Keggin-type POMs Research Articles

Switchable Redox and Thermo-Responsive Supramolecular Polymers Based on Cyclodextrin-Polyoxometalate Tandem.

Supramolecular polymers built from stimuli-responsive host-guest interactions represent an attractive way of tailoring smart materials. Herein, we exploit the chaotropic effect of polyoxometalates and related host-guest properties to design unconventional polymer systems with reversible redox and thermo-responsive sol-gel transition. These supramolecular networks result from the association of cyclodextrin-based oligomers and Keggin-type POMs acting as electro-active crosslinking agents. The structure and the dynamics of such self-assembly systems have been investigated using a multiscale approach involving MALDI-TOF, viscosity measurements, cyclic voltammetry, 1H-NMR (1D and DOSY), and Small-Angle X-ray Scattering. Our results reveal that the chaotropic effect corresponds to a powerful and efficient force that can be used to induce responsiveness in hybrid supramolecular oligomeric systems.

Synthesis and Properties of Preyssler-type Polyoxotungstates Formed by the Interaction Between Polyoxometalate and Lanthanide Metals

POMs modified with lanthanide metals combine the advantages of POMs and lanthanide metals, and show promising properties in the fields of luminescence, magnetism, catalysis, etc. Comparing with the classical Keggin-type POMs, the Preyssler-type POMs with larger size exhibit a rich spatial structure evolution, thereby offering potential for the synthesis of polyoxometalates with excellent properties. In this paper, the Preyssler-type POMs K12.5Na1.5[NaP5W30O110] (P5W30) were selected as the research object, five polyoxometalates KNa{[Tb4(H2O)32](NaP5W30O110)}·24H2O(1), KNa{[Eu4(H2O)32](NaP5W30O110)}·19H2O(2), KNa{[La4(H2O)24](NaP5W30O110)}·22H2O(3), KNa{[Gd4(H2O)30](NaP5W30O110)}·20H2O(4), KNa{[Sm4(H2O)24](NaP5W30O110)}·21H2O(5) were designed and synthesized with different lanthanide metals by hydrothermal synthesis method. The effects of pH value, stoichiometric ratio and reaction temperature on the synthesis were summarized. The crystal structures of five polyoxometalates were described, and the coordination environment of lanthanide metals was analyzed. PXRD, infrared spectra, thermogravimetric and fluorescence properties of these compounds were studied.

Five new multifunctional inorganic–organic hybrids based on Keggin-type polyoxometalates and a 3,5-di(1H-imidazol-1-yl)benzoic acid ligand

The tripodal ligand 3,5-di(1H-imidazol-1-yl)benzoic acid was used to synthesize five new inorganic–organic hybrid materials based on Keggin-type POMs, which is of great significance in the field of environmental monitoring.

One-step fabrication of TiO2@polyoxometalates@α-Fe2O3 one-dimensional tandem heterojunctions for conductometric acetone sensors

Herein, we provide a new strategy to construct TiO2 @POMs@ α-Fe2O3 one-dimensional tandem heterojunctions materials utilizing one-step coaxial electrospinning. Three series of TiO2 @POMs@ α-Fe2O3 core-middle-shell nanoribbons with different Keggin-type POMs are synthesized. The tandem heterogeneous interfaces between the three layers are continuously distributed in a one-dimensional line along the nanoribbons. Gas sensing performances of the nanoribbons are investigated. The gas sensors show significantly improved sensitivity and selectivity to acetone compared with both composite nanoribbons and POMs-free nanoribbons. The enhancement can be due to the addition of POMs electron acceptors, the construction of tandem heterojunctions and the one-dimensional nano-structure, which can together accelerate electrons transfer and reduce their recombination. Besides, the effects of different kinds and contents of POMs on gas sensing performance are studied. The results show that the response value of the optimal sensor to 100 ppm acetone can reach 14.81, which is 2.9 times of that of TiO2 @ α-Fe2O3 double-layered nanoribbons. This work provides a novel strategy for developing high performance gas sensors by constructing tandem heterojunctions as well as introducing POMs electron acceptor, and offers new insight into the development of POMs-based gas sensors.

Two silver–containing polyoxometalate–based inorganic–organic hybrids as heterogeneous bifunctional catalysts for construction of C–C bonds and decontamination of sulfur mustard simulant

The construction of C–C bonds and degradation of chemical warfare agents (CWAs) are two central organic conversions for future sustainable development. Herein, two remarkable silver-containing polyoxomolybdates-based hybrids (Ag-POMos), [Ag(L)(H2O)2]2[HPMo12O40] (1) and [Ag2(L)2]2[HPMo10V2O40]·4DMSO (2) (L ​= ​4,6-bis(2-pyridyl)-2-aminopyrimidine) were designed and prepared by using silver cation, polyoxomolybdates and multiple N-donors ligand (L). The structure of compound 1 is constructed from an independent Ag center and a keggin-type POMs; compound 2 is composed of two independent Ag centers and a keggin-type POMs, which displays intriguing POM-incorporated 1-D chain structure with hydrogen bonding. Additionally, Ag-POMos catalysts can be severed as the efficient bifunctional catalysts for the three-component Mannich reaction to achieve the construction of C–C bonds and decontamination of sulfur mustard 2-chloroethyl ethyl sulfide (CEES). As bifunctional heterogeneous catalyst, particularly compound 2, exhibits satisfying catalytic performances in the Mannich reaction (conv. 99%; sele. 99%) and the oxidation of CEES (conv. 96%; sele. 96%). Notedly, after three runs, compound 2 maintains excellent catalytic properties, demonstrating its remarkable cycling stability.

A sensitive colorimetric sensor for glutathione on the basis of the oxidase-like activity of polyoxometalate-based helical compound and its nanocomposite with SWNT-COOH

Here, a new Keggin-type POMs based supramolecular compound with the 4-fold meso-helixes, [PW12O40]4MI·3H2O (PWMI) (MI = 2-methylimizole), was successfully synthesized and characterized by routine method. As one new kind of nanozymes, compound PWMI was found to possess oxidase-like activity for the first time, namely, it can effectively catalyze the oxidation of the substrate 3,3′,5,5′-tetramethylbenzidine (TMB). With functionalized single-walled carbon nanotubes (SWNTs-COOH) introduced into PWMI, the oxidase-like activity of nanocomposite PWMI/SWNTs-COOH was further enhanced. More importantly, the PWMI/SWNTs-COOH-based colorimetric sensor exhibits the fastest response time (30 s) among all reported nanozymes for glutathione (GSH) detection to date, to the best our knowledges, and the limit of detection (LOD) as low as 0.017 μM, and the linear range as wide as 5 to 150 μM.

Basicity of isostructural porous ionic crystals composed of Nb/Ta-substituted Keggin-type polyoxotungstates

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW12O40]5−, [SiW11NbO40]5−, and [SiW11TaO40]5− ) are synthesized.

Two POM-based compounds containing Zn-capped Keggin anions as decent heterogeneous catalysts for sulfur oxidation and CO2 cycloaddition reactions.

Carbon dioxide (CO2) and the combustion of sulfide in gasoline are the main causes of air pollution. A great deal of attention has been paid to solving the problem and the catalytic reaction seems to be a decent choice. Due to the high-density of Lewis acidic active sites, polyoxometalates are undoubtedly an ideal choice for the sulfur oxidation reaction. With the reasons foregoing, two novel Zn-capped polyoxometalate-based organic-inorganic hybrids, {[α-PMoV2MoVI10O39(OH)Zn2][bbbm]3}·0.5C2H5OH (1) and TBA2{[ε-PMoV8MoVI4O37(OH)3Zn4][phim]3} (2) ((where bbbm = 1-(4-imidazol-1-ylbutyl) imidazole) and phim = 2-phenylimidazole) were successfully obtained by hydrothermal synthesis. In the two compounds, the N-donor ligands in a monodentate or bidentate coordination mode are directly connected to the Keggin anions by Zn-capped atoms, forming an extended one-dimensional chain. It is noteworthy that compound 2 ends up with an interesting spiral infinite chain possibly thanks to the TBA+ cations residing in gaps as structure-directing agents. Simultaneously, the catalytic properties indicate that compounds 1 and2 as efficient heterogeneous catalysts display a decent catalytic activity in the sulfur removal process. Especially, 2 enabled satisfying catalytic oxidation of dibenzothiophene (DBT) to produce more valuable dibenzothiophene sulfone (DBTO2) at 55 °C, and the conversion almost reached 99%. Besides, compound 2 also shows satisfactory catalytic effectiveness in the oxidation of various epoxides in the CO2 cycloaddition reaction, which suggests that compound 2 has the potential to function as a dual functional material with tremendous prospects in sulfur oxidation and carbon dioxide cycloaddition for the first time.

Keggin-type polyoxometalates as Cu(II) chelators in the context of Alzheimer's disease.

Two Keggin polyoxometalates were used as new copper ligands to counteract the effects of CuII(Amyloid-β) interaction. Their ability to remove CuII from CuII(Amyloid-β), to stop CuII(Amyloid-β) induced formation of reactive oxygen species and to restore apo-like self-assembly of CuII(Amyloid-β) was shown.

Roles of Organic Fragments in Redirecting Crystal/Molecular Structures of Inorganic-Organic Hybrids Based on Lacunary Keggin-Type Polyoxometalates.

Lacunary polyoxometalates (LPOMs) are key precursors for the synthesis of functional POMs. To date, reviews dedicated to behavioral studies of LPOMs often comprise the role of metal ions, including transition metal (TM) and rare earth (RE) ions, in extending and stability of high-nuclearity clusters. In contrast, the role of organic ligands in the structures and properties of lacunary-based hybrids has remained less explored. In this review, we focus on the role of organic fragments in the self-assembling process of POM-based architectures and discuss relationships between the nature and structure of organic ligand and properties such as the topology of hybrid inorganic–organic material in RE and TM-RE heterometallic derivatives of lacunary Keggin-type POMs. The effects of organic fragment in mixed ligand hybrids are also briefly reviewed.

A new polycatenated framework encapsulated Keggin-type silicotungstate crystalline compound with bifunctional electrochemical performances

A new polycatenated framework encapsulated Keggin-type silicotungstate crystalline compound, L0.5[Cu2L3.5(SiW12O40)] (1) (L = 1,4-bis(1-imidazol-yl)-2,5-dimethyl benzene), has been prepared by solvothermal conditions. A new 3D polycatenaned framework was obtained by the 1D track chains and 2D grid layers catenating with each other, and the Keggin-type POMs are encapsulated in host framework. Compound 1 shows a rare case of 3D polycatenated framework encapsulated Keggin-type polyoxometalate. The electrocatalytic efficiency is ca. 179.2% containing 10 mM NO2−. Meanwhile, 1-GCE shows higher specific capacitance of 159.2 F g−1 (3 A g−1) than the majority of the reported POM-based crystalline compounds.

Photocurrent generation from visible light irradiation of covalent polyoxometalate–porphyrin copolymers

Four hybrid polyoxometalate–porphyrin copolymer films were obtained by the electrooxidation of zinc-β-octaethylporphyrin (ZnOEP) or 5,15-ditolylporphyrin (H2T2P) in the presence of organosilyl functionalized Keggin-type POMs TBA3[PW11Si2O40C26H16N2] (abbreviated Py-PW11Si2-Py) and Dawson-type POMs TBA6[P2W17Si2O62C26H16N2] (abbreviated Py-P2W17Si2-Py) bearing two remote pyridyl groups. The electropolymerization process of the four copolymers was monitored by EQCM. The obtained copolymers were characterized by UV/Vis spectroscopy, X-ray photoelectron spectroscopy, electrochemistry, and AFM. Their impedance properties (EIS) were studied and their photovoltaic performances were also investigated by photocurrent transient measurements under visible light irradiation. These studies showed a correlation between impedance and photovoltaic performances, the films based on Dawson type POMs and Zn porphyrins giving the best results. This last system displayed one of the best photocurrent efficiency for a reported POM photosensitized hybrid.

Surface-grafted Europium and Erbium Complexes of the 12-Tungstosilicate Heteropolyoxometalate: A Synthetic and Structural Investigations

The hydrothermal synthesis of two new inorganic-organic hybrid materials in which PDA (PDA = 1,10-phenanthroline-2,9-dicarboxylate) is connected to a well-defined Keggin-type polyoxometalate anion (POM) and a Ln(III) cation (POM = [SiW12O40]4−; Ln = Eu3+ (1), Er3+ (2)), are reported. These hybrids are characterized by elemental analyses, IR spectra and single-crystal X-ray diffraction. Single-crystal X-ray analysis reveals that both compounds have been assembled by hydrogen bonding and anion-π interactions among POM, PDA and lattice water units into a three-dimensional (3D) supramolecular structure. Compound 1 is composed of two different cationic layers of Eu(III) clusters with the POMs located in cavities that are created by the Eu-PDA groups. Compound 2 consists of [SiW12O40]4– Keggin-type POMs, where a single {W3O13} triad is decorated by a trinuclear Er(III)-complex.

Synergistic Effect of POMCPs and PPy for Enhancing Visible-Light Photocatalytic Activity and High Quantum Yields

A new Keggin-type POMs based Cu-bib (bib = 1,4-bis(imidazol-1-ylmethyl)benzene) coordination polymer with appropriate surface areas and rich active sites, [Cu(bib)][H3PW12O40]·(Hbib)2·2H2O (Cu(bib)PW12), was successfully synthesized and structurally characterized. To improve the photocatalytic activity of Cu(bib)PW12, polypyrrole (PPy) was loaded on its surface through a facile in situ oxidation polymerization process. Under the irradiation of visible light, the composites (Cu(bib)PW12/PPy (A and B) exhibited excellent photocatalytic activities for the degradation of methylene blue than pure Cu(bib)PW12 crystal, PPy and their mechanically mixing (Cu(bib)PW12/PPy (A′ and B′), respectively. Catalytic mechanism study indicates that the enhancement of photocatalytic activities of Cu(bib)PW12/PPy can be attributed to the higher separation efficiency of the photogenerated electron–hole pair on the interface of PPy and Cu(bib)PW12. These results suggest that loading PPy onto the surface of POMCPs would be a feasible strategy to enhance the photocatalytic activity of POMCPs.

Polyoxometalates as bifunctional templates: engineering metal oxides with mesopores and reactive surfaces for catalysis

For the first time, Keggin type POMs were found to be excellent bifunctional templates for generating not only mesopores but also reactive surfaces in metal oxides.

Polyoxometalate-Based Metal-Organic Frameworks as Visible-Light-Induced Photocatalysts.

Two stable 3D polyoxometalate-based metal-organic frameworks (PMOFs), [CuI12(trz)8(H2O)2][α-SiW12O40]·2H2O (1) and [CuI12(trz)8Cl][α-PW12O40] (2) (Htrz = 1- H-1,2,4-triazole) based on Keggin-type POMs were successfully obtained and fully characterized. The basic building units of the two PMOFs are [CuI12(trz)8], but polyoxoanion (POA) template effect leads to different structures and properties: 1 represents an interesting example that [α-SiW12O40]4- locate in the nine-membered Cu-trz rings through Cu···O weak interactions to form a 3D framework, whereas 2 shows a 3D structure constructed from 2D bilayer cationic network [CuI12(trz)8Cl]3+ and [α-PW12O40]3- lying in the adjacent layers via Cu···O weak interactions. PMOF 1 as unusual visible-light photocatalyst exhibit significantly enhanced photocatalytic activity under visible-light and excellent stability during the photocatalysis process for recovering and recycling, as well as photocatalytic hydrogen evolution activity.

Cu-containing Keggin-type polyoxometalates-based organic-inorganic hybrids with double electro-catalytic behaviors

Four new organic-inorganic hybrids consisting of Keggin-type polyoxometalates: [Cu5(bimpy)5(α-BW12O40)]·4H2O (1), [Cu4(bimpy)4(α-SiW12O40)]·2H2O (2), [Cu4(bimpy)4(α-HPMo12O40)2]·2H2O (3), [Cu2(bimpy)4(H2O)2(α-HPW12O40)2]·8H2O (4) (bimpy = 2,5-bis(1H-imidazol-1-yl)pyridine), have been hydrothermally synthesized. Compounds 1–4 are constructed from Cu/bimpy segments modified different types of Keggin POMs. The 1D double chains of compound 1 are featured by {-Cu/bimpy-POM-Cu/bimpy-}n chains and {-Cu-bimpy-Cu-}n metal-organic chains; compound 2 with 1D “ladder-like” structure stemmed from {-Cu-bimpy-Cu-}n wave-like chains and α-SiW12 clusters; In compound 3, [Cu4(bimpy)4]4+ motifs are linked by α-PMo12 clusters to give rise to a (3,4)-connected two-dimensional architecture with the (83)(86) topology, while compound 4 has a (3,4,5)-connected 3D framework with the (42,6)(42,6,83)(42,65,83) topology. Cyclic voltammetries of compounds 1–4 show discrepant double electro-catalytic properties for reduction of nitrite and oxidation of ascorbic acid owing to variant Keggin-type POMs and Cu/bimpy complexes.

MoO2@Cu@C Composites Prepared by Using Polyoxometalates@Metal-Organic Frameworks as Template for All-Solid-State Flexible Supercapacitor

We have synthesized a composite of MoO2@Cu@CusingathermolysistemplateofPOM@MOFcrystal(POM=polyoxometalates, MOF=Metal⿿organic frameworks), in which the Keggin-type Mo-based POMs with negative charges exist as secondary building units of Cu-based MOF. In addition, the material is confirmed to exhibit an excellent capacity of 28.33mAhg⿿1 at a current density of 1Ag⿿1, which is much higher than MoO2-based supercapacitors reported before. Moreover, we have fabricated a symmetrical flexible all-solid-state device with the MoO2@Cu@C electrode using PVA-KOH gel polymer as electrolyte and separator, and it shows outstanding sTable performance retaining 91% of its highest charge capacity maintained after 5000 cycles. The present study displayed a novel thermolysis template in the term of POM@MOF and provided an accessible foundation for investigations of MoO2-based supercapacitors.

Can tetrazole-functionalized ligands realize the role-control of Keggin-type POMs in hybrid frameworks?

Two tetrazole-functionalized flexible ligands, bis(1-methyl-1H-tetrazol-5-yl)sulfane (bmps) and 5,5′-((2,2-bis(((1-methyl-1H-tetrazol-5-yl)thio)methyl)propane-1,3-diyl)bis(sulfanediyl))bis(1-methyl-1H-tetrazole) (bpbb), have been designed and used for the role-control of Keggin-type POMs in hybrid frameworks during self-assembly processes. With the isolation of two pairs of new compounds [Cu2(bmps)2(H2O)3(PMo12O40)]·(OH)·2.5H2O (1) and [Ag3(bmps)3(PMo12O40)]·H2O (3); [Cu4(bpbb)2Cl(PW12O40)] (2) and [Ag3(bpbb)(H2O)2(PW12O40)]·4H2O (4), we demonstrate here that it is feasible to achieve the role-control of POMs under hydrothermal conditions. In 1 and 3, the use of the simple and short bmps ligands favours the assembly of discrete and/or low-dimensional metal–organic moieties, and the POMs are utilised as building units to complete the construction of the whole structures. For 2 and 4, the large bpbb ligands facilitate the formation of porous metal–organic networks, in which the POMs are utilised as templates.

K7[CoIIICoII(H2O)W11O39]: a molecular mixed-valence Keggin polyoxometalate catalyst of high stability and efficiency for visible light-driven water oxidation

Water-soluble K7[CoIIICoII(H2O)W11O39] (1) was tested as the first cobalt-containing Keggin polyoxometalate catalyst for efficient O2 production via both visible-light driven and thermal water oxidation. Under the optimal photocatalytic conditions [photoirradiation at λ ≥ 420 nm, [Ru(bpy)3]Cl2 as the photosensor, Na2S2O8 as the oxidant in borate buffer (pH = 9.0)],the turnover number (TON) can reach as high as 360, the initial quantum yield and the initial turnover frequency (TOF) for the first 60 seconds was 27% and 0.5 s−1, respectively. Variables of the photocatalytic reaction, including catalyst concentrations, buffer types and concentrations, pHs, dye concentrations, oxidant concentrations, etc., were systemically studied. The oxygen atoms of the evolved oxygen came from water, as confirmed by isotope-labeled experiments. In the thermal water oxidation, the TON and oxygen yield were measured to be 15 and 60%, respectively. The stability of 1 was tested and confirmed with multiple experiments (laser flash photolysis, DLS, CV, FT-IR, EDX, and catalyst recycling) within the photocatalytic water oxidation duration, which ruled out the possibility that neither the free Co2+ ions were present in the reaction solution nor were cobalt oxide/hydroxide nanoparticles in situ formed from the assumed decomposition of 1. All the evidence stated here collectively supports that 1 is the true molecular catalyst. In addition, the recycled sample was reused for water oxidation catalysis and showed similar catalytic behaviors (kinetics and activity) to that of the freshly prepared catalyst. No insoluble forms where found when the borate buffer solution of 1 was aged for 2 months, whereas its analogue, K8[CoIICoII(H2O)W11O39] (4), is rapidly decomposed to a blue-purple cobalt oxide precipitate in borate buffer. The stability difference between 1 and 4 indicates that the +3 oxidation state of the central cobalt in 1 plays a vital role in maintaining the structural integrity. A series of other Keggin-type POMs, such as K6[CoIIW12O40] (2), K5[CoIIIW12O40] (3), K6[SiCoII(H2O)W11O39] (5), and K5[PCoII(H2O)W11O39] (6), were also evaluated for their catalytic activity by comparison with 1. Our study demonstrates that the unique structural features of the mixed-valent central and peripheral cobalt atoms are essential for 1 to maintain both catalytic stability and efficiency.