The Preparation, Characterization, and Pressure-Influenced Dihydrogen Interactions of Tetramethylphosphonium Borohydride

Tetramethylphosphonium borohydride was synthesized via an ion metathesis reaction in a weakly-coordinating aprotic environment. [(CH(3))(4)P]BH(4), in contrast to related [(CH(3))(4)N](+) compounds which tend to crystallize in a tetragonal system, adopts the distorted wurtzite structure (P6(3)mc), resembling some salts containing analogous ions of As and Sb. [(CH(3))(4)P]BH(4) decomposes thermally in several endo- and exothermic steps above ca. 240 °C. This renders it more stable than [(CH(3))(4)N]BH(4), with a lowered temperature of decomposition onset by ca. 20 °C and solely exothermic processes observed. Raman spectra measured at the 0–10 GPa range indicate that a polymorphic transition occurs within 0.53–1.86 GPa, which is further confirmed by the periodic DFT calculations. The latter suggests a phase transition around 0.8 GPa to a high-pressure phase of [(CH(3))(4)N]BH(4). The P6(3)mc phase seems to be destabilized under high pressure by relatively closer dihydrogen interactions, including the C–H…H–C contacts.