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Dynamics of Si−H−Si Bridges in Agostically Stabilized Silylium Ions

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journal contribution
posted on 19.02.2009, 00:00 by Samat Tussupbayev, Georgii I. Nikonov, Sergei F. Vyboishchikov
The silylium ion [C6(SiMe2)(SiHMe2)5]+ offers an amazing example of multiple Si···H interactions. It exhibits a symmetric Siα−H−Siα motif supported by two additional Siβ−H···Siα agostic interactions. This cation is highly fluctional in NMR spectra at room temperature due to shift of the hydride bridge. The DFT calculations show that the hydride shift is related to internal rotation of silyl groups. We performed NMR, static DFT, and dynamics studies of this process and found two possible mechanisms, associated with internal rotation of either β- or γ-silyls. The energy barrier is largely caused by the silyl internal rotation, whereas the hydride transfer itself is intrinsically quite easy. The γ-silyl rotation is somewhat more favorable than the β-silyl rotation. Vibrational dynamics of the cation is also discussed.