxo cluster and the protein, both of which also influence the stability of the resulting assemblies. As a result, the rate of formation, size, and stability of the supramolecular assemblies can be tuned by controlling the electrostatic interactions between the cluster and the protein (e.g., through varying the ionic strength of the solution), thereby paving the way toward biomaterials with tunable assembly and disassembly properties.

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.

Aryl selenonium vs. aryl sulfonium counterions in polyoxometalate chemistry: the impact of Se+ cationic centers on the photocatalytic reduction of dichromate.

A selenonium organic counter ion has been used in polyoxometalate chemistry to develop a new aryl selenonium polyoxometalate (POM) hybrid, and its photocatalytic properties have been explored in comparison with an aryl sulfonium POM-hybrid counterpart for the first time. The chalcogenonium counterions, namely, methyldiphenylsulfonium trifluoromethane sulfonate (MDPST) and methyldiphenylselenonium trifluoromethane sulfonate (MDPSeT), and their octamolybdate ([Mo8O26]4-) hybrids, 1 and 2, with the general formula (C13H13X)4[Mo8O26] (where X = S for 1 and Se for 2) were synthesized and characterized. Hybrids 1 and 2 vary in their chalcogenonium cationic center (S+vs. Se+), which enabled a direct comparison of their photocatalytic properties as a function of the cationic center. The photocatalytic activities of hybrids 1 and 2 were tested using the reduction of dichromate (Cr2O72-) as a model reaction under UV irradiation. A 99% photocatalytic reduction of Cr2O72- with a rate constant of 0.0305 min-1 was achieved with hybrid 2, while only a 67% reduction with a rate constant of 0.0062 min-1 was observed with hybrid 1 in 180 minutes. The better catalytic performance of hybrid 2 may be correlated to the larger atomic radii of Se than S, which helps in better stabilizing the photogenerated electron-hole (e--h+) pair on the POM cluster by polarizing its lone pair more efficiently compared to S. The catalytic recyclability was tested for up to 4 cycles using hybrid 2, and up to 98% reduction was obtained even after the 4th cycle. Recyclability tests and control experiments also indicated the generation of some elemental Se through possible cleavage of some C-Se bonds of MDPSe under prolonged UV exposure during catalysis, and the Se thus generated was found to contribute to the catalytic reduction of dichromate. This study, therefore, opens new avenues for aryl selenonium moieties and their POM hybrids for potential catalytic applications.

The effect of ionic versus covalent functionalization of polyoxometalate hybrid materials with coordinating subunits on their stability and interaction with DNA.

Inorganic-organic hybrid materials that combine both Polyoxometalates (POMs) and metal ion coordinating subunits (CSUs) represent promising multifunctional materials. Though their individual components are often biologically active, utilization of hybrid materials in bioassays significantly depends on the functionalization method and thus resulting stability of the system. Quite intriguingly, these aspects were very scarcely studied in hybrid materials based on the Wells-Dawson POM (WD POM) scaffold and remain unknown. We chose two model WD POM hybrid systems to establish how the functionalization mode (ionic vs. covalent) affects their stability in biological medium and interaction with nucleic acids. The synthetic scope and limitations of the covalent POM-terpyridine hybrids were demonstrated and compared with the ionic Complex-Decorated Surfactant Encapsulated-Clusters (CD-SECs) hybrids. The nature of POM and CSU binding can be utilized to modulate the stability of the hybrid and the extent of DNA binding. The above systems show potential to behave as model cargo-platforms for potential utilization in medicine and pharmacy.

Covalent organic-inorganic polyoxometalate hybrids in catalysis.

Polyoxometalates (POMs) are a class of compounds known for the vast range of tunable structures and properties available, leading to applications in areas such as catalysis, energy, and advanced medicine. The ability to covalently functionalize POMs with organic components has been investigated extensively to tune the physical and chemical properties of the resulting hybrid materials. These hybrids, where the organic entity is covalently attached to the POM-core ( Class II hybrid POMs) result in a vast library of promising customizable catalytic systems, displaying tunable properties with a high level of synergy between the polyanion and the organic component. A number of Class II hybrids have been investigated for a wide range of catalytic applications, and here, we give a brief overview of Class II hybrids of the p-block elements and their applications in catalysis.

Self-assembly of a unique triangle-like tungstovanadate containing pentagonal {(WO7)W3(SnR)2} cluster.

In aqueous solution, a novel triangle-like tungstovanadate estertin derivative K10H10.5[(W4O15(H2O)2){(SnCH2CH2COO)2(V0.75W10.75/V0.25O39)}{{(SnCH2CH2COO)2(μ-OH)}2(SnCH2CH2COO)(VW10O37)}2]·31H2O ((SnR)8-V3W35, R = CH2CH2COO) was assembled by a conventional synthetic method. (SnR)8-V3W35 is composed of one [VW11O39]7- ({VW11}) and two [VW10O37]9- ({VW10}) units connected by eight [Sn(CH2)2COO]2+ groups and a {W4O19} cluster. Interestingly, there exists a pentagonal bipyramid WO7 polyhedral center surrounded by two SnCO5 and three WO6 octahedra, forming a pentagonal {(WO7)W3(SnR)2} cluster in this polyoxometalate (POM), which is also the first example of a pentagonal structure formed by transition metals (TMs) and main group organometals in the POM family. Furthermore, the structure of this organic-inorganic hybrid POM also exhibits the largest number of organotin groups introduced into the POM system. It was characterized with various physico-chemical and spectroscopic methods, including X-ray single crystal and powder diffraction analysis, 119Sn and 51V NMR, IR, thermal gravimetric analysis (TGA), etc. In addition, the catalytic activity of (SnR)8-V3W35 as a mimic of peroxidase was evaluated using o-phenylenediamine (OPD) as a peroxidase substrate. The major factors influencing the oxidation reaction such as pH, the dosage of (SnR)8-V3W35, and concentrations of OPD and H2O2 were mainly studied. (SnR)8-V3W35 exhibits good peroxidase-like catalytic activity. From another perspective, the successful acquisition of (SnR)8-V3W35 further proves the instability and easy reassembly characteristics of TM-sandwich-type tungstovanadates, which also provides a new assembly strategy for synthesizing POM-estertin derivatives.

Highly luminescent soft aggregates and films assembled by amphiphilic polyoxometalate complex in a polymerizable aprotic ionic liquid

Hybrid co-assembly from lanthanide-containing polyoxometalates (POMs) and surfactants could efficiently optimize the formed aggregate luminescent properties and structural diversity. Meanwhile, compared to conventional solvents, aprotic ionic liquids (ILs) are not only excellent assembly media but also ideal solvents for luminescent lanthanide compounds. Therefore, in this work, the luminescent amphiphilic POM complex, namely surfactant-encapsulated POM hybrid (SEP) was initially prepared mainly through electrostatic interaction from polyanionic lanthanide-containing POM, Na9(EuW10O36)·32H2O (EuW10) and cationic surfactant, 1-octadecyl-3-methylimidazolium bromide ([C18mim]Br, OB). Then, highly luminescent lamellar micro-fiber aggregates were assembled by SEP in a polymerizable imidazolium-IL solvent, i.e. 1-vinyl-3-butyl imidazolium terafluoroborate ([Vbim]BF4). After morphology and structure characterizations for soft aggregates in detail, a lamellar aggregation arrangement mechanism was proposed. Further through in situ photopolymerization for such aggregate matrices, the flexible and organized films with enhanced luminescent properties were obtained. Our work indicates that the rational design of polymerizable organized aggregate matrices in ILs could greatly improve the photophysical properties of such luminescent soft materials and also expand their practical applications.

Investigations of switchable non-linear optical response inpyridyl containing porphyrin@Anderson polyoxometalate hybrid by Z-scan measurements

Investigations of switchable non-linear optical response inpyridyl containing porphyrin@Anderson polyoxometalate hybrid by Z-scan measurements

Accordion-like polyoxometalate hybrid architectures for capacity-dense and flexible Zn-Ion battery cathodes

Polyoxometalates (POMs) have received significant attention as functional materials owing to their tunable structures and sizes, versatile electronic properties, and multiple electrochemical redox behaviours. However, the hierarchical architecture of POMs and their unique Zn storage mechanism have yet to be explored for Zn-ion battery (ZIB) applications. Herein, we demonstrate an accordion-like hierarchical architecture of POM-based hybrids (L-rGO/PPy@POM) via a redox-relay Heck reaction, where multi-layered reduced graphene oxide (rGO)/polypyrrole (PPy) hybrid nanosheets are assembled with ultrafine POM nanoparticles through specific interaction. As confirmed through in situ and ex situ spectroscopic methods, the Mo=O bond is more implicated in Zn2+ insertion than the two Mo–O single bonds and two edge-sharing and corner-sharing Mo–Mo bonds. Owing to a fast and facile redox kinetics, the ZIB cells with l-rGO/PPy@POM delivered high specific capacity of 269.3 mAh g−1 at 100 mA g−1, high-rate capacity of 68.0 mAh g−1 at 10,000 mA g−1, and 80.4% of capacity retention over 22,000 cycles in water-in-bisalt electrolytes. Furthermore, flexible quasi-solid-state ZIB full cells with l-rGO/PPy@POM cathode achieved the maximum volumetric energy and power densities of 51.1 mWh cmcell−3 and 1159.9 mW cmcell−3, demonstrating long-term cyclic stability and functional operation of various electronic devices.