Dynamics of Si-H-Si bridges in agostically stabilized silylium ions.

The silylium ion [C(6)(SiMe(2))(SiHMe(2))(5)](+) offers an amazing example of multiple Si...H interactions. It exhibits a symmetric Si(alpha)-H-Si(alpha) motif supported by two additional Si(beta)-H...Si(alpha) 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 beta- or gamma-silyls. The energy barrier is largely caused by the silyl internal rotation, whereas the hydride transfer itself is intrinsically quite easy. The gamma-silyl rotation is somewhat more favorable than the beta-silyl rotation. Vibrational dynamics of the cation is also discussed.