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Characterization of hydrogen-substituted silylium ions in the condensed phase

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Science  12 Jul 2019:
Vol. 365, Issue 6449, pp. 168-172
DOI: 10.1126/science.aax9184

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An acidic route to silicon cations

The simplest silicon cation, with a central Si atom bonded to just three hydrogens, has long eluded bulk synthesis. Wu et al. now report a straightforward route to this molecule by reacting a carborane acid with phenyl silane, producing benzene and the silylium carborane ion pair. A similar protocol offered efficient syntheses of primary and secondary silyl cations through acidic cleavage of Si–phenyl or Si–H bonds. All three products, characterized crystallographically and in solution, manifested weak coordination to bromine substituents of the carboranes.

Science, this issue p. 168

Abstract

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.

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