Metal hydride–hydrazine borane: Towards hydrazinidoboranes or composites as hydrogen carriers

In the present work, the behavior of hydrazine borane N2H4BH3 in the presence of alkali/alkaline-earth hydrides is investigated. (i) Hydrazine borane N2H4BH3 is readily destabilized by an alkali hydride MH (M=Li, Na, K). The electronic properties of M drive the reactivity of MH1 towards N2H4BH3. KH is the most reactive (at 25 °C, ΔrH = −70.25 kJ mol−1) while K is the least electronegative and the biggest element. Hydrazinidoboranes MN2H3BH3 form. (ii) Hydrazine borane N2H4BH3 is destabilized by MHx (x = 2, 3; M=Mg, Ca, Al). In comparison to pristine N2H4BH3, better dehydrogenation properties are found: MgH2 has a catalytic effect; CaH2 strongly destabilizes N2H4BH3; and, unstable AlH3 is able to destabilize N2H4BH3 under heating. Though the synthesis of hydrazinidoboranes M(N2H3BH3)x is difficult, the mixtures MHx–N2H4BH3 leads to composites. The most efficient composite is CaH2–N2H4BH3. The aforementioned hydrazinidoboranes and composites may have potential as solid-state hydrogen storage materials. This is discussed herein. •Electronic properties of M (Li, Na, K) drive the reactivity of MH towards N2H4BH3.•KH is the most reactive: ΔrH = −70.25 kJ mol−1 for KH + N2H4BH3 → KN2H3BH3 + H2.•N2H4BH3 is destabilized by MHx (x = 2, 3; M=Mg, Ca, Al), leading to composites.•The most efficient composite as solid-state hydrogen carrier is CaH2–N2H4BH3.•The synthesis of hydrazinidoboranes M(N2H3BH3)x (x = 2, 3) is however difficult.