The mechanism of the rhodium(I) catalyzed dehydrocoupling of di-n-hexylsilane was investigated by isolating and characterizing products of stoichiometric reactions, by preparing a series of rhodium(I) phosphine precatalysts and silane substrates and measuring catalytic activity, and by observing catalytic reaction mixtures directly using 31P{ 1H} NMR spectroscopy. Catalyst initiation was found to occur via oxidative addition of silane to a rhodium centre, followed by reductive elimination of a chlorosilane generating an unsaturated rhodium hydride complex, the putative active catalyst fragment. The series of precatalysts screened for catalytic activity include [Rh(PPh3)3Cl] (1), [Rh(PPh3)2(µ-Cl)]2 (2). [Rh(dppe)(µ-Cl)]2 (3) [Rh(dppb)(µ-Cl)]2, (4) [Rh(COD)(µ- Cl)]2 (5), [Rh(PPh3)3H] (6), [Rh(PPh3)4H] (7), [Rh(xantphos)(COD)(Cl)]. (8). Of these eight precatalysts the two hydride complexes (6 and 7) displayed the highest catalytic activity. The flexibility of chelating bis(phosphine) ligands was found to be important for catalyst activity. Of the chelating bis(phosphine) rhodium complexes (3, 4, and 8), 4 displayed the highest catalytic activity. The catalyst resting state for all precatalysts containing the monodentate PPh3 ligand was found to be trans-[Rh(PPh3)2LX] when catalytic reactions were observed in situ by 31P{1H} NMR spectroscopy. The substitution and steric bulk of the silane substrate was found to influence reactivity as well. Of the substrates studied, the least bulky primary n-hexylsilane was most reactive. By monitoring the reaction over by 31P{1H} NMR spectroscopy, decomposition of the catalyst was found to involve phosphine dissociation. Decomposition was found to occur more slowly for precatalysts containing chelating bis(phosphine) ligands than for precatalysts containing monodentate phosphine ligands.

This thesis describes the catalysed addition of X-H bonds (X = S, O and N) to alkynes using a range of novel rhodium(I) and iridium(I) complexes containing hybrid bidentate phosphine-pyrazolyl, phosphine-imidazolyl and phosphine-N heterocyclic carbene (NHC) donor ligands. The synthesis of novel bidentate phosphine-pyrazolyl, phosphine-imidazolyl (P-N) and phosphine-NHC (PC) donor ligands and their cationic and neutral rhodium(I) and iridium(I) complexes [M(P N)(COD)]BPh4, [M(PC)(COD)]BPh4, [Ir(P-N)(CO)2]BPh4 and [M(P-N)(CO)Cl] were successfully performed. An unusual five coordinate iridium complex with phosphine-NHC ligands [Ir(PC)(COD)(CO)]BPh4 was also obtained. Seventeen single crystal X-ray structures of these new complexes were determined. A range of these novel rhodium and iridium complexes were effective as catalysts for the addition of thiophenol to a variety of alkynes. Iridium complexes were more effective than rhodium analogues. Cationic complexes were more effective than neutral complexes. Complexes with hybrid phosphine-nitrogen donor were more effective than complexes containing bidentate nitrogen donor ligands. An atom-economical, efficient method for the synthesis of cyclic acetals and bicyclic O,O-acetals was successfully developed based on the catalysed hydroalkoxylation. Readily prepared terminal and non-terminal alkyne diols were cyclised into bicyclic O,O-acetals in quantitative conversions in most cases. The efficiency of a range of rhodium and iridium complexes containing bidentate P-N and PC donor ligands as catalysts for the cyclisation of 4-pentyn-1-amine to 2-methyl-1-pyrroline varied significantly. The cationic iridium complexes with the bidentate phosphine-pyrazolyl ligands, [Ir(R2PyP)(COD)]BPh4 (2.39-2.42) were extremely efficient as catalysts for this transformation. Increasing the size of the substituent on or adjacent to the donor led to improvement in catalytic activity of the corresponding metal complexes. The mechanism of the catalysed hydroalkoxylation was proposed to proceed by the initial activation of the alkyne via ?? coordination to the metal centre. The ?? binding of both aliphatic and aromatic alkynes to [Ir(PyP)(CO)2]BPh4 (2.44) was observed by low temperature NMR and no reaction between 2.44 and alcohols was observed. In contrast, the facility in which thiol and amine oxidatively added to 2.44 led the proposal that in the hydrothiolation and hydroamination reaction, the catalytic cycle commences with the activation of the X-H bond (X = S, N) by an oxidative addition process.

The mechanism of the rhodium(I) catalyzed dehydrocoupling of di-n-hexylsilane was investigated by isolating and characterizing products of stoichiometric reactions, by preparing a series of rhodium(I) phosphine precatalysts and silane substrates and measuring catalytic activity, and by observing catalytic reaction mixtures directly using 31P{ 1H} NMR spectroscopy. Catalyst initiation was found to occur via oxidative addition of silane to a rhodium centre, followed by reductive elimination of a chlorosilane generating an unsaturated rhodium hydride complex, the putative active catalyst fragment. The series of precatalysts screened for catalytic activity include [Rh(PPh3)3Cl] (1), [Rh(PPh3)2(µ-Cl)]2 (2). [Rh(dppe)(µ-Cl)]2 (3) [Rh(dppb)(µ-Cl)]2, (4) [Rh(COD)(µ- Cl)]2 (5), [Rh(PPh3)3H] (6), [Rh(PPh3)4H] (7), [Rh(xantphos)(COD)(Cl)]. (8). Of these eight precatalysts the two hydride complexes (6 and 7) displayed the highest catalytic activity. The flexibility of chelating bis(phosphine) ligands was found to be important for catalyst activity. Of the chelating bis(phosphine) rhodium complexes (3, 4, and 8), 4 displayed the highest catalytic activity. The catalyst resting state for all precatalysts containing the monodentate PPh3 ligand was found to be trans-[Rh(PPh3)2LX] when catalytic reactions were observed in situ by 31P{1H} NMR spectroscopy. The substitution and steric bulk of the silane substrate was found to influence reactivity as well. Of the substrates studied, the least bulky primary n-hexylsilane was most reactive. By monitoring the reaction over by 31P{1H} NMR spectroscopy, decomposition of the catalyst was found to involve phosphine dissociation. Decomposition was found to occur more slowly for precatalysts containing chelating bis(phosphine) ligands than for precatalysts containing monodentate phosphine ligands.

Aqueous-biphasic organometallic catalysis is, as illustrated by the industrial hydroformylation of propene and butene, one of the most promising ways to overcome the intrinsic problem of catalyst separation in organometallic catalysis. However, for poorly water-soluble substrates, mass transfer limitations bring the reaction rate below any that could be economically viable, greatly limiting the scope of this elegant technology. We have studied three different strategies to overcome this limitation. We developed additives that speed up the reaction whilst retaining fast phase separation and good metal retention. Evidence suggests that those additives affect the reaction by forming emulsions with poor stability under the reaction conditions These emulsions increase the interfacial surface area but break after settling for a short time. We also developed ligands that allow the catalyst to be reversibly transported between an aqueous and an organic phase upon addition and removal of carbon dioxide. This allows the reaction to be carried out under homogeneous conditions, only limited by intrinsic kinetics, and the catalyst to be separated by aqueous extraction triggered by carbon dioxide. The catalyst can be returned to a fresh organic phase by flushing out the carbon dioxide. By applying this methodology for the hydroformylation of medium chain length alkenes, very high reaction rates were obtained and the catalyst could be recycle three times with excellent retention of activity and low metal leaching. This methodology could also be reversed with the reaction being carried out in an aqueous phase in the presence of carbon dioxide and extracting the catalyst into an organic solvent using nitrogen flushing. Finally, we briefly investigated the use of an oscillatory baffled reactor as a mean for mass transfer improvement for aqueous-biphasic hydroformylation. This new type reactor did not improve the performance of the system under the investigated conditions, but may require less energy input for equivalent agitation and mixing.

In this thesis, graphene (i.e. monolayer carbon film) and carbon clusters supported on a transition metal surface are systematically studied by local probe techniques, with respect to their structures, electronic properties and formation mechanisms. The main tools used are low-temperature scanning tunnelling microscopy and spectroscopy (STM and STS), which are introduced in Chapter 2. The mechanism of the resonance tunnelling at electron energies higher than the work function of the surface is discussed in detail, and a qualitative explanation of the Gundlach oscillations in the corresponding spectroscopy is presented. Epitaxial graphene synthesised on the Rh(111) surface by ethylene dehydrogenation is investigated by STM in Chapter 4. Such carbon film exhibits a hexagonal Moiré pattern due to a lattice mismatch between graphene and the rhodium substrate. The periodicity and local registries of the graphene/Rh(111) superstructure are carefully analysed. Based on a thorough discussion about the “commensurate vs. incommensurate” nature of the Moiré pattern in surface science field, the graphene/Rh(111) system is identified to have a non-simple-commensurate superstructure. The surface electronic properties and geometric buckling of graphene/Rh(111) are investigated by resonance tunnelling spectroscopy (RTS) and density functional theory (DFT) calculations in Chapter 5. Spectroscopy measurements reveal a modulation of the electronic surface potential (or work function Φ) across the supercell of epitaxial graphene. Based on the microscopy/spectroscopy data and the extended DFT calculations, we examined the electronic coupling of the various local C-Rh registries, and identified both experimentally and theoretically the local atomic configurations of maximum and minimum chemical bonding between graphene and the rhodium substrate. We studied in Chapter 6 the growth mechanism of graphene on Rh(111) at elevated temperatures. This part starts by investigating the dehydrogenation of ethylene into ethylidyne. When the dehydrogenation process is complete, monodispersed carbon species, identified as 7C6, are found to dominate the cluster population on the rhodium terraces. A significant coalescence of the 7C6 clusters into graphene islands occurs at temperatures higher than 873 K. The structural and electronic properties of the 7C6 carbon clusters are examined by high-resolution STM and STS, and compared with coronene molecules, i.e. the hydrogenated analogues of 7C6. DFT calculations are further used to explain the stability of 7C6 supported on the Rh(111) surface, and also the structural characteristics of such magic-sized carbon clusters.

The carbonylation of methanol to acetic acid is one of the most important applications in homogeneous catalysis. The first chapter comprises a review on the mechanistic studies into the catalytic cycle of the ‘Monsanto process’ and includes some of the most prominent studies into the use of phosphines in the rhodium-catalysed carbonylation of methanol. The second chapter of this thesis reports on an investigation into the application of rhodium complexes containing several C4 bridged diphosphines, namely BINAP, dppb, dppx and dcpb as catalysts for hydrogen tolerant methanol carbonylation. An investigation into the structure, reactivity and stability of pre-catalysts and catalyst resting states of these complexes has also been carried out. The origin of this hydrogen tolerance is explained based on the differing reactivities of the Rh acetyls with hydrogen gas, and by considering the structure of the complexes. In the third chapter I report on an investigation into how electronic properties and coordination mode affect the elimination of phosphonium salts from rhodium complexes. The stability of a range of monodentate, bidentate and tridentate rhodium-phosphine complexes was tested. I also report on the formation of a novel bidentate complex containing a partially quaternised TRIPHOS ligand and investigate the mechanism of formation using 13CH3I. Strong evidence is also presented supporting a dissociative mechanism as the means of phosphine loss from the rhodium centre. In the final chapters I report an investigation into the stability of rhodium-aminophosphine ligand complexes and into increasing the solubility of potential rhodium pre-catalysts through the use of amine-containing phosphine ligands.

Le travail de thèse propose d’éclaircir les dynamiques qui régissent les réactions d’hydrogénation du dioxyde d’azote (NO2) se déroulant à la surface de catalyseurs modèles de platine, de rhodium et de leur alliage Pt-Rh. Une meilleure compréhension de la réaction catalytique en conditions réactionnelles permettrait, à terme, un meilleur contrôle de la réaction. De manière similaire, le comportement du catalyseur permettrait d’orienter la synthèse de catalyseurs afin d’en augmenter la sélectivité et/ou activité. La structure de ces catalyseurs ainsi que l’analyse des processus sont effectuées à l’échelle nanométrique grâce à l’utilisation de microscopies à effet de champ :la microscopie ionique à effet de champ (FIM) et la microscopie d’émission d’électrons par effet de champ (FEM). La réaction NO2+H2 étudiée à 390 K sur le platine permet de mettre en évidence la présence de différents domaines réactionnels :dynamique monostable, oscillations périodiques auto-entretenues, oscillations périodiques bimodales, ainsi que des oscillations bruitées. Malgré la présence importante de fluctuations à l’échelle du nanomètre, les traitements de signaux mettent en évidence une importante robustesse qui se traduit par un temps de corrélation qui s’étend sur plusieurs centaines de périodes. Les données donnent matière à la reconstruction de l’attracteur dynamique consistant en un cycle limite. La pression d’hydrogène est le paramètre de contrôle qui est varié de sorte à provoquer l’apparition d’oscillations selon une bifurcation de type homocline dans ce cas-ci. Des mesures à haute-vitesse d’acquisition démontrent que l’ignition des différentes faces réactives s’effectue de manière désynchronisée, et la vitesse de propagation est de l’ordre de ~2 μm/s. Au sein d’une seule face, à l’échelle du nanomètre, des propagations de fronts d’ondes chimiques peuvent également être observées à une vitesse de ~2 μm/s, en accord avec les vitesses analysées lors d’expériences menées à l’échelle du micromètre et du millimètre. Sur base des observations, un mécanisme réactionnel de production d’H2O a été proposé. La réaction sur le rhodium à 450 K engendre également des oscillations périodiques qui diffèrent par une robustesse plus faible et par l’apparition d’un cycle limite selon une bifurcation de Hopf. Des mesures exploratoires à 500 K font ressortir la présence de chaos dans le système. Finalement, l’alliage Pt-Rh utilisé comme catalyseur permet d’obtenir des oscillations à 425 K de période comprise entre celles observées sur les deux métaux purs. L’ensemble des expériences et des résultats obtenus à l’échelle du nanomètre permet pour la première fois de valider la théorie des systèmes dynamiques à une telle échelle. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished