The C5SiMe7+ cation: pyramidal, bicyclic, or cyclohexadienyl?

The monosila analogue of the C6Me7+ cation can be accessed by hydride abstraction from (pentamethylcyclopentadienyl)dimethylsilane, Cp*SiMe2H. Treatment of this material with triphenylmethylium tetrakis(pentafluorophenyl)borate in dichloromethane at -50 degrees C produced a single cationic species stable for a day or longer. The presence of a single resonance for the ring methyls and a single resonance for the silyl methyls in both the 1H and the 13C spectra indicated either a static structure with fivefold symmetry or a dynamic structure in which the dimethylsilyl group equilibrates rapidly among positions. Density function theory calculations found a minimum whose structure had the unsymmetrical silabicyclo[3.1.0]hexenyl structure. Calculated 1H, 13C, and 29Si chemical shifts for this structure were in good agreement with the observed values for this structure. Another calculated minimum was the silacyclohexadienyl structure, but its calculated chemical shifts were very different from those observed. We conclude that the cation has the bicyclic structure but rapidly interconverts the ring positions to produce the very simple NMR spectra.