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. [(CH3)(4)P]BH4, in contrast to related [(CH3)(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. [(CH3)(4)P]BH4 decomposes thermally in several endo- and exothermic steps above ca. 240 & DEG;C. This renders it more stable than [(CH3)(4)N]BH4, with a lowered temperature of decomposition onset by ca. 20 & DEG;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 [(CH3)(4)N]BH4. The P6(3)mc phase seems to be destabilized under high pressure by relatively closer dihydrogen interactions, including the C-H horizontal ellipsis H-C contacts.

Automated summary

Tetramethylphosphonium borohydride was synthesized in a weakly-coordinating aprotic environment through an ion metathesis reaction. Unlike related compounds, it has a distorted wurtzite structure and is more stable, decomposing at a lower temperature with only exothermic processes observed. Raman spectra and DFT calculations confirm the occurrence of a polymorphic transition and a phase transition to a high-pressure phase for [(CH3)(4)N]BH4.