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

Abstract

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 Δ G_d²⁹⁸(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 (Δ G_d²⁹⁸(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.