Lewis base -boranes and base-alanes: Molecular building blocks for the synthesis of boron -containing clusters and solid state materials

Lewis base-boranes (e.g. Et$\\sb3$N$\\cdot$BH$\\sb3$) and base-alanes (e.g. Et$\\sb3$N$\\cdot$AlH$\\sb3$) are versatile synthetic reagents which have been utilized in a variety of chemical transformations. In these species a transient Lewis acid fragment such as BH$\\sb3$ or AlH$\\sb3$ is stabilized by acting as an electron-pair acceptor to a Lewis base donor. This donor-acceptor relationship provides a source of reactive Br or AlR (R = H, halogen, alkyl, etc.) moieties which are then available for substitution, insertion or elimination chemistry. The objective of this dissertation was to explore the utility of Lewis acid-base adducts as molecular building blocks for the expansion of small carborane clusters, and as molecular precursors to boron-based solid-state materials. Major accomplishments of this work are summarized below: (1) The first viable synthetic routes to the small closo-carborane 2,3-Et$\\sb2$C$\\sb2$B$\\sb5$H$\\sb5$ were developed. These pathways are based on cluster expansion reactions of nido-2,3-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb6$ utilizing a variety of base-borane adducts. Thus, reaction of nido-2,3-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb6$, Na$\\sp+$ (nido-2,3-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb5$) $\\sp-$ or Na$\\sp+$Li$\\sp+$ (nido-2,3-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb4$) $\\sp{2-}$, with Et$\\sb3$N$\\cdot$BH$\\sb3$, (CH$\\sb3)\\sb2$S$\\cdot$BH$\\sb3$ or (CH$\\sb3)\\sb2$S$\\cdot$BHBr$\\sb2$, respectively, all resulted in the formation of closo-2,3-Et$\\sb2$C$\\sb2$B$\\sb5$H$\\sb5$ in good yield. (2) The first chemical studies of closo-2,3-Et$\\sb2$C$\\sb2$B$\\sb5$H$\\sb5$ were undertaken, resulting in the production and structural characterization of the first eighteen-electron seven-vertex nido polyhedral boranes. For example, reaction of closo-2,3-Et$\\sb2$C$\\sb2$B$\\sb5$H$\\sb5$ with Li(BEt$\\sb3$)H in the presence of (CH$\\sb3)\\sb4$NCl was found to result in the formation of the first seven-vertex nido-carborane anion (CH$\\sb3)\\sb4$N$\\sp+$ (nido-3,4-Et$\\sb2$C$\\sb2$B$\\sb5$H$\\sb6$) $\\sp-$. (3) New high-yield synthetic routes aluminacarboranes were developed based on cluster expansion reactions of nido-2,3-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb6$ employing base-alanes. Thus reaction of nido-2,3-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb6$ with Et$\\sb3$N$\\cdot$AlH$\\sb3$ at 0$\\sp\\circ$C produced the aluminum-bridging species nido-4,5-$\\mu$-AlH$\\sb 2\\cdot$NEt$\\sb3$-2,3-Et$\\sb2$C$\\sb 2$B$\\sb4$H$\\sb5$, at 50$\\sp\\circ$C gave the seven-vertex nido species nido-6-AlH$\\cdot$NEt$\\sb3$-3,4-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb4$ and at 70$\\sp\\circ$C, with an additional equivalent of nido-2,3-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb6$ produced the bis(carboranyl) compound commo-Al$\\cdot$NEt$\\sb3$- ((6-Al$\\cdot$NEt$\\sb3$-3,4-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb4$) (4$\\sp\\prime$,5$\\sp\\prime$-$\\mu$-Al$\\cdot$NEt$\\sb 3$-2$\\sp\\prime$,3$\\sp\\prime$-Et$\\sb2$C$\\sb2$B$\\sb4$H$\\sb5$)), all in good yields. (4) Lewis base-boranes were utilized as molecular precursors to boron-based ceramic materials. The Lewis base-borane (CH$\\sb3)\\sb2$S$\\cdot$BHBr$\\sb2$ was shown to be an effective molecular precursor for the formation of bulk powders and ceramic fiber coatings of boron nitride. (Abstract shortened with permission of author.)