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-d(4)) 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. (C) 1997 Elsevier Science S.A.