Uranium oxides are the most common form of uranium in both nuclear waste and in the environment. As such, the uranyl dioxo framework (UO2)2+, and its strong U≡O triple bond interactions, dominate the research. It is well-established the oxo ligands are thermodynamically robust and kinetically inert, thus making the uranyl fragment very unreactive. Interestingly, microbial activation of uranyl is a vital component in the sequestration of uranium in uranium-contaminated environments, likely requiring functionalization of the normally unreactive oxo ligands. However, there is little evidence available for the mechanism of this process and model systems describing this chemistry are scarce.
In contrast to uranium oxide chemistry, the chemistry of uranium and the softer Group 16 elements (e.g., S, Se, Te) is substantially less developed even though it has been proposed that separation of radiotoxic fission products in spent nuclear fuel would be greatly enhanced by employment of soft-donor extractants. This postulation has been rationalized on the basis of increased covalency within actinide-chalcogenide bonds. As such, developing soft-donor, heteroatom-substituted analogues of the uranyl moiety will allow for greater insight into the extent of f-orbital participation, and thus, covalency, within uranium-ligand multiple-bonding frameworks and facilitate the design of novel waste extractants.
This first part of this dissertation describes the activation of uranyl via reductive silylation. By employing strongly donating equatorial co-ligands, the nucleophilic nature of the oxo-ligands is exploited by addition of Me 3SiI. This substrate promotes reduction to U(V) and silylation of both oxo ligands. Subsequent addition of Lewis bases results in facile reduction to U(IV) and clearly displays the complete transformation of the uranyl fragment to better described 'alkoxide-like' complexes.
The second part of the dissertation attempts to expand the library of uranium-chalcogenide complexes, targeting molecules which exhibit multiple-bonding ligation modes reminiscent of uranyl. In this regard, a highly reducing U(III) starting material was employed and found to readily activate chalcogen sources to yield several complexes which exhibit terminally-bound chalcogenide ligands. These complexes were fully characterized to better understand the electronic nature of the uranium chalcogenide bonds.
|Advisor:||Hayton, Trevor W.|
|Commitee:||Butler, Alison, Ford, Peter C., Zakarian, Armen|
|School:||University of California, Santa Barbara|
|School Location:||United States -- California|
|Source:||DAI-B 74/08(E), Dissertation Abstracts International|
|Subjects:||Chemistry, Inorganic chemistry|
|Keywords:||Chalcogenides, Multiple bonding, Selenium, Tellurium, Uranium, Uranyl|
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