The solution structure of (Me3Si)3CSiBr--an ab initio/NMR study

Area of Science:

• Organosilicon Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• The synthesis of stable silylenes, silicon analogs of carbenes, is a significant challenge in organosilicon chemistry.
• Characterizing transient silylene species in solution requires advanced computational and spectroscopic techniques.

Purpose of the Study:

• To computationally investigate the structure and NMR properties of the first solution-stable halosilylene.
• To reconcile discrepancies between calculated and experimental NMR chemical shifts for silylenes.
• To provide guidance for future identification of silylenes using NMR spectroscopy.

Main Methods:

• Ab initio calculations including IGLO SOS-DFPT PW91/B2//B3LYP/6-31+G(d)) were employed.
• Density Functional Theory (DFT) was used to model potential silylene structures and their (29)Si NMR chemical shifts.
• Comparison of calculated (29)Si NMR chemical shifts with experimental data.

Main Results:

• The calculated (29)Si NMR chemical shift for free (Me(3)Si)(3)CSiBr (446 ppm) did not match the experimental value (106 ppm).
• Calculated shifts for the (Me(3)Si)(3)CSiBr(2) anion (124 ppm) and its complexes with Li(+) (117 and 134 ppm) agreed with the experimental signal.
• A range of calculated (29)Si NMR chemical shifts (200-900 ppm) for alkylsilylenes was presented.

Conclusions:

• The experimental NMR signal at 106 ppm is likely due to a halosilylene anion complex, not the free halosilylene.
• Computational modeling is crucial for accurate characterization of reactive intermediates like silylenes.
• The provided data serves as a valuable reference for identifying novel silylenes in future synthetic studies.