From Borateto Borohydride: Achieving a High-YieldSynthesis of Magnesium Borohydride Enabled by Cation Tuning

Since the discovery of magnesium borohydride, its synthesis has predominantly relied on the borane method or ion exchange method to this day. However, these approaches have been hampered by the use of highly toxic, explosive, and expensive raw materials. In this study, we reported a low-cost and high-yield synthesis method of solvent-free Mg­(BH₄)₂ (yield: ∼80%, time: 45 min) through a dry mechanochemical reaction between MgH₂ and hydrated borate Zn₂B₆O₁₁·3.5H₂O at room temperature. Unlike the traditional focus on the triggering role of transition metal or rare earth elements in regents, we proposed a new perspective involving cation tuning using nonboride-forming elements. This approach facilitates rapid breaking of the B–O bonds and the formation of the B–H bond without side effects of consuming generated [BH₄]⁻ resulting in the formation of oxides or borides. Additionally, the intermediate Mg­(B<i>_<i>y</i></i>H<i>_<i>z</i></i>)<i>_n</i> during the ball milling process was confirmed by solid-state nuclear magnetic resonance. This work provides a low-cost method for preparing α-, β-, or γ-Mg­(BH₄)₂ with high yield under different solutions of extraction and desolvation conditions, which highlights the contribution of the ds-block element and limited solvent usage and finally broadens the Mg­(BH₄)₂ synthesis route from borate to borohydride.