The coordination- and organometallic chemistry of uranium complexes bearing the non-carbocyclic ancillary ligand XA2 (4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) has been developed as a major focus of this thesis. A number of air-sensitive actinide chloro complexes and alkyl derivatives featuring reactive An–C bonds were prepared, and investigated using a variety of structural and spectroscopic analytical techniques, including X-ray diffraction, NMR spectroscopy, elemental analysis, and electrochemical methods. The research described in this thesis serves to expand the currently underdeveloped, fundamental chemistry of actinide complexes supported by non-carbocyclic (i.e. non-cyclopentadienyl) ligands. For example, the use of the prototypical xanthene-based ligand XA2 has led to neutral dialkyl uranium(IV) complexes which a) react with alkyl anions to yield anionic trialkyl ‘ate’ complexes, b) C–H activate neutral pyridines to yield organouranium(IV) species featuring cyclometalated pyridine-based ligands, and c) react with Lewis acids to yield rare examples of cationic monoalkyl uranium(IV) complexes featuring coordinated arene ligands. By altering the nature of the arene solvent/ligand, latent catalytic ethylene polymerization behaviour has also been unlocked in cationic XA2 uranium and thorium complexes, and this development may offer industrial relevance. Additionally, new NON-donor ligand designs featuring bulky terphenyl-based substituents (the "XAT" ligand) as well as 1-adamantyl groups (the "XAd" ligand) have been developed; a family of crystallographically-characterized dipotassium XAT complexes have been prepared which feature unprecedented potassium–alkane interactions, and the XAd ligand has been employed for the development of new organometallic thorium chemistry. The developments described in this thesis contribute to an emerging field and delineate new reactivities and structural motifs, providing important steps forward in organoactinide chemistry. / Thesis / Doctor of Philosophy (PhD)

<p>While crucial advancements have been made in understanding transition metal−nitrogen interactions, the actinides have not been studied in such depth as their transition metal counterparts. Uranium has shown to catalyze the Haber−Bosch process to produce NH₃ but more attention has turned to transition metals such as iron due to their low cost and accessibility. It is thought that transition metal imido species are essential intermediates to this process; therefore, it is critical to understand NH bond cleavage and formation on the metal. To study the potential that uranium has, it is important to bridge the knowledge gap of uranium with its transition metal counterparts and further understand NH bond cleavage and formation on the metal to make the suspected imido intermediate.</p> <p>Redox neutral methods have been popular and effective for synthesizing uranium imido complexes such as starting with a uranium(IV) amide and deprotonating it with a base to yield its respective uranium(IV) imido. It was of interest to understand if the bisTp* uranium(III) system would be amenable to a deprotonation pathway. To test this, the reactivity of Tp*₂UBn with bulky 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)benzenamine (terpy-aniline) and sterically smaller p-toluidine (ptol-aniline) was explored to first synthesize uranium(III) anilido species. Following successful synthesis, their reactivity is explored to yield respective uranium(IV) imido species by oxidative deprotonation.</p> <p>In addition to redox neutral methods, synthetic processes that rely on redox reactions at the uranium center have also been successful but are less common since the starting material must be a stable, low-valent uranium species. Our group has explored this method to make uranium(IV) imido species where the addition of 1 equivalent of organic azide to trivalent Tp*₂UBn or one equivalent of organic azide and potassium graphite to Tp*₂UI results in the formation of uranium(IV) imido species. The downside to this is azides are explosive and their synthesis could inhibit synthesis of diverse complexes. A redox method that eliminated usage of explosive azides is of interest so the reactivity of hydrogen atom transfer (HAT) reagents, Gomberg’s dimer or the 2,4,6-tri-tBu-phenoxy radical (·OMes*), with uranium(III) anilido complexes of varying steric bulk and electronic profile was explored. Conversion to their respective uranium(IV) imido species was achieved and this method was also explored with uranium(III) amides smaller than a phenyl since their respective azide are too dangerous to synthesize.</p> <p>Following isolation of uranium(III) anilido complexes and exploring reactivity it was of interest to understand how they compare to phosphorus analogues and how reactivity and interactions might be similar. Reactivity of Tp*₂UBn with phosphines of various steric bulk and electronic profile allowed for the isolation of uranium(III) phosphido complexes and their reactivity showed to be different than previously explored uranium(III) anilido counterparts. The electronic differences of the pnictogens were also observed in the crystal structures.</p> <p>With the differences in reactivity and electronic effects between the nitrogen and phosphorous complexes having been observed, our curiosity expanded to explore more uranium-pnictogen interactions. Therefore, synthesis of bis-substituted arsine and bis-substituted phosphine ligands were conducted for reactivity with Tp*₂UBn. Preliminary data reveals these bonds are more unstable and reactive relative to uranium(III) anilido species, likely due to the electronic mismatch between oxophilic uranium and soft pnictogens. Where applicable, compounds were characterized by multinuclear NMR spectroscopy, infrared spectroscopy, electronic absorption spectroscopy, single crystal X-ray crystallography, and quantum chemical calculations.</p>

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The chemistry of actinides in solution is a very important aspect of the nuclear fuel cycle, especially as the energy needs of the world continue to increase. However, the radio-active nature of the actinides makes experimentation very difficult and dedicated expensive instruments are required. In addition, the disposal of radio-active waste materials requires a proper understanding of their chemistry at a molecular level. To tackle the problem, and to underpin the experimental studies, in this thesis we have studied the redox chemistry and disproportionation mechanism of actinyl complexes in solution using state-of-the art computational methods. Reduction potentials of actinyl complexes in solution have been estimated in solution using density functional theory (DFT) approaches. Solvation effects were included in the quantum chemistry calculations with the conductor like polarisable continuum model (CPCM) solvation method. First of all, we have validated our computational method by studying a variety of solute cavity definitions within the CPCM solvation model and assessed the performance of a range of DFT functionals to suitable to accurately describe the actinide chemistry in solution. Penta-valent uranyl(V) ions are unstable and readily disproportionate; in this study we have explored outer-sphere electron transfer and disproportionation mechanisms to determine the stability of these ions in solution. We have found that the process of outer-sphere disproportionation is unlikely to occur in non-aqueous solutions, such as DMSO, DMF, DCM, acetonitrile and pyridine, when the uranyl(V) ion is bound with a multi-dentate organic ligand. However, our computational results hypothesise that the presence of a trace of water in the experimental conditions can promote a disproportionation reaction by protonating the uranyl(V) ‘yl’ oxygen atoms and then the electron transfer process would proceed through either inner or outer sphere mechanism. In addition, the effect of alkali metal cations on the outer-sphere disproportionation mechanisms was also studied. Overall it has been shown that DFT can be used to accurately predict the redox properties of actinyl complexes in solution and thus contributing for an effective and efficient design of nuclear material separations, proper as well as safer radioactive waste disposal.