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Drawing inspiration from nature where enzymes containing multi-metallic active sites are ubiquitous, chemists have designed various ligands to bind more than one metal in precise structural arrangements. In Chapter One, a class of binucleating Schiff base pyrrole (Pacman) macrocycles which are both straightforward to synthesise and can be varied systematically to alter the metal environment and intermetallic separation are introduced, along with the state-of-the-art in this area. Previously reported complexes of these ligands with late transition metals, lanthanides and actinides are also reviewed. The results and discussion chapters of the thesis focus on the isolation and investigation of previously unexplored early transition-metal Pacman complexes and present new advances in low oxidation state uranium Pacman chemistry. In Chapter Two, binuclear chromium(II) complexes of two Schiff base macrocycles, H4LMe and H4LA are described. [Cr2(LMe)] features an ortho-phenyl spacer between the macrocycle donor compartments whereas the Cr(II) ions are separated by a larger anthracenyl spacer in [Cr2(LA)]. Both compounds have been characterised in solution and the solid state. Reactivity studies were carried out for [Cr2(LMe)]. Reactions of [Cr2(LMe)] with isocyanides and triphenylphosphine oxide were investigated leading to the isolation of the contrasting co-ordination compounds [Cr2(OPPh3)2(LMe)] and [Cr2(μ-CNR)(LMe)] (R = xylyl, tBu). Oxidation of [Cr2(LMe)] with I2 yields the Cr(III)/Cr(III) Pacman products [Cr2(μ- I)(I)(THF)(LMe)] and [Cr2(μ-I)(py)2(LMe)][I] when carried out in THF or pyridine, respectively. Cr(III) alkyl compounds are obtained by reaction of [Cr2(μ-I)(I)(THF)(LMe)] with the non-reducing alkyl transfer reagents MgBrEt and ZnEt2. When ZnEt2 in toluene is employed, one zinc cation is incorporated in the molecular cleft, whereas use of MgBrEt in THF yields the simple chromium alkyl complex [{Cr(Et)}2(endo-THF)(LMe)]. One ethyl group may be abstracted from [{Cr(Et)}2(endo-THF)(LMe)] by [CPh3][B(C6F5)4] to form a cationic alkyl complex. The activity of both the neutral and cationic alkyl species towards ethylene was investigated. Conclusions are discussed at the end of the chapter. Previously, investigation of low oxidation state uranium Pacman chemistry has been confined to the smaller macrocycle, H4LMe, and frequently resulted in the formation of insoluble polymeric materials that were intractable and challenging to analyse. In Chapter Three, metallation of the larger macrocycle, H4LA, with UI3 to generate a single soluble species is described, although this product could not be isolated or characterised in the solid state. A new synthesis of [U(BH4)3(THF)2] from UI3 and NaBH4 affords an alternative U(III) precursor to UI3. Metallation of H4LA using a sodium base and U(BH4)3(THF)2 yields the ionic product [Na(THF)4][{U(BH4)}2(μ-BH4)(THF)2(LA)] which was characterised in solution and the solid state. Reaction of this compound with KO(C6H2(tBu)3) forms the ligand substitution product [{U(OAr)}2(endo-BH4K)(THF)2(LA)] which undergoes selective reaction with excess S8 or CS2 to form [{U(OAr)}2(μ-S2)(LA)] and [{U(OAr)}2(μ-S)(LA)] respectively. It was discovered that the [U(BH4)3(THF)2] metallation strategy could be successfully extended to H4LMe to form [Li(THF)4][{U(BH4)}2(μ-BH4)(LMe)]. Protonolysis of the borohydride ligands of the complexes of the two different macrocycles was investigated using the weak acid [HNEt3][BPh4]. NMR spectroscopy indicated that both exo BH4 − groups in both complexes can be successively removed to generate neutral and cationic complexes but these were not isolated. Metallation of H4LA with UCl4 forms the ionic product [Li(THF)4][{U(Cl)}2(μ-Cl)3(LA)]. Various ligand substitution reactions were attempted but the only structurally characterised product was [{U(OtBu)(Cl)}{U(OtBu)(py)}(μ-Cl)(LA)], formed by reaction with KOtBu. Conclusions are discussed at the end of the chapter. Experimental and characterising data are provided in Chapter Four.

This work has shown that nitrogen dioxide will selectively remove uranium hexafluoride from a mixture containing the hexafluorides of uranium, molybdenum and tungsten via the formation of nitrylium hexafluorouranate(V). The production of lower uranium fluorides and elemental sulphur or sulphur fluorides from many of the attempted preparations of uranium(VI) thiofluorides was predicted from a thermodynamic study of the stabilities of MC1nF6-n (M = U, Mo, W; n = 1-5), MOnF6-2n' MSnF6-2n (M = U, Mo, W; n = 1 or 2) with respect to decomposition and disproportionation. Sorption of the liquid hexafluorides on various potential sorbents was investigated. Encouraging results were obtained for cryolite. Salts such as ARuF6 (A = K, Rb, NH4) were synthesised by direct interactions between AF and RuF5 in AHF. These reactions were found to require the presence of a polar solvent. It was found that ruthenium pentafluoride in liquid uranium hexafluoride could be removed by the Introduction of trace quantities of water.

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Attenuation of uranium (U) and rare earth elements (REEs) has been observed in stream and wetland sediments, but the geochemical and mineralogical processes involved in sequestering these elements in natural systems are not well understood. The decommissioned Bicroft Uranium Mine near Bancroft, ON uses a modified stream and wetland system to reduce the concentrations of U and other metals in tailings pond effluent to levels below the Provincial Water Quality Objectives. The Bicroft Mine was operated from 1957 to 1963, and processed low-grade (~0.17 wt% U3O8), disseminated U hosted by pegmatite dykes in amphibolite gneiss, forming 2,284,421 tonnes of tailings, deposited into two tailings impoundments. The Bicroft site has since operated as a passively attenuating stream and wetland remediation system for 55 years, demonstrating the potential longevity and viability for long-term sequestration of U and REEs through natural attenuation. To identify U and REE hosts and their stability in the natural environment, colloids, tailings, stream and wetland sediments were analyzed using various methods, including tangential flow filtration (TFF), ICP-ES/MS, scanning electron microscopy, and synchrotron techniques (bulk and µXANES, µXRF, and µXRD). The results show that Fe- and Mn-oxyhydroxides, goethite, and birnessite are the main mineral hosts for U and REEs in both the colloids and sediments. In addition, detrital grains of U- and REE-bearing minerals were found >200 m downstream in colloids and wetland sediments, showing the potential for long range transport of colloids and particulates in the stream system. Seasonal influences on the stability of trace metals in sediments were observed, and may demonstrate the limitations of passively attenuating remediation systems as a viable method for attenuation. Changes in the redox state of the stream system were observed to influence the attenuation of U and REEs, however, changes in the redox state with depth in the wetland sediments were not observed, and showed no direct influence on the attenuation of U and REEs. The results of this study will help to develop better monitoring strategies for U tailings sites and should reduce the impacts of future U mining operations. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2013-05-06 07:44:03.327

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Density Functional Theory (DFT) is the most widely used theoretical tool for studying actinide complexes amongst the many available computational methods. However, the best choice of functional for theoretical actinide science is still not completely clear. Thus, benchmarking studies have been performed in this thesis in order to evaluate the performance of modern DFT applied to actinide systems. The first chapter is an introductory chapter which gives the background and the methods applied in this thesis. The second chapter is a part of the actinide spectroscopy Round-Robin test (RRT). The performance of DFT using different quantum chemistry programs with identical DFT functionals has been investigated by studying five U(VI) acetate complexes. The experimental data from the other clusters of RRT are used as reference for the quantum chemical calculations. The performance of a total of 22 different DFT functionals for small uranium complexes has been further investigated in Chapter 3. These functionals are compared by calculating geometries, vibrations frequencies, and reaction enthalpies against experimental data and high level ab initio CCSD(T) calculations. The last chapter presents a summary of the works in this thesis as well as directions for future studies. / May 2016