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

Sixty R 3 SiX/solvent (S) and R 2 HSiX/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 R 3 SiX/S and R 2 HSiX/S as well as different temperatures. With the help of measured δ 29 Si and δ 13 C chemical shifts as well as 1 J Si - C and 2 J Si - P coupling constants, typical NMR parameters for R 3 SiX and R 2 HSiX, R 3 Si(S) +, R 2 HSi(S) +, and R 2 HSi(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 Me 3 Si(S) n + and Me 2 HSi(S) n + complexes in different solutions. NMR measurements and ab initio calculations clearly show that R 3 Si(S) + and R 2 HSi(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 - n H n Si + cations with n ≥ 1. This is a result of internal (hyperconjugative) stabilization of R 3 Si + 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 R 3 Si + and R 2 HSi + ions.