Synthesis Mechanism of Alkali Borohydrides by Heterolytic Diborane Splitting

Similar to alane in alanates, borane species are assumed to be the mass transport intermediate in the hydrogen storage reaction MH + B + 3/2H2 ⇔ MBH4 with M = Li and Na. One possible substep of this reaction is the interaction of diborane with the alkali hydride. In this paper, we unravel the synthesis mechanism of alkali borohydrides by solid−gas reaction of alkali hydrides and diborane gas by H/D isotope labeling of the reaction educts (e.g., LiD + B2H6). The labeling enables us to trace the hydrogen/deuterium atoms in the borohydride product by Raman scattering and in the gas by infrared spectrometry measurements. We conclude that, during the LiBH4 synthesis from LiH, the entire BH4− unit is transferred from the diborane to the Li+ cation. This provides clear evidence for the heterolytic splitting of diborane on alkali hydrides and implies exchange of BH4− with H− ions of the underlying hydride. The detection of Li−H bonds at the surface of newly formed LiBH4 confirms the importance of H− defects for the synthesis of borohydrides.