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, i Pr, t Bu, 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 t Bu and Ad substituents. NMR spectra of the i Pr-, t Bu-, 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 t Bu-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 ΔG d 298(exptl) = 0.3(5) kcal mol–1 from NMR experiments for all-meta t Bu-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 (ΔG d 298(exptl) = 2.1(6) kcal mol–1) being more stable than its t Bu 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.

A Universal Quantitative Descriptor of the Dispersion Interaction Potential

“…However, in recent years, there was an upsurge in the appreciation of dispersion in small molecules in solution. Several studies addressed the concomitant compensation compared to the gas phase and found that dispersion is largely, but not fully, compensated . Concurrently, the introduction of so‐called dispersion energy donors (DEDs) was proposed as a design principle to tune interactions in solution .…”

“…Concurrently, the introduction of so‐called dispersion energy donors (DEDs) was proposed as a design principle to tune interactions in solution . Consequently, this new paradigm, inter alia, was successfully applied to isolate stable hexaphenylethane derivatives having very long C−C bonds and to guide the ligand design of CuH‐catalyzed hydroamination reactions for increased reactivity …”

Some Recent Advances in the Design and Use of Molecular Balances for the Experimental Quantification of Intramolecular Noncovalent Interactions of π Systems

“…and Schreiner et al. ) are beyond the scope of this review. As indicated in the title, this article is mainly focused on noncovalent interactions of π systems, such as those of aromatic and heteroaromatic rings and double bonds.…”

A Universal Quantitative Descriptor of the Dispersion Interaction Potential

“…However, in recent years, there was an upsurge in the appreciation of dispersion in small molecules in solution. Several studies addressed the concomitant compensation compared to the gas phase and found that dispersion is largely, but not fully, compensated . Concurrently, the introduction of so‐called dispersion energy donors (DEDs) was proposed as a design principle to tune interactions in solution .…”

“…Concurrently, the introduction of so‐called dispersion energy donors (DEDs) was proposed as a design principle to tune interactions in solution . Consequently, this new paradigm, inter alia, was successfully applied to isolate stable hexaphenylethane derivatives having very long C−C bonds and to guide the ligand design of CuH‐catalyzed hydroamination reactions for increased reactivity …”