High Yield Autoclave Synthesis of pure M₂B₁₂H₁₂ (M = Na, K)

Metal dodecaborates (MB₁₂H₁₂) are a versatile class of materials used in polymer chemistry and cancer treatment and are promising candidates as electrolytes for solid-state batteries. However, a general and scalable approach has not yet been developed for producing high-purity B₁₂H₁₂^2- derivatives. In this work, we report a simple, efficient, and environmentally benign solvothermal method to prepare diffraction and ¹¹B NMR pure Na₂B₁₂H₁₂ (85% yield) and K₂B₁₂H₁₂ (84% yield). This new synthetic approach is based on the use of the borane dimethyl sulfide complex (DMS·BH₃) and borohydrides (NaBH₄, KBH₄) heated at different temperatures in diglyme in an autoclave. It was found that high-purity Na₂B₁₂H₁₂·diglyme solvate is obtained via an intermediate formation of B₃H₈^-, B₉H₁₄^-, and B₁₁H₁₄^-, which are all soluble in diglyme. Heating under vacuum is shown to be efficient for removing the coordinated diglyme, allowing the formation of unsolvated Na₂B₁₂H₁₂. Autoclave synthesis starting from KBH₄ directly yields solvent-free K₂B₁₂H₁₂, and ball-milling KBH₄ prior to the synthesis enabling us to significantly improve the final yield. The new synthetic method paves the way for large-scale synthesis of MB₁₂H₁₂ derivatives, enabling to envisage a wider scope of practical applications.