Chemistry of polynuclear transition-metal complexes in ionic liquids

Transition-metal chemistry in ionic liquids (IL) has achieved intrinsic fascination in the last few years. The use of an IL as environmental friendly solvent, offers many advantages over traditional materials synthesis methods. The change from molecular to ionic reaction media leads to new types of materials being accessible. Room-temperature IL have been found to be excellent media for stabilising transition-metal clusters in solution and to crystallise homo- and heteronuclear transition-metal complexes and clusters. Furthermore, the use of IL as solvent provides the option to replace high-temperature routes, such as crystallisation from the melt or gas-phase deposition, by convenient room- or low-temperature syntheses. Inorganic IL composed of alkali metal cations and polynuclear transition-metal cluster anions are also known. Each of these areas will be discussed briefly in this contribution.     

Synthesis of polyoxovanadates via “chimie douce”

A large variety of new polyoxovanadates have been synthesized during the past few years by sol–gel chemistry or hydrothermal methods. These wet chemistry methods offer many advantages compared to the usual solid state syntheses. New open structures have been obtained from aqueous precursors. They result from the self-assembling of ionic species in the solution.Vanadium oxide gels and sols, V₂O₅·nH₂O, are formed around the point of zero charge (pH≈2). They have a ribbon-like structure and exhibit a liquid crystal behavior. These mesophases are similar to those currently observed with nematic polymers. Xerogel layers deposited from V₂O₅·nH₂O gels exhibit some preferred orientation and behave as versatile host structures for intercalation giving new hybrid organic–inorganic nanocomposites.Layered structures are formed around pH≈7 in the presence of large organic cations. They are built of mixed valence polyoxovanadate planes made of [VO₅] pyramids and [VO₄] tetrahedra. Organic cations lie between the oxide layers where they interact with the negative oxygen of the VO double bonds.Anions can behave as templating agents. Hollow cluster shells are formed around anions that remain encapsulated within the negatively charged polyvanadate cage. Large cations only behave as counter ions for the formation of a neutral crystalline network.It appears that the molecular structure of V^V precursors depends mainly on pH, but the way they self-assemble may be governed by other ionic species in the solution.     

Scintillation of Sol-Gel Derived Lutetium Orthophosphate Doped with Rare Earth Ions

In this paper, the synthesis, the characterization and the scintillation properties of LuPO₄ doped, with several concentrations of Ce³⁺, Eu³⁺ and Tb³⁺ ions, are presented. These materials have been synthesized by sol-gel process. The purity of powders has been verified by X-Ray diffraction and the results confirm the xenotime structure of all the materials. A thermogravimetric analysis allows the obtention of informations on the crystallisation of LuPO₄ and the study of its evolution from the amorphous to crystalline form. The morphology of the powders has been studied by Scanning Electron Microscopy and shows that the powders are constituted of small particles with narrow size distribution. Optical properties have been studied in order to determine the scintillation performances of these materials. The optima are obtained for Ce³⁺, Eu³⁺ and Tb³⁺ concentration of respectively 0.1, 10 and 5% with high scintillation yields. This study thus confirms the potentialities of these materials as scintillators.     

Anionic Magnesium and Calcium Hydrides: Transforming CO into Unsaturated Disilyl Ethers

The synthesis, characterisation and reactivity of two isostructural anionic magnesium and calcium complexes is reported. By X-ray and neutron diffraction techniques, the anionic hydrides are shown to exist as dimers, held together by a range of interactions between the two anions and two bridging potassium cations. Unlike the vast proportion of previously reported dimeric group 2 hydrides, which have hydrides that bridge two group 2 centres, here the hydrides are shown to be “terminal”, but stabilised by interactions with the potassium cations. Both anionic hydrides were found to insert and couple CO under mild reaction conditions to give the corresponding group 2 cis-ethenediolate complexes. These cis-ethenediolate complexes were found to undergo salt elimination reactions with silyl chlorides, allowing access to small unsaturated disilyl ethers with a high percentage of their mass originating from the C₁ source CO.     

Study of Nanocrystalline BiMnO3‐‐PbTiO3: Synthesis,Structural Elucidation,and Magnetic Characterization of the Whole Solid Solution

In the last ten years, the study and the search for new multiferroic materials have been a major challenge due to their potential applications in electronic technology. In this way, bismuth‐containing perovskites (BiMO₃), and particularly those in which the metal M position is occupied by a magnetically active cation, have been extensively investigated as possible multiferroic materials. From the point of view of synthesis, only a few of the possible bismuth‐containing perovskites can be prepared by conventional methods but at high pressures. Herein, the preparation of one of these potential multiferroic systems, the solid solution xBiMnO₃‐(1?x)PbTiO₃ by mechanosynthesis is reported. Note that this synthetic method allows the oxides with high x values, and more particularly the BiMnO₃ phase, to be obtained as nanocrystalline phases, in a single step and at room temperature without the application of external pressure. These results confirm that, in the case of Bi perovskites, mechanosynthesis is a good alternative to high‐pressure synthesis. These materials have been studied from the point of view of their structural characteristics by precession electron diffraction and magnetic property measurements.     

Caesium salts of titanium,zirconium and thorium cyanoferrates(II)

The synthesis of cyanoferrates of quadrivalent metals and the sorption of caesium ions by these materials have been studied. It has been found that the compositions of the compounds and their properties are strongly influenced by the degree of hydrolysis of the metal. These is no sorption of caesium by Zr⁴⁺ and Th⁴⁺ cyanoferrates, while the compounds of oxygencontaining bivalent complex cations exhibit behaviour similar to that of the other bivalent metals. The sorption of caesium by (TiO)₂R is govermed by an ion-exchange mechanism; with (ZrO)₂R changes occur in the crystal lattice, and (ThO)₂R exhibits a low affinity towards caesium.     

Silica self-assembly and synthesis of microporous and mesoporous silicates

The microstructure of silica in basic aqueous solutions containing organic cations and prepared from monomeric precursors is reviewed and interpreted within the context of classical ideas of self-assembly of molecular aggregates. The solution properties can be understood by using the pseudo-phase separation approach coupled to the acid-base chemistry of silanol groups and the Poisson-Boltzmann equation. The silica nanoparticles frequently observed in these systems have a core-shell structure with silica in the core and the organic cations at the shell. Individual particles are observed when the forces between particles are repulsive-as is the case for small cations such as tetramethylammonium or tetrapropylammonium-and extended structures such as M41S materials are formed when the forces are attractive--as is the case for surfactants such as cetyltrimethylammonium. These ideas are useful to understand the evolution of zeolite synthesis gels from nucleation to crystal growth. Although at room temperature the silica and the organic cations are segregated, upon heating the organic cations are embedded within the particles. This transformation signals the onset of structure direction whereby the size and geometry of the organic cation induce changes in the structure of silica that may lead to zeolite nuclei.     

Competitive demethylation and substitution in N,N,N-trimethylanilinium fluorides

Fluorination of aromatic compounds by nucleophilic displacement of trimethylanilinium salts by fluoride is a commonly used reaction for radiotracer synthesis. Though the liberated trimethylamine is thought to be an excellent leaving group for this type of S_NAr reaction, scattered reports show that amine demethylation (reverse Menschutkin reaction) sometimes dominates over substitution, particularly when relatively electron rich fluoroarenes are the desired targets. Here we provide systematic experimental and theoretical studies of trimethylanilinium demethylation and substitution. Results from these studies highlight the limits of this leaving group in fluoroarene synthesis and have important ramifications for the design of nucleophilic fluorinating agents featuring ammonium cations.     

Crystallisation behaviour of high density polyethylene blends with bimodal molar mass distribution 1. Basic characteristics and isothermal crystallisation

Isothermal crystallisation of high density polyethylene (HDPE) blends and their parent polymers was investigated. The blends having broad bimodal molar mass distributions and various compositions were prepared by blending a high molar mass (M_w=330 kg/mol, M_w/M_n=4.8) and a low molar mass HDPE (M_w=34 kg/mol, M_w/M_n=10) in different ratios in xylene solution. The blends and their parent components were characterised by size-exclusion chromatography, dynamic rheological and density measurements. Crystallisation kinetics were studied using a polarised light microscope equipped with an in-house built hot stage and by differential scanning calorimetry. The Avrami theory was applied for crystallisation kinetics analysis. Such crystallisation kinetics parameters as nucleation rate, nucleation density, the Avrami index and cystallisation rate contant were determined for the blends and their parent polymers.According to the results obtained an increasing polydispersity of the sample had a slight increasing effect on the Avrami index, indicating gain in prevalence of the thermal nucleation over the athermal one. In all samples nucleation density increased continuously during crystallisation verifying that the presence of a certain thermal nucleation was typical for all the materials studied. Both the crystallisation rate constant and the nucleation rate decreased with increasing molar mass of the sample. The nucleation density increased proportionally to the increase in average molar mass and the values were larger at lower crystallisation temperatures.The formed supermolecular structure was found to be sensitive to the blend composition and crystallisation temperature. Irregular banded or non-banded spherulites were observed in the materials. Banding of spherulites was typical for the samples having higher average molar mass. The superstructures observed in this work were smaller and vaguer than the superstructures reported in the earlier studies of polyethylene materials having similar average molar mass but narrow molar mass distribution.     

Synthesis,Structure, and Reactivity of Metal Complexes with Alkoxysilylmethyl Ligands

The synthesis and properties of the new metal complexes 1 , 6 – 12 with alkoxysilylmethy substituents (RO–SiR′₂–CH₂–ML_n) is described. The complexes 14 , 15 and 18 with a chloromethylsiloxy ligand were also prepared. These molecules should serve as starting compounds for the synthesis of metallasilaoxetanes. Several reactions which should lead to these new metallacycles have been performed, but it was never possible to isolate them or to proof their existence spectroscopically. However, chloride abstraction from (C₅H₅)₂Ti(Cl)CH₂Si(CH₃)₂O^tBu ( 7 ) by silver cations led to the activation of the Si–O–R group. This indicates an interaction of the oxygen atom with the metal atom, but there was no proof for the intermediate formation of a four membered metallacycle.     

Mechanism of CIT-6 and VPI-8 crystallization from zincosilicate gels

The crystallisation of CIT-6, a large-pore zincosilicate with the framework topology of zeolite Beta and synthesised from clear hydrogels that contain, tetraethylammonium (TEA+), Li+ and Zn2+ cations, proceeds initially through the formation of an amorphous solid that incorporates all the initial Zn species. Nucleation of the *BEA phase is effected by reorganisation of the amorphous phase, whereas crystal growth involves the incorporation of soluble species also. A highly crystalline CIT-6 material is obtained after 164 h of synthesis at 140 degrees C. Scanning electron microscopy (SEM) shows that this sample exhibits two different types of crystals: well-defined pseudo-cubic crystals and rounded crystals. The latter has a broad crystal-size distribution. If crystallisation is continued with longer synthesis times, the VPI-8 crystalline phase appears, and a new population of needle-shaped crystals is detected in the SEM images. This new crystalline phase is nucleated on the surface of the rounded CIT-6 crystals, which disappear as the crystallisation progresses, while no changes are observed in the population of pseudo-cubic CIT-6 crystals. At higher crystallisation temperatures these phase transformations are accelerated, and the formation of VPI-8 is favoured over that of CIT-6.     

Comparison of the crystallisation and solid state reaction methods for the preparation of rare-earth orthophosphates

Two easy laboratory methods for preparation of rare-earth orthophosphates (crystallisation from phosphoric acid solution and solid-state reaction with (NH₄)₂HPO₄) were compared on the basis of the products’ properties with a focus on their application as new inorganic pigments. The preparation method has a significant influence on optical properties. The samples prepared by crystallisation have lighter and less rich colour and also change colour more under sunlight irradiation. The surface properties analysed by SEM and presence of a greater amount of phosphoric acid in the crystallisation procedure influence the pH and resistivity of aqueous extract of products and thus their corrosion-inhibition properties. The optical properties of rare-earth orthophosphates and their preliminary corrosion tests show their potential application as highly efficient corrosion-inhibition pigments.     

A computational study of proton transfer and solvent effect on nitroamino[1,3,5]triazine-based ammonium energetic salts

We have performed dispersion corrected density functional theory calculations to study the proton transfer in the gas-phase and solvent effects on the structural transformation for a series of nitrogen-rich energetic salts. Proton transfer was observed from the cations to anions within all the salts in the gas-phase and resulted into neutral hydrogen-bonding complexes; however, they were stabilized as ionic structures in the liquid state with solvation energies in the range of ?37.72–69.37 kJ/mol. An increment by 4–9 Debye in the dipole moment was found when the salts went from the gas to solution. Moreover, these ionic salts exhibited relatively high densities in the range of 1.63–1.96 g/cm³ desirable for energetic materials. A combination of NH₃OH⁺ to the cation and ?NO₂ or ?NF₂ group to the anion can improve efficiently the detonation performance. Most of the ammonium, hydroxyammonium, and hydrazinium salts were promising competitive explosives and could be used as potential targets for synthesis.     

Synthesis of BaZrS3 and BaHfS3 Chalcogenide Perovskite Films Using Single-Phase Molecular Precursors at Moderate Temperatures

Chalcogenide perovskites have garnered interest for applications in semiconductor devices due to their excellent predicted optoelectronic properties and stability. However, high synthesis temperatures have historically made these materials incompatible with the creation of photovoltaic devices. Here, we demonstrate the solution processed synthesis of luminescent BaZrS₃ and BaHfS₃ chalcogenide perovskite films using single-phase molecular precursors at sulfurization temperatures of 575 °C and sulfurization times as short as one hour. These molecular precursor inks were synthesized using known carbon disulfide insertion chemistry to create Group 4 metal dithiocarbamates, and this chemistry was extended to create species, such as barium dithiocarboxylates, that have never been reported before. These findings, with added future research, have the potential to yield fully solution processed thin films of chalcogenide perovskites for various optoelectronic applications.     

Time‐Resolved In Situ MAS NMR Monitoring of the Nucleation and Growth of Zeolite BEA Catalysts under Hydrothermal Conditions

Time‐resolved ¹³C, ²³Na, ²⁷Al, and ²⁹Si MAS NMR has been applied in situ for monitoring the hydrothermal synthesis of zeolite BEA. Isotopic labelling with ²⁹Si and ¹³C isotopes has been used to follow the fate of siliceous species and structure directing agent ((¹³CH₃−CH₂)₄NOH). Two mechanistic pathways, namely solution‐mediated and solid–solid hydrogel rearrangement have been distinguished for two synthesis procedures studied. The mechanisms of structure‐directing behavior of TEA⁺ cations in two reaction pathways have been elucidated. The results show that multinuclear MAS NMR can serve as a superior tool for monitoring hydrothermal synthesis of various solids including zeolites, zeotypes, mesoporous materials, metal–organic frameworks and so on and for the design of novel outstanding materials for different applications.     

Synthesis and Catalytic Activity of Atrane-type Hard and Soft Lewis Superacids with a Silyl,Germyl, or Stannyl Cationic Center

The synthesis and isolation of atrane-type molecules 1 E⁺ (E=Si, Ge, or Sn) having a cationic group 14 elemental center are reported. The cations 1 E⁺ act as hard and soft Lewis superacids, which readily interact with various hard and soft Lewis basic substrates. The rigid atrane framework stabilizes the localized positive charge on the elemental center and assists the formation of the well-defined highly coordinated states of 1 E⁺. The cations were applied to the hydrodefluorination, Friedel-Crafts reaction, alkyne cyclization, and carbonyl reduction as Lewis acid catalysts. Most notably, [ 1 Si][ClO₄] exhibits unique chemoselectivity that depends on a solvent in the competitive reaction of silyl enol ether with a mixture of benzaldehyde dimethyl acetal and benzaldehyde. Our findings indicate the potential of hard and soft Lewis superacids in organic synthesis.     

Two-component polymer films of palladium and fullerene with covalently linked crown ether voids: effect of cation binding on the redox behavior

Redox active films have been generated via electrochemical reduction in a solution containing palladium(II) acetate and fulleropyrrolidine with covalently linked crown ethers, viz., benzo-15-crown-5 and benzo-18-crown-6. In these films, fullerene moieties are covalently bonded to palladium atoms to form a polymeric network. Films show ability to coordinate alkali metal cations from the solution. Therefore, in solutions containing salts of alkali metal cations, benzo-15-crown-5-C₆₀/Pd and benzo-18-crown-6-C₆₀/Pd films are doped with cations coordinated by crown ether moiety and anions of supporting electrolyte which enter the film to balance positive charge. These films are electrochemically active in the negative potential range due to the reduction of the fullerene moiety. Reduction of the polymer is accompanied by the transport of supporting electrolyte ions between solution and solid phase. In solution containing alkali metal salts, the process of film reduction is accompanied by the transport of anions from the film to the solution. In the presence of tetra(alkyl)ammonium salts, transport of cations from the solution to the film takes place during the polymer reduction.     

Insights on the Structural Chemistry of Hydrocalumite and Hydrotalcite-like Materials: Investigation of the Series Ca2M(OH)6Cl·2H2O (M: Al, Ga, Fe, and Sc) by X-Ray Powder Diffraction

Hydrotalcite and hydrocalumite are two close minerals belonging to the layered double hydroxide family. Both structures are based on positive brucite-like layers alternating with layers containing anions and water molecules. Most of synthetic (LDHs) are hydrotalcite-like materials. On the other hand, the hydrocalumite structure type is rare for those less broad in composition, typically Ca²⁺ and Al³⁺ in the hydroxide layers. In order to get further insight into the conditions of stabilization of this structure type, we have undertaken the synthesis and the structural characterization by powder X-ray diffraction of the series Ca₂M³⁺(OH)₆Cl·2H₂O(M³⁺:Al³⁺, Ga³⁺, Fe³⁺ and Sc³⁺). The incorporation of Sc³⁺ ions is quite original. All phases crystallize in the rhombohedral space group R-3 resulting in a three-layers polytype. The main consequence of the replacement of Al³⁺ cations by large M³⁺ cations is a compression of the octahedral layers like it proceeds in hydrotalcite-like materials. The existence of Sc-containing phase allows us to say that it is the size of Ca²⁺ ions and the pronounced anisotropy of coordination spheres around Ca²⁺ and M³⁺ which are responsible for the ordered distribution of cations in hydrocalumite-like materials.     

Metallasupramolecular architectures, an overview of functional properties and applications

The synthesis of metallasupramolecular architectures, such as two-dimensional squares, triangles and polygons, and three-dimensional cages and polyhedra, has attracted much interest in the past decade. These structures are designed to have novel specific shapes and dimensions with interesting functional properties. In this overview the functional properties of metallasupramolecular architectures are highlighted with emphasis on potential applications such as catalysis, cavity-directed synthesis and sensing, that can be performed with these materials.     

Chemical sensors based on π-conjugated organic molecules and gold nanoparticles

Scientists have developed techniques for synthesizing and characterizing many new materials including conjugated small molecules, polymers and gold particles protected by conjugated organic chromophores for testing specific sensing properties in the past decade. Still, the design and synthesis or supermolecular systems fabrication of novel materials with controlled sensing properties is a significant and ongoing challenge within nanoscience and nanotechnology. Recently, our group has successfully constructed a series of chemosensors using small organic molecules, conjugated polymers and gold nanoparticles for real-time detection of specific analytes. The chemosensors show high selectivity and sensitivity in the detection of cations and biologic analytes and thus are potentially promising for applications in sensing assay system. In this review, recent sutdies on the design, synthesis and photo-physical properties of novel materials and construct of chemosensors are summarized with an emphasis on the development in our groups in recent years. Supported by the National Natural Science Foundation of China (Grant Nos. 20531060, 20721061 & 20873155), and the National Basic Research 973 Programme of China (Grant No. 2007CB936401)