Anne Baudouin

Dinitrogen Dissociation on an Isolated Surface Tantalum Atom

Both industrial and biochemical ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong triple bond of dinitrogen. Such multimetallic cooperation for dinitrogen cleavage is also the general rule for dinitrogen reductive cleavage with molecular systems and surfaces. We have observed cleavage of dinitrogen at 250°C and atmospheric pressure by dihydrogen on isolated silica surface–supported tantalum(III) and tantalum(V) hydride centers [(≡Si-O)2TaIII-H] and [(≡Si-O)2TaVH3], leading to the TaV amido imido product [(≡SiO)2Ta(=NH)(NH2)]: We assigned the product structure based on extensive characterization by infrared and solid-state nuclear magnetic resonance spectroscopy, isotopic labeling studies, and supporting data from x-ray absorption and theoretical simulations. Reaction intermediates revealed by in situ monitoring of the reaction with infrared spectroscopy support a mechanism highly distinct from those previously observed in enzymatic, organometallic, and heterogeneous N2 activating systems.

17O NMR gives unprecedented insights into the structure of supported catalysts and their interaction with the silica carrier.

Flame silica was surface-labeled with (17)O, through isotopic enrichment of both siloxanes and silanols. After heat treatment at 200 and 700 °C under vacuum, the resulting partially dehydroxylated silica materials were investigated by high-field solid-state (1)H and (17)O NMR. More specifically, MQ MAS and HMQC sequences were used to probe the (17)O local environment. In a further step, these (17)O-tagged supports were used for the preparation of supported catalysts by reaction with perhydrocarbyl transition metal derivatives (zirconium tetraalkyl, tantalum trisalkyl-alkylidene, and tungsten trisalkyl-alkylidyne complexes). Detailed (17)O 1D and 2D MQ and HMQC MAS NMR studies demonstrate that signals in the Si-OH, Si-O-Si, and Si-O-metal regions are highly sensitive to local structural modifications, thanks to (17)O wide chemical shift and quadrupolar constant ranges. Experimental results were supported by DFT calculations. From the selective surface labeling, unprecedented information on interactions between supported catalysts and their inorganic carrier has been extracted.

Synthesis and characterization of ionic liquids based upon 1-butyl-2,3-dimethylimidazolium chloride/ZnCl2

Trialkylimidazolium chlorozincate molten salts resulting from the combination of zinc chloride and 1-butyl-2,3-dimethylimidazolium chloride, [BMMI][Cl], have been prepared with a mole percent of ZnCl2, R (R = nZnCl2/nZnCl2 + n[BMMI][Cl]) equal to 0, 0.1, 0.25, 0.33, 0.5, 0.66, 0.75. Their analyses by DSC, 13C, 1H and 35Cl solid state and solution NMR, and mass spectrometry (ESI, MS/MS) are consistent with the presence of [BMMI][Cl] and [BMMI][ZnCl3] for R < 0.5; pure [BMMI][ZnCl3] for R = 0.5, and [BMMI][ZnCl3] with [BMMI][Zn3Cl7] for R > 0.5. Infrared spectra realized in the presence of pyridine show that the Lewis acidity of ZnCl2–[BMMI][Cl] increases with R. High temperature (110 °C) 13C and 35Cl NMR experiments on neat [BMMI][ZnCl3] (R = 0.5) evidenced that its structure varies with time from [BMMI][ZnCl3] to [BMMI⋯Cl⋯ZnCl2].

Mononuclear Ruthenium Hydride Species versus Ruthenium Nanoparticles: The Effect of Silane Functionalities on Silica Surfaces

The area of nanoparticle synthesis has gained interest in the past few years due to their potential and application in areas such as microelectronics and selective catalysis. In the case of metal nanoparticles, their properties are often related to their size and shape, and therefore controlling their growth has been an area of active research for decades. In the specific case of supported metal nanoparticles, some control is possible by changing the precursor, the method of impregnation, the nature of oxide support, and the final decomposition method. Using perhydrocarbyl complex precursors provides some advantages because of their ease of decomposition under H2, which leads to metal surfaces free of strong ligands such as CO or Cl . The particle mean size can be somewhat controlled by the choice of the support, which directs the migration of the zero-valent ensembles in the process of the crystal growth, but the resulting particles are usually large (>1 nm). Even if the organometallic precursor is first grafted to the support to ensure a high dispersion, treatment under H2 at high temperatures leads to the cleavage of the M O bond and to aggregation, yielding large supported metal particles. Controlling or even avoiding the aggregation process on oxide supports by surface organometallic chemistry tools would lead to smaller nanoparticles of a few atoms (<1 nm) or analogues of the early transition-metal surface hydrides, which could be of great interest for catalytic applications. Recently, Pelzer et al. have shown that the treatment of a pentane solution of [RuACHTUNGTRENNUNG(cod) ACHTUNGTRENNUNG(cot)] (cod=cyclooctadiene, cot=cyclooctatriene) under 3 bars of H2 in the presence of octylsilane yields soluble 2-nm nanoparticles stabilized by direct Ru Si bonds. Here, we show that tuning silica by adding surface Si H bonds generates a support that avoids aggregation of the metal during the treatment under H2 at high temperatures of grafted [Ru ACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(cot)], yielding stable silica-supported mononuclear ruthenium hydride species ACHTUNGTRENNUNG[Ru-H] (Scheme 1). This system will be compared with what is usually obtained on silica support, namely Ru nanoparticles, RuP/SiO2. [6] The solid ACHTUNGTRENNUNG[Ru-H] is typically prepared by treatment under H2 (2055 equiv) at 300 8C for 24 h of solid ACHTUNGTRENNUNG[Ru-L] obtained by reaction of [Ru ACHTUNGTRENNUNG(cod) ACHTUNGTRENNUNG(cot)] (0.3–0.6 equiv/SiH) with a silica covered with surface silanes, [( SiO)SiMe2H] (0.24 mmol per g or ca. 0.7 SiH per nm). During the treatment under H2 at 300 8C, about 10 1 equivalents of CH4/ Ru are formed, in agreement with the hydrogenolysis of some hydrocarbon species. TEM analysis on the resulting light brown solid ACHTUNGTRENNUNG[Ru-H] does not show the presence of Ru particles, even for 0.85 wt% Ru loading (see Figure S1 in the Supporting Information). This is in sharp contrast with what is obtained by treating [Ru ACHTUNGTRENNUNG(cod) ACHTUNGTRENNUNG(cot)] adsorbed on silica under H2 at 300 8C, which affords homogeneously dispersed Ru particles, RuP/SiO2 (0.73 wt%), with a mean size of 2.1 0.3 nm (see Figure S2 and S3 in the Supporting Information). In fact, while [Ru ACHTUNGTRENNUNG(cod) ACHTUNGTRENNUNG(cot)] is only physisorbed on silica (SiO2 (700), ca. 0.7 OH per nm ) and can be removed by simple washing with pentane, [Ru ACHTUNGTRENNUNG(cod) ACHTUNGTRENNUNG(cot)] reacts with [( SiO)SiMe2H] (1.65 equiv) as evidenced by the disappearance of the yellow color of the solution during impregnation to yield an orange-brown solid ACHTUNGTRENNUNG[Ru-L] . This solid contains 1.52wt% of Ru, that is 0.15 mmol of Ru per g of solid, in agreement with the grafting of all the [Ru ACHTUNGTRENNUNG(cod) ACHTUNGTRENNUNG(cot)] loaded. [a] R. Berthoud, A. Baudouin, Dr. J.-M. Basset, Dr. J.-P. Candy, Dr. C. Cop=ret Universit= de Lyon, Institut de Chimie de Lyon, C2P2 LCOMS ESCPE Lyon, 43, Bd. du 11 Novembre 69616 Villeurbanne (France) Fax: (+33)472431795 E-mail : coperet@cpe.fr [b] Dr. B. Fenet Centre de RMN, Universit= Lyon 1 Claude Bernard, ESCPE Lyon, 43, Bd du 11 Novembre, 69616 Villeurbanne Cedex (France) [c] Dr. W. Lukens Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA) [d] Dr. K. Pelzer Department for Inorganic Chemistry, Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4-6, 14195 Berlin (Germany) Supporting information for this article is available on the WWW under http://www.chemistry.org or from the author.

Synthesis and characterisation of O-6-alkylthio- and perfluoroalkylpropanethio-α-cyclodextrins and their O-2-, O-3-methylated analogues

The synthesis of twelve alkylthio- or perfluoroalkylpropanethio-α-cyclodextrin derivatives and their O-2-, O-3-methylated analogues are described. The coupling reaction involves firstly the basic in situ hydrolysis of alkylperfluoropropane isothiouronium iodide or alkylisothiouronium bromide, then reaction with an α-cyclodextrin modified at the C-6 position by an iodine or methylsulfonyl group. The interfacial properties of these new compounds have been determined by the studies of their mono-molecular layer at the air–water interface.

Supported cationic complexes: selective preparation and characterization of the well-defined electrophilic metallocenium cation [SiO–B(C6F5)3]−[Cp*ZrMe2(Et2NPh)]+ supported on silica

The reaction of Cp*ZrMe3, 1, with the heterogeneous activator [[triple bond]SiO-B(C6F5)3]- [HNEt2Ph]+, 2, has been investigated to generate, by an irreversible process of methane elimination, the well-defined cationic silica-supported metallocenium species [[triple bond]SiO-B(C6F5)3]- [Cp*ZrMe2(NEt2Ph)]+, 3, as an active olefin polymerisation catalyst.

On-Demand Cyclophanes: Substituent-Directed Self-Assembling, Folding, and Binding

A family of p-cyclophanes based on bis- or tetrafunctionalized 1,4-bisthiophenol units linked by disulfide bridges was obtained by self-assembly on a gram scale and without any chromatographic purification. The nature of the functionalities borne by these so-called dyn[4]arenes plays a crucial role on their structural features as well as their molecular recognition abilities. Tuning these functions on demand yields tailored receptors for cations, anions, or zwitterions in organic or aqueous media.