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.
Anthony J. Fernandes
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Characterization of hydrogen-substituted silylium ions in the condensed phase
The Weakly Coordinating Tris(trichlorosilyl)silyl Anion
Abstract
Closely following the procedure for the preparation of the base-stabilized dichlorosilylene complex NHCDipp⋅SiCl2 reported by Roesky, Stalke, and co-workers (Angew. Chem. Int. Ed. 2009, 48, 5683–5686), a few crystals of the salt [NHCDipp−H⋅⋅⋅Cl⋅⋅⋅H−NHCDipp]Si(SiCl3)3 were isolated, aside from the reported byproduct [NHCDipp−H+⋅⋅⋅Cl−], and characterized by X-ray crystallography (NHCDipp=N,N-di(2,6-diisopropylphenyl)imidazo-2-ylidene). They contain the weakly coordinating anion Si(SiCl3)3−, which was also obtained in high yields upon deprotonation of the conjugate Brønsted acid HSi(SiCl3)3 with NHCDipp or PMP (PMP=1,2,2,6,6-pentamethylpiperidine). The acidity of HSi(SiCl3)3 was estimated by DFT calculations to be substantially higher than those of other H-silanes. Further DFT studies on the electronic structure of Si(SiCl3)3−, including the electrostatic potential and the electron localizability, confirmed its low basicity and nucleophilicity compared with other silyl anions.
The Brønsted acidity of the H-silane HSi(SiCl3)3 is extremely high, namely more than 43 orders of magnitude higher than that of HSiMe3 and even slightly higher than that of HCl in MeCN. Its deprotonation with an N-heterocyclic carbene provides the conjugate base Si(SiCl3)3−, a weakly coordinating anion.
C−F Bond Activation by Silylium Cation/Phosphine Frustrated Lewis Pairs: Mono-Hydrodefluorination of PhCF3, PhCF2H and Ph2CF2
Abstract
Single defluorination of aryl polyfluoromethyl functionalities is achieved by both intra- and intermolecular silylium cation/phosphine Lewis pairs. Phosphine-captured aryl fluoromethyl cations are then treated with Brønsted base to complete the first mono-hydrodefluorinations of PhCF3, Ph2CF2, and PhCF2.
Reactions of CF3: Silylium/phosphine- based frustrated Lewis pairs are shown to effect the selective abstraction of fluoride ion from aryl-CF3 and CF2 groups, ultimate converting CF3 to CF2H to CFH2 fragments.
Cyclic (Amino)(Phosphonium Bora-Ylide)Silanone: A Remarkable Room-Temperature-Persistent Silanone
Abstract
A silanone substituted by bulky amino and phosphonium bora-ylide substituents has been isolated in crystalline form. Thanks to the exceptionally strong electron-donating phosphonium bora-ylide substituent, the lifetime at room temperature of the silanone is dramatically extended (t1/2=4 days) compared to the related (amino)(phosphonium ylide)silanone VI (t1/2=5 h), allowing easier manipulation and its use as precursor of new valuable silicon compounds. The interaction of silanone with a weak Lewis acid such as MgBr2 increases further its stability (no degradation after 3 weeks at room temperature).
A silanone substituted by bulky amino and phosphonium bora-ylide substituents has been synthesized and isolated in crystalline form. Thanks to the exceptionally strong electron-donating phosphonium-bora-ylide substituent, the lifetime is dramatically extended (t1/2=4 days). The interaction with a weak Lewis acid such as MgBr2 increases further its stability, and no degradation of silanone was observed after 3 weeks at room temperature.