The Development of New Routes to Half-Sandwich Metallatricarbadecaborane Complexes, Triflate-Functionalized Polyboranes and Carboranes, and Ammonia Borane Hydrogen Release
The three main goals of the research discussed in this dissertation were (1) to expand and characterize the first extensive series of half-sandwich metallatricarbadecaborane complexes, (2) to synthesize and explore the reactivity of 5-TfO-B10H13, and (3) to investigate metal-catalyzed hydrogen release from ammonia borane in ionic liquids. The major achievements are outlined below. Chapter 1 describes the synthesis and structural characterization of the first series of group 9 tricarbadecaboranyl half-sandwich complexes. The tricarbadecaboranyl ligands in these complexes exhibit properties distinct from their cyclopentadienyl analogs, including (1) strong electron-withdrawing properties, (2) the ability to readily form slipped-cage structures, and (3) the ability to stabilize lower metal oxidation states. The unique properties of these complexes mean they could find potential applications as, for example, catalysts for alkyne oligomerizations, polymer and dendrimer building blocks, and synthetic precursors for medicinal therapies. Chapter 2 discusses the development of several high yield pathways to 5-TfO-B10H13. The new routes to 5-TfO-B10H13 allowed for a systematic exploration of its reactivity and potential use as a starting material for the syntheses of other important polyboranes. The combined reactions of the synthesis of 5-TfO-B10H13, the formation of its Lewis acid-base adduct, 5-TfO-6,9-(Me2S)2-B10H11, and the alkyne-insertion and base-promoted cage closure reactions of 5-TfO-6,9-(Me2S)2-B10H11, now provide the first pathways to B-triflate-substituted ortho-carboranes and decaborates. Chapter 3 reports that the metal-catalyzed hydrogen release from ammonia borane (AB) in an ionic liquid produces a significant increase in initial H2-release. However, the metal-catalysts affect only the rate of loss of the first H2-equivalent, with the uncatalyzed and catalyzed rates becoming equal following that point. NMR studies indicate that the mechanism of AB H2-release of these metal-catalyzed systems in ionic liquids appears to be similar to that observed in organic solvents, but differs from that observed in only ionic liquids.