Polyimide/polyoxometalate copolymer thin films: synthesis,thermal and dielectric properties

A mono‐lancunary keggin‐type decatungstosilicate (SiW₁₁) polyoxometalate (POM) modified by γ‐aminopropyltriethoxysilane (KH550) was incorporated into polyimide (PI) through copolymerization. Nuclear magnetic resonance (NMR), fourier transition infrared spectroscopy (FTIR), and wide angle X‐ray diffraction (WAXD) were used to characterize the structure and composition of the polyoxometalate–organosilane hybrid (SiW₁₁KH550) and PI/SiW₁₁KH550 copolymers. The differential scanning calorimetry (DSC) studies indicate that the glass transition temperature (T_g) of PI/SiW₁₁KH550 copolymers increases from 330°C (for neat PI) to 409°C (for the copolymer sample with 10 wt% of SiW₁₁KH550). Dielectric measurement showed that both the dielectric constant and the dielectric loss for the copolymer thin films decreased with the increase in SiW₁₁KH550 content, and the dielectric constant and dielectric loss values decreased to 2.1 and 3.54 × 10^?3, respectively, for the copolymer sample with 10 wt% of SiW₁₁KH550. The incorporation of SiW₁₁KH550 into polymer matrices is a promising approach to prepare PI films with a low dielectric constant and low dielectric loss. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

Preparation,characterization, and electrical properties of dual‐emissive Langmuir‐Blodgett films of some europium‐substituted polyoxometalates and a platinum polyyne polymer

A new series of organometallic/inorganic composite Langmuir‐Blodgett (LB) films consisting of a rigid‐rod polyplatinyne polymer coordinated with 2,7‐bis(buta‐1,3‐diynyl)‐9,9‐dihexylfluorene (denoted as PtP) as the π‐conjugated organometallic molecule, an europium‐substituted polyoxometalate (POM; POM = Na₉EuW₁₀O₃₆, K₁₃[Eu(SiW₁₁O₃₉)₂] and K₅[Eu(SiW₁₁O₃₉)(H₂O)₂]) as the inorganic component, and an amphiphilic behenic acid (BA) as the auxiliary film‐forming agent were prepared. Structural and photophysical characterization of these LB films were achieved by π–A isotherms, absorption and photoluminescence spectra, atomic force microscopy imaging, scanning tunneling microscopy, and low‐angle X‐ray diffraction. Our experimental results indicate that stable, well‐defined, and well‐organized Langmuir and LB films are formed in pure water and POM subphases, and the presence of Eu‐based POM in the subphase causes an area expansion. It is proposed that a lamellar layered structure exists for the PtP/BA/POM LB film in which the POM and PtP molecules can lay down with the interfacial planes. Luminescence spectra of the prepared hybrid LB films show that near‐white emission spectra can be obtained due to the dual‐emissive nature of the mixed PtP/POM blends. These Pt‐polyyne‐based LB films displayed interesting electric conductivity behavior. Among them, PtP/BA/POM 13‐layer films showed a good electrical response, with the tunneling current up to ±100 nA when the voltage was monitored between ?1 and 7 V. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 879–888, 2010     

Polyoxometalate Ionic Liquids as Self‐Repairing Acid‐Resistant Corrosion Protection

Corrosion is a global problem for any metallic structure or material. Herein we show how metals can easily be protected against acid corrosion using hydrophobic polyoxometalate‐based ionic liquids (POM‐ILs). Copper metal disks were coated with room‐temperature POM‐ILs composed of transition‐metal functionalized Keggin anions [SiW₁₁O₃₉TM(H₂O)]^n? (TM=Cu^II, Fe^III) and quaternary alkylammonium cations (C_nH_{2 n+1})₄N⁺ (n=7–8). The corrosion resistance against acetic acid vapors and simulated “acid rain” was significantly improved compared with commercial ionic liquids or solid polyoxometalate coatings. Mechanical damage to the POM‐IL coating is self‐repaired in less than one minute with full retention of the acid protection properties. The coating can easily be removed and recovered by rinsing with organic solvents.     

Keggin‐Type Polyoxometalate‐Based Metal–Organic Networks for Photocatalytic Dye Degradation

The reaction of Keggin‐type polyoxometalate (POM) units, transition‐metal (TM) ions, and a rigid bis(imidazole) ligand (1,4‐bis(1‐imidazolyl)benzene (bimb)) in a hydrothermal environment led to the isolation of four new POM‐based metal–organic networks, [H₂L][CuL][SiW₁₂O₄₀]?2 H₂O ( 1 ), [H₂L]₂[Co(H₂O)₃L][SiW₁₁CoO₃₉]?6 H₂O ( 2 ), KH[CuL]₂[SiW₁₁CoO₃₉(H₂O)]?2 H₂O ( 3 ), and [CuL]₄[GeW₁₂O₄₀]?H₂O ( 4 ; L=bimb). All four compounds were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X‐ray diffraction, and single‐crystal X‐ray diffraction. Compounds 1 and 3 are new 3D networks with 1D channels. Compounds 2 and 4 contain 2D networks, which further stack into 3D supramolecular networks. The contributions of pH value, the negative charge of the POM, and the TM coordination modes to the construction of 3D networks were elucidated by comparing the synthetic conditions and structures of compounds 1 – 4 . The photocatalytic properties of compounds 1 – 4 were investigated using methylene blue (MB) degradation under UV light. All compounds showed good catalytic activity and structural stability. The possible catalytic mechanism was discussed on the basis of active‐species trapping experiments. The different photocatalytic activities of compounds 1 – 4 were explained by comparison of the band gaps of different POM species and different packing modes of POM units in these hybrid compounds.     

Organosilyl/‐germyl Polyoxotungstate Hybrids for Covalent Grafting onto Silicon Surfaces: Towards Molecular Memories

Organosilyl/‐germyl polyoxotungstate hybrids [PW₉O₃₄(tBuSiO)₃Ge(CH₂)₂CO₂H]^3? ( 1 a ), [PW₉O₃₄(tBuSiO)₃Ge(CH₂)₂CONHCH₂C?CH]^3? ( 2 a ), [PW₁₁O₃₉Ge(CH₂)₂CO₂H]^4? ( 3 a ), and [PW₁₁O₃₉Ge(CH₂)₂CONHCH₂C≡CH]^4? ( 4 a ) have been prepared as tetrabutylammonium salts and characterized in solution by multinuclear NMR spectroscopy. The crystal structure of (NBu₄)₃ 1 a? H₂O has been determined and the electrochemical behavior of 1 a and 2 a has been investigated by cyclic voltammetry. Covalent grafting of 2 a onto an n‐type silicon wafer has been achieved and the electrochemical behavior of the grafted clusters has been investigated. This represents the first example of covalent grafting of Keggin‐type clusters onto a Si surface and a step towards the realization of POM‐based multilevel memory devices.     

A novel biological active multilayer film based on polyoxometalate with pendant support-ligand

A novel nanosized biological active multilayer film composed of polyoxometalate (POM) anion α-[SiW₁₁O₃₉Co(H₂PO₄)]⁷⁻(abbr. SiW₁₁Co-PO₄) and poly(diallyldi methylammonium chloride) (abbr. PDDA) was fabricated by layer-by-layer self-assembly (LBL). The composition and growth processes of the films have been determined by X-ray photoelectron spectra (XPS) and ultraviolet-visible absorption spectra (UV). The composite film was formed by the alternate adsorption of SiW₁₁Co-PO₄ and PDDA, and the deposition process was quantitative and highly reproducible from layer to layer. The morphology of the film was studied by atomic force microscopy (AFM), which showed that the film was relatively uniform and smooth, and POM anions aggregated into nanoclusters distributing on the surface uniformly. The film exhibited favorable electrochemical behavior of POM indicated by cyclic voltammetry (CV). The film can immobilize the DNA molecules via Mg²⁺-bridging medium.     

Biofunctionalization of Polyoxometalates with DNA Primers,Their Use in the Polymerase Chain Reaction (PCR) and Electrochemical Detection of PCR Products

The bioconjugation of polyoxometalates (POMs), which are inorganic metal oxido clusters, to DNA strands to obtain functional labeled DNA primers and their potential use in electrochemical detection have been investigated. Activated monooxoacylated polyoxotungstates [SiW₁₁O₃₉{Sn(CH₂)₂CO}]^8? and [P₂W₁₇O₆₁{Sn(CH₂)₂CO}]^6? have been used to link to a 5′‐NH₂ terminated 21‐mer DNA forward primer through amide coupling. The functionalized primer was characterized by using a battery of techniques, including electrophoresis, mass spectrometry, as well as IR and Raman spectroscopy. The functionality of the POM‐labeled primers was demonstrated through hybridization with a surface‐immobilized probe. Finally, the labeled primers were successfully used in the polymerase chain reaction (PCR) and the PCR products were characterized by using electrophoresis.     

Covalent Attachment of Anderson‐Type Polyoxometalates to Single‐Walled Carbon Nanotubes Gives Enhanced Performance Electrodes for Lithium Ion Batteries

Single‐walled carbon nanotubes (SWNTs) covalently functionalized with redox‐active organo‐modified polyoxometalate (POM) clusters have been synthesized and employed as electrode materials in lithium ion batteries. The Anderson cluster [MnMo₆O₂₄]^9? is functionalized with Tris (NH₂C(CH₂OH)₃) moieties, giving the new organic–inorganic hybrid [N(nC₄H₉)₄]₃[MnMo₆O₁₈{(OCH₂)₃CNH₂}₂]. The compound is then covalently attached to carboxylic acid‐functionalized SWNTs by amide bond formation and the stability of this nanocomposite is confirmed by various spectroscopic methods. Electrochemical analyses show that the nanocomposite displays improved performance as an anode material in lithium ion batteries compared with the individual components, that is, SWNTs and/or Anderson clusters. High discharge capacities of up to 932 mAh g^?1 at a current density of 0.5 mA cm^?2 can be observed, together with high long‐term cycling stability and decreased electrochemical impedance. Chemisorption of the POM cluster on the SWNTs is shown to give better electrode performance than the purely physisorbed analogues.     

Suzuki coupling reaction catalyzed heterogeneously by Pd(salen)/polyoxometalate compound: another example for synergistic effect of organic/inorganic hybrid

A hybrid compound consisting of palladium(salen) [salen = N,N′‐bis(salicylidene)ethylenediamine] complex covalently linked to a lacunary Keggin‐type polyoxometalate, K₈[SiW₁₁O₃₉](POM), was synthesized and characterized by FT‐IR, elemental analysis, inductively coupled plasma and diffuse reflectance UV–visible spectroscopic methods. The hybrid, [Pd(salen)–POM], was investigated in the Suzuki cross‐coupling in EtOH/H₂O under mild reaction conditions. In comparison to the corresponding organic and inorganic moiety, the hybrid has shown greatly improved catalytic activity, and much higher yields toward coupling products were obtained with a low catalyst loading for various aryl halides, including unreactive and sterically hindered ones. The catalyst also exhibited prominent recyclable performance and no obvious loss of activity was observed after six consecutive runs. Copyright © 2013 John Wiley & Sons, Ltd.     

New Polyoxometalates Containing Hybrid Polymers and Their Potential for Nano‐Patterning

Two new polyoxometalate (POM)‐based hybrid monomers (Bu₄N)₅(H)[P₂V₃W₁₅O₅₉{(OCH₂)₃CNHCO(CH₃)C?CH₂}] ( 2 ) and (S(CH₃)₂C₆H₄OCOC(CH₃)=CH₂)₆[PVMo₁₀O₄₀] ( 5 ) were developed by grafting polymerizable organic units covalently or electrostatically onto Wells–Dawson and Keggin‐type clusters and were characterized by analytical and spectroscopic techniques including ESI‐MS and/or single‐crystal X‐ray diffraction analyses. Radical initiated polymerization of 2 and 5 with organic monomers (methacryloyloxy)phenyldimethylsulfonium triflate (MAPDST) and/or methylmethacrylate (MMA) yielded a new series of POM/polymer hybrids that were characterized by ¹H, ³¹P NMR and IR spectroscopic techniques, gel‐permeation chromatography as well as thermal analyses. Preliminary tests were conducted on these POM/polymer hybrids to evaluate their properties as photoresists using electron beam (E‐beam)/extreme ultraviolet (EUV) lithographic techniques. It was observed that the POM/polymer hybrid of 2 with MAPDST exhibited improved sensitivity under EUV lithographic conditions in comparison to the MAPDST homopolymer resist possibly due to the efficient photon harvesting by the POM clusters from the EUV source.     

Hydrothermal Combination of Trilacunary Dawson Phosphotungstates and Hexanickel Clusters: From an Isolated Cluster to a 3D Framework

Three novel hexa‐Ni‐substituted Dawson phosphortungstates [Ni₆(en)₃(H₂O)₆(μ₃‐OH)₃(H₃P₂W₁₅O₅₆)] ? 14 H₂O ( 1 ), [Ni(enMe)₂(H₂O)][Ni₆(enMe)₃(μ₃‐OH)₃(H₂O)₆(HP₂W₁₅O₅₆)] ? 10 H₂O ( 2 ), and [Ni(enMe)₂]₃[Ni(enMe)₂(H₂O)][Ni(enMe)(H₂O)₂][Ni₆(enMe)₃(μ₃‐OH)₃(Ac)₂(H₂O)(P₂W₁₅O₅₆)]₂ ? 6 H₂O ( 3 ) (en=ethylenediamine, enMe=1, 2‐diaminopropane, Ac=CH₃COO^?) have been made under hydrothermal conditions and were characterized by IR spectroscopy, elemental analysis, diffuse reflectance spectroscopy, thermogravimetric analysis, powder X‐ray diffraction, and single‐crystal X‐ray diffraction. The common structural features of compounds 1 – 3 contain the similar hexa‐Ni‐substituted Dawson polyoxometalate (POM) units that can be viewed as a [Ni₆(μ₃‐OH)₃]⁹⁺ cluster capping on a [P₂W₁₅O₅₆]^12? fragment. Compounds 1 and 2 are two isolated clusters, whereas compound 3 is the first 3D POM framework constructed from hexa‐Ni‐substituted Dawson POM units and Ni(enMe) complex bridges. The preparations of compounds 1 – 3 not only indicate that triangle coplanar Ni₆ clusters are very stable fragments in both trivacant Keggin and trivacant Dawson POM systems, but also offer that the hydrothermal technique can act as an effective strategy for making novel Dawson‐type high‐nuclear transition‐metal cluster substituted POMs by combination of lacunary Dawson precusors with transition‐metal cations in the tunable role of organic ligands. In addition, magnetic measurements illustrate that there exist overall ferromagnetic interactions in compound 3 .     

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity,Stability, and Temperature‐ Dependent Activation Properties

Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs₄[α‐SiW₁₂O₄₀]?n H₂O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was ?67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Au^δ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.     

Photochemical Reduction of Carbon Dioxide Catalyzed by a Ruthenium‐Substituted Polyoxometalate

A polyoxometalate of the Keggin structure substituted with Ru^III, ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉] in which ⁶Q=(C₆H₁₃)₄N⁺, catalyzed the photoreduction of CO₂ to CO with tertiary amines, preferentially Et₃N, as reducing agents. A study of the coordination of CO₂ to ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉] showed that 1) upon addition of CO₂ the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g_x=2.146, g_y=2.100, and g_z=1.935), and 3) the ¹³C NMR spectrum had a broadened peak of bound CO₂ at 105.78 ppm (Δ_1/2=122 Hz). It was concluded that CO₂ coordinates to the Ru^III active site in both the presence and absence of Et₃N to yield ⁶Q₅[Ru^III(CO₂)SiW₁₁O₃₉]. Electrochemical measurements showed the reduction of Ru^III to Ru^II in ⁶Q₅[Ru^III(CO₂)SiW₁₁O₃₉] at ?0.31 V versus SCE, but no such reduction was observed for ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉]. DFT‐calculated geometries optimized at the M06/PC1//PBE/AUG‐PC1//PBE/PC1‐DF level of theory showed that CO₂ is preferably coordinated in a side‐on manner to Ru^III in the polyoxometalate through formation of a Ru? O bond, further stabilized by the interaction of the electrophilic carbon atom of CO₂ to an oxygen atom of the polyoxometalate. The end‐on CO₂ bonding to Ru^III is energetically less favorable but CO₂ is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp² hybridization state. Formation of a O₂C–NMe₃ zwitterion, in turn, causes bending of CO₂ and enhances the carbon sp² hybridization. The synergetic effect of these two interactions stabilizes both Ru–O and C–N interactions and probably determines the promotional effect of an amine on the activation of CO₂ by [Ru^III(H₂O)SiW₁₁O₃₉]^5?. Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO₂ and Et₃N. A mechanistic pathway for photoreduction of CO₂ is suggested based on the experimental and computed results.     

Synthesis of α‐Dawson‐Type Silicotungstate [α‐Si2W18O62]8− and Protonation and Deprotonation Inside the Aperture through Intramolecular Hydrogen Bonds

The design of structurally well‐defined anionic molecular metal–oxygen clusters, polyoxometalates (POMs), leads to inorganic receptors with unique and tunable properties. Herein, an α‐Dawson‐type silicotungstate, TBA₈[α‐Si₂W₁₈O₆₂] ? 3 H₂O ( II ) that possesses a ?8 charge was successfully synthesized by dimerization of a trivacant lacunary α‐Keggin‐type silicotungstate TBA₄H₆[α‐SiW₉O₃₄] ? 2 H₂O ( I ) in an organic solvent. POM II could be reversibly protonated (in the presence of acid) and deprotonated (in the presence of base) inside the aperture by means of intramolecular hydrogen bonds with retention of the POM structure. In contrast, the aperture of phosphorus‐centered POM TBA₆[α‐P₂W₁₈O₆₂]?H₂O ( III ) was not protonated inside the aperture. The density functional theory (DFT) calculations revealed that the basicities and charges of internal μ₃‐oxygen atoms were increased by changing the central heteroatoms from P⁵⁺ to Si⁴⁺, thereby supporting the protonation of II . Additionally, II showed much higher catalytic performance for the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde than I and III .     

Polyoxometalate‐Based Entangled Coordination Networks Induced by an Extended Bis(triazole) Ligand

The introduction of an extended bridging bis(triazole) ligand, that is, 4,4′‐bis(1,2,4‐triazol‐1‐ ylmethyl)biphenyl (BBPTZ), into the hydrothermal reaction system containing transition metal ions and Keggin‐type polyoxometalates (POMs) led to the isolation of three new organic–inorganic hybrid entangled coordination networks, [Cu^I₂Cu^II(BBPTZ)₆][SiW₁₂O₄₀]?12 H₂O ( 1 ), [Ni(BBPTZ)₂(H₂O)][H₂SiW₁₂O₄₀]?11 H₂O ( 2 ), and [Ni₂(BBPTZ)₄(H₂O)₂][SiW₁₂O₄₀]?3 H₂O ( 3 ). All three compounds were characterized by elemental analysis, IR spectroscopy, TG analysis, powder X‐ray diffraction, and single‐crystal X‐ray diffraction. Compound 1 contains a 2‐D POM‐based metal–organic layer entangled with 1‐D ladder‐like metal–organic chains. The adjacent 2‐D networks are parallel to each other, further stacking into a 3‐D supramolecular framework with 1‐D channels. Compound 2 exhibits a 1‐D cantilever‐type loop‐containing chain. The Keggin‐type POMs act as the cantilever groups, leading to the adjacent catilever‐type chains interwaving together to form a 3‐D supramolecular open framework with two types of channels. Compound 3 possesses a 3‐D open framework based on 2‐D metal–organic undulated layer and Keggin‐type POM clusters. Three sets of such frameworks further interpenetrate with each other to form an interesting three‐fold interpenetrating framework. The photocatalytic activities of compounds 1–3 for the decomposition of methylene blue (MB) under UV light have been investigated.     

Hybrid Coordination Networks Constructed from ɛ‐Keggin‐Type Polyoxometalates and Rigid Imidazole‐Based Bridging Ligands as New Carriers for Noble‐Metal Catalysts

Three hybrid coordination networks that were constructed from ?‐Keggin polyoxometalate building units and imidazole‐based bridging ligands were prepared under hydrothermal conditions, that is, H[(Hbimb)₂(bimb){Zn₄PMo^V8Mo^VI₄O₄₀}] ? 6 H₂O ( 1 ), [Zn(Hbimbp)(bimbp)₃{Zn₄PMo^V8Mo^VI₄O₄₀}] ? DMF ? 3.5 H₂O ( 2 ), and H[Zn₂(timb)₂(bimba)₂Cl₂{Zn₄PMo^V8Mo^VI₄O₄₀}] ? 7 H₂O ( 3 ) (bimb=1,4‐bis(1‐imidazolyl)benzene, bimbp=4,4′‐bis(imidazolyl)biphenyl, timb=1,3,5‐tris(1‐imidazolyl)benzene, bimba=3,5‐bis(1‐imidazolyl)benzenamine). All three compounds were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and single‐crystal X‐ray diffraction. The mixed valence of the Mo centers was analyzed by XPS spectroscopy and bond‐valence sum calculations. In all three compounds, the ?‐Keggin polyoxometalate (POM) units acted as nodes that were connected by rigid imidazole‐based bridging ligands to form hybrid coordination networks. In compound 1 , 1D zigzag chains extended to form a 3D supramolecular architecture through intermolecular hydrogen‐bonding interactions. Compound 2 consisted of 2D curved sheets, whilst compound 3 contained chiral 2D networks. Because of the intrinsic reducing properties of ?‐Keggin POM species, noble‐metal nanoparticles were loaded onto these POM‐based coordination networks. Thus, compounds 1 – 3 were successfully loaded with Ag nanoparticles, and the corresponding composite materials exhibited high catalytic activities for the reduction of 4‐nitrophenol.     

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?.     

Electrochemical‐Reduction‐Assisted Assembly of a Polyoxometalate/Graphene Nanocomposite and Its Enhanced Lithium‐Storage Performance

Herein, we present an electrochemically assisted method for the reduction of graphene oxide (GO) and the assembly of polyoxometalate clusters on the reduced GO (rGO) nanosheets for the preparation of nanocomposites. In this method, the Keggin‐type H₄SiW₁₂O₄₀ (SiW₁₂) is used as an electrocatalyst. During the reduction process, SiW₁₂ transfers the electrons from the electrode to GO, leading to a deep reduction of GO in which the content of oxygen‐containing groups is decreased to around 5 %. Meanwhile, the strong adsorption effect between the SiW₁₂ clusters and rGO nanosheets induces the spontaneous assembly of SiW₁₂ on rGO in a uniformly dispersed state, forming a porous, powder‐type nanocomposite. More importantly, the nanocomposite shows an enhanced capacity of 275 mAh g^?1 as a cathode active material for lithium storage, which is 1.7 times that of the pure SiW₁₂. This enhancement is attributed to the synergistic effect of the conductive rGO support and the well‐dispersed state of the SiW₁₂ clusters, which facilitate the electron transfer and lithium‐ion diffusion, respectively. Considering the facile, mild, and environmentally benign features of this method, it is reasonable as a general route for the incorporation of more types of functional polyoxometalates onto graphene matrices; this may allow the creation of nanocomposites for versatile applications, for example, in the fields of catalysis, electronics, and energy storage.     

The hydroxyl radical reaction rate constant and products of 3,5‐dimethyl‐1‐hexyn‐3‐ol

A bimolecular rate constant,k_DHO, of (29 ± 9) × 10^?12 cm³ molecule^?1 s^?1 was measured using the relative rate technique for the reaction of the hydroxyl radical (OH) with 3,5‐dimethyl‐1‐hexyn‐3‐ol (DHO, HC?CC(OH)(CH₃)CH₂CH(CH₃)₂) at (297 ± 3) K and 1 atm total pressure. To more clearly define DHO's indoor environment degradation mechanism, the products of the DHO + OH reaction were also investigated. The positively identified DHO/OH reaction products were acetone ((CH₃)₂C?O), 3‐butyne‐2‐one (3B2O, HC?CC(?O)(CH₃)), 2‐methyl‐propanal (2MP, H(O?)CCH(CH₃)₂), 4‐methyl‐2‐pentanone (MIBK, CH₃C(?O)CH₂CH(CH₃)₂), ethanedial (GLY, HC(?O)C(?O)H), 2‐oxopropanal (MGLY, CH₃C(?O)C(?O)H), and 2,3‐butanedione (23BD, CH₃C(?O)C(?O)CH₃). The yields of 3B2O and MIBK from the DHO/OH reaction were (8.4 ± 0.3) and (26 ± 2)%, respectively. The use of derivatizing agents O‐(2,3,4,5,6‐pentalfluorobenzyl)hydroxylamine (PFBHA) and N,O‐bis(trimethylsilyl)trifluoroacetamide (BSTFA) clearly indicated that several other reaction products were formed. The elucidation of these other reaction products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible DHO/OH reaction mechanisms based on previously published volatile organic compound/OH gas‐phase reaction mechanisms. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 534–544, 2004