Phenyl introduced ammonium borohydride: synthesis and reversible dehydrogenation properties

Novel borohydrides of para- and meta-bis-(ammonium)-benzene (m/p-BABB), possessing theoretical hydrogen storage capacities of 10.1 wt%, have been synthesized upon metathesis of p/m-benzenediamine dihydrochloride (p/m-BDADC) and lithium borohydride through simple ball-milling. It was found that the dehydrogenation of the BABBs proceeds in the temperature range of 50–200 °C via a multistep mechanism similar to that of NH3BH3 and [NH4]+[BH4]−, releasing 8.35 and 6.52 wt% hydrogen from p- and m-BABB, respectively. Solid state 11B MAS NMR studies indicate that the dehydrogenation process of p-BABB and m-BABB follows a pathway similar to that of [NH4]+[BH4]−via [BH2(NH3)2]+[BH4]− (DADB). The residues obtained after releasing ∼6 equiv. H2 are mainly N,N′-phenyl substituted polyborazylenes. The dehydrogenated products can be converted back to N,N′-phenyl substituted ammonia-boranes by treatment with hydrazine sulfate [(N2H4)2·H2SO4] in liquid ammonia (NH3) for one week at 45 °C, demonstrating the potential of this class of material for regenerable chemical hydrogen storage.