Complexation of Ammonia Boranes with Al³⁺

Ammonia borane, NH₃BH₃ (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 Al³⁺ in Al­(BH₄)₃·AB endothermically dehydrogenates to a single product identified as Al­(BH₄)₃·NHBH, with the potential for direct rehydrogenation of AB. Here we explore the reactivity of AB-derived RNH₂BH₃ (R = −CH₃, −CH₂−) with AlX₃ salts (X = BH₄^–, Cl^–), aiming to extend the series to different anions and to enlarge the stability window for Al­(BH₄)₃·NRBH. Three novel complexes were identified: Al­(BH₄)₃·CH₃NH₂BH₃ having a molecular structure similar to that of Al­(BH₄)₃·AB but different dehydrogenation properties, as well as [Al­(CH₃NH₂BH₃)₂Cl₂]­[AlCl₄] and [Al­(NH₂CH₂CH₂NH₂)­(BH₄)₂]­[Al­(BH₄)₄], rare examples of Al³⁺ 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.