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

Area of Science:

• Organometallic Chemistry
• Silicon Chemistry
• Carbocation Analogues

Background:

• The C6Me7+ cation is a well-known organometallic compound.
• Exploring analogues with different central atoms can reveal unique chemical properties.
• Monosila analogues offer insights into bonding and reactivity compared to their carbon counterparts.

Purpose of the Study:

• To synthesize and characterize the monosila analogue of the C6Me7+ cation.
• To determine the structural and dynamic properties of the synthesized cation.
• To compare experimental findings with theoretical predictions.

Main Methods:

• Hydride abstraction from (pentamethylcyclopentadienyl)dimethylsilane (Cp*SiMe2H) using triphenylmethylium tetrakis(pentafluorophenyl)borate.
• Nuclear Magnetic Resonance (NMR) spectroscopy (1H, 13C, 29Si) for structural elucidation.
• Density Functional Theory (DFT) calculations for structural optimization and chemical shift prediction.

Main Results:

• Successful synthesis of a stable cationic species, the monosila analogue of C6Me7+.
• NMR spectra indicated either a static fivefold symmetry or rapid dynamic equilibration of the dimethylsilyl group.
• DFT calculations identified a stable silabicyclo[3.1.0]hexenyl structure with good agreement to experimental NMR data.
• A silacyclohexadienyl structure was computationally feasible but inconsistent with observed spectra.

Conclusions:

• The synthesized cation adopts a bicyclic silabicyclo[3.1.0]hexenyl structure.
• Rapid interconversion of ring positions explains the simplified NMR spectra.
• This study expands the understanding of organosilicon cations and their unique structural dynamics.