Efficient Syntheses of 5-X-B₁₀H₁₃ Halodecaboranes via the Photochemical (X = I) and/or Base-Catalyzed (X = Cl, Br, I) Isomerization Reactions of 6-X-B₁₀H₁₃

High yield syntheses of the 5-X-B10H13 (5X) halodecaboranes have been achieved through the photochemical (X = I) or base-catalyzed (X = Cl, Br, I) isomerization reactions of their 6-X-B10H13 (6X) isomers. 5I was obtained in 80% isolated yield upon the UV photolysis of 6I. Treatment of 6X (X = Cl, Br, I) with catalytic amounts of triethylamine at 60 °C led to the formation of 78:22 (Cl), 82:18 (Br), and 86:14 (I) ratio 5X/6X equilibrium mixtures. The 5X isomers were then separated from these mixtures by selective crystallization (Br and I) or column chromatography (Cl), with the supernatant mixtures in each case then subjected to another round of isomerization/separation to harvest a second crop of 5X. The combined isolated yields of pure products after two cycles were 71% 5-Cl-B10H13, 83% 5-Br-B10H13, and 68% 5-I-B10H13. The previously proposed structures of 5-Br-B10H13 and 5-I-B10H13 were crystallographically confirmed. Deprotonation of 6X and 5X with 1,8-bis(dimethylamino)naphthalene (PS) resulted in the formation of [PSH+][6X−] and [PSH+][5X−]. Density functional theory-gauge-independent atomic orbital (DFT/GIAO) calculations and crystallographic determinations of [PSH+][6Cl−] and [PSH+][6Cl−] confirmed bridge-deprotonation at a site adjacent to the halogen-substituted borons. NMR studies of the 6-Br-B10H13 isomerization induced by stoichiometric amounts of PS showed that following initial deprotonation to form 6-Br-B10H12−, isomerization occurred at 60 °C to form an equilibrium mixture of 6-Br-B10H12− and 5-Br-B10H12−. DFT calculations also showed that the observed 5-X-B10H13/6-X-B10H13 equilibrium ratios in the triethylamine-catalyzed reactions were consistent with the energetic differences of the 5-X-B10H12− and 6-X-B10H12− anions. These results strongly support a mechanistic pathway for the base-catalyzed 6X to 5X conversions involving the formation and subsequent isomerizations of the 6X− anions. While triethylamine did not catalyze the isomerization reactions of either 6-(C6H13)-B10H13 or 6,9-(C6H13)2-B10H12, it catalyzed the isomerization of 6-X-9-(C6H13)-B10H12 to 5-X-9-(C6H13)-B10H12 resulting from halo, but not alkyl rearrangement. Comparisons of the chemical shift values found in the temperature-dependent 11B NMR spectra of 6Cl− and 6F− with DFT/GIAO chemical shift calculations indicate the fluxional behavior observed for these anions results from a process involving hydrogen migration around the open face that leads to the averaging of some boron resonances at higher temperatures.