Solvated Silylium Cations: Structure Determination by NMR Spectroscopy and the NMR/Ab Initio/IGLO Method

Sixty R3SiX/solvent (S) and R2HSiX/S systems with R ) methyl, ethyl, butyl and S ) methylene chloride, DMPU, DMSO, sulfolane, HMPA, acetonitrile, pyridine, N-methylimidazole, and triethylamine were investigated with the help of NMR spectroscopy for different concentration ratios of R3SiX/S and R2HSiX/S as well as different temperatures. With the help of measured δ29Si and δ13C chemical shifts as well as JSi-C and JSi-P coupling constants, typical NMR parameters for R3SiX and R2HSiX, R3Si(S), R2HSi(S), and R2HSi(S)2 were established and discussed to distinguish between possible silylium cation-solvent complexes and equilibria between them. In addition, the NMR/ab initio/IGLO method (based on the continuum solvent model PISA and IGLO-PISA chemical shift calculations) was used to determine geometry, stability, and other properties of Me3Si(S)n and Me2HSi(S)n complexes in different solutions. NMR measurements and ab initio calculations clearly show that R3Si(S) and R2HSi(S) complexes with tetracoordinated Si are formed with solvents (S) more nucleophilic than methylene chloride while complexes with two S molecules and a pentacoordinated Si atom can only be found for R3-nHnSi cations with n g 1. This is a result of internal (hyperconjugative) stabilization of R3Si by alkyl groups and external stabilization by S coordination, as well as of steric factors involving R and S. Complex binding energies are in the range of 40-60 kcal/mol, which is significantly different from complex binding energies in the gas phase. In all cases investigated, (weakly) covalent bonds between Si and S are formed that exclude any silylium cation character for the solvated R3Si and R2HSi ions.