Applications des polyoxométalates en catalyse hétérogène

The aim of this work was preparation and characterization of catalysts based on polyoxometalates and their use in various catalytic reactions in heterogenous conditions. Methane C-H activation on silica-supported polyoxometalates was shown already at room temperature. Methoxy species [SiMo12O40(CH3)]3- from the 13C-enriched methane adsorption at 200°C on the surface of a silicadispersed silicomolybdic acid was detected by means of 13C SS NMR. Its hydrolysis led to methanol formation, thus completing the catalytic cycle. After 13C-enriched MeOH adsorption presence of two distinct methoxy species on the surface of polyoxometalates was shown, located on terminal (single coordinated) and bridging (double coordinated) oxygen atoms and resulting in the resonances at 58 and 77 ppm in 13C SS NMR. Grafting of PtMe2COD on the surface of various polyoxometalate supports led to methane or combined methane and ethane release, explained by means of oxidative addition/reductive elimination mechanism on metal centers. Ammonium salts of phosphotungstic acid catalyzed efficiently n-butane to isobutane skeletal isomerisation at mild conditions (225 °C, atmospheric pressure). Successful heterogenization of copper catalysts, active in enantioselective C-H carbene insertion reactions, on polyoxometalate supports have been shownL’objectif de la thèse était la préparation et la caractérisation des catalyseurs hétérogènes à base de polyoxométalates. L’étude mécanistique d’oxydation du méthane jusqu’au méthanol a montré que sur des polyoxométalates supportés sur la silice l’activation C-H a lieu déjà à la température ambiante. L’adsorption du méthane-13C sur H4SiMo12O40 supporté, suivie par RMN solide a mis en évidence la création de l’espèce méthoxy [SiMo12O40(CH3)]3-. Le cycle catalytique est complété par l’hydrolyse de cette espèce - méthanol est formé et une molécule de l’eau recrée la structure du départ de polyoxométalate. L’adsorption du méthanol-13C sur des polyoxométalates a montré la création de deux types des espèces methoxy, localisées sur des atomes d’oxygènes terminaux ou pontants est caractérisées par deux signaux RMN distincts – à 58 et 77 ppm, respectivement. En greffant un complexe de platine PtMe2COD sur les sels de césium de polyoxometalates, le dégagement du méthane ou de la mélange du méthane et de l’éthane a été observé et expliqué par la séquence de l’addition oxydative du proton de polyoxometalate au centre métallique, couplage C-H ou C-C et finalement l’élimination réductrice et libération d’une molécule de gaz. Sels d’ammonium de l’acide phosphotungstique H3PW12O40 ont été montrées de catalyser l’isomérisation du n-butane a l’isobutane dans des conditions douces (225°C, 1 atm.). Composé du cuivre Cu(OTf)2 sur la surface des sels inorganiques des polyoxometalates donne des catalyseurs très actifs en insertion des carbènes aux liaisons C-H des éthers cycliques

Synergistic catalysis: highly diastereoselective benzoxazole addition to Morita–Baylis-Hillman carbonates

An expedited method has been developed for the diastereoselective synthesis of highly functionalized alkyl-azaarene systems with good yields and high diastereoselectivities (>15 : 1 dr). The methodology includes a synergistic catalysis event involving organometallic (10 mol% AgOAc) activation of an alkyl azaarene and Lewis base (10 mol% DABCO) activation of a Morita–Baylis–Hillman carbonate. The structure and relative configuration of a representative product were confirmed by X-ray analysis

Quenching of lanthanide emission by intervalence charge transfer in crystals containing closed shell transition metal ions

The quenching of the luminescence originating from the excited states P-3(0) and D-1(2), of Pr3+ and D-5(3) and D-5(4) of Tb3+ has been studied in oxide crystals containing closed shell transition metal ions, such as titanates, vanadates, niobates, and tantalates. It has been shown that the emission from these excited states can be quenched by an intervalence charge transfer mechanism. The temperature dependence of the emission intensities has allowed estimating indicative activation energies for the crossover to the intervalence charge transfer state. In the case of Tb3+, the quenching gives rise to relatively short decay times for the D-5(4) state

Powder diffraction and synchrotron radiation, a powerful tool in the investigation of new compounds : CdBr2(3−fluoroaniline)2CdBr_2(3-fluoroaniline)_2 and CdBr2(4−fluoroaniline)2CdBr_2(4-fluoroaniline)_2

Using synchrotron powder diffraction data, two new complexes of CdBr2 with 3-fluoroaniline and 4-fluoroaniline were fully characterised with regard to their structure. Their crystal structures were solved and refined by means of powder diffraction techniques. The investigated materials belong to the group of hybrid inorganic–organic coordination polymers. The structures of both compounds are created by the same type, infinite chains formed by CdBr4N2 octahedra. Each cadmium cation is surrounded by four Br− anions, and two N atoms from two fluoroaniline molecules. The obtained structural data enable us to investigate the influence of the position of the F atom in the aniline ring on the crystal structure, as well as to interpret its thermal stability