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 ¹J_Si-C and ²J_Si-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 R_3-nH_nSi⁺ cations with n ≥ 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.