Gallic Acid Dimer As a Double π–Hole Donor: Evidence from X-ray, Theoretical Calculations, and Generalization from the Cambridge Structural Database

In this work, we demonstrate that the centrosymmetric eight-membered supramolecular ring R22(8) that is formed upon dimerization of benzoic acids has a marked tendency to establish π–hole interactions with electron-rich atoms. We have used the Cambridge Structural Database to demonstrate the preference of carboxylic acid dimers to form donor–acceptor interactions involving π–holes located at the C atoms above and below the molecular plane. Moreover, we have carried out DFT calculations (PBE0-D3/def2-TZVP) to investigate the geometric and energetic features of these interactions and how they are affected by the substituents of the aromatic ring. Finally, as an example we report the synthesis and X-ray characterization of a solvate of gallic acid with dioxane, where two molecules of dioxane are located above and below the eight-membered supramolecular ring, forming two symmetrically equivalent O···C π–hole interactions.