Strong Intramolecular Solvation of Silyl Cations

To maximize stabilizing interactions between the empty 3p7t(Si) orbital and the electron lone pairs at the N atoms of the two substituents -CH2-NR2, the R2Si group in 41 adopts a conformation, in which it is perpendicular to the plane of 

Calculated properties of silyl cations with and without internal coordination. a 

Corriu and co-workers [124] pointed out that the results of the NMR measurements depend on the counterion used when generating bidentated cations of type 41. In the case of a strongly interacting counterion (41 with R2Si+ = PhMeSi+ and Z = NMe2), 13C NMR shifts are in line with an exchange process. 

However, in the case of more weakly interacting counterion such as TPFPB the NMR data are in line with the formation of a pentacoordinated siliconium ion. [124] It was concluded that the stability of the pentacoordinated ion strongly depends on solvent and counterions. Specifically, methanol seems to stabilize the tetracoordinated form extensively. [124] 

The quantum chemical studies suggest that interactions between the CH2-NH2 ligands and R2Si+ in cations 41 resemble those between a N-containing solvent (e.g., an amine) and a R2HSi+ cation. In both cases it is possible that a pentacoordinated Si atom is formed. [43,44] 

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