Alkali and alkaline-earth metal borohydride hydrazinates: synthesis, structures and dehydrogenation

Abstract

Four new borohydride hydrazinates, including NaBH₄·NH₂NH₂, LiBH₄·1/2NH₂NH₂, LiBH₄·1/3NH₂NH₂ and Mg(BH₄)₂·3NH₂NH₂, were synthesized. NaBH₄·NH₂NH₂ and Mg(BH₄)₂·3NH₂NH₂ possess monoclinic and trigonal structures, respectively, while LiBH₄·1/2NH₂NH₂ and LiBH₄·1/3NH₂NH₂ exhibit orthorhombic and monoclinic structures. The effects of composition on the dehydrogenation of hydrazinates were investigated. It is demonstrated that cations with high Pauling electronegativity hold hydrazine strongly in the vicinity of borohydride and result in direct dehydrogenation at elevated temperatures. Specifically, Mg(BH₄)₂ hydrazinates can directly generate hydrogen upon heating it under a flow of Ar; on the other hand, the Li and Na counterparts lost part or all of the hydrazine components under the same condition. In addition, reducing NH₂NH₂ content in the complexes leads to improved dehydrogenation properties. Mechanistic investigation of Mg(BH₄)₂ hydrazinates using isotopic labelling indicates that hydrogen desorption is via homogeneous dissociation of N–N bond of NH₂NH₂ followed by the establishment of B–N bond and combination of H^δ+ (N) and H^δ− (B).