6- and 5-Halodecaboranes: Selective Syntheses From ClOSO-B10H10(2-) and Use as Polyborane Building Blocks
Variable Temperature NMR
Decaborane halogenated in the 6-position has been synthesized in high yields via the super-acid induced cage-opening reactions of closo-B10H10(2-) salts. These 6-halogenated compounds were then isomerized to their 5-substituted isomers through base catalysis. The isomerization was driven by the energy differences between the anionic-forms of each respective isomer. These reactions provided 5-halodeboranes in high yields. The bridging-hydrogens of the halodecaboranyl anions were fluxional at a range of temperatures. Variable-temperature NMR studies supported computationally proposed fluxional mechanisms. Both 5- and 6-halodecaboranes were reacted with alcohols yielding boranyl ethers. The mechanisms of substitution, where reactions with 6- and 5-halodecaboranes yielded 5- and 6-boranyl ethers, respectively, were explained computationally and confirmed through isotopic-labeling studies. The regeneration of the polymeric products of ammonia-borane dehydrogenation was carried out through a process that included digestion of the polymer, complexation of the digestate with a base, reduction of B-X bonds to B-H bonds, and finally displacement of the base with ammonia. While digestion schemes proved unable to digest all forms of the dehydrogenated materials, portions of the polymer digested to boron-trihalides were quantitatively regenerated to ammonia borane, with complete separation and collection of by-products.