Th(IV) and U(IV)-Carboxylates: Species Formation, Stability, and Reactivity
Vanagas, Nicole A.
Knope, Karah E.
This dissertation focuses on the synthesis and characterization of An(IV)-carboxylates, (An = Th and U), with the aim of gaining a more thorough understanding on how the identity of the organic ligand influences species formation. Twenty-one compounds have been synthesized using a range of mono- and dicarboxylates that vary in pKa and sterics. The structural differences are governed by the metal center, the nature of the organic ligand, and solution conditions. Systematic modification of the synthetic conditions allowed us to examine the influence of metal to ligand ratio, reaction time, pH, solvent system, and temperature on the structural units formed. The structures of the resulting compounds varied with respect to metal ion nuclearity, ligand binding modes, and dimensionality. Inspection of the structural chemistry provided a handle with which the effects of hydrolysis and condensation, ligand complexation, and solution conditions on An(IV) could be assessed.Species formation is governed largely by metal ion oxidation state, as well as hydrolysis and condensation and ligand complexation. Due to their relevance to separations chemistry as well as environmental systems, actinide-carboxylate interactions have received significant interest. However, only a limited number of carboxylates have been studied. In this work, actinide complexation by a series of carboxylates varying in pKa and sterics were examined to assess the effects that such modifications have on species formation, stability, and reactivity. Differences that arise due to the identity of the metal center (i.e. Th(IV), U(IV)) were also investigated. The first two chapters present an introduction to An(IV)-carboxylate structural chemistry and the factors that influence speciation. In Chapter 3, the influence of furoate and 2,5-furan dicarboxylate on the structural chemistry of Th(IV) compounds isolated from aqueous solution are described. Chapter 4 examines the influence of synthetic conditions on U(IV)-2-furoate structural chemistry and identifies trends in Th(IV) and U(IV) structural chemistry. The solution and solid-state structural chemistry of Th(IV) and U(IV)-4 hydroxybenzoates are examined in Chapter 5 to further examine trends in periodicity. Finally, reactivity of U(IV) hexamers as precursors for the formation of UO2 and the influence of synthetic conditions on particle size is presented in Chapter 6.
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