Facile synthesis of anhydrous alkaline earth metal dodecaborates MB12H12 (M = Mg, Ca) from M(BH4)2

Abstract

Metal dodecaborates M_2/nB₁₂H₁₂ are among the dehydrogenation intermediates of metal borohydrides M(BH₄)_n with a high hydrogen density of approximately 10 mass%, the latter is a potential hydrogen storage material. There is therefore a great need to synthesize anhydrous M_2/nB₁₂H₁₂ in order to investigate the thermal decomposition of M_2/nB₁₂H₁₂ and to understand its role in the dehydrogenation and rehydrogenation of M(BH₄)_n. In this work, anhydrous alkaline earth metal dodecaborates MB₁₂H₁₂ (M = Mg, Ca) have been successfully synthesized by sintering M(BH₄)₂ (M = Mg, Ca) and B₁₀H₁₄ in a stoichiometric molar ratio of 1 : 1. Thermal decomposition of MB₁₂H₁₂ shows multistep pathways with the formation of H-deficient monomers MB₁₂H_12−x containing icosahedral B₁₂ skeletons and is followed by the formation of (MB_yH_z)_n polymers. Comparison of the thermal decomposition of MB₁₂H₁₂ and M(BH₄)₂ suggests different behaviours of the anhydrous MB₁₂H₁₂ and those formed from the decomposition of M(BH₄)_n.