13 research outputs found

    Dissymmetrical U-Shaped π-Stacked Supramolecular Assemblies by Using a Dinuclear CuI Clip with Organophosphorus Ligands and Monotopic Fully π-Conjugated Ligands

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    International audienceReactions between the U-shape binuclear CuI complex A bearing short metal-metal distances and the cyano-capped monotopic π-conjugated ligands 1-5 carrying gradually bulkier polyaromatic terminal fragments lead to the formation of π-stacked supramolecular assemblies 6-10 respectively in 50-80 % yields. These derivatives have been characterized by multinuclear NMR spectroscopy and X-ray diffraction studies. Their solid state structures show the selective formation of U-shaped supramolecular assemblies in which two monotopic π-conjugated systems present large (6,7,9) or medium (8,10) intramolecular π-overlap revealing π-π interactions. These assemblies self-organize into head-to-tail π-stacked dimers that in turn self-assemble affording infinite columnar π-stacks. The nature, the extentand the complexity of the intermolecular contacts within the head-to-tail π-stacked dimer depends on the nature of the terminal polyaromatic fragment carried by the cyano-capped monotopic ligand but it does not alter the result of the self-assembling process. These results demonstrate that the dinuclear molecular clip A bearing short metal-metal distance allows selective supramolecular assembling processes driven by the formation of intra- and intermolecular short π-π interactions in the resulting self-assembled structures demonstrating that their shape is not only dictated by the symmetry of the building blocks. This approach opens perspectives toward the formation of extented π-stacked columns based on dissymmetrical and functionnal π-conjugated systems

    Mechanistic studies of the gold-catalyzed aerobic oxidation of bulky hydrocarbons

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    L’objectif de ce travail est de proposer un mĂ©canisme rĂ©actionnel pour l’oxydation aĂ©robie d’hydrocarbures lourds, alcanes et alcĂšnes, catalysĂ©e par les nanoparticules d’or. La co-oxydation du stilbĂšne et du mĂ©thylcyclohexane est utilisĂ©e comme rĂ©action modĂšle afin de comprendre les nombreux mĂ©canismes mis en jeu. Le criblage initial d’une large gamme de catalyseurs d’or nous permet de mettre en Ă©vidence des effets de support dans cette rĂ©action et d’établir un cahier des charges bien dĂ©fini pour l’élaboration d’un catalyseur de rĂ©fĂ©rence. Une mĂ©thode chimique simple est mise au point pour prĂ©parer un tel catalyseur. Ce catalyseur, optimisĂ© pour les rĂ©actions en milieux organiques apolaires, est ensuite utilisĂ© pour rĂ©aliser des Ă©tudes macrocinĂ©tiques en variant de nombreux paramĂštres expĂ©rimentaux : tempĂ©rature, concentration des rĂ©actifs, quantitĂ© de catalyseur. Un intermĂ©diaire rĂ©actionnel clef, l’hydroperoxyde de mĂ©thylcyclohexane, est identifiĂ©. AprĂšs dosage, l’étude de l’évolution de sa concentration au cours du temps dans les diffĂ©rentes conditions de rĂ©action permet de valider le mĂ©canisme rĂ©actionnel existant et de mieux comprendre l’importance de certaines Ă©tapes Ă©lĂ©mentairesThe aim of this work is to propose a mechanism for the gold-catalyzed aerobic oxidation of bulky hydrocarbons, alkanes and alkenes. The co-oxidation of stilbene and methylcyclohexane is used as a model reaction to study different mechanisms which can take place simultaneously. After an initial screening of different gold catalysts in this reaction, essential characteristics of a reference catalyst for organic reactions in apolar media are identified. Based on these requirements, a straight-forward, chemical bottom-up method is designed to prepare this reference catalyst. This catalyst is used for macro-kinetic studies of the co-oxidation by modifying experimental parameters, such as temperature, alkane/alkene ratio, and reactants initial concentration. One key reaction intermediate, 1-methylcyclohexyl hydroperoxide, is identified. After titration, following the evolution of the concentration of this intermediate with time under the various reaction conditions considered validates the existing mechanism and highlight the importance of some elementary steps of the co-oxidatio

    Epoxidation of olefins with molecular oxygen as the oxidant using gold catalysts supported on polyoxometalates

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    4 We report a highly efficient green process for the epoxidation of various olefins using polyoxometalate-supported gold nanoparticles as catalysts and using oxygen as the terminal oxidant. The nanoparticles were prepared through thermal reduction of Au(III) and were characterised by TEM and XPS. The elemental composition of the catalyst was also determined using ICP. The results show that higher calcination temperatures give a higher degree of reduction, but also result in some sintering of the particles. The catalytic epoxidation reaction may be carried out with or without a solvent and with air or molecular oxygen as the stoichiometric oxidant and using t-butyl hydroperoxide as an initiator. The method shows consistently high conversions and >90% selectivity to epoxide formation for norbornene and cyclooctene oxidation. The catalysts show little deactivation with time and are easily recovered by filtration and can be reused with little or no loss in activity and selectivity

    Gold Nanoparticles Supported on Passivated Silica: Access to an Efficient Aerobic Epoxidation Catalyst and the Intrinsic Oxidation Activity of Gold

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    Well-defined and perfectly dispersed [(square SiO)Au(1)] surface species supported on silica have been obtained via surface organometallic chemistry and transformed upon mild reduction (H(2), 300 degrees C) into small (1.8 +/- 0.6 nm) Au particles supported on silica passivated with SiMe(3) functionalities. improved performance in liquid-phase aerobic epoxidation has been achieved, and the intrinsic activity of gold in oxidation is revealed
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