Gauging the Steric Effects of Silyl Groups with a Molecular Balance

We present an experimental
and computational study of a cyclooctatetraene
(COT)-based molecular balance disubstituted with commonly used silyl
groups. Such groups often serve as protecting groups and are typically
considered innocent bystanders. Our motivation here is to determine
the actual steric effects of such groups by employing a molecular
balance. While in the unfolded 1,4-valence isomer the silyl groups
are far apart (dσ–σ ≥ 5.15 Å), the folded 1,6-isomer is affected greatly
by noncovalent interactions due to close σ–σ contacts
(dσ–σ ≤ 2.58
Å). In order to investigate the thermodynamic equilibrium between
the 1,6- and 1,4-valence isomers, we employed temperature-dependent
nuclear magnetic resonance measurements. Additionally, we assessed
the nature of attractive 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 of theory. We identified
London dispersion interactions as the main contributor to the molecular
stability of the folded states, whereas Pauli exchange repulsion and
a resulting internal strain favor the unfolded diastereomer.