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 ( P 6 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 P 6 3 mc phase seems to be destabilized under high pressure by relatively closer dihydrogen interactions, including the C-H…H-C contacts.