Hydrolysis of the Borohydride Anion BH4−: A 11B NMR Study Showing the Formation of Short-Living Reaction Intermediates including BH3OH−

Abstract

In hydrolysis and electro-oxidation of the borohydride anion BH4−, key reactions in the field of energy, one critical short-living intermediate is BH3OH−. When water was used as both solvent and reactant, only BH3OH− is detected by 11B NMR. By moving away from such conditions and using DMF as solvent and water as reactant in excess, four 11B NMR quartets were observed. These signals were due to BH3-based intermediates as suggested by theoretical calculations; they were DMF·BH3, BH3OH−, and B2H7− (i.e., [H3B−H−BH3]− or [H4B−BH3]−). Our results shed light on the importance of BH3 stemming from BH4− and on its capacity as Lewis acid to interact with Lewis bases such as DMF, OH−, and BH4−. These findings are important for a better understanding at the molecular level of hydrolysis of BH4− and production of impurities in boranes synthesis.