Facile Solvent-Free Synthesis of Anhydrous Alkali Metal Dodecaborate M₂B₁₂H₁₂ (M = Li, Na, K)

Metal dodecaborate, widely regarded as an obstacle of the rehydrogenation of high-density hydrogen storage materials metal borohydrides M­(BH₄)_n, is generally synthesized using liquid-phase process followed by a careful dehydration process. In this study, we propose a new and facile solvent-free synthesis process of dodecaborates using B₁₀H₁₄ with a low melting point of 99.6 °C as a boron source. As a case study, our first challenge focused on the syntheses of anhydrous M₂B₁₂H₁₂ (M = Li, Na, and K) by heat treatment of starting materials (a) 2MH + 1.2B₁₀H₁₄ or (b) 2MBH₄ + B₁₀H₁₄ at 200–450 °C conditions, which have been proved to be successful for the first time by X-ray diffraction (XRD), Raman, and NMR analysis. Starting materials (b) 2MBH₄ + B₁₀H₁₄ shows better reactivity than that of (a) 2MH + 1.2B₁₀H₁₄, which demonstrates that synthesis of anhydrous M₂B₁₂H₁₂ by heat treatment of 2MBH₄ + B₁₀H₁₄ is a feasible solvent-free process.