Gauging the Steric Effects of Silyl Groups with a Molecular Balance

We present an experimental and computational studyof a cyclooctatetraene (COT)-based molecular balance disubstitutedwith commonly used silyl groups. Such groups often serve asprotecting groups and are typically considered innocent bystanders.Our motivation here is to determine the actual steric effects of suchgroups by employing a molecular balance. While in the unfolded 1,4-valence isomer the silyl groups are far apart (d Sigma-Sigma >= 5.15 A), thefolded 1,6-isomer is affected greatly by noncovalent interactions dueto close Sigma-Sigma contacts (d Sigma-Sigma <= 2.58 A). In order to investigate thethermodynamic equilibrium between the 1,6- and 1,4-valenceisomers, we employed temperature-dependent nuclear magneticresonance measurements. Additionally, we assessed the nature ofattractive and repulsive interactions in 1,6-disilyl-COT derivatives via a combination of local energy decomposition analysis (LED)and symmetry-adapted perturbation theory (SAPT) at the DLPNO-CCSD(T)/def2-TZVP and sSAPT0/aug-cc-pVDZ levels oftheory. We identified London dispersion interactions as the main contributor to the molecular stability of the folded states, whereasPauli exchange repulsion and a resulting internal strain favor the unfolded diastereomer.

Automated summary

In this study, we investigated the actual steric effects of commonly used silyl groups in cyclooctatetraene (COT) molecules through experimental and computational methods. We found that the interactions between silyl groups greatly affect the stability of the molecular configuration.