Sizing the role of London dispersion in the dissociation of all-meta tert-butyl hexaphenylethane

Abstract

The structure and dynamics of enigmatic hexa(3,5-di-<i>tert</i>-butylphenyl)ethane was characterized <i>via</i> NMR spectroscopy for the first time. Our variable temperature NMR analysis demonstrates an enthalpy-entropy compensation that results in a vanishingly low dissociation energy (Δ<i>G</i>298d = -1.60(6) kcal mol^-1). An <i>in silico</i> study of increasingly larger all-<i>meta</i> alkyl substituted hexaphenylethane derivatives (Me, ⁱPr, ^<i>t</i> Bu, Cy, 1-Ad) reveals a non-intuitive correlation between increased dimer stability with increasing steric crowding. This stabilization originates from London dispersion as expressed through the increasing polarizability of the alkyl substituents. Substitution with conformationally flexible hydrocarbon moieties, <i>e.g.</i>, cyclohexyl, introduces large unfavourable entropy contributions. Therefore, using rigid alkyl groups like <i>tert</i>-butyl or adamantyl as dispersion energy donors (DED) is essential to help stabilize extraordinary bonding situations.