published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

A successful transition from fossil fuels to renewable energies such as wind and solar will require the implementation of high-energy-density storage technologies. Promising energy storage technologies include lithium-ion batteries, metal-air batteries, and hydrogen production via photoelectrochemical water splitting. While these technologies differ substantially in their mode of operation, they often involve transition-metal oxides as a component. Thus, fundamental materials research on metal oxides will continue to provide much needed advances in these technologies. In this thesis, the electrochemical and electrocatalytic properties of Fe- and Mn-substituted layered LiNiO₂ materials were investigated. These materials were prepared by heating mixed nitrate precursors in O₂ atmosphere at 700-850 °C for 12 h with intermediate grindings. The products were chemically delithiated with NO₂BF₄, and the delithiated samples were annealed at moderate temperatures in order to transform them to a spinel-like phase. Samples were characterized by inductively coupled plasma analysis and Rietveld refinement of the X-ray diffraction patterns, which were found to be in reasonably close agreement regarding lithium stoichiometry. Spinel-like materials were found to possess an imperfect spinel structure when heated at lower temperatures and a significant amount of NiO impurity was formed when heated to higher temperatures. This structural disorder was manifested during electrochemical cycling -- only Mn-rich compositions showed reversible capacities at a voltage of around 4.5 V. The layered materials exhibited significant capacity loss upon cycling, and this effect was magnified with increasing Fe content. These materials were further investigated as catalysts for the oxygen evolution reaction (OER). All samples containing Mn exhibited low OER activity. In addition, delithiation degraded catalyst performance and moderate temperature annealing resulted in further degradation. Because delithiation significantly increased surface area, activities were compared to the relative to BET surface area. Li₀.₉₂Ni₀.₉Fe₀.₁O₂ exhibited significantly higher catalytic activity than Li₀.₈₉Ni₀.₇Fe₀.₃O₂. This prompted testing of Li[subscript x]Ni₁₋[subscript y]Fe[subscript y]O₂ (y = 0, 0.05, 0.1, 0.2, and 0.3) samples. It was found that a Fe content of approximately 10% resulted in the highest OER activity, with decreased activities for both larger and smaller Fe contents. These results were found to be consistent with studies of Fe substituted nickel oxides and oxyhydroxides, suggesting a similar activation mechanism. / text

Transition metal oxides have been extensively studied during past decades. The purpose of this research was to synthesize new or little characterised transition metal oxides using high-pressure/high-temperature (HPHT) techniques. Various ternary vanadium oxides have been synthesised at ambient and high pressure conditions. All compounds have been studied by neutron and laboratory X-ray powder diffraction and magnetisation measurements. In some cases resistivity and synchrotron X-ray powder diffraction measurements were also carried out. The MnVO3 perovskite containing localized 3d5 Mn2+ and itinerant 3d1 V4+ states has been synthesised at 8 GPa and 1100°C. MnVO3 crystallises in Pnma space group (a = 5.2741(6) Å, b = 7.4100(11) Å, and c = 5.1184(8) Å at 300 K) and is metallic at temperatures of 2 – 300 K and at pressures of up to 67 kbar. Synchrotron X-ray powder diffraction study on the combined sample of several high pressure products showed slight variation in the stoichiometry of MnVO3. Incommensurate Mn spin order was discovered in the neutron powder diffraction measurements, which reveal a (0.29 0 0) magnetic vector below the 46 K spin ordering transition, and both helical and spin density wave orderings are consistent with the diffraction intensities. Electronic structure calculations show large exchange splittings of the Mn and V 3d bands, and (kx 0 0) crossings of the Fermi energy by spin up and down V 3d bands may give rise to Ruderman-Kittel-Kasuya-Yosida coupling of Mn moments, in addition to their superexchange interactions. The new compound CoVO4 has been discovered in a high pressure synthesis experiment. Magnetic susceptibility measurement, synchrotron X-ray and neutron powder diffraction studies were carried out. Refinements of the synchrotron X-ray and neutron data show CoVO4 to crystallise in space group Pbcn (a = 4.5012(2) Å, b = 5.5539(3) Å, and c = 4.8330(2) Å at 300 K (synchrotron X-ray data)). The magnetic susceptibility measurement reveals that Co3+ is most likely in a low spin state in CoVO4. Monoclinic brannerite type CoV2O6 was synthesised in ambient pressure. Neutron powder diffraction measurements were carried out and an antiferromagnetic order with an a x b x 2c supercell was observed below TN = 15 K. High spin Co2+ moments of magnitude 4.77(4) μB at 4 K lie in the ac plane and are ferromagnetically coupled within chains of edge-sharing CoO6 octahedra parallel to b axis. No structural transition is observed down to 4 K, but a magnetostriction accompanying antiferromagnetic order at TN = 15 K was discovered. A field-induced 1/3 magnetisation plateau and corresponding changes in the magnetic structure were studied by carrying out neutron powder diffraction measurements at 2 K in applied magnetic fields of 0, 2.5 and 5.0 T. Three collinear magnetic phases were observed as field increases; the above antiferromagnetic state with propagation vector (0 0 ½), a ferrimagnetic (¯⅓ 1 ⅓) phase, and a (0 0 0) ferromagnetic order. Co2+ moments of 4.4 - 5.0 μB have a large orbital component and are aligned close to the c-axis direction in all cases. Spin-lattice coupling leads to a magnetostriction and volume expansion as field increases. The ferrimagnetic phase accounts for the previously reported 1/3 magnetisation plateau, and demonstrates that monoclinic CoV2O6 behaves as an accidental triangular antiferromagnetic lattice in which further frustrated orders may be accessible. Orthorhombic columbite-type NiV2O6 and CoV2O6 compounds were synthesised at 6 GPa and 900°C. Metamagnetism and magnetic transitions were found in magnetic measurements. Powder neutron diffraction studies in zero and applied field were carried out. Both compounds were refined in space group Pbcn and the following lattice parameters were obtained at 300 K, CoV2O6: a = 13.4941(20) Å, b = 5.5736(9) Å, and c = 4.8082(8) Å and NiV2O6: a = 13.3725(17) Å, b = 5.5344(7) Å, and c = 4.8162(7) Å. Neutron powder diffraction studies in zero field did not reveal any magnetic peaks for either of the compounds but magnetic order emerges in applied fields between 1 and 4 T.

The electronic structure of a variety of metal-heteroatom multiply bonded complexes, including some active alkyne metathesis catalysts, have been investigated using He(I) and He(II) ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and Fenske-Hall molecular orbital calculations. Utilizing this electronic structure information, confirmation of the proposed mechanism for the alkyne metathesis reaction which involves formation of a metallacyclobutdienyl intermediate was ascertained. Also, the important relationships between metallatetrahedral and metallacyclobutadienyl complexes, both of which have been mentioned as possible intermediates in the alkyne metathesis reaction and for which examples have been prepared and isolated, are discussed in significant detail. In the final chapters the electronic structure of some corresponding metal-nitrogen triply bonded complexes are discussed as well as the results probing the charge distribution in metal-heteroatom multiply bonded systems as determined by the XPS experiment.

Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.

This thesis documents the development of novel synthetic methodologies for the incorporation of sulfur dioxide into organic molecules employing the amine-sulfur dioxide complex DABSO (vide infra). These developed processes serve to access a range of sulfonyl-containing (-SO₂-) compounds including sulfones and sulfonamides, via sulfinic acid precursors. <b>Chapter 1</b> provides an overview of the synthesis and applications of sulfonyl-containing compounds and the organic chemistry of sulfur dioxide. A comprehensive introduction to the developed uses of sulfur dioxide surrogates in organic chemistry is given. The synthetic utility of metal sulfinates towards accessing sulfonyl-containing compounds is also discussed. <b>Chapter 2</b> details the development of a one-pot sulfone synthesis via metal sulfinates generated from organometallic reagents and DABSO. Alkyl, alkenyl and (hetero)aryl sulfinates prepared from organolithium and Grignard reagents can be efficiently coupled with a range of electrophiles to access a range of products including diaryl, aryl-heteroaryl and &beta;-hydroxy sulfones. <b>Chapter 3</b> describes an array-compatible, one-pot sulfonamide synthesis employing metal sulfinates and N-chloroamines as in situ-generated intermediates. This employs DABSO and sodium hypochlorite (bleach) as simple reagents and organolithium, organozinc and Grignard reagents along with amines as readily-accessible building blocks. The robust nature of this methodology and its potential application in discovery chemistry is demonstrated with a 65-compound array synthesis. <b>Chapter 4</b> documents the development of a palladium-catalysed sulfination reaction of boronic acids to access a range of sulfonyl-containing compounds. This involved the establishment of a one-pot/one step synthesis of sulfones leading to the discovery of a redox-neutral, ligand-free sulfination procedure using DABSO and palladium(II) catalysis. Sulfinic acid derivatives can be generated and subsequently trapped in situ with a variety of electrophiles to furnish sulfones and sulfonamides. <b>Chapter 5</b> summarises the research and the potential future work. <b>Chapter 6</b> provides experimental details and data.

This thesis is concerned with a systematic study of phosphine-containing transition metal complexes and cluster compounds by high resolution solid state nmr spectroscopy, using the techniques of magic angle spinning and cross polarisation. Previous work has indicated the potential of the solid state nmr technique to investigate a variety of materials: this is considered in the introduction to this thesis, and the reasons for choosing to study transition metal phosphine compounds are discussed. The analysis of spinning sidebands to obtain the principle values of the shielding tensor is examined to determine how well the calculated values represent the true values. Simulations of slow MAS spectra are proposed as a means of testing and refining the calculated tensor components before attempting to correlate the shielding with structural parameters. The results of a study of a series of crystalline phosphine-containing complexes and clusters are presented. The spectra are interpreted on the basis of the known crystal structures: in some cases separate resonances can be resolved in the solid state spectra from the distinct phosphine environments of a cluster framework, and from inequivalent sites in the unit cell. Information is obtained from the isotropic shifts, scalar couplings and the chemical shift anisotropy. Many of the compounds are fluxional in solution, some even at low temperature: whereas a number of these are found to be rigid in the solid state at room temperature, some of the crystalline compounds retain their fluxionality. Investigations of the species formed when transition metal carbonyl clusters are anchored to oxide supports were carried out. These show the presence of several distinct phosphorus-containing species, some of which are not consistent with the simple attachment of the cluster to the surface. The unique importance of the solid state technique is demonstrated in the study of these supported species.

This Thesis describes the synthesis and characterisation of a series ofbisphenolate supported samarium borohydride, amide and zwitterionic rare earth complexes and their ability to effect the ring opening polymerisation (ROP) of cyclic esters and methylmethacrylate (MMA). Chapter 1 introduces ROP from both an industrial and an academic perspective and describes in detail the research in this area, with emphasis on rare earth initiators. The lanthanide elements and the bisphenolate ligand are also introduced. Chapter 2 describes the synthesis and characterisation ofbisphenolate supported samarium borohydride and silylamide complexes. Chapter 3 describes the ability of a selection of samarium borohydride and amide complexes to effect the ROP of the cyclic esters s-caprolactone (f-CL) and rac- lactide (rac-LA). Emphasis is placed on the effect that the nature of the bisphenolate pendant arm and the initiating moiety has on the polymerisation process. Chapter 4 describes the synthesis and characterisation of rare earth zwitterionic complexes and the ability ofa range of these complexes to effect the ROP of s-Cl. and rac-lactide. Mechanistic aspects ofthe ROP process will be discussed, as will the ability of these complexes to yield amide functionalised poly(rac-LA). Chapter 5 describes the ability ofbisphenolate samarium borohydride complexes to initiate the polymerisation of MMA. The experimental work conducted as part of this study is further supported computationally by calculations at the DFT level, both aspects will be described. Aspects concerning the synthesis and characterisation of the related borohydride derivative [Sm(N2siMe3NNPY)(BH4)2Li]oo will also be emphasised. Chapter 6 contains full experimental and characterising data for all 0 f the new compounds reported in this Thesis. Appendices A- T contain tables of selected crystallographic data for all new crystallographically characterised complexes described in this Thesis (partially on CD).

Doctor of Philosophy / Department of Chemistry / Christopher J. Levy / A series of chiral titanium and manganese complexes with helix-directing salen ligands have been prepared, characterized and studied. Their structures displayed as a chiral helical motif as expected. And it was also found that all M(salen) units were exclusively M-helimeric in the solid state, except Ti(cyclohexyl-benz[a]anthryl) as P-helix. This may be due to the energy difference between P and M helice, which enables crystal packing forces to control and drive the molecular structure. This is also in agreement with the previous computational studies that the M configuration predominates in THF solution. All metal centers adopt a cis-β octahedral geometry except in Mn(binapthyl-phenanthryl-salen). Most of M(salen) complexes in this work afforded μ–oxo dinuclear helicates, instead of the expected monohelicate, except Mn(binapthyl-phenanthryl-salen), which is bridged by a third salen ligand. The titanium salt affected the complex solution behavior. In the presence of Cl[superscript]-, only mononuclear species was found by ESI-MS, while both di- and mononuclear species was found in MeOH in the presence of –O[superscript]iPr. The NMR spectra of Ti(salen) indicated one major species with cis-β geometry exist in most solution, which could be monomer or dimer, except Ti(binapthyl-salen). No counterions have been found in the solid state of Mn(salen) complexes in this work, but they affected the ligand decomposition in the solution in Mn(binapthyl-phenanthryl-salen). The Mn(salen) complexes could effectively and enatioselectively catalyze the asymmetric epoxidation of somoe trans, cis and terminal olefin, and various oxidants were employed.