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 delta(29)Si and delta(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 greater than or equal to 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.