From Exothermic to Endothermic Dehydrogenation – Interaction of Monoammoniate of Magnesium Amidoborane and Metal Hydrides

Metal amidoborane ammoniate such as Mg­(NH2BH3)2·NH3 and Ca­(NH2BH3)2·nNH3 with n = 1, 2, release NH3 predominantly
and endothermically
at low temperatures in an open-system. However, a strong exothermic
reaction occurs when these ammoniates were dehydrogenated in a closed-system,
where the adducted NH3 take part in the reaction. Our approach
in tailoring the thermodynamic properties of Mg­(NH2BH3)2·NH3, by replacing its adducted
NH3 with amide was successful, yielding a composite consisting
of bimetallic amidoborane and Mg­(NH2)2. Crystal
structures of bimetallic amidoboranes, i.e., Na2Mg­(NH2BH3)4 and K2Mg­(NH2BH3)4 were identified and solved. Significant
improvement in the dehydrogenation thermodynamic was observed in the
composite system as compared to the pristine Mg­(NH2BH3)2·NH3, i.e., the dehydrogenation
enthalpies were altered from an exothermic to an endothermic one.
In addition, the detection of bimetallic amidoboranes in the composites
urges detailed investigation on pristine bimetallic amidoborane, to
which later we found that Na2Mg­(NH2BH3)4 also dehydrogenated endothermically at the identical
temperature range (ca. 150–170 °C) with that of composite
systems. Similar activation barriers were observed in Na2Mg­(NH2BH3)4 and composite systems,
suggesting that metal hydride mediation may be the internal barrier
that dominates the kinetic barrier of the composite system. First-principles
calculations also showed that the thermodynamic stability of metal
amidoborane (MNH2BH3, MAB) increases with decreasing
Pauling electronegativity of the metal. Based on the calculated results,
a reactant stabilization approach was proposed, which suggests that
forming a stable reactant is an effective way of reducing the exothermicity
of the dehydrogenation of metal amidoborane.