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, ethy l, butyl and S = methylene chloride, DMPU, DMSO, sulfolane, HMPA, acet onitrile, pyridine, N-methylimidazole, and triethylamine were investig ated with the help of NMR spectroscopy for different concentration rat ios of R(3)SiX/S and R(2)HSiX/S as well as different temperatures. Wit h the help of measured delta(29)Si and delta(13)C chemical shifts as w ell as (1)J(Si-C) and (2)J(Si-P) coupling constants, typical NMR param eters for R(3)SiX and R(2)HSiX, R(3)Si(S)(+), R(2)HSi(S)(+), and R(2)H Si(S)(2)(+) were established and discussed to distinguish between poss ible silylium cation-solvent complexes and equilibria between them. In addition, the NMR/ab initio/IGLO method (based on the continuum solve nt model PISA and IGLO-PISA chemical shift calculations) was used to d etermine geometry, stability, and other properties of Me(3)Si(S)(n)(+) and Me(2)HSi(S)(n)(+) complexes in different solutions. NMR measureme nts 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 resul t of internal (hyperconjugative) stabilization of R(3)Si(+) by alkyl g roups and external stabilization by S coordination, as well as of ster ic factors involving R and S. Complex binding energies are in the rang e of 40-60 kcal/mol, which is significantly different from complex bin ding energies in the gas phase. In all cases investigated, (weakly) co valent bonds between Si and S are formed that exclude any silylium cat ion character for the solvated R(3)Si(+) and R(2)HSi(+) ions.