Influence of the solvent and of the counteranion on the structure of silyl cations stabilized by a terdentate aryldiamine ligand

Solution NMR studies of silyl cations ArSiMe₂]⁺X⁻ (X = I, CF₃SO₃) incorporating the terdentate aryl diamine ligand Ar - C₆H₃₋ 2,6-(CH₂NMe₂)₂ have been carried out in a protic solvent (methanol-d₄) and in an aprotic solvent (CD₂Cl₂). This study has shown that the structure of these silyl cations is highly dependent on the solvent. In CD₂Cl₂, the silyl cation is five-coordinated owing to the coordination of one NMe₂ group and of the anion to the silicon centre which gives rise to a dissymmetric structure. On the other hand, in CD₃OD there is no coordination of the anion, but the silyl cation is also probably five-coordinated due to the coordination of the solvent to the silicon atom which is supported by the X-ray analysis of the compound 9. With the weakly nucleophilic anion BPh₄⁻ in CD₂Cl₂, in addition to the silyl cation previously described, another five-coordinated silyl cation resulting from the coordination of both NMe₂ groups to the Si centre was postulated.