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Expanding the Fundamental Chemistry of Thorium Through the Synthesis and Reactivity of the First Molecular Complexes of Th(II) and New Classes of Th(III) and Th(IV) Compounds

Abstract

This dissertation describes synthetic investigations aimed to expand the fundamental chemistry of thorium. Chapter 1 describes the radical reactivity of thorium-ligand bonds with the molecular radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). Organothorium bonds are found to be very reactive with TEMPO and reveal previously unobserved reaction pathways for thorium. Chapter 2 discusses the isolation and characterization the first molecular complexes which contain thorium in the formal +2 oxidation state: [K(2.2.2-cryptand)]{[C5H3(SiMe3)2]3Th} and [K(18-crown-6)(THF)2]{[C5H3(SiMe3)2]3Th}. Physical, spectroscopic, and theoretical studies of these new Th2+ ions conclude they the first examples of a [Rn]6d2 electronic ground state in an isolable metal complex of any kind. The 6d2 configuration is what is expected for the radioactive fourth row transition metal ions such as Rf 2+ and Db3+. Chapters 3 and 4 describe the reactivity of the new Th2+ complexes, with comparisons to the analogous Th3+ compounds where appropriate. Substrates investigated include H2, [Et3NH][BPh4], cyclooctatetraene, iodomethane, iodoethane, benzyl bromide, chlorbenzene, fluoropentane, and benzyl isocyanide. Th2+ is shown to affect both one- and two-electron reductions of substrates. The isolation of new Th3+ complexes, including the first mixed-valent thorium complex and first hydride of Th3+, [K(18-crown-6)(Et2O)]{[C5H3(SiMe3)2)2ThH2]}2, is described, as well as evidence for the in situ generation of a transient Th2+ complex with the (C5Me4H)33− ligand set. Chapter 5 involves the synthesis and reactivity of the sterically-crowded Th3+ complex (C5Me5)3Th. Other sterically-crowded Th4+ complexes are also isolated and their reactivity is reported. Substrates examined include iodomethane, diiodide, chlorobenzene, trimethylaluminum, dihydrogen, and carbon monoxide. Investigations with carbon monoxide led to the formation of [(C5Me5)3Th(CO)][BPh4], the first room temperature stable carbonyl complex of thorium. Chapter 6 describes an initial investigation into the reactivity of the Th3+ complexes (C5Me4H)3Th and [C5H3(SiMe3)2]3Th with nitric oxide (NO). Two structurally different bimetallic complexes are isolated, namely [(C5Me4H)2Th]2(μ-ONNO), and {[C5H3(SiMe3)2]2Th(μ-O)}2. Chapter 7 describes the synthesis of the nitrile-solvated [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me, Ph, tBu) complexes, which are the first examples of linear metallocenes of thorium containing cyclopentadienyl ligands. Chapter 8 presents preliminary results on other synthetic attempts to isolate low-valent thorium compounds. Evidence of transient low-valent species is described and factors leading to the instability are discussed.

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