Dibenzosilanorbornadienyl Cations and Their Fragmentation into Silyliumylidenes

The terphenyl-substituted
dibenzosilanorbornadienyl cation 11 was synthesized and
isolated in the form of its [B­(C6F5)4]− salt. The salt
was characterized by NMR spectroscopy supported by quantum mechanical
computations and by an XRD analysis of a corresponding acetonitrilium
salt. The thermal fragmentation of 11[B­(C6F5)4] in benzene results in the high-yield
formation of diphenylterphenylsilylium borate 17[B­(C6F5)4]. High-lying intermediates in this
process are solvent-complexed terphenylsilyliumylidene 8 and the hydrogen- and phenyl-substituted silylium ion 20. The formation of silylium ion 20 by reaction of silyliumylidene 8 with the solvent benzene demonstrates the high potential
of this four valence electron species in C–H bond activation
reactions. In addition, the instability of the hydrogen-substituted
silylium ion 20 in benzene opens new mechanistic perspectives
particular for dihydrogen activation by silyl cationic frustrated
Lewis pairs and in general for the dihydrogen activation by strong
Lewis acids.