Alkyl groups as electron density donors in π-hole bonding

Whereas sp(2) and sp-hybridized carbon compounds have been described as electron-rich species able to interact with electron-deficient moieties to form pi and sigma-hole bonds, to the best of our knowledge there is no report in the literature of sp(3) carbons acting as Lewis bases in hole-bonds. By means of a combined structural and computational analysis we have demonstrated the unexpected ability of alkyl groups to act as electron density donors in pi-hole bonding. We focused on the interaction between alkyl groups and typical Lewis bases, namely ER3 (E = B, Al, Ga, In, Tl) molecular frameworks. Structural database searches revealed a clear directionality for these short contacts. The attractive nature of the interaction was confirmed by "atoms in molecules" (AIM) and "non-covalent interactions" (NCI) analysis. The interaction strength was evaluated at the MP2/aug-cc-pVTZ level of theory on several models and its nature further investigated by MEP, NBO and EDA analysis. In terms of energy, they are comparable to other p-hole and weak hydrogen bonds, which opens the door to their use in crystal engineering and supramolecular design.