Probing the Delicate Balance between Pauli Repulsion and London Dispersion with Triphenylmethyl Derivatives

The long-known, ubiquitously present,
and always attractive London
dispersion (LD) interaction was probed with hexaphenylethane (HPE)
derivatives. A series of all-meta hydrocarbyl [Me, iPr, tBu, Cy,
Ph, 1-adamantyl (Ad)]-substituted triphenylmethyl (TPM) derivatives
[TPM–H, TPM–OH, (TPM–O)2, TPM•] was synthesized en route, and several derivatives
were characterized by single-crystal X-ray diffraction (SC-XRD). Multiple
dimeric head-to-head SC-XRD structures feature an excellent geometric
fit between the meta-substituents; this is particularly
true for the sterically most demanding tBu and Ad substituents. NMR spectra of the iPr-, tBu-, and Cy-derived trityl
radicals were obtained and reveal, together with EPR and UV–Vis
spectroscopic data, that the effects of all-meta alkyl
substitution on the electronic properties of the trityl scaffold are
marginal. Therefore, we concluded that the most important factor for
HPE stability arises from LD interactions. Beyond all-meta tBu-HPE we also identified the hitherto
unreported all-meta Ad-HPE. An intricate mathematical
analysis of the temperature-dependent dissociation constants allowed
us to extract ΔGd298(exptl)
= 0.3(5) kcal mol–1 from NMR experiments for all-meta tBu-HPE, in good agreement
with previous experimental values and B3LYP-D3­(BJ)/def2-TZVPP­(C-PCM)
computations. These computations show a stabilizing trend with substituent
size in line with all-meta Ad-HPE (ΔGd298(exptl) = 2.1(6) kcal mol–1) being more stable than its tBu congener. That is, large, rigid, and symmetric hydrocarbon
moieties act as excellent dispersion energy donors. Provided a good
geometric fit, they are able to stabilize labile molecules such as
HPE via strong intramolecular LD interactions, even in solution.