Polyoxometalate-based Hybrid Compound Research Articles

Formation, Reactivity, and Catalytic Behavior of a Keggin Polyoxometalate/Bipyridine Hybrid in the Epoxidation of Cyclooctene with H2O2.

The present study further explores the behavior of polyoxometalate-based hybrid compounds as catalysts for liquid-phase cyclooctene epoxidation with H2O2. Precisely, it unveils the nature of the relevant active species derived from the hybrid based on Keggin polyoxometalate (POM) and bipyridines (bpy) of formula (2,2'-Hbpy)3[PW12O40] (1). Whereas (i) it is generally accepted that the catalytic oxidation of organic substrates by H2O2 involving Keggin HPAs proceeds via an oxygen transfer route from a peroxo intermediate and (ii) the catalytically active peroxo species is commonly postulated to be the polyperoxotungstate {PO4[W(O)(O2)2]4}3- complex (PW4), we show that the studied epoxidation reaction seems to be more sophisticated than commonly reported. During the catalytic epoxidation, 1 underwent a partial transformation into two oxidized species, 2 and 3. Compound 3 corresponding to 2,2'-bipyridinium oxodiperoxotungstate of formula [WO(O2)2(2,2'-bpy)] was shown to be the main species responsible for the selective epoxidation of cyclooctene since 2 (in which the POM is associated with a protonated mono-N-oxide derivative of 2,2'-bpy of formula (2,2'-HbpyO)3[PW12O40]) exhibited no activity. The structures of 1, 2, and 3 were solved by single-crystal X-ray diffraction and were independently synthesized. The speciation of 1 was monitored under catalytic conditions by 1H and 1H DOSY NMR spectroscopies, where the formation in situ of 2 and 3 was revealed. A reaction mechanism is proposed that highlights the pivotal, yet often underestimated, role of H2O2 in the reached catalytic performances. The active species responsible for the oxygen transfer to cyclooctene is a hydroperoxide intermediate species that is formed by the interaction between the anionic structure of the catalyst and H2O2. The latter operates as a "conservative agent" whose presence in the catalytic system is required to prevent the catalysts from deactivating irreversibly.

Two bimetal-doped (Fe/Co, Mn) polyoxometalate-based hybrid compounds for visible-light-driven CO2 reduction.

Two polyoxometalate (POM)-based hybrid compounds have been successfully designed and constructed by the hydrothermal method with molecular formulas [K(H2O)2FeII0.33Co0.67(H2O)2(DAPSC)]2{[FeII0.33Co0.67(H2O)(DAPSC)]2[FeII0.33Co0.67(H2O)4]2[Na2FeIII4P4W32O120]}·21.5H2O (1), and [Na(H2O)2FeII0.33Mn0.67(H2O)2(DAPSC)]2{[FeII0.33Mn0.67(H2O)(DAPSC)]2[FeII0.33Mn0.67(H2O)4]2[Na2FeIII4P4W32O120(H2O)2]}·24H2O (2) (DAPSC = 2,6-diacetylpyridine bis-(semicarbazone)), respectively. Structural analysis revealed that 1 and 2 consisted of metal-organic complexes containing DAPSC ligands with dumbbell-type inorganic clusters, iron-cobalt (iron-manganese) and some other ions. By utilizing a combination of strongly reducing {P2W12} units and bimetal-doped centres the CO2 photoreduction catalytic capacity of 1 and 2 was improved. Notably, the photocatalytic performance of 1 was much better than that of 2. In CO2 photoreduction, 1 exhibited CO selectivity as high as 90.8%. Furthermore, for 1, the CO generation rate reached 6885.1 μmol g-1 h-1 at 8 h with 3 mg, and its better photocatalytic performance was presumably due to the introduction of cobalt and iron elements to give 1 a more appropriate energy band structure. Further recycling experiments indicated that 1 was a highly efficient CO2 photoreduction catalyst, which could still possess catalytic activity after several cycles.

Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate-Tris(bipyridine)ruthenium Hybrid Compounds.

The development of visible-light photocatalysts for the selective oxidative coupling of amines to imines is an area of great interest. Herein, four hybrid compounds based on polyoxometalate anions and tris(bipyridine)ruthenium cations, Ru(bpy)3[M6O19] (M = Mo, W) 1-2, [Ru(bpy)3]2[Mo8O26] 3, [Ru(bpy)3]2[W10O32] 4, are prepared and characterized by X-ray diffraction (single-crystal and powder), elemental analysis, energy-dispersive X-ray spectroscopy (EDS) analysis, infrared (IR) spectroscopy, and solid diffuse reflective spectroscopy. Single-crystal structural analysis indicates that polyoxometalate anions and tris(bipyridine)ruthenium cations interact with each other through extensive hydrogen bonds in these compounds. These hybrid species with strong visible-light-harvesting abilities and suitable photocatalytic energy potentials show excellent photocatalytic activity and selectivity for the oxidation of amines to imines at room temperature in air as an oxidant. Among them, compound 1 with the [Mo6O19]2- anion has the highest catalytic activity, which can swiftly convert >99.0% of benzylamine into N-benzylidenebenzylamine with a selectivity of 98.0% in 25 min illumination by a 10 W 445 nm light-emitting diode (LED). Its turnover frequency reaches 392 h-1, which is not only better than the homogeneous catalyst [Ru(bpy)3]Cl2 but also much superior to those achieved over most of reported heterogeneous catalysts. Moreover, it shows a wide generality for various aromatic amines, accompanied by the advantages of good recyclability and stability. The photocatalytic oxidation mechanism of amines to the corresponding imines over polyoxometalate-based hybrid compounds was fully investigated.

Exceptional structure flexibility and adaptive catalytic desulfurization of a cyclic decanuclear polyoxoalkoxyvanadate (Ⅲ)

Removal of notorious organic sulfur-containing compounds to produce low-sulfur-content diesel is of great significance in developing modern industry and addressing environmental protection, but the key challenge is to seek recyclable heterogeneous catalysts with high efficiency. Herein, an intelligent regulation structural polyoxometalate-based hybrid compound with a ten-membered ring cluster has been synthesized via facile solvothermal method, formulated as [VIII10(μ2-OCH3)20(CH3CO2)10] (V10). Single crystal diffraction shows that every two adjacent vanadium ions in V10 are connected by methoxides and acetate group to construct a ten-membered wheel ring. It is noteworthy that the center of the ring can exhibit different opening and closing states through the disordered methoxy groups with rotational/oscillatory flexibility without disrupting the main structure of the ring, being analogous to IRIS stop of an optical camera. Notably, sulfur oxidation experiments indicated that V10 as heterogeneous catalyst displayed excellent catalytic activity, selectivity and stability for the catalytic oxidation of dibenzothiophene (DBT) at mild condition, which outperforms most of reported vanadium oxygen clusters. Benefiting from the multi-modal transformation of the inner ring structure, the catalyst can adaptively adjust the position of methoxy groups to make the sulfides more accessible to the active site, thus facilitating the conversion of DBT up to >99% in 40 ​°C. This study affords a novel perspective for the development of polyoxovanadates with better performance.

Polyoxometalate based hybrid compound as a pre-catalyst for electrocatalytic water reduction at neutral pH

Polyoxometalate based hybrid compound as a pre-catalyst for electrocatalytic water reduction at neutral pH

Assembly and luminescent property of a new polyoxometalate-based hybrid compound showing entangled double helical and porous motifs

A new polyoxometalate (POM)-based compound, (H3O)2[CuII3(btb)6][H4W12O40]2∙2H2O (btb = 1,4-bis(1,2,4-triazol-1-yl)butane) was hydrothermally synthesized, and characterized by elemental analyses, FT-IR spectrum, thermogravimetry analysis, powder X-Ray diffraction and single X-ray diffraction. Compound 1 is an unusual example of POM-based hybrid that consists of both helical and porous structural features. Moreover, luminescent property of compound 1 is investigated in the solid state at room temperature. In contrast to free btpe ligand, compound 1 exhibits an enhanced luminescent property, attributed to an internal heavy metal effect.

Synthesis and PPy loading for enhanced visible-light photocatalytic activity of new POMOFs containing silver chains

A new polyoxometalate based hybrid compound containing Ag-Ag chain, [Ag29(trz)18][SiWV7WVI5O40] (Ag29SiW12), was synthesized by the simple one-step hydrothermal reaction of silver nitrate, 1, 2, 3- triazole (trz), and [SiW12O40]4- polyanion. Single crystal X-ray diffraction analysis shows that the [SiW12O40]4- polyanions as coordinating guests were successfully encapsulated into the metal-organic framework host matrix. The Keggin [SiW12O40]n- polyanions as templates and trz molecules as the small and delicate ligands play the decisive factors to the formation of silver chain. In addition, to improve the photocatalytic activity of the new compound Ag29SiW12, its polypyrrole (PPy) composite (PPy@Ag29SiW12) has been prepared and exhibits excellent photocatalytic activity (93.1% for MB and 48.8% for RhB) and selectivity adsorption (16.2 mg/g for MB and 1.60 mg/g for RhB) for organic dyes under the visible light radiation.

Isomeric organic ligand dominating polyoxometalate-based hybrid compounds: synthesis and as electrocatalysts and pH-sensitive probes

By introducing isomeric organic ligands into polyoxometalate (POM) systems, two new POM-based hybrid compounds, [Cu6(m-pyttz)2(H2O)][HPMo12O40] (1) and [Ag3(p-H2pyttz)(p-Hpyttz)Cl][H2PMo12O40]·6H2O (2) (m-/p-H2pyttz = 3-(pyrid-3/4-yl)-5(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl), have been hydrothermally synthesized and characterized. Single-crystal structural analysis shows the m-pyttz ligands link CuI ions to generate a two-dimensional layer with hanger-like rhombus, which is pillared by the PMo12 anions in 1. Compound 2 exhibits a three-dimensional supramolecular framework, in which PMo12 anions are building blocks facilitating the extension of the whole structure. The influence of the coordination modes of m-/p-H2pyttz on the structures is discussed in detail. Furthermore, electrochemical properties of 1 and 2 have been studied and they display excellent electrocatalytic activities toward the reduction of nitrite and hydrogen peroxide and pH-dependent electrochemical behaviors.

Two mixed-addenda Nb/W polyoxometalate-based hybrid compounds containing multicopper units: synthesis, structures, and electrochemical and magnetic properties.

Two mixed-addenda Nb/W polyoxometalate (POM)-based hybrid compounds, [CuICu(μ3-OH)(H2O)6(trz)3]2(PW9Nb3O40)·13H2O (1) and [CuICu(μ3-OH)(H2O)4(Htrz)(trz)3]2(PW9Nb3O40)·13H2O (2), were synthesized under hydrothermal conditions. Single-crystal X-ray diffraction revealed that compound 1 contains planar triangular tricopper {Cu(μ3-OH)(trz)3} units, and these tricopper units link to each other to form a 1D anti-parallel chain. CuI ions connect the 1D chains by the coordination of trz ligands to form 2D layers in the ab plane. Compound 2 contains the same tricopper {Cu(μ3-OH)(trz)3} units as well as bicopper {Cu(Htrz)2} units. The tricopper units also form 1D chains and the bicopper units connect the 1D chains to form similar 2D layers and finally construct the 3D framework. In both compounds, each Nb/W mixed-addenda Keggin {PW9Nb3O40} unit links eight tricopper units to generate the final 3D structure. Magnetism studies indicate anti-ferromagnetic exchange interactions among the CuII ions within the tricopper units, with JCu-Cu values of -209.3 and -200.8 cm-1 in 1 and 2, respectively.

Two Keggin polyoxometalate-based hybrid compounds with different helix: Syntheses, structure and catalytic activities

By adjusting pH value, two new Keggin polyoxometalate-based hybrid compounds with different helical structure, [Ag6(btp)(pyttz)6·(HSiMo12O40)2]·4H2O (1) and [Ag2(pyttz)4·(H2SiMo12O40)2]·(TMA)2·4H2O (2), (pyttz=3-(pyrid-4-yl)-5-(1H-1,2,4–3-yl)-1,2,4-triazolyl, btp=5,5′-di(pyridin-4-yl)-1H,1′H-3,3′-bi(1,2,4-triazole), TMA=tetramethylammonium), were successfully isolated by hydrothermal reaction and structurally characterized. Structural analysis shows the nature of the inorganic meso-helical channel in compound 1 and 2D homological helical layer in compound 2. It is also interesting that a new organic ligand btp is formed in situ by the coupling of N-Heterocycle in 1. In addition, the catalytic activities of 1 and 2 as solid-acid catalyst are explored, and the results indicate that 1 and 2 are a kind of highly-efficient and promising solid catalysts. Additionally, the proposed generation mechanism of btp in situ is also discussed.

“Pinwheel” subunit-directed polyoxometalate-based hybrid compound [Cu(bimb)2]2[β-Mo8O26] with both helixes and chiral layers

An unprecedented hybrid octamolybdate-based compound with both chiral and helical structure, consisting of a pair of enantiomorphous chiral “pinwheel” subunits, has been synthesized and its photocatalytic properties have been investigated.

Facile fabrication of self-assembly polyoxometalate-type hybrid material through supermolecular interactions

A novel kind of self-assembly polyoxometalate-type hybrid material, [TEAPS]8P2W16V2O62, bearing of sulfo-group grafted ammonium (TEAPS) and vanadium-substituted heteropoly anions has been prepared. This compound has a layered-shape structure in total and it can process self-assembly to nano-size particle and chrysanthemum-like assemblies through supermolecular interactions. Moreover, from cyclic voltammetry studies, this kind of polyoxometalate-based hybrid compounds exhibit unique electrochemical properties that the vanadium in the complexes can process reduction in anhydrous environment, which is unlikely in water solution as a simple hydrated ion because of the water protonation about the substrate.

Structural and catalytic performance of a polyoxometalate-based hybrid compound

By introducing long rigid ligands into polyoxometalate (POM) systems, a new POM-based hybrid compound with CDS topology, {[Cu(bpe)]2(H2SiMo12O40)}·(bpe)·2H2O (1) (bpe = bis(4-pyridyl)ethylene), has been hydrothermally synthesized and characterized by physical methods. Single-crystal X-ray diffraction analysis shows that the ladder-like 1-D structure of 1 is formed from Keggin clusters modified by the wavy ([Cu(bpe)]n) chains. Compound 1 contains two types of tunnels along the a axis and the [1–10] axis, which are occupied by isolated bpe molecules. Compound 1 bulk-modified carbon paste electrode (1-CPE) displays good electrocatalytic activity toward the reduction of nitrite and bromate, and photocatalytic activities indicate that 1 presents good degradation activity and may be a photocatalyst to decompose some organic dyes.

Structural and catalytic performance of a polyoxometalate-based hybrid compound

By introducing long rigid ligands into polyoxometalate (POM) systems, a new POM-based hybrid compound with CDS topology, {[Cu(bpe)]2(H2SiMo12O40)}·(bpe)·2H2O (1) (bpe = bis(4-pyridyl)ethylene), has been hydrothermally synthesized and characterized by physical methods. Single-crystal X-ray diffraction analysis shows that the ladder-like 1-D structure of 1 is formed from Keggin clusters modified by the wavy ([Cu(bpe)]n) chains. Compound 1 contains two types of tunnels along the a axis and the [1–10] axis, which are occupied by isolated bpe molecules. Compound 1 bulk-modified carbon paste electrode (1-CPE) displays good electrocatalytic activity toward the reduction of nitrite and bromate, and photocatalytic activities indicate that 1 presents good degradation activity and may be a photocatalyst to decompose some organic dyes.

Assembly of Six Polyoxometalate-Based Hybrid Compounds from a Simple Supramolecule to a Complicated Pseudorotaxane Framework via Tuning the pH of the Reaction Systems

Three pairs of inorganic–organic hybrid compounds respectively based on [Mo6O19]2–/[Mo8O26]4–, [PW12]3– and [P2W18]6– anions were obtained at different pH values under hydrothermal conditions, namely, [H2bib][Mo6O19] (1), CuII(bib)1.5(H2O)(β-Mo8O26)0.5 (2), [Hbib]2[HPW12O40] (3), [bib0.5][CuII(bib)3CuI(bib)0.5(PW12O40)] (4), [H2bib][CuII(bib)2.5(H2O)(H2P2W18O62)]·2H2O (5), and [CuIbib][(CuIbib)2(H2KP2W18O62)]·4H2O (6), (bib = 1,4-bis(1-imidazol-yl)-2,5-dimethylbenzene). Their structures have been determined by single-crystal X-ray diffraction analyses and characterized by infrared spectra (IR), elemental analyses, and powder X-ray diffraction (PXRD) patterns. Compounds 1 and 3 are zero-dimensional (0D) monomers, in which the protonated bib ligands as countercations combine with [Mo6O19]2–/[PW12O40]3– anions by the intermolecular hydrogen bonding interactions forming a supramolecular layer. In 2, the (4, 4) two-dimensional (2D) layer is formed by β-[Mo8O26]4– anions as double-dentate inorganic ligands to l...

Reversible Dehydration in Polyoxometalate-Based Hybrid Compounds: A Study of Single-Crystal to Single-Crystal Transformations in Keggin-Type Germanotungstates Decorated with Copper(II) Complexes of Tetradentate N-Donor Ligands

Hydrothermal reaction of the Keggin-type [α-GeW11O39]8– anion with copper(II) complexes of the tetradentate N-donor ligands N,N′-dimethyl-N,N′-bis-(pyridine-2-ylmethyl)-1,2-diaminoethane (bpmen) or N,N′-dimethyl-N,N′-bis-(pyridine-2-ylmethyl)-1,3-diaminopropane (bpmpn) results in three new hybrid compounds [Cu(bpmen)(H2O)][GeW12O40{Cu(bpmen)}]·3.5H2O (1), [GeW12O40{Cu(bpmpn)(H2O)}2]·3H2O (2), and [Cu(bpmpn)(H2O)][GeW12O40{Cu(bpmpn)}]·8H2O (3). All compounds have been characterized by elemental analyses and infrared spectroscopy, and their structures have been established by single-crystal X-ray diffraction. Both 1 and 3 contain monodecorated polyanions and additional complex cations, but their layered structures show noticeable differences. For 1, the polyanions are arranged in rows whose stacking generates intralamellar cavities where complex cations and lattice water molecules are hosted, whereas double-layers of hybrid anions pillared by the cations occupying the interlamellar space are observed for 3. In the case of 2, the packing of trans-didecorated clusters generates honeycomb-like metalorganic layers and perpendicular hexagonal channels where Keggin moieties are nested. A combination of thermogravimetric and variable temperature powder X-ray diffraction analyses show that dehydration of 1 and 2 results in thermally stable, crystalline phases. Dehydrations are fully reversible upon exposure to air and proceed via single-crystal to single-crystal transformations implying rupture/formation processes in the copper(II) bonding and consequent geometrical and conformational modifications in the complexes. Transformations have been followed by single-crystal X-ray diffraction, allowing for the structures of the anhydrous phases [GeW12O40{Cu(bpmen)}2] (1a) and [GeW12O40{Cu(bpmpn)}2] (2a) to be determined. Dehydration of 1 into 1a with consequent formation of neutral, didecorated clusters does not result in the collapse of the intralamellar cavities but rather in an open-framework structure with slit-shaped micropores. Structural variations between 1 and 2 and their corresponding anhydrous phases are reflected in the electron paramagnetic resonance spectra.

Hydrothermal Synthesis, Characterization and Electrocatalytic/Photocatalytic Activities of New Polyoxometalate Based Hybrid Compound

In this paper, we report the hydrothermal synthesis and physicochemical characterization of a complicated high-dimensional and high-connected POM-based hybrid compound, namely, [Ag3(pytz)2(H2O)]2.[HAgGeMo12O40]·H2O, which has been synthesized by reactions of pyridyltetrazoles and Keggin POMs. X-ray diffraction and crystallographic analyses reveals that the new compound exhibits POMs chains pillaring transition-metal complexes, which represents the highest coordination number of Keggin POMs up to date. Moreover, the new compound bulk-modified carbon-paste electrode (1-CPE) displays good electrocatalytic activity toward the reduction of nitrite, and the result of the photocatalytic activities indicates that the new compound presents better degradation activity and may be a potential photocatalyst to oxidative decomposition of rhodamine B dye in the presence of crystals [Ag3(pytz)2(H2O)]2·[HAgGeMo12O40]·H2O catalyst under ultraviolet-light irradiation.

Hydrothermal Synthesis and Crystal Structure of a Meso-Helical Chain Based on Lindqvist Polyoxometalates

A new Lindqvist polyoxometalate-based hybrid compound with a helical chain structure, [Cu(bipy)2][W6O19] (1), (bipy = 2;20`-bipyridine), has been hydrothermally synthesized and characterized by elemental analysis, infrared spectroscopy, thermogravimetry, and single-crystal X-ray diffraction. The compound crystallizes in the space group C2/c of the monoclinic system. In 1, the [W6O19]2- anions are connected alternately with [Cu(bipy)2]2+ subunits to form a meso-helical chain. To our knowledge, this structure represents the first example of a helical chain structure consisting of the hexatungstate anion [W6O19]2-. The electrochemical properties of the title compound have been studied.

Unusual in situ synthesis of an N-heterocycle ligand in the self-assembly of polyoxometalate-based hybrid compound

Abstract An unusual coordination compound, [Cu4(dmpz)2(mi)2H3PMoVI8MoV4O40]·6H2O (dmpz = 2,3-dimethylphenazine, mi = 2-methyl-1H-imidazole) (1), has been hydrothermally synthesized and characterized by elemental analysis, IR, XPS and single crystal X-ray diffraction. Compound 1 consists of dmpz and mi organic ligands, CuI cations and [H3PMoVI8MoV4O40]4 − polyoxoanions. Interestingly, the dmpz organic ligands are from in situ reaction of 1,2-diaminobenzene (dab) and 1,2-benzenedicarboxylic acid (bdc).

Application of Tetrazole-Functionalized Thioethers with Different Spacer Lengths in the Self-Assembly of Polyoxometalate-Based Hybrid Compounds

Three metal-organic networks based on Keggin-type polyoxometalates (POMs) have been hydrothermally synthesized by tuning the spacer lengths of bis(tetrazole)-functionalized thioether ligands and structurally characterized: [Cu(4)(bmtm)(4)][SiW(12)O(40)]·2H(2)O (1), [Cu(4)(bmte)(3.5)][SiW(12)O(40)] (2), and [Cu(4)(bmtp)(4)][SiW(12)O(40)] (3) [bmtm = 1,1'-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)methane, bmte = 1,2-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)ethane, and bmtp = 1,5-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)pentane]. The spacer lengths and sulfhydryl of bis(tetrazole)-functionalized thioether ligands play important roles in the final framework formation, as shown by X-ray diffraction analysis. In compound 1, with the connection of a N,S bridge of bmtm, two kinds of binuclear Cu(I) units are formed and linked to construct a one-dimensional (1D) chain. The [SiW(12)O(40)](4-) (SiW(12)) cluster provides four terminal O atoms linking four binuclear units to generate a two-dimensional layer with (8(3))(2)(8(5)·10) topology. In compound 2, centrosymmetric octameric moieties composed of two equivalent tetrameric Cu(I) units are bridged by bmte ligands to form a 1D chain. The SiW(12) clusters show an unusual (2,8)-connected mode to connect with the 1D chain and construct a four-connected three-dimensional (3D) network with 5(3)·6(2)·7 topology. Compound 3 exhibits a rare 3D host framework with a type of large cavity and two types of small windows. The SiW(12) clusters as templates are strongly cemented into the large cavities and completely encircled by small windows. Furthermore, the compound 2 bulk-modified carbon-paste electrode (2-CPE) displays good electrocatalytic activity toward the reduction of nitrite.