A promising hydrogen-storage compound

Ammonia triborane (shown) is being touted as "a promising new material for chemical hydrogen-storage applications" by Larry G. Sneddon and Chang Won Yoon of the University of Pennsylvania (J. Am. Chem. Soc., DOI: 10.1021/ja064526g). The first synthesis of this triborane, reported in 1959, involved an explosive starting material. The Pennsylvania group has now found a safe, convenient way to synthesize it by oxidizing the air-stable salt Bu₄N⁺B₃H₈^- (Bu = butyl) with iodine in 1,2-dimethoxyethane (glyme). The resulting (glyme)B₃H₇ adduct is then treated with anhydrous ammonia to give the desired product, NH₃B₃H₇, in high yield. Since Bu₄N⁺B₃H₈^- can be readily prepared, "the large-scale, safe synthesis of NH₃B₃H₇ is now possible," the researchers say. They found that ammonia triborane can be rapidly hydrolyzed in aqueous solutions containing acids or metal catalysts to yield almost eight equivalents of H₂. Since the reaction also forms mixtures of borates, the real utility of this hydrogen-storage system will depend on developing methods to regenerate ammonia triborane from these borates, the team notes.