Synthesis of Silylium and Germylium Ions by a Substituent Exchange Reaction

The
reaction of diarylalkylsilanes and -germanes with trityl cation
in the presence of a weakly coordinating anion to give the corresponding
triarylsilylium or -germylium ions was investigated. This reaction
provides a facile access to a larger range of these sterically highly
hindered Lewis acids. The factors that promote the substituent exchange
were studied, and significant effects of the substituent, of the solvent,
and of the group 14 element were revealed. A combined solid-state
XRD and NMR investigation of the tris­(pentamethylphenyl) silylium
borate [(Me5C6)3Si]2[B12Cl12] disclosed the trigonal planar coordination
environment of the silicon atom in this silylium ion. NMR investigations
indicate for 2,4,6-triisopropylphenyl-substituted silylium and germylium
ions the onset of C–H···E+ three-center
interactions (E = Si, Ge) between the distant CH bond of the isopropyl
group and the positively charged group 14 element atom.