Closely Approaching the Silylium Ion (R3Si+)

Area of Science:

• Inorganic Chemistry
• Organosilicon Chemistry
• Crystal Engineering

Background:

• Silylium cations (R3Si+) are highly reactive intermediates with significant positive charge on silicon.
• The nucleophilicity of counterions plays a crucial role in stabilizing these transient species.
• Understanding the degree of silylium character is key to controlling reactivity.

Purpose of the Study:

• To synthesize and characterize a novel tri-isopropyl silyl species with a highly non-nucleophilic carborane anion.
• To quantify the extent of silylium cation character in this new compound through structural and spectroscopic analysis.
• To compare the electronic properties with previously reported silyl cations.

Main Methods:

• Single-crystal X-ray diffraction to determine the precise atomic arrangement and bond angles.
• Silicon-29 nuclear magnetic resonance (NMR) spectroscopy to probe the electronic environment of the silicon atom.

Main Results:

• The crystal structure of i-Pr3Si(Br6-CB11H6) exhibits a C-Si-C angle of 117 degrees, approaching the 120 degrees planarity of a pure silylium ion.
• Silicon-29 NMR chemical shift of 109.8 ppm indicates a higher degree of silylium character compared to toluene-solvated analogs (
• The brominated carborane anion (Br6-CB11H6-) is identified as a highly effective, weakly coordinating counterion.

Conclusions:

• The studied tri-isopropyl silyl species demonstrates unprecedented silylium cation character.
• The results challenge previous interpretations of bonding in related systems, suggesting significant covalent pi interactions.
• This work provides a benchmark for highly electrophilic silicon species and their stabilization.