No description available.

No description available.

Le but de ce travail de thèse est d’étudier et de rationaliser, à l’aide de mécanismes réactionnels, le comportement sous irradiation de différents matériaux argileux. Les systèmes d’intérêt ont d’abord été le talc synthétique, qui est le prototype d’un matériau non gonflant. Sous irradiation par des électrons accélérés, la production de dihydrogène dans ce système, uniquement due aux groupes hydroxyle de surface, est du même ordre de grandeur que celle obtenue dans l’eau liquide. Ce rendement est divisé par 30 dans le cas du talc naturel de Luzenac, mettant ainsi en évidence l’importance des impuretés comme capteurs des précurseurs du dihydrogène. Des smectites synthétiques, qui sont des matériaux gonflants, ont ensuite été étudiées.Les résultats mettent en évidence la radiolyse de l’eau confinée dans l’espace interfoliaire, qui conduit à des rendements de production de H2 pouvant être deux à trois fois supérieurs à ceux mesurés dans l’eau. Ils sont similaires pour la saponite et la montmorillonite, montrant que la localisation de la charge foliaire ne joue qu’un rôle mineur. Enfin, l’étude des hydroxydes doubles lamellaires ou argiles anioniques, prouve que, dans ce cas, c’est la nature de l’anion dans l’espace interlamellaire qui pilote la réactivité. En parallèle à ces mesures, des expériences de spectroscopie paramagnétique de l’électron ont permis de proposer des mécanismes réactionnels. Enfin, tous les résultats obtenus sont d’intérêt dans le contexte du stockage des déchets radioactifs. / The aim of this PhD thesis is to study and understand, by proposing reaction mechanisms, the behavior under irradiation of various clay materials. The systems of interest were first synthetic talc, which is the prototype of a non-swelling material. Under irradiation by accelerated electrons, the production of dihydrogen in this system, due solely to surface hydroxyl groups, is of the same order of magnitude as the one obtained in liquid water. This yield is divided by 30 in the case of natural talc from Luzenac, thus highlighting the importance of the impurities as scavengers of the precursors of dihydrogen. Synthetic smectites, which are swelling materials, were then studied.The results evidence the radiolysis of water confined in the interlayer space, leading to H2 yields which may be two to three times higher than those measured in water. Moreover, they are similar for montmorillonite and saponite, evidencing that the charge location plays only a minor role. Finally, the study of double layered hydroxides or anionic clays shows that, in this case, the nature of the anion in the interlamellar space controls the reactivity. Parallel to these measurements, electron paramagnetic spectroscopy experiments have enabled proposing reaction mechanisms. Finally, all these results are of interest in the context of the disposal of radioactive waste.

No description available.

Un pont de dihidrogen (dihydrogen bond,DHB) és un tipus de pont d'hidrogen atípic que s'estableix entre un hidrur metàl·lic i un donador de protons com un grup OH o NH. Els ponts de dihidrogen són claus en les característiques geomètriques i altres propietats de compostos que en presenten tan de molècules petites com el dímer de NH3BH3, com d'estructures superiors més complicades com complexes metàl·lics o sòlids. Poden ser útils aplicats a certes molècules o síntesis moleculars per a obtenir nous materials amb propietats o característiques fetes a mida. El treball d'aquesta tesi està orientat a millorar la comprensió dels ponts de dihidrogen, aprofundint en certs aspectes de la seva naturalesa atòmica/molecular utilitzant mètodes teòrics basats en la química física quàntica. / A dihydrogen bond (or DHB) is a kind of unconventional hydrogen bond, established between a metal hydride bond and a proton donor like OH or NH. They are the key to important structure features and properties in compounds which have them. They can be responsible for the specific geometry not only of small molecules like the NH3 BH3 dimer, but also of higher structures like metallic complexes or solids. It is in this fashion that dihydrogen bonds can be profitable, up to a plausible extent, when they can be used in certain molecules or certain syntheses to obtain a new material with particular or even tailored properties or geometries. The work developed in this thesis is aimed to have a deeper understanding of dihydorgen bonds, deepening on certain aspects using theoretical methods.

L’objectif principal de cette étude est d’évaluer le rôle de l’activité de bactéries hydrogénotrophes et ferri-réductrices sur le processus de corrosion anoxique en utilisant des indicateurs géochimiques. Il est considéré que le couple redox H2/Fe(III) est un moteur important pour les activités bactériennes qui peuvent ainsi affecter les vitesses de corrosion par la déstabilisation des couches de passivation (i.e. magnétite, Fe3O4). Les résultats indiquent que la magnétite de synthèse est déstabilisée en présence de bactéries hydrogénotrophes et ferri-réductrices due à la réduction du Fe(III) structural couplée à l’oxydation de l’H2. La quantité de Fe(III) bioréduit est augmentée en présence de concentrations croissantes en H2 dans le système: 4% H2 < 10% H2 < 60% H2. De plus, les résultats indiquent que la réaction de corrosion est différente selon la composition de la solution et la surface de contact de l’échantillon métallique (poudre de fer ou coupon en acier au carbone). Les produits de corrosion solides sont différents pour chaque échantillon étudié: vivianite, sidérite et chukanovite sont les principales phases minérales identifiées dans les expériences avec de la poudre de fer, tandis que vivianite et magnétite sont identifiées en présence de coupons en acier au carbone. Les résultats montrent que la vitesse de corrosion est quasiment deux fois plus importante en présence de bactéries après 5 mois de réaction. Cette étude apporte une nouvelle approche sur la compréhension des phénomènes de biocorrosion, l’identification des mécanismes physico-chimiques et la détermination des paramètres contrôlant la vitesse de corrosion. / The main objective of this study is to evaluate the role of hydrogenotrophic and IRB activities on anoxic corrosion process by using geochemical indicators. It is assumed that the redox couple H2/Fe(III) is an important driver for bacterial activities potentially affecting the corrosion rate by destabilization of passive layers (i.e. magnetite, Fe3O4). Our results indicate that synthetized Fe3O4 is destabilized in the presence of hydrogenotrophic IRB due to structural Fe(III) reduction coupled to H2 oxidation. The extent of Fe(III) bioreduction is notably enhanced with the increase in the H2 concentration in the system: 4% H2 < 10% H2 < 60% H2. Moreover, the results indicate that corrosion extent changes according to the solution composition and the surface of metallic sample (iron powder and carbon steel coupon). The solid corrosion products are different for each sample: vivianite, siderite and chukanovite are the main mineral phases identified in the experiments with iron powder, while vivianite and magnetite are identified with carbon steel coupons. Our results demonstrate that corrosion rate is increased almost two-fold in the presence of bacteria after 5 months of reaction. This study gives new insights regarding the understanding of biocorrosion phenomena, identification of physicochemical mechanisms, and determination of key parameters controlling the corrosion rate.

No description available.

This thesis work is based on an investigation of intermediates involved in various metal mediated catalytic reactions such as hydrogenation, hydroboration, functionalization of methane etc. An intermediate dictates the energetics of the catalytic cycle of these reactions. Therefore, it is important to study such types of intermediates in order to design a better catalyst. These intermediates are called σ-complexes in which a σ-bond is coordinated to the metal center at some stage of the reaction coordinate. These species are rarely stable at ambient conditions which create difficulties in exploring their chemistry. Our aim is to study the effect of ancillary ligands on the coordination properties of a σ-bond ligand. We chose two different classes of σ-complexes – one contains a B–H σ-bond as a ligand, i.e., σ-borane complex and another contains a H–H σ-bond as a ligand, i.e., σ-dihydrogen complex. Both M–H–B and M–H2 interactions are 3-center-2-electron coordination bonds comprised of two bonding components. One is σ-donation, which is present in both and another is π-back donation from the metal center, which is negligible in the σ-borane complexes contrary to the σ-dihydrogen complexes. The bonding characteristics of M–H–B and M–H2 interactions suggest that an electron deficient metal center is necessary to study the σ-borane complexes with reasonable stability. Thus, we selected an early transition metal, i.e., Cr(0) bearing arene and CO ancillary ligands, for studying the σ-borane complexes. On the other hand, the cis-dihydrogen/hydride and cis-dihydrogen chloride complexes were studied on a late transition metal center, i.e., Ru(II) bearing phosphine and N–N bidentate ligands. Ammonia-borane is known to be a potential hydrogen storage material. Therefore, we picked up the catalytic dehydrogenation reaction of this compound and intended to investigate the interaction between a metal center and the BH σ-bonds of amine-boranes. We characterized the σ-borane complexes [(η6-arene)Cr(CO)2(η1-H–BH2•NMe3)] (arene = fluorobenzene, benzene, and mesitylene), and observed an interesting correlation between the electronics and stability of these species. This was the first report of σ-borane systems possessing an η6-arene ligand. A prototype homobimetallic σ-borane complex, [(η6-C6H5CH2NMe2•BH2–HCr(CO)5)Cr(CO)3] was characterized using single crystal X-ray crystallography. An intramolecular σ-borane complex, (η1-(η6-C6H5CH2NMe2•BH2–H))Cr(CO)2 was found to possess an interesting chelation of the η6-arene, and BH coordination sites of its amine-borane moiety with the Cr(0) center. These σ-borane complexes showed an interesting dynamics in the binding interface between the metal center and the borane ligand. Free energy of activation (ΔG#) for this process was estimated to be 30-40 kJ/mol. To explore certain σ-dihydrogen complexes we investigated the chemistry of cis-dihydrogen/hydride complexes of Ru(II) bearing phosphine and N-N bidentate ligands cis,trans-[RuH(η2-H2)(PPh3)2(N-N)][OTf] (N-N = 2, 2′-bipyridyl, 2, 2′-bipyrimidine) in detail. In those cases, we established that the adjacent hydride ligand has large influence on the dihydrogen coordination. The η2-H2 and hydride ligands showed a single 1H NMR spectral signal due to fast site exchange among each other. We established the mechanism and calculated the free energy of activation (ΔG# = 8-13 kJ/mol) of this dynamics. These complexes were found to be stable at ambient conditions although, a labile dihydrogen ligand is present in the coordination sphere of the metal center. In fact, we could obtain the single crystals of cis,trans-[RuH(η2-H2)(PPh3)2(bpy)][OTf]. The molecular structure of a σ-complex in which a σ-bond (before it gets completely formed or broken) acts as a ligand is what fascinates this area in chemistry. A cis-dihydrogen chloride complex, cis,trans-[RuCl(η2-H2)(PPh3)2(bpm)][OTf] was characterized unambiguously using NMR spectroscopy. The H-H distance (dHH) for the η2-H2 ligand of these complexes were estimated to be 0.9-1.0 Å. We attempted to observe some σ-methane species spectroscopically at low temperatures. Unfortunately, these species were quite unstable for exhibiting the NMR spectral signals even at low temperatures. Nevertheless, we investigated the reactivity of cis,trans-[RuHX(PPh3)2(N-N)] (X = H, Cl; N-N = 2, 2′-bipyridyl, 2, 2′-bipyrimidine) towards a methylating agent, CH3OTf. This reaction resulted in methane evolution by the combination of the hydride ligand of a Ru(II) complex and the CH3+ moiety of CH3OTf. This reaction was carried out in a sealed tube inside a NMR probe at ~183 K and monitored for a long period of time; however, the methane bound metal species was not observed. Perhaps, the longevity of this class of σ-methane complex falls below the NMR time scale.