The unifying objective of this work is the study of model systems that will assist in the development of new metal catalysts capable of converting carbon monoxide and hydrogen (syngas) into organic oxygenates at mild temperature and pressure. The selective catalytic transformation of carbon monoxide and hydrogen to liquid fuels and chemical feedstocks represents a major "Grand Challenge" of catalysis science. A core objective is the study of a macrocycle that is related to a porphyrin ligand in order to circumvent some of the disadvantages of utilizing porphyrins in catalysis. The rhodium complex of the macrocycle dibenzotetramethylaza[14]annulene ([(tmtaa)Rh]2]) was reacted with a series of small molecules relevant to CO reduction and hydrogenation. Several complexes were formed that demonstrated the ability to partially reduce and hydrogenate carbon monoxide, including the dirhodium ketone (Rh-C(O)-Rh) and an example of a thermodynamically favorable metal formyl complex ((tmtaa)Rh-C(O)H). An important feature of this work is the measurement of thermodynamic data to provide experimental benchmarks for obtaining key species in CO reduction and hydrogenation. A thorough study of the (tmtaa)Rh system will help in identifying structural features that assist or hinder CO hydrogenation. All reactions are monitored by 1H NMR which permits determination of solution thermodynamics from equilibrium constants obtained by NMR peak integrations. DFT calculations have supplemented experimental results by providing estimates to compare with the experimentally determined thermodynamic data. These computations also provide insight into the structures and thermodynamics of species that cannot be observed directly such as short lived intermediates and thermodynamically unfavorable products. Heterobimetallic complexes of (tmtaa)Rh* with a second metal radical have been utilized to attempt to convert CO to organic products. Reactivity with CO and H2 has been accomplished, with most of these metal systems providing (tmtaa)Rh-C(O)H and M-H as final products. These systems are now ideally set up for catalysis in which M-H can deliver hydrogen to reduce and hydrogenate the rhodium formyl unit. Performing these reactions at high pressures of CO/H2 or with a more sterically hindered analog of tmtaa may provide the conditions necessary for catalysis. Reactivity, thermodynamic and computational studies have been used to analyze and compare bond energies in this current work with previous studies accomplished in the Wayland group. These comparisons permit further understanding into the factors that control bond strength, guiding future studies and allowing tuning of bond strengths based on choice of conditions for a catalytic process. This research has resulted in a more complete understanding of the factors that control the favorability of various intermediates in CO reduction and hydrogenation and the application of these results can be used to guide the next generation of metal ligand systems that will yield organic oxygenates at mild conditions. / Chemistry

This thesis presents new research on the topics of oxygen evolution catalysts inspired by Photosystem II’s active site, the oxygen evolving complex (OEC). The approach uses a 2-pyrine methanol scaffolding ligand. A second project involves the synthesis of green burn rate modifiers for inducing a catalytic burn rate effect in solid propellants and replacing lead with a friendlier metal that is as effective. The shift toward renewable energy sources has led to increased attention to new methods of storing and harnessing energy. Solar and wind energy are becoming cost-effective and will continue to improve. A problem with such renewables is peak production times for electricity on off peak usage. Peak electricity use is in the evening, but peak renewable production is earlier in the day. This creates a mismatch in pairing availability of electricity in the grid and its usage. The solution to this is the storage of excess energy. Currently, excess energy produced from all inputs, such as coal and nuclear, are used to pump water vertically as a means of energy storage. This water stores energy as potential energy like a dam. When the energy is needed, the water is allowed to flow downward through a turbine. This process is very energy inefficient and wastes a lot of energy through friction and heat loss. The need for an efficient and scalable storage method is desired to replace the methods currently in place. Hydrogen gas can be used instead of this method, by burning or using hydrogen in an electrochemical cell. Water can be used as a feedstock for hydrogen generation. Electrolytically generated hydrogen gas is scalable for energy grid storage, but efficiency problems in electrolytic water splitting continue to hinder greater adaptation of the technology. An electrocatalyst is needed to bring hydrogen generation to the forefront of grid storage. Nature has developed an enzyme that produces molecular oxygen and protons from water, known as Photosystem II (PSII). The interest in Photosystem II is to better understand the mechanism the oxygen evolving complex (OEC) undergoes when it transitions through its various oxidation states, and to explore catalysts that are functionally similar to OEC to both understand the OEC and design better catalysts for oxygen evolution. Taking inspiration from nature with the CaMnO5 cluster as the active site in PSII, an ideal catalyst could evolve oxygen at an anode while readying the protons for hydrogen evolution at a cathode. This is the motivation for our exploration of hemicubane motifs with manganese and calcium and exploring solubility and reactivity in water. A functionally related but structurally and compositionally distinct enzyme in oxygen metabolism is cytochrome c oxidase. This enzyme uses a copper and Fe-heme active site to reduce oxygen to water. A binuclear copper site located away from this active site reduces the cytochrome c to then allow the electron to transfer through to the active site. The oxygen reduction seems to occur between the Cu and Fe-heme sites, and not the adjacent Fe-heme site. In multi-copper oxidases, three coppers bind oxygen and a fourth copper site transfers electrons to the other coppers. A second project of this thesis is the exploration of alternative metals and the isolation of said metal ions with stabilizers in solid propellants. Metal ions create a phenomenon in rocket fuel known as burn rate modification, with the ability to increase the surface burn rate and pressure of the propellant upon burning. These characteristics combine for increased velocity and acceleration of the rocket. The most commonly used burn rate modifier (BRM) has been lead-based, whose use in the manufacturing process is toxic. Lead is being replaced by other metal BRMs, but all alternatives shorten shelf life of the propellant by increasing radical chain decomposition. In chapter 3, we describe the preparation and isolation of a Mn3 trimer, that was synthesized in inert atmosphere and anhydrous solvents. The trimer has a distorted octahedral and two distorted trigonal bipyramidal coordination environments for the MnII ions. Two ligands coordinate to open sites on the two distorted trigonal bipyramidal metal ions, that could coordinate solvents. In chapter 4 we describe the preparation and isolation of Mn hemicubanes that are water soluble and coordinate with water. Over time the cluster oxidizes from exposure to water, indicating some interaction with an oxidant that can cause the alcohol arm of the ligand scaffolding to oxidize to a carboxylate. This phenomenon does not occur with a different ligand scaffolding, it only occurs with a (4-(dimethylamino)pyridin-2-yl)methanol. The Mn4 clusters, when doped with calcium exhibit electrochemical stability and catalytic activity in water. Mn4 clusters are stabilized by the presence of calcium in solution, but do not prevent the oxidation of the cluster over longer time frames. In chapter 5, we describe electrochemical experiments coupled with oxygen experiments to determine cluster reactivity in water. It is found that the (4-(dimethylamino)pyridin-2-yl)methanol scaffolding yields water soluble clusters capable of oxidation. Bulk electrolysis with coulometry showed very little oxygen or hydrogen peroxide formation. The cluster is a poor catalyst for oxidizing water. It has been shown that doping the solution with CaII stabilizes the catalyst depositing on the electrode. This new catalyst does not generate additional deposition with every sweep, or with a constant positive potential. The 2-pyridine methanol showed similar reactivity with water oxidation, and with CaII present. The 2-pyridine methanol cluster does not oxidize over time in solution though. In chapter 6 we describe analogous attempts to synthesize cobalt, calcium, and copper clusters and their isolation. Calcium formed mononuclear complexes with the ligand hydroxide groups still protonated, whereas cobalt formed hemicubanes clusters with bridging alkoxide ligand arms. Cobalt also formed two different scaffolding motifs; one motif is similar to the chapter 4 Mn ion hemicubanes, and a new cluster motif, where the pyridyl nitrogens bonded to the same corner Co, and the alkoxide arms formed the cluster. This left coordination sites in the middle of the cluster, where solvent would ligate the complex. In chapter 7 we describe the replacement of the pyridyl groups of 2-pyridine methanol with five-membered heterocyclic rings, and their reactions with manganese and cobalt precursors. These reactions yielded degraded ligand products, protonated ligands with an anion, or a polymer with an unknown origin for the ligand scaffolding. The five membered ring-based ligand scaffolds thus proved to be too unstable for the formation of clusters. In chapter 8, we describe the electrochemical experiments of a CuII dimer that showed promise for catalytic oxygen reduction. We present preliminary evidence that a synthetic Cu-Cu complex is able to reduce oxygen when in a Cu(I) state exposed to oxygen. The dimer shows catalytic activity in acetonitrile with oxygen present and when oxygen is removed the catalytic wave is absent. The dimer is robust and does not decompose during electrochemical experiments, though the signatures the dimer shows in the absence of water indicate water is binding to the dimer and interacting with it. In chapter 9 we present the combination of a stabilizer with feasible metal catalysts to produce a copper complex that is coordinated with stabilizers with two outer sphere oxidizers. The synthesis of a CuII complex surrounded by n-phenyl urea produced a CuI tetrahedral acetonitrile complex with a perchlorate counteranion. These two reactions are competitive to each other, where the formation of the ACN complex is much easier and hinders the synthesis of the CuII complex. The CuII complex is square planar, with open sites in the z-axis, but has perchlorate in proximity to the open coordination site. This complex could be used as a BRM that has a localized stabilizer, while the other Cu-ACN complex could be a potential primary explosive. These topics all involve coordination chemistry and their application to build better catalysts for their respective fields. Oxygen evolution enables better access to molecular oxygen for hospitals and a source of hydrogen for molecular hydrogen. Oxygen reduction enables the ability to drive electrochemical cells to consume oxygen to generate energy from the reaction with hydrogen. Stabilizing burn rate modifiers allows for the expansion of potential metals as catalysts for propulsion. / 08/30/2029

<p>Series of rare-earth intermetallic compounds were synthesized and studied.</p> <p>The Gd₄Ge_3-<em>x</em><em>Pn_x</em> (<em>Pn</em> = P, Sb, Bi; <em>x</em> = 0.5 - 3) series of compounds were synthesized to explore the stability of the non-existent Gd₄Ge₃ binary through partial <em>Pn</em> substitutions to increase valence electron concentrations. Electronic band structure calculations were performed to elucidate the relationships between the hypothetical "Gd₄Ge₃" and Gd₅Ge₄ binaries. All Gd₄Ge_3-<em>x</em><em>Pn_x</em> phases order ferromagnetically with relatively high Curie temperatures of 234 to 356 K.</p> <p>The Gd₅Ge_4-<em>x</em>P<em>_x</em> phases were synthesized to explore the effects of both atomic size and valence electron concentration differences between Ge and P atoms. Partial substitution of P for Ge atoms occur on the interslab site, which causes the interslab distances to increase. In Gd₅Ge_4-<em>x</em>P<em>_x</em>, only a small amount of P substitution (<em>x</em> = 0.25) is required to induce ferromagnetic ordering. The appearance of a Griffiths phase is also discussed.</p> <p>The temperature dependence of the different <em>RE</em>₂Fe₁₇ (<em>RE</em> = Gd - Ho) phases was tracked by reciprocal space images generated from single crystal X-ray diffraction.</p> <p>GdCo₄B was synthesized by arcmelting and tri-arc techniques to confirm the presence of magnetostriction. Single crystal and powder X-ray diffraction, dilatometer and magnetic measurements were done.</p> / Master of Science (MSc)

No description available.

A detailed programme of research is presented to design, build and operate a high resolution h?hz5000 curved crystal Johann-type x-ray spectrometer for the waveband below 13A. The spectrometer is used to observe line emission features from different laboratory x-ray sources. Characteristics of the Johann geometry are described with emphasis given to the properties of sensitivity, dispersion, resolving power and waveband. The tolerance of the instrumental parameters is defined for successful high spectral resolution operation. The key feature of the spectrometer is the unique crystal bending device which can generate a high quality cylindrical curvature of radius R=150?5000mm. The crystal focusing alignment and testing procedures are evaluated. Choice of crystals suitable for the observation programme is discussed together with analysis techniques for interpretation of the x-ray spectral line profiles. The instrument is optimised for time-integrated and time-resolved ion temperature measurements of UKAELA DITE Tokamak at the Culham Fusion Laboratory. X-ray line emission results from medium Z He-like and H-like impurity ions are presented for different plasma conditions. Density sensitive He-like and Li-like Aluminium ion satellite emission features are studied for intense transient laser produced plasmas at the Central Laser Facility, SERC Rutherford Appleton Laboratory. The peak plasma electron density of 0.1 time solid density is estimated from these line intensity ratios and is in good agreement with Stark line width measurements. X-ray emission from beam-foil interactions is observed on the Folded Tandem accelerator of the Nuclear Physics Department, Oxford University. The proposed improvement in the intrinsic spectral line broadening due to the accelerator is investigated by high resolution axial beam measurements of the He-like Silicon and H-like Neon n=2 transitions. The Lyman-a intensity ?-ratio and wavelength separation ?hFS is studied for the fine-structure of Hydrogenic Neon, Magnesium, Aluminium and Silicon. The fine-structure separation is compared with the Dirac theory and other experimental data, while the possible mechanisms giving rise to the non-statistical value of the ?-ratio are analysed.

A simplified description is given of the operation of a multiwire proportional chamber (MWPC) in the soft X-ray imaging application. Expressions are developed to allow the calculation of the distribution of induced charge on the cathodes of an MWPC. With extensions to permit direct comparison, the calculations are subjected to detailed experimental verification. A generalised, approximate formulation of the distribution with one independent parameter is described. The prediction of cathode system position response using the theoretical distributions is demonstrated. The available MWPC position readout methods are reviewed, and where possible their differential non-linearity is measured experimentally. A new position-sensitive cathode of good linearity and spatial resolution is presented. The effect of the wires of an MWPC on its imaging performance is briefly considered. An attempt is made to assess the contribution to MWPC spatial resolution of the range of the electrons produced initially by an X-ray absorption event in argon-methane mixtures. In conclusion, the important causes of MWPC imaging imperfection are noted and classified.

Alkali-metal diuranates (VI) and dineptunates (VI) have been prepared by controlled thermal decomposition of well-characterised alkali-metal uranyl (VI) and neptunyl (VI) triacetates having the desired stoicheiometricalkali-metal to actinide ratio. The compounds in the series MI2MVI207 (MI = Na - Cs incl., MVI = U or Np) have been obtained in this way. This preparative method failed to provide further evidence for the existence of the controversial ternary oxide, Li2U207. Isolation of the analogous neptunium complex, Li2Np207, was not achieved. Enthalpies of formation have been derived from the enthalpies of solution in 1 mol dm-3HC1 and auxiliary thermodynamic data. The values for Na2U207 and Cs2U207 are in good agreement with those assessed by Cordfunke and O'Hare and thus confirm the reliability of the method of preparation of such mixed oxides, as compared to the 'classical' methods which involve heating an actinide oxide with an alkali-metal binary oxide, nitrate or carbonate. The values for K2U207 and Rb2U207 are previously unreported. Confirmation of an earlier assessment of the enthalpy of formation of Na2Np207 and preliminary values for the enthalpies of formation of K2Np207, Rb2Np207 and Cs2Np207 have been obtained.

The general theme of this thesis is the effect of solvents on the initial state and transition state contributions. This has involved kinetic and thermodynamic measurements. The kinetics were examined for a variety of inorganic reactions with the aim of investigating rate laws, products of reaction (where possible) and suitability for an initial state-transition state analysis. The following reactions were followed:- (a) 2,2'- Bipyridyltetracarbonylmolybdenum (0) and various related compounds with cyanide and other nucleophiles; (b) manganese and rhenium carbonyl halides with cyanide and amino-acids; (c) potassium tetrachloroplatinate (II) with cyanide; (d) hexadentate Schiff base complexes of iron(II) with cyanide and with hydroxide. An extension of mercury (II) catalysis was carried out in the examination of solvent effects on such catalysis, and on the hydrolysis reaction, of the pharmaceutical fenclorac (?-3-dichloro-4-cyclohexyl-benzeneacetic acid). The thermodynamics and solubility measurements have enabled the calculation of Gibbs free energies of transfer. This has been done for potassium cyanide in non-aqueous and in mixed aqueous media, also for potassium tetrachloroplatinate (II) and 2,2'-bipyridyltetracarbonyl-molybdenum (0) in limited ranges of solvents and solvent mixtures. From this [special character omitted](non electrolytes), directly, and (ions), using standard extrathermodynamic assumptions, may be obtained to get data on the initial state. These together with the kinetic data permit an initial state - transition state dissection of solvent effects on reactivity. Such an analysis was successfully carried out for 2,2'-bipyridyltetracarbonylmolybdenum (0) and potassium tetrachloro-platinate(II). In connection with the reaction of the iron(II)-hexadentate Schiff base complex with cyanide and hydroxide, it was necessary to carry out an X-ray structure determination to be certain the central moiety of the hexadentate ligand was in the form -NHCH2CH2-NH rather than the diimine form -N=CH CH=N-.

Solvation trends of ternary iron(II)-cyanide-diimine complexes in binary aqueous media are examined using spectroscopic and thermodynamic techniques. The observed trends are discussed in terms of the solvent-solute interactions present, and how the relative importance of these interactions varies with the technique and the complex. Crystallographic data for two of these structures are reported and show the effect of the solvate molecules on the structure. Pressure effects on MLCT frequencies are investigated, and the correlation between piezochromic and solvatochromic effects for mono- and binuclear iron(II) and iron(III)-diimine complexes is shown. Pressure and solvent modification of the rate of oxidation of a series of iron(II)-diimine complexes are interpreted and discussed in terms of the solvation changes that occur upon transition state formation. Complementary partial molar volume data for these complexes are also reported, and combined to form a volume profile for one reactant pair. The structure of a binuclear iron(II)-diimine complex is examined in the solid state and in solution. Solvation trends in binary aqueous media are reported for this binuclear cation, and comparisons are made with mononuclear iron(II) and cobalt(III), and with binuclear cobalt(III) complexes. Kinetics of base hydrolysis of several binuclear iron(II)-diimine complexes are discussed in terms of the structural differences between these complexes. The effects of added organic cosolvents on the rate constants for one of these binuclear complexes is also reported. Spectroscopic evidence for a ligand substituted intermediate is found. The structure of such intermediates is discussed with respect to existing data for mononuclear iron(II)-diimine complexes. Finally, a preliminary chemical and electrochemical redox investigation is made on the most suitable binuclear iron(II)-diimine complex in light of the base hydrolysis reactivity patterns established.

The reaction between WF6 and C6F5NH2 is found to proceed via iminolysis. 19F NMR analysis of acetonitrile (MeCN) solutions show the product to be a mixture containing [C6F5NH3]+[WF5(NC6F5)]-, [C6F5NH3]+[W2F9(NC6F5)2]-, [C6F5NH3]+F- and the unique mixed imido-, oxo-species [O=WF4-F-WF4(NC6F5)] - formed from traces of oxyfluoride impurities. The 19F nmr spectrum of a trifluoroacetic acid (TFA) solution showed the presence of the dimeric anion salt and fluorination products. An X-ray crystallographic study of a single crystal of [C6F5NH3]+[W2F9(NC6F5)2]-, grown from TFA, revealed the compound to adopt a triclinic (space group P1)structure, with two formula units in the unit cell, each with a slightly differing geometry about the bridging fluorine atom. The reaction of Me3SiNCO with MF6 (M= Mo,W) was examined. The products are assigned as MF5NCO3 from IR spectra recorded. The reaction between MoF6 and (Me3Si)2NH results in the formation of MoF4NH. Addition of MeCN to MoF4NH produces a 1:1 adduct, MoF4NH.MeCN. Analysis of the IR spectra of the two compounds indicates that MoF4NH adopts a cis-fluorine bridged chain structure, and MoF4NH.MeCN adopts a pseudo-octahedral (C4), monomeric stucture in the solid state. WOF4 reacts with Me3SiNCO to form WOF3NCO, identified from its mass spectrum. The hexafluorides MF6 (M= Mo,Re), when allowed to react with tetrakis-tri-methylsilylhydrazine (TTSH) undergo reduction to the pentafluoride in approximately 80% yield. Tungsten hexafluoride is believed to react with TTSH to form the diazene compound, WF4N-MWF4.