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.