6- and 5-halodecaboranes: Selective syntheses from closo- boron hydride anions and use as polyborane building blocks

Decaborane halogenated at the 6-position (6-X-B10H13 , X = Cl, Br, I) was synthesized through cage-opening reactions of closo-B 10H102-- induced by treatment with hydrogen halides absorbed in ionic liquid mixtures known to greatly enhance HX acidity. The previously unknown member of the 6-halogenated series, 6-F-B10H 13, was synthesized in excellent yields by reaction of the closo-B 10H102-- with triflic acid in the presence of 1-fluoropentane. Triflic acid also induced the cage-opening of closo-B 10H102-- to 6-Cl-B10H 13 when performed in CH2Cl2.;The 6-halogenated isomers were used as starting materials in the syntheses of 5-X-B10H13 (X = Cl, Br, I). Base catalyzed isomerization reactions yielded equilibrium mixtures of 5-X-B10H13 and 6-X-B10H13 with ratios dictated by the free energy difference between the respective anions, 5-X-B10H12 -- and 6-X-B10H12--, strongly favoring the 5-substituted isomer. Pure 5-X-B10 H13 was isolated in good yields by selective crystallization or by chromatography. These syntheses represent a significant step forward in the chemistry of this isomer, and provide the first path by which these compounds may be synthesized in useful quantities.;Calculations on the 5- and 6-halogenated anions identified fluxional processes by which bridging hydrogens move about the open face of the cage, explaining the results of variable-temperature NMR studies on these compounds. Variable-temperature NMR was also used to validate similar processes previously proposed for the parent B10H13--.;As proof of the utility of 5-X-B10H13 and 6-X-B 10H13 as starting materials for further functionalization a new class of boranyl ether compounds, selectively substituted at the 6- or 5-position, were synthesized using the reaction of the halogenated species with alcohols. A range of alcohols, with various pendant functional groups, were tethered to the cage via a rare B-O-C organic/inorganic ether linkage. The regiochemistry of the reaction was unique in that reaction of 5-X-B 10H13with alcohols selectively yielded 6-RO-B10H 13 ethers, and 6-X-B10H13 with the same alcohols yielded 5-RO-B10H13 compounds. A plausible reaction mechanism explaining this regiochemistry was found using DFT calculations, which subsequent experiments with deuterated starting materials supported.