An ammonia-stabilized mixed-cation borohydride: synthesis, structure and thermal decomposition behavior

Abstract

We demonstrate the synthesis, crystal structure and thermal decomposition behavior of a novel ammonia-stabilized mixed-cation borohydride where the NH₃ groups enable the coexistence of Li and Mg cations as an “assistant”. Li₂Mg(BH₄)₄·6NH₃, which is comprised of orderly arranged Mg[NH₃]₆²⁺ ammine complexes and Li₂[BH₄]₄²⁻ complex anions, was synthesized by the mechanochemical reaction between Mg(BH₄)₂·6NH₃ and LiBH₄. This novel compound crystallizes in a tetragonal P4₃2₁2 (No. 96) structure with lattice parameters a = b = 10.7656(8) Å and c = 13.843(1) Å with very short dihydrogen bonds, which determine a very low onset temperature of 80 °C for hydrogen release and are also responsible for the nucleation of Li₂Mg(BH₄)₄·3NH₃ as a decomposition intermediate. Mechanistic investigations on the thermal decomposition showed that the H^δ+–H^δ− combination in the ammonia-stabilized mixed-cation borohydride was significantly enhanced due to the strengthened Mg–N bonds. Upon heating, 11.02 moles of H₂ (equivalent to 11.1 wt%) and 3.07 moles of NH₃ are evolved from one mole of Li₂Mg(BH₄)₄·6NH₃ with a three-step reaction. The insights into the formation mechanism of ammonia-stabilized mixed-cation borohydride and the role played by NH₃ group are very useful as a guideline for the design and synthesis of novel B–N-based materials with high hydrogen content.