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

Abstract

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.