Interaction d'un faisceau d'ions lourds avec un plasma cree par un laser CO2 : simulation de l'experience et mise au point d'un tube ionographique

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Theorical study of coupling reactions catalyzed by transition metals

Ce travail consiste en l'étude théorique DFT des mécanismes de couplage. Le couplage déshydrogénant de stannanes implique la formation in situ d'un stannylène, formation possible grâce à l'inertie de la paire libre et au caractère acide de Lewis des stannylènes. Le couplage entre un carboxylate aromatique et un arène Ar-H commence par la décarboxylation en 2 étapes (isomérisation et désinsertion de CO2) sur un complexe de Pd(II) et se poursuit par une étape de CMD. Celle-ci est facilitée par la présence de fluors en ortho sur Ar-H essentiellement en raison du renforcement de la liaison M-C qu'ils induisent. Le couplage entre un arène et un alcyne catalysé par Ni(0)L présente une étape originale de transfert d'hydrogène de l'arène sur l'alcyne concerté avec la création de deux liaisons M-C (M-aryl et M-vinyl). Cette étape est à nouveau associée à une barrière d'activation plus basse en présence de fluors sur l'arène. L'hydroboration avec ouverture de cycle d'alkylidenecyclopropane en présence de Rh(I) nécessite la création de deux sites vacants pour que l'étape souhaitée d'ouverture de cycle soit favorisée par rapport à une étape d'élimination réductrice C-B.This work is a theoretical study with DFT method of coupling reactions. The dehydrogenating coupling of stannanes proceeds via the formation of stannylene. This is made possible because of the chemical inertness of the lone pair and the Lewis acid character of stannylene. Coupling an aromatic carboxylate and an arene Ar-H on a Pd(II) complex is initiated by a 2-step reaction (isomerization and decarboxylation). The CMD reaction that follows is facilitated by ortho fluorine subtituents on the arene ArH because of the strengthening of the resulting Pd-C bond. The coupling between an arene and an alkyne by a Ni(0)L catalyst starts by an unusual hydrogen transfer from the arene to the alkyne, concerted with the formation of two NiC (aryl and vinyl) bonds. The energy barrier of this elementary step is lowered by fluorine subtituents on the arene. The hydroboration with ring opening of alkylidenecyclopropane in presence of Rh(I) complex is feasible only with two empty coordination sites during the key ring-opening step. Under these conditions, the ring opening is favored over the reductive elimination C-B.MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Hydrofluoroarylation of Alkynes with Ni Catalysts. C-H Activation via Ligand-to-Ligand Hydrogen Transfer, an Alternative to Oxidative Addition

International audienceThe mechanism of the hydrofluoroarylation of alkynes, RC≡CR, by nickel phosphine complexes, described by Nakao et al. ( Dalton Trans. 2010, 39, 10483), was studied by density functional theory (DFT) calculations. The oxidative addition of a C-H bond of partially fluorinated benzenes, C6FnH6-n (n = 0-5) to a Ni(0) phosphine complex is reversible, but the oxidative addition of a C-F bond yields a stable product via a high-energy barrier. A pathway via the Ni(II) hydride complex is eliminated on the basis of a calculated H/D kinetic isotope effect (KIE) that does not agree with the measured value. An alternate pathway was determined, using as reactant a Ni(phosphine)(alkyne) complex that is shown to be the major species in the reactive media under the catalytic conditions. This pathway is initiated by arene coordination to the Ni alkyne complex followed by proton transfer from the σ-C-H bond of the coordinated arene to the alkyne as the C-H activation step. Analysis of the charge distribution shows that the alkyne is strongly negatively charged when coordinated to the Ni(phosphine) species, which favors a C-H activation as a proton transfer, similar to that in CMD and AMLA but not previously seen between hydrocarbyl ligands for electron rich metals. The C-H activation step thus represents an example of a general class of mechanism that we term ligand-to-ligand hydrogen transfer (LLHT). The product of this reaction is a nickel(vinyl)(aryl) complex, which rearranges to place the aryl and vinyl groups cis to one another before undergoing reductive elimination of the arylalkene. An analysis of the calculated turnover frequencies shows that the rate-determining states that control the energy span are the alkyne complex + free arene and the transition state for the vinyl-aryl complex trans-to-cis rearrangement. The calculated KIE agrees with the observed lack of isotope effect. Analysis of the effects of fluorine substituents shows that the Ni-C(aryl) bond energies control the energy barriers for the arene C-H activation step and the energy spans. A correlation between bond dissociation energies for the Ni-C(aryl) bond and the arene C-H bond follows the behavior presented previously ( J. Am. Chem Soc. 2009, 131, 7817), in which the effects of ortho fluorine substituents are dominant. Consequently, fluorine substitution of the arene, especially at the ortho positions, strengthens the Ni-C bond and increases the TOF. The LLHT mechanism described here may also apply to nickel-catalyzed C-H activation reactions with other substrates

Importance of palladium–carbon bond energies in direct arylation of polyfluorinated benzenes

International audienceFagnou et al. reported direct arylation reactions that use palladium catalysts to couple Ar1–X to Ar2–H with the aid of a coordinated base. These reactions are particularly favourable for polyfluorinated arenes Ar2–H (see S. I. Gorelsky, D. Lapointe and K. Fagnou, J. Am. Chem. Soc. 2008, 130, 10848). In this paper, we show by means of a DFT analysis how the energetics and activation energies vary with fluorine substitution and examine the structures of intermediates and transition states. The reactant is modelled by Pd(OAc)(Ph)(PMe3)(DMA) (DMA = dimethylacetamide). The sequence consists of (a) replacement of DMA by arene, (b) Concerted Deprotonation Metallation (CMD), (c) decoordination of AcOH, (d) reductive elimination of biaryl. Many of the variations are dominated by the number of fluorine substituents ortho to the C–H bond and fall into three groups labelled accordingly: Set0Fo, Set1Fo, and Set2Fo. In the first step a coordinated solvent is replaced by the arene. The arenes of Set0Fo and Set1Fo coordinate in a conventional η2-CHdouble bond, length as m-dashCH mode, whereas the arenes of Set2Fo coordinate in an η1-CH mode assisted by an Ocdots, three dots, centeredH–C hydrogen bond from the coordinated acetate. Both the energy barriers to CMD and the product energies fall into the three typical sets with the highest barrier and highest product energy being for Set0Fo. They correlate more satisfactorily with the variations in Pd–C bond energies than with the C–H acidities. The barriers to reductive elimination from Pd(Ph)(ArF)(PMe3)(AcOH) increase systematically from Set0Fo to Set2Fo as the Pd–C bond becomes stronger in a regular fashion from Set0Fo to Set2Fo. Again there is a strong correlation between the energy barriers to reductive elimination and the Pd–C bond energies. It is found overall that the key aspects of the reactions are: (a) the lowering of the energy of the CMD step by the ortho fluorine substituents, (b) the regioselective activation of C–H bonds ortho to fluorine which is also determined at the CMD step, (c) the decoordination of AcOH, which maintains the transition state for reductive elimination at low Gibbs free energy. The presence of fluorine increases the effectiveness of the reaction in the sense of points a and b via the increasing strength of the palladium-carbon bond

Nanostructured particles by controlled precipitation techniques Example of nickel and cobalt hydroxides.

Numéro spécial suite au congrès EMRS Fall meeting, Warsaw, Poland, 4 -8 septembre 2006. disponible en ligne

Facile Interconversion of [Cp2(Cl)Hf(SnH3)] and [Cp2(Cl)Hf(μ-H)SnH2]: DFT Investigations of Hafnocene Stannyl Complexes as Masked Stannylenes

International audienceSn two-step: Dehydrocoupling of stannanes by the d0 complex [Cp2(Cl)HfH] preferentially occurs by two successive reactions (see scheme): -bond metathesis to form [Cp2(Cl)Hf(SnH3)] and subsequent stannylene transfer into the Cp2(Cl)HfSnH3 bond. [Cp2(Cl)Hf(SnH3)] readily isomerizes to a species possessing a reactive stannylene unit, [Cp2(Cl)Hf(-H)SnH2], thus making the stannylene-transfer reaction energetically feasible