Made available in DSpace on 2014-10-09T12:30:23Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:06:15Z (GMT). No. of bitstreams: 1 01260.pdf: 1861297 bytes, checksum: d3c9b48544bf2c99079376c242d689d8 (MD5) / Tese (Doutoramento) / IPEN/T / Swiss Fed. Inst. Techn., Lausanne, Switzerland

CHAPTER ONE reviews the literature, discussing aspects of transition metal mediated asymmetric catalysis in the presence of enantiomerically pure ligands. CHAPTER TWO discusses the asymmetric addition of dialkyl-zinc reagents to aromatic aldehydes. The work presented is particularly concerned with the design and construction of enantiomerically pure oxazoline ligands tethered to alcohols These ligands have proved effective in the acceleration of the alkylation reaction and are able to influence good levels of asymmetric induction in the resultant secondary alcohol products CHAPTER THREE examines the electronic (and steric) effects of enantiomerically pure oxazoline ligands for the palladium catalysed allylic substitution reaction. Using ligands possessing two electronically different donor atoms, it is possible to create electronic distortion upon the intermediate allyl complex. In doing so it is possible to direct nucleophilic addition to one carbon centre preferentially to the other, resulting in asymmetric induction. Manipulation of these ligands enables control in the extent of electron distortion inflicted upon the allyl complex and consequently influences the levels of enantioselectivity observed. CHAPTER FOUR investigates the ability of hydrolytic enzymes to kinetically resolve a series of allylic acetates, under varying conditions. Lipases appeared superior to esterases for the substrates employed. In particular cis-3-acetoxy-5-carbomethoxycyclohexene was smoothly resolved m high yield and enantioselectivity. CHAPTER FIVE reports on the potentiality of a dynamic resolution of allylic acetates, using hydrolytic enzymes in the presence of a palladium catalyst. A proposed mechanism is discussed. Initial results are promising, however, the sensitivity of the reaction is realised and optimisation of conditions still needs to be addressed.

Three new tetrafluoropyridyl-substituted cyclopentadienes were synthesized. Reactions of pentafluoropyridine (C5F5N) with sodium cyclopentadienide (NaCp) in THF with excess NaH present afforded mixtures of (4-tetrafluoropyridyl)cyclopentadiene (1), 1,3-bis(tetrafluoropyridylcyclopentadiene) (2), and 1,2-bis(tetrafluoropyridylcyclo-pentadiene) (3). Selectivity for mono- and diarylation was controlled by varying the reaction time. Each of the three cyclopentadienes (1, 2, and 3) was converted to its corresponding substituted sodium cyclopentadienide (4, 5, and 6, respectively) by treatment with NaH in THF. Reaction of the monoarylated sodium cyclopentadienide (4) with M(CO)5Br in THF (M = Mn, Re) afforded the corresponding substituted CpM(CO)3 complexes (7Mn and 7Re). The diarylated sodium cyclopentadienides (5 and 6) likewise afforded the diarylated complexes 8Mn, 8Re, 9Mn, and 9Re. Infrared spectroscopic measurements of [(C5F4N)nC5H5-n]M(CO)3 (M = Mn, Re; n = 0 - 2) revealed an increase of about 6 cm-1 in the A-symmetric C-O stretching mode per C5F4N group, which is significantly higher than the average increase (4 cm-1) found earlier for C6F5 groups. Reaction of 2 equiv of 4 with FeBr2 in THF afforded the 1,1'-diarylated ferrocene (10). Analogous reactions starting with 5 and 6 afforded tetraarylated ferrocenes (11 and 12, respectively). Reaction of 2 equiv of 4 with ZrCl4 afforded (C5F4NCp)2ZrCl2 (13), whereas the reaction of CpZrCl3 with 1 equiv of 4 afforded (C5F4NCp)CpZrCl2 (14). Metallocene (13) was found to be moderately active for ethylene/1-hexene copolymerization (1 atm of C2H4, toluene solvent, methylalumoxane cocatalyst). / Master of Science

An exploration of some early transition metal (oxy) fluoride systems using solvothermal techniques has been carried out. 30 novel materials have been synthesised, which fall into three classes based on different metal centres; vanadium (21), molybdenum (5) and niobium (4). Some of these also contain secondary metal centres, namely copper (22) and zinc (1). Simultaneously, the relationship between the SHG values and the crystal structures of the hilgardites family members Pb₂B₅O₉Cl, Pb₂B₅O₉Br, Sr₂B₅O₉Cl and Ba₂B₅O₉Cl was investigated. In particular, the Pb–containing members of the hilgardite family of borate halides exhibit an abnormally large non–linear optical response, which was analysed based on neutron powder diffraction. Using solvothermal synthesis in HF–containing media, 21 novel vanadium oxyfluorides containing interesting structural features, were synthesised at 160˚C using a range of organo-amine compounds as a ligand, template, linker or structure directing agent. The architectures of the crystal structures may be categorised into; four clusters including monomeric vanadium units, five clusters including vanadium dimers, eight 1–D chains, two 2–D layers and two 3–D networks. ‘Composition–space’ diagrams with three components were used to study the effect of stoichiometry changes of reactants and to map out the crystallisation fields. The combination of early (Nb⁵⁺, Mo⁶⁺) and late (Cu²⁺) transition metals with different organo-amines produced nine novel compounds incorporating monomers, chains and 2– D interpenetrated networks. The chains and layers were synthesised from a systematic series of reactions at 160˚C and can be subdivided into four pairs, the topologies of which are essentially unique to each ligand, containing in each case a Cu–based cationic species, but alternately either [MoO₂F₄]²⁻ or [NbOF₅]²⁻, in an isomorphous manner, as the anionic moiety. The overall structures of these materials reflect the influence of the organo–amine ligands. The materials have been studied for their magnetic properties and characterised by thermogravimetric analysis, Rietveld refinement and elemental analysis where relevant.

A dissertation submitted to the Faculty of Science University of the Witwatersrand, in partial fulfilment of the requirements for the degree of master of science (MSc) School of physics, University of Witwatersrand, 2017. / Intensive study on structural, electronic and optical properties of bulk transition metal dichalcogenides and dipnictogenides (MX2; where M = V, Nb and X = S, Se, Te, P) was undertaken. A relative stability test was done to determine the most stable ground state configuration via calculation of total ground state energy and volume which was fitted to the third order Birch-Murnaghan equation of state to extract lattice parameters. Cohesive energies of the above mentioned MX2 compounds and their elemental solids were then computed from which formation energies were acquired based on their respective equations of reaction between reactants and product. Its significance was to aid in determining if a material is energetically stable. Elastic constants were predicted from which mechanical properties i.e bulk, Young’s and shear moduli and consequently Poisson’s ratio were resolved by feeding the stiffness matrix onto online elastic tensor analysis tool which facilitated verification of their mechanical stability based on the well-known Born stability conditions which varies from one crystal system to another, at a later stage phonon dispersion curves were plotted after performing phonon calculation based on phonopy code to verify if the materials of concern are dynamically stable. After a material had fulfilled all the above stability tests, its structural study was initiated using various functionals. Functional that described best the structural properties of each individual compound considered was then applied in exploring its electronic and optical properties whose motivation was to find out the most stable phase as well as gauge if these materials could be used in various fields that suits their mechanical and optical properties. Furthermore, from carefully calculated optical spectra, plasma frequencies were analyzed which indicated the possibility of applying a material in plasmonic related fields. In addition to above, other factors to be considered when selecting a given electrode material that are crucial for optoelectronics are good chemical and thermal stabilities, high transparency and excellent conductivity. / XL2018

Thesis (MScEng) -- Stellenbosch University , 2001. / ENGLISH ABSTRACT: The electrochemical analysis of selected metal ions with the newly biodegradable chelating agents, lminodisuccinic acid (IDS) and Polyaspartic acid (PASP) was performed by cyclic voltammetry using a thin mercury film glassy carbon microelectrode. IDS and PASP are of interest from an environmental perspective. They were introduced as alternatives to chelants and dispersants. The thermodynamic stability constants of these selected metal complexes were determined by cyclic voltammetry (CV). A comparison with similar data obtained using potentiometric methods shows a good correlation between the two methods. This reaffirms that CV is a useful and facile means for evaluating metal-ligand complex thermodynamic data. Furthermore, our results were used to validate the predicted models obtained using an Equilibrium Speciation model JESS program. Capillary Electrophoresis (CE) was used to separate metal-IDS and metal-PASP complexes at different pH's. The separation of these chelates was achieved within 7 minutes. The obtained results were compared with speciation and a reasonable agreement was observed, although the separation ofmetal-PASP showed poor broad peaks. / AFRIKAANSE OPSOMMING: Die elektrochemiese analise van geselekteerde metaal-ione met nuwe biodegredeerbare chelate, Iminodisuksien suur (IDS) en Polyaspartiese suur was gedoen met sikliese voltametrie deur gebruik te maak van 'n dun laag kwik glasagtige koolstof mikroelektrode. Vanaf'n omgewingsstandpunt is IDS en PASP van belang. Hulle was voorgestel as alternatiewe vir chelate en verspreidingsagente. Die termodinamies stabiliteits konstantes van hierdie uitgesoekte metal komplekse was bepaal deur middel van sikliese voltametrie (CV). 'n Vergelyking met soortgelyke data, wat met potensiometriese metodes bepaal was, dui 'n goeie korrelasie aan tussen die twee metodes bepaal was. Dit bevestig die feit dat CV 'n bruikbare en maklike manier is om metal ligand komplekse se termodiniese data te evalueer. Die resultate was ook gebruik om die voorspelde model, wat verkry was deur gebruik te maak van "Equilibrium Speciation model JESS program, te bevestig. Kapillere elektroforese (CE) was gebruik om metal-IDS en metal-PASP komplekse by verskillende pH's te skei. Die skeiding van hierdie chelate was binne sewe minute verkry. Hierdie resultate was vergelyk met die spesiasie data en 'n aanvaarbare ooreenstemming was waargeneem, alhoewel die skeiding van die Metaal-PASP swak pieke toon.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

No description available.

No description available.

A series of new planar complexes with the dianion of 2,3-quinoxalinedithiol ligand has been prepared. The complexes have been characterized from the study of their analyses, magnetic moment, conductance, polarograms, electron spin resonance spectra, and electronic spectra, and compared with the available data on the corresponding maleonitriledithiolene and toluene-3,4-dithiolene complexes.