Classical Keggin Intercalated into Layered Double Hydroxides: Facile Preparation and Catalytic Efficiency in Knoevenagel Condensation Reactions

The family of polyoxometalate (POM) intercalated layered double hydroxide (LDH) composite materials has shown great promise for the design of functional materials with numerous applications. It is known that intercalation of the classical Keggin polyoxometalate (POM) of [PW₁₂O₄₀]^3? (PW₁₂) into layered double hydroxides (LDHs) is very unlikely to take place by conventional ion exchange methods due to spatial and geometrical restrictions. In this paper, such an intercalated compound of Mg_0.73Al_0.22(OH)₂ [PW₁₂O₄₀]_0.04?0.98 H₂O (Mg₃Al‐PW₁₂) has been successfully obtained by applying a spontaneous flocculation method. The Mg₃Al‐PW₁₂ has been fully characterized by using a wide range of methods (XRD, SEM, TEM, XPS, EDX, XPS, FT‐IR, NMR, BET). XRD patterns of Mg₃Al‐PW₁₂ exhibit no impurity phase usually observed next to the (003) diffraction peak. Subsequent application of the Mg₃Al‐PW₁₂ as catalyst in Knoevenagel condensation reactions of various aldehydes and ketones with Z‐CH₂‐Z′ type substrates (ethyl cyanoacetate and malononitrile) at 60 °C in mixed solvents (V_2‐propanol:V_water=2:1) demonstrated highly efficient catalytic activity. The synergistic effect between the acidic and basic sites of the Mg₃Al‐PW₁₂ composite proved to be crucial for the efficiency of the condensation reactions. Additionally, the Mg₃Al‐PW₁₂‐catalyzed Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate demonstrated the highest turnover number (TON) of 47 980 reported so far for this reaction.     

The First 3‐Connected SrSi2‐Type 3D Chiral Framework Constructed from {Ni6PW9} Building Units

A novel 3‐connected SrSi₂‐type 3D chiral framework constructed from hexa‐Ni^II‐cluster‐substituted polyoxometalate (POM) units [Ni(enMe)₂]₃[WO₄]₃[Ni₆(enMe)₃(OH)₃PW₉O₃₄]₂?9H₂O ( 1 ) (enMe=1,2‐diaminopropane) has been made from a hydrothermal synthetic method. This POM represents the first 3D framework based on {Ni₆PW₉} units and {WO₄} connectors.     

Synthesis and characterization of hybrid materials based on 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid and Dawson-type tungstophosphate K7[H4PW18O62]·18H2O and K6[P2W18O62]·13H2O

In this study, we synthesized hybrid materials using well-Dawson polyoxometalates (POMs), K₇[H₄PW₁₈O₆₂]·18H₂O or K₆[P₂W₁₈O₆₂]·13H₂O and a room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]). K, W, P and CHN elemental analysis showed that one mole of [H₄PW₁₈O₆₂]⁷⁻ reacts with 6 moles of BMIM⁺ and one mole of [P₂W₁₈O₆₂]⁶⁻ reacts with 4 moles of BMIM⁺ to form, respectively, K[BMIM]₆H₄PW₁₈O₆₂ and K₂[BMIM]₄P₂W₁₈O₆₂. X-ray diffraction illustrated amorphous structure of the hybrid materials. FT-IR spectra showed the presence of both 1-butyl-3-methylimidazolium cation and the Dawson anion. TG analysis displayed a relative thermal stability of the hybrid materials compared to the parents Dawson POMs. Cyclic voltammetry showed that the reduction peak potentials of the Dawson anion in the hybrid materials shift towards negative values and the shift is more pronounced for K[BMIM]₆H₄PW₁₈O₆₂ compared to K₂[BMIM]₄P₂W₁₈O₆₂. This was attributed to a decrease in the acidity of the Dawson POM anion in the hybrid material.     

Structural Characterization of the Complex between Hen Egg‐White Lysozyme and ZrIV‐Substituted Keggin Polyoxometalate as Artificial Protease

Successful co‐crystallization of a noncovalent complex between hen egg‐white lysozyme (HEWL) and the monomeric Zr^IV‐substituted Keggin polyoxometalate (POM) (Zr1 K1), (Et₂NH₂)₃[Zr(PW₁₁O₃₉)] ( 1 ), has been achieved, and its single‐crystal X‐ray structure has been determined. The dimeric Zr^IV‐substituted Keggin‐type polyoxometalate (Zr1 K2), (Et₂NH₂)₁₀[Zr(PW₁₁O₃₉)₂] ( 2 ), has been previously shown to exhibit remarkable selectivity towards HEWL hydrolysis. The reported X‐ray structure shows that the hydrolytically active Zr^IV‐substituted Keggin POM exists as a monomeric species. Prior to hydrolysis, this monomer interacts with HEWL in the vicinity of the previously identified cleavage sites found at Trp28‐Val29 and Asn44‐Arg45, through water‐mediated H‐bonding and electrostatic interactions. Three binding sites are observed at the interface of the negatively charged Keggin POM and the positively charged regions of HEWL at: 1) Gly16, Tyr20, and Arg21; 2) Asn44, Arg45, and Asn46; and 3) Arg128.     

Grafting of Secondary Diolamides onto [P2W15V3O62]9− Generates Hybrid Heteropoly Acids

The Dawson tungstovanadate [P₂W₁₅V₃O₆₂]^9? can be grafted to secondary diolamides. The electron‐withdrawing character of the polyanion increases the acidity of the amide proton, leading to an organo‐polyoxometalate, which can be used as a Brønsted organocatalyst. High‐field NMR and DFT modeling indicate that the amide proton stays on the nitrogen and that the exalted acidity derives from the interaction between the organic and inorganic parts of the organo‐polyoxometalate. The amide‐inserted vanadotungstates thus form a new family of (hybrid) heteropolyacids, offering new perspectives for the application of POM‐based catalysis in organic synthesis.     

Structural stability index of heteropoly- and isopoly-anions—III. Application to site-vacant fragments of keggin and dawson polyanions α-[PW12O

For some site-vacant species of Keggin and Dawson polyanions α-[PW₁₂O₄₀]^3? and α-[P₂W₁₈O₆₂]^6? containing hypothetic     

Polyplatinayne/polyoxometalate composite Langmuir‐Blodgett films: Preparation,structural characterization,and potential optoelectronic applications

A new family of organometallic/inorganic composite Langmuir‐Blodgett (LB) films consisting of rigid‐rod polyplatinayne polymer ([Pt‐T(OMe‐Fl)T]_∞ or [Pt‐T(F‐Fl)T]_∞, where the triplet bonds are abbreviated by T, and fluorene ring by Fl) as the π‐conjugated organometallic molecule, polyoxometalate (POM, POM = K₃PMo₁₂O₄₀, H₃PW₁₂O₄₀, or H₆P₂W₁₈O₆₂) of the Keggin and Dawson structures as the inorganic component, and dimethyldioctadecylammonium bromide (DODA) or a mixture of octadecanoic acid (OA) and docosanoic acid (DA) as the auxiliary film‐forming agent were prepared and characterized by π–A isotherms, UV–Vis absorption spectra, photoluminescence spectra, atomic force microscopy imaging, scanning tunneling microscopy, and low‐angle X‐ray diffraction. Our experimental results indicate that steady, even, and ordered Langmuir and LB films are formed in pure water and polyoxometalate subphases. It was shown that the POM molecules are probably embedded inside the polyplatinayne molecules in the LB film structure and they can quench the luminescence of the Pt polyyne. These Pt‐polyyne based LB films display interesting electric conductivity behavior. [Pt‐T(OMe‐Fl)T]_∞/DODA/HPW₁₂ monolayer film shows a good electrical conductivity, and the tunneling current amounts to ±100 nA when the voltage is set at ±8 V. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3193–3206, 2008     

Diphosphoryl-functionalized Polyoxometalates: Structurally and Electronically Tunable Hybrid Molecular Materials

Herein, we report the synthesis and characterization of a new class of hybrid Wells–Dawson polyoxometalate (POM) containing a diphosphoryl group (P₂O₆X) of the general formula [P₂W₁₇O₅₇(P₂O₆X)]⁶⁻ (X=O, NH, or CR¹R²). Modifying the bridging unit X was found to impact the redox potentials of the POM. The ease with which a range of α-functionalized diphosphonic acids (X=CR¹R²) can be prepared provides possibilities to access diverse functionalized hybrid POMs. Compared to existing phosphonate hybrid Wells–Dawson POMs, diphosphoryl-substituted POMs offer a wider tunable redox window and enhanced hydrolytic stability. This study provides a basis for the rational design and synthesis of next-generation hybrid Wells–Dawson POMs.     

A Mono‐Cobalt Substituted Wells–Dawson Phosphotungstate with an Antenna Ligand

A mono‐cobalt substituted Wells–Dawson polyoxometalate with an antenna ligand linked to the Co^II atom, was prepared by reaction of the mono‐vacant Wells–Dawson precursor [P₂W₁₇O₆₁]^10– with a imidazole‐cobalt complex by using the bench method. It was isolated as the imidazole salt: (HIm)₇H[P₂W₁₇O₆₁Co(Im)] · 4H₂O ( 1 ) (Im = imidazole). Compound 1 was characterized by elemental analysis, IR and UV/Vis spectroscopy, TG analysis, cyclic voltammetry and single‐crystal/powder X‐ray diffraction. This is the first example of the 3d transition metal mono‐substituted Wells–Dawson polyoxometalate with an antenna ligand.     

Pickering Emulsion Stabilized by Catalytic Polyoxometalate Nanoparticles: A New Effective Medium for Oxidation Reactions

Decyl‐, dodecyl‐, and tetradecyltrimethylammonium cations were combined with the catalytic polyoxometalate [PW₁₂O₄₀]^3? anion to give spherical and monodisperse nanoparticles that are able to stabilize emulsions in the presence of water and an aromatic solvent. This triphasic liquid/solid/liquid system, based on a catalytic surfactant, is particularly efficient as a reaction medium for epoxidation reactions that involve hydrogen peroxide. The reactions proceed at competitive rates with straightforward separation of the phases by centrifugation. Such catalytic “Pickering” emulsions combine the advantages of heterogeneous catalysis and biphasic catalysis without the drawbacks (e.g., catalyst leaching or separation time).     

Visible‐Light‐Induced Water Oxidation Mediated by a Mononuclear‐Cobalt(II)‐Substituted Silicotungstate

A mononuclear‐cobalt(II)‐substituted silicotungstate, K₁₀[Co(H₂O)₂(γ‐SiW₁₀O₃₅)₂] ? 23 H₂O (POM‐ 1 ), has been evaluated as a light‐driven water‐oxidation catalyst. With in situ photogenerated [Ru(bpy)₃]³⁺ (bpy=2,2′‐bipyridine) as the oxidant, quite high catalytic turnover number (TON; 313), turnover frequency (TOF; 3.2 s^?1), and quantum yield (Φ_QY; 27 %) for oxygen evolution at pH 9.0 were acquired. Comparison experiments with its structural analogues, namely [Ni(H₂O)₂(γ‐SiW₁₀O₃₅)₂]^10? (POM‐ 2 ) and [Mn(H₂O)₂(γ‐SiW₁₀O₃₅)₂]^10? (POM‐ 3 ), gave the conclusion that the cobalt center in POM‐ 1 is the active site. The hydrolytic stability of the title polyoxometalate (POM) was confirmed by extensive experiments, including UV/Vis spectroscopy, linear sweep voltammetry (LSV), and cathodic adsorption stripping analysis (CASA). As the [Ru(bpy)₃]²⁺/visible light/sodium persulfate system was introduced, a POM–photosensitizer complex formed within minutes before visible‐light irradiation. It was demonstrated that this complex functioned as the active species, which remained intact after the oxygen‐evolution reaction. Multiple experimental parameters were investigated and the catalytic activity was also compared with the well‐studied POM‐based water‐oxidation catalysts (i.e., [Co₄(H₂O)₂(α‐PW₉O₃₄)₂]^10? (Co₄‐POM) and [Co^IIICo^II(H₂O)W₁₁O₃₉]^7? (Co₂‐POM)) under optimum conditions.     

Multisensor system for determination of polyoxometalates containing vanadium at its different oxidation states

The electronic tongue (ET) multisensor system has been employed for the detection of metal-oxygen cluster anions (polyoxometalates) containing vanadium (IV/V) atoms. Sensitivity of a variety of potentiometric chemical sensors with plasticized polyvinyl chloride and chalcogenide glass membranes was evaluated with respect to vanadyl/vanadate ions, decavanadate and a series of Keggin-type polyoxometalates (POM) such as α-[SiW₁₁V^IVO₄₀]⁶⁻, α-[SiW₁₁V^VO₄₀]⁵⁻, α-[BW₁₁V^IVO₄₀]⁷⁻, α-[BW₁₁V^VO₄₀]⁶⁻, α-[PW₁₁V^IVO₄₀]⁵⁻ and α-[PW_12−nV_n^VO₄₀]⁽³⁺ⁿ⁾⁻ (n = 1, 2, 3). Sensor's responses to vanadium complexes were evaluated in the pH range of 2.4-6.5 and a set of sensors appropriate for detecting a variety of vanadium species was selected. Such sensor array was able to distinguish different vanadium complexes allowing their simultaneous quantification in binary (V(IV)/V(V)) mixtures. The vanillyl alcohol oxidation with α-[SiW₁₁V^VO₄₀]⁵⁻ was monitored using ET to evaluate the capacity of proposed analytic system to detect simultaneously V(IV)/V(V) in POM under dynamic equilibrium. ET was demonstrated to be a promising tool for the discrimination and quantification of vanadium-containing POMs at different oxidation states. In particular, such a system could represent a significant interest for the mechanistic studies of redox reactions with POMs.     

Determination of the stability constant of cobalt-substituted mono-lacunary Keggin-type polyoxometalate and its electrocatalytic water oxidation performance

The anion form of a cobalt-substituted mono-lacunary Keggin-type polyoxometalate (POM) complex in aqueous solution was identified as [Co(H₂O)(H₃PW₁₁O₃₉)]^2? and its stability constant was determined to be 5.19 × 10⁴ by spectrophotometry. Additionally, the electrocatalytic performances toward water oxidation of different concentrations of [Co(H₂O)(H₃PW₁₁O₃₉)]^2? were investigated in the neutral phosphate buffer solution, and it was found that [Co(H₂O)(H₃PW₁₁O₃₉)]^2? anion itself has little activity; the activity of [Co(H₂O)(H₃PW₁₁O₃₉)]^2? solution actually results from dissociated [Co(H₂O)₆]²⁺. This work provides a simple way to understand the origin of catalytic water oxidation activity of a POM-based complex.     

Organoboron‐Functionalization Enables the Hierarchical Assembly of Giant Polyoxometalate Nanocapsules

The aggregation of molecular metal oxides into larger superstructures can bridge the gap between molecular compounds and solid‐state materials. Here, we report that functionalization of polyoxotungstates with organo‐boron substituents leads to giant polyoxometalate‐based nanocapsules with dimensions of up to 4 nm. A “lock and key” mechanism enables the site‐specific anchoring of aromatic organo‐boronic acids to metal‐functionalized Dawson anions [M₃P₂W₁₅O₆₂]^9? (M=Ta^V or Nb^V), resulting in unique nanocapsules containing up to twelve POM units. Experimental and theoretical studies provide initial insights into the role of the organo‐boron moieties and the metal‐functionalized POMs for the assembly of the giant aggregates. The study therefore lays the foundations for the design of organo‐POM‐based functional nanostructures.     

Uranium(IV) polyoxotungstophosphates

Two tris(oxouranium)-substituted Keggin and Dawson sandwich-type tungstophosphate heteropolyanions Na₁₂[(UO)₃(H₂O)₆(PW₉O₃₄)₂]·21 H₂O (1) and Na₁₈[(UO)₃(H₂O)₆(P₂W₁₅O₅₆)₂]·27 H₂O (2) have been prepared by reaction of uranium sulphate with [PW₉O₃₄]⁹⁻ and [P₂W₁₅O₅₆]¹²⁻, respectively, in aqueous media at 4.7 pH. The products were characterized by elemental and thermal analyses, IR, UV-Vis spectroscopy and magnetical susceptibility. The results of these studies suggest that the compounds obtained from Keggin and Dawson trilacunary anions are 2∶3 sandwich-type complexes and both exhibit a square antiprismatic stereochemistry for uranium(IV) with retention of polyoxometallate parent structure.     

One‐pot synthesis of trifluoromethylated benzimidazolines catalyzed by phosphotungstic acid with a low catalyst loading

The inexpensive Keggin‐type polyoxometalate, i.e. H₃PW₁₂O₄₀ was found to be an effect catalyst for the condensation‐cyclization reaction of 1,2‐phenylenediamines and trifluoromethyl ketones to synthesize trifluoromethylated heterocycles, including benzimidazolines, benzoxazolines and benzothiazolines. Only 1 mol% of H₃PW₁₂O₄₀ was required in this work, and the synergistic effect of proton and polyanion was vital for the reaction. Significantly, the POM catalyst could be easily recovered by using a biphasic solvent system (H₂O/toluene, V/V = 1:5), and reused at least for four times without significant loss in activity.     

Electrotransport, sorption, and photochemical properties of nanocluster polyoxomolybdates with a toroidal structure

The electrotransport, sorption, and photochemical properties of polyoxometalate NH₄)₃₂[MO ₁₁₀ ^VI MO ₂₈ ^V O₄₁₆H₆(H₂O)₅₈(CH₃CO₂)₆] · xH₂O (x ?? 250) with a toroidal structure are studied. Conditions for studying these characteristics are selected on the basis of data on the stability of POM, in dependence on the concentration and acidity of the medium and the processes of thermal decomposition. A set of independent physical and chemical methods of analysis is used. The possibility of the complexation of toroidal POM with polymer molecules is established. The photostabilizing effect of this POM on polymer molecules is revealed.     

A series of heterometallic 3d-4f polyoxometalates as single-molecule magnets

Three sandwich-like [Ln₂Fe₂(B-α-FeW₉O₃₄)₂]^10? clusters (Ln₂Fe₄, Ln = Dy (1), Ho (2), and Y (3)) were obtained by reacting Na₉[B-α-SbW₉O₃₃], Ln₂O₃, FeCl₃·6H₂O and KH₂PO₄. The [B-α-FeW₉O₃₄]^11? units were formed via the in situ conversion of lacunary polyoxometalates (POM) [B-α-SbW₉O₃₃]^9? and the Ln³⁺ ions were generated from the slow dissolution of Ln₂O₃, both of which play important roles in the synthesis of Ln₂Fe₄. Ln₂Fe₄ is the first 3d-4f cluster assembled from d-metal heteroatom-containing POM. The Dy₂Fe₄ cluster exhibits single-molecule magnet properties with an 80 K energy barrier in an optimal DC field. Cyclic voltammetry tests and controlled-potential coulometry experiments show that the polyoxometalate Fe heteroatom in clusters 1–3 is also electrochemically active.     

Synthesis,Crystal Structure and Electrochemistry of Two Complexes （YLn）3＋[PW12O40]3-（L=Me2SO,Me2NCHO）

The reaction of α-H3[PW12O40] with Y（NO3）3 in the presence of DMF or DMSO leads to two complexes with the formulae {Y（DMSO）7}·PW12O40（1） and {[Y（DMF）7]2PW12O40}·PW12O40（2）. The crystal structures indicate that complex 1 consists of discrete [YLn]3＋ cations and α-Keggin heteropolyanions [PW12O40]3-, whereas, in complex 2, donor-acceptor interaction results in a cation-anion-cation triplet. In addition, the electrochemical behavior of the two complexes indicates the usual successive reduction processes of the W atoms in the anions.     

Synthesis and Disassembly/Reassembly of Giant Ring‐Shaped Polyoxotungstate Oligomers

The disassembly and reassembly of giant molecules are essential processes in controlling the structure and function of biological and artificial systems. In this work, the disassembly and reassembly of a giant ring‐shaped polyoxometalate (POM) without isomerization of the monomeric units is reported. The reaction of a hexavacant lacunary POM that is soluble in organic solvents, [P₂W₁₂O₄₈]^14?, with manganese cations gave the giant ring‐shaped POM [{γ‐P₂W₁₂O₄₈Mn₄(C₅H₇O₂)₂(CH₃CO₂)}₆]^42?. This POM is a hexamer of manganese‐substituted {P₂W₁₂O₄₈Mn₄} units, and its inner cavity was larger than any of those previously reported for ring‐shaped polyoxotungstates. It was disassembled into monomeric units in acetonitrile, and the removal of the capping organic ligands on the manganese cations led to reassembly into a tetrameric ring‐shaped POM, [{γ‐P₂W₁₂O₄₈Mn₄(H₂O)₆}₄(H₂O)₄]^24?.