Simple Route to [PSH][B₉H₁₄] and a Contemporary Study of Its Solid-State Dynamic Behavior

The 1,8-bis(dimethylamino)naphthalenium ([PSH]⁺) decaborane salt, [PSH][B₁₀H₁₃], has been found to react in ethanol to form [PSH][B₉H₁₄] (1), affording a simple route to the synthesis of the arachno-nonaborate anion. This new polyhedral salt is characterized by NMR spectroscopy and X-ray diffraction. The measurement of diffusion coefficients by NMR methods demonstrates that the [PSH]⁺ cation and the [B₉H₁₄]^- anion form ion pairs in a non-coordinating solvent such as CH₂Cl₂, whereas in CD₃CN the formation of ion pairs was not observed. Insights into the long-known low-energy dynamic behavior, which involves the bridging and endo-terminal hydrogen atoms, are elucidated using DFT calculations. Salts [PSH][B₉H₁₄] (1) and [PSH][B₉H₁₄]·0.5CHCl₃ (solvated, 1·0.5CHCl₃) have also been studied by X-ray diffraction analysis. A solid-state NMR study has demonstrated that K[B₉H₁₄] and [PSH][B₉H₁₄] (1) undergo significantly different motion regimes, being a low-energy, weakly temperature-dependent process for 1, which may be ascribed to some type of low-amplitude reorientation of the whole boron cages. This process may be the mechanism for the low- to-room-temperature order-disorder hidden transition found by X-ray analysis.