Synthesis of Aminodiborane From Ammonia Borane Using Different Lewis Acids: A Competing Reaction for Diborane Formation with Boron Trihalides (BX3, X=Cl, Br)

Abstract

The dehydrogenation of ammonia borane in the presence of a variety of Lewis acids such as ICl, IBr, Br₂, CuCl₂, AlCl₃, GaCl₃, InCl₃, and [Ph₃C]BF₄ at a concentration of 7.5 mol % was effective in selectively producing aminodiborane (μ-NH₂B₂H₅, ADB) at 80 °C. Compounds such as ICl, IBr, Br₂, AlCl₃, and GaCl₃ at the same concentration could also generate ADB at lower temperatures of 35 °C and 50 °C. In contrast, BX₃ (X=Cl, Br) at the same concentration of 7.5 mol % was found to give exclusively B₂H₆. Further, selective synthesis of diborane or ADB was achieved by adjusting the stoichiometry of the boron trihalides. A concentration of 7.5 mol % (upto 1 equivalent) of BBr₃ favored the formation of B₂H₆, while 1 mol % BBr₃ predominantly yielded ADB. Interestingly, both ADB and B₂H₆ facilitated the reduction of acetanilides. A mechanism has been proposed for both diborane and ADB formation using these Lewis acids.