Winning Strategy toward Acentric Crystals: Transverse Dipole Moment Molecules

Imines obtained by condensation of 4-hydroxybenzohy-drazide with carbonyl compounds are a rare example of non-chiral compounds showing a remarkable tendency to crystallize in acentric and polar space groups, as we underlined in a previous study of imines obtained by condensation with aliphatic ketones. Here, we present an extensive study of imines obtained by condensation with aromatic aldehydes and demonstrate that over a representative set of 60 crystal structures, the overall occurrence of acentric phases in this class of compounds is 47%. This figure is remarkably higher than the overall 22% frequency of acentric structures in the Cambridge Structural Database and pinpoints a successful strategy toward the synthesis of acentric crystals. In the crystal structures of the acentric imines, complex chains of H-bonded molecules are generally present, with formation of 2D H-bonded networks, similar to other classes of compounds described in the literature with a propensity to form acentric crystals. Another reason for the high success in achieving acentric and polar structures in this class of compounds can be related to the presence of strong transverse bond dipole moments. This feature allows a polar packing of the long molecular axes while keeping the bond dipole moments antiparallel.