Complexation of Ammonia Boranes with Al3.

Abstract

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.