Using the Lewis Acid Me₃Si−F−Al(OR^F)₃ To Prepare Phosphino‐Phosphonium Cations with the Least‐Coordinating Anion [(R^FO)₃Al−F−Al(OR^F)₃]⁻

Abstract

By reaction of two equivalents of Me₃ Si-F-Al(OR^F )₃ 1 with an equimolar amount of PPh₂ Cl, the salt [Ph₂ P-PPh₂ Cl]⁺ [(R^F O)₃ Al-F-Al(OR^F )₃ ]^- 2 is prepared smoothly in 91 % yield (NMR, XRD). The synthesis of [Ph₂ P-PPh₃ ]⁺ [(R^F O)₃ Al-F-Al(OR^F )₃ ]^- 3 is best achieved by a two-step reaction: first, two equivalents of 1 react with one PPh₃ to give [Me₃ Si-PPh₃ ]⁺ [(R^F O)₃ Al-F-Al(OR^F )₃ ]^- 4 (NMR, XRD), which, upon reaction with PPh₂ Cl, yields pure 3 and Me₃ SiCl (NMR, XRD). Typically, a stoichiometry of two equivalents of 1 with respect to one equivalent of the chloride donor should be used. Otherwise, the residual strong Lewis acidity of the [(R^F O)₃ Al-F-Al(OR^F )₃ ]^- anion in the presence of the [F-Al(OR^F )₃ ]^- anion-that forms with less than two equivalents of 1-leads to further chloride exchange reactions that complicate work-up. This route presents the easiest way to introduce the least-coordinating [(R^F O)₃ Al-F-Al(OR^F )₃ ]^- anion into a system. We expect a wide use of this route in all areas, in which chloride-bond heterolysis in combination with very weakly coordinating anions is desirable. Additionally, we performed calculations on the bond dissociation mechanisms of [R₂ P-PMe₃ ]⁺ and the isoelectronic Me₂ P-SiMe₃ and Me₂ Si-PMe₃ in dependence of the solvent permittivity. These calculations show, especially for the neutral reference compounds, a heavy influence of the solvent on the dissociation mechanism, which is why we suggest investigating these properties in solution instead of gas phase.