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
1JSi-C and
2JSi-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 ≥ 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.