Control of hydrogen release and uptake in amine borane molecular complexes: thermodynamics of ammonia borane, ammonium borohydride, and the diammoniate of diborane

Molecular complexes of Lewis acid-base pairs can be used to activate molecular hydrogen for applications ranging from hydrogen storage for fuel cells to catalytic hydrogenation reactions. In this paper, we examine the factors that determine the thermodynamics of hydrogen activation of a Lewis acid-base pair using the pedagogical examples of ammonia borane (NH3BH3, AB) and ammonium borohydride ([NH4][BH4], ABH(2)). At ambient temperatures, ABH(2) loses hydrogen to form the Lewis acid-base complex AB, suggesting that free energy drives the reaction to release hydrogen. However, direct measurement of the reaction enthalpy is not straightforward given the complex decomposition pathways leading to the formation of the diammoniate of diborane ([NH3BH2NH3][BH4], DADB). In this work, we compare two approaches for deriving the thermodynamic relationships among AB, DADB, and ABH(2).