Complexation of Ammonia Boranes with Al.

Ammonia borane, NHBH (AB), is very attractive for hydrogen storage; however, it dehydrogenates exothermally, producing a mixture of polymeric products with limited potential for direct rehydrogenation. Recently, it was shown that AB complexed with Al in Al(BH)·AB endothermically dehydrogenates to a single product identified as Al(BH)·NHBH, with the potential for direct rehydrogenation of AB. Here we explore the reactivity of AB-derived RNHBH (R = -CH, -CH-) with AlX salts (X = BH, Cl), aiming to extend the series to different anions and to enlarge the stability window for Al(BH)·NRBH. Three novel complexes were identified: Al(BH)·CHNHBH having a molecular structure similar to that of Al(BH)·AB but different dehydrogenation properties, as well as [Al(CHNHBH)Cl][AlCl] and [Al(NHCHCHNH)(BH)][Al(BH)], rare examples of Al making part of the cations and anions simultaneously. The latter compounds are of interest in the design of novel electrolytes for Al-based batteries. The coordination of two ABs to a single Al atom opens a route to materials with higher hydrogen content.