UC Irvine

This dissertation describes synthetic, structural, spectroscopic, reactivity, and theoretical investigations into the factors that stabilize thorium and uranium compounds in the low oxidation states +2 and +3. Extensions of the actinide work to rare-earth metals and bismuth are also included. The importance of understanding this aspect of actinide metal chemistry is described in Chapter 1. Chapter 2 describes a theoretical study using density functional theory (DFT) on U(II) complexes analyzing how the local coordination geometry around the uranium center influences the electronic configuration. U(II) species with planar coordination geometries favor 5f36d1 electron configurations, while non-planer geometries appear to favor 5f4 configurations. Chapter 3 describes electrochemical determination of the Th(IV)/Th(III), Th(III)/Th(II), and U(III)/U(II) reduction potentials in tris(cyclopentadienyl) complexes with complementary spectroelectrochemical measurements. It was found that the reduction potentials trend with ligand donation strength. In addition, the study revealed that the Th(IV)/Th(III) and Th(III)/Th(II) reduction potentials were quite similar which led to the synthesis of Th(II) compounds directly from Th(IV) precursors.Chapters 4–6 detail reduction studies of heteroleptic U(III) complexes to prepare new U(II) compounds and understand how to stabilize this low oxidation state. Within these Chapters are UV-visible and EPR spectroscopic studies and DFT calculations on newly synthesized compounds to probe the electronic structure of the U(II) species. The heteroleptic species appear less stable than the homoleptic analogs despite having similar steric properties and reduction potentials. Chapter 7 describes the reaction of U(III) compounds with organoazides which led to the observation of a U(V) intermediate and a new mechanistic proposal for the formation of U(VI) imides. Such an intermediate had not been observed in this type of reaction and the results redefine the mechanism for the reduction of azides by uranium compounds. Chapters 8 and 9 are crystallographic studies on U(III) compounds containing cyclopentadienyl and iodide ligands, showing the many structural variations that can exist with simple organoactinide compositions. Chapters 10 and 11 describe the synthesis of new Th(III) compounds with the C5H4SiMe3 (Cp′) and C5H3(SiMe3)2 (Cp″) ligands and a new disproportionation route to Th(II) complexes that was discovered. Chapter 10 describes extensive efforts to characterize the elusive Cp′3Th. In Chapter 11, reactions of Cp″3Th with simple salts are described that lead to formation of Th(II) complexes by disproportionation. An unusual reduction of benzene by “[Cp″3ThMe]1−” is also reported. Chapter 12 extends the electrochemistry method described in Chapter 3 to the Cp′3Ln series and some (C5Me4H)3Ln compounds. It was found that the reduction potentials for the Cp′3Ln series were found to be practically identical across the Ln series (excluding Eu, Sm, Tm, and Yb). The remaining Chapters describe the coordination chemistry of the underutilized nitrogen heterocycles trimethyltriazacyclohexane and trimethyltriazacyclononane with the rare-earth metals (Chapter 13), the actinide metals (Chapter 14), and bismuth (Chapter 15). Appendix A describes attempts to form Th(II) complexes by reduction of C5Me4H-ligated Th(III) complexes. Appendix B describes DFT studies on a bimetallic yttrium system with benzoxazole-derived ligands. Appendices C and D summarize work with the bimetallic (CpAnYH)2 system [CpAn = (C5H3SiMe3)2SiMe2] and uranium borohydride coordination chemistry, respectively. Appendix E summarizes DFT studies on various complexes not yet mentioned in other sections of this dissertation. Finally, Appendix F contains a chronological list of crystal structures obtained throughout this work.