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

Metal dodecaborates (MxB12H12) 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 B12H122– 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 MxB12H12 derivatives, enabling to envisage a wider scope of practical applications