Spectroscopic and Computational Assessment of Silicon’s Electrophilicity in Phosphinosilylium Cations

An experimental method of determining the electrophilicity of silicon in phosphinosilylium cations is reported and compared to Djukic’s DFT method of computing relative intrinsic silylicity, Π. We also establish linear correlations between silicon electrophilicity and 29Si NMR chemical shifts, DFT-computed silylicities, and the Tolman electronic parameter of the phosphine. These correlations were not universal, as deviations were observed for the most sterically hindered phosphines. Intermolecular silylium transfer experiments between phosphinosilyliums and added phosphines provided a thermodynamic assessment of the electrophilic character at silicon. This confirmed that relative intrinsic silylicity (Π) and 29Si NMR chemical shifts are useful parameters for semiquantitatively determining the electrophilicity of the silyl group in a Lewis pair.