Isolation of halogen-substituted silylium ions

The existence and intermediacy of halogen-substituted silylium ions have been the subject of speculation for decades. These elusive reactive intermediates are synthetically attractive because of their computationally predicted super Lewis acidity and their relevance in several synthetic transformations such as recycling of waste from the Müller-Rochow process and hydrodefluorination of per- and polyfluoroalkyl substances. Here we report the generation and characterization of all halogen-substituted silylium ions of type [Alk₂XSi(HCB₁₁H₅Br₆)] (X = F, Cl, Br or I; Alk = Me, Et, iPr or tBu). While the established Corey hydride transfer reaction fails to make such ions in the condensed phase, the protolysis of the halosilanes Alk₂XSi‒LG (LG = H or Ph) using Reed's superacidic benzenium ion [H(C₆H₆)]⁺[HCB₁₁H₅Br₆]^- provides practical access. The full series of counteranion-stabilized iPr₂XSi⁺ cations is isolated and crystallographically characterized. The obtained halogen-substituted silylium ions are more Lewis acidic than their known trialkyl- and hydrogen-substituted congeners, as verified by quantitative assessment of their fluoride ion affinities using density functional theory calculations.