Characterization of hydrogen-substituted silylium ions in the condensed phase.

Hydrogen-substituted silylium ions are long-sought reactive species. We report a protolysis strategy that chemoselectively cleaves either an Si-C(sp2) or an Si-H bond using a carborane acid to access the full series of [CHB11H5Br6]--stabilized R2SiH+, RSiH2 +, and SiH3 + cations, where bulky tert-butyl groups at the silicon atom (R = tBu) were crucial to avoid substituent redistribution. The crystallographically characterized molecular structures of [CHB11H5Br6]--stabilized tBu2HSi+ and tBuH2Si+ feature pyramidalization at the silicon atom, in accordance with that of tBu3Si+[CHB11H5Br6]- Conversely, the silicon atom in the H3Si+ cation adopts a trigonal-planar structure and is stabilized by two counteranions. This solid-state structure resembles that of the corresponding Brønsted acid.