UCLA

This dissertation describes the exploration of icosahedral carboranes chalcogenides containing remarkably stable exopolyhedral B-S, B-Se and B-Te bonds. Specifically, we assess their synthesis, reactivity, and ability to act as tunable ligands for metal chalcogenide materials.Chapter 1 provides a brief overview of carbon-based compounds containing C-S, C-Se, and C-Te bonds and their applications in organic chemistry, chemical biology, organometallic chemistry, and self-assembled materials. The synthesis and characteristic properties of icosahedral boron-rich clusters (B12H122-, C2B10H12) are also discussed in addition to the synthesis and applications of clusters featuring exopolyhedral B-S, B-Se, and B-Te bonds. Chapter 2 demonstrates the potential for Pd(II)-based precatalysts designed for rapid reduction to the catalytically active Pd(0) species in cross-coupling reactions with carboranes iodinated at the electron-rich boron vertices. Kumada-type cross-coupling between iodocarboranes and Grignard reagents with fast reaction times (< 2h) has been established. Suprisingly, this method is entirely selective for boron-iodine bonds with no substitution observed and brominated boron vertices. Chapter 3 explores the synthesis of boron vertex-centered radical precursors featuring exopolyhedral B-[B] ([B]: -B(OH)2, -BF3K) bonds and the reactivity of said carboranyl radicals. The introduction of exopolyhedral boron-based substituents to the electron-rich boron vertices is made possible by the palldium-catalyzed borylation of iodinated carboranes with a similar catalytic system implemented in chapter 2. The formed B-[B] bond is oxidatively unstable and can be heterolytically cleaved under electrochemical conditions or by inorganic oxidants. The transient carboranyl radical can undergo substitution mechanisms in the presence of several radical traps, affording products with exopolyhedral B-O, B-S, B-Se, B-Te, and B-C bonds. Chapter 4 is an assessment of the electrophilic and nucleophilic reactivity of meta-carboranes containing exopolyhedral B-Se and B-Te bonds at the electron-rich boron vertices. Electrophilic selenyl(II), tellurenyl(II), and tellurenyl(IV) chlorides were prepared by the treatment of the respective biscarboranyl dichalcogenides with thionyl chloride. Despite the electron-rich environment imparted by the carborane, the selenyl(II) and tellurenyl(IV) reagents show good electrophilic reactivity in the presence of carbon-based nucleophiles (e.g. Girgnard reagents, alkene, alkyne, enolate). Futhermore, the isolation of an electron-rich boron-connected carborane tellurol has been reported for the first time, with remarkable air stability. The electron-rich carborane selenols and tellurols have been shown as competent nucleophiles in nucleophilic aromatic substitutions as well. Chapter 5 highlights the potential for electron-rich carborane chalcogenols as electronically tunable, though sterically invariant, ligands for photoluminescent self-assembled metal chalcogenide materials. By changing between meta- and ortho-carborane based chalcogenols the relative carborane dipole to the ligating substituent, in this case a selenolate or thiolate group, can be tuned both in orientation and magnitude. This tuning has then been used to modulate both the crystalline morphology and photophysical properties of copper(I) chalcogenolate microcrystals. In the case of one material, composed of copper(I) and meta-carboranyl selenolate, key structural information was obtained by applying microcrystal electron diffraction techniques, a first for this class of materials. The structural characterization revealed that the material was composed of an unprecedented Cu4Se4, with steric protection afforded by the meta-carboranyl substituents.