Dilemmas in crystallization.: The 'unusual' behavior of trans-1,5-dichloro-9,10-diethynyl-9,10-dihydroanthracene-9,10-diol and the more 'normal' behavior of its pseudopolymorphs with dipolar aprotic solvents

The compound trans-1,5-dichloro-9,10-diethynyl-9,10-dihydroanthracene-9,10-diol (DDDA) has an inversion center as the only molecular symmetry element and yet does not occupy an inversion center in the centrosymmetric space group that it adopts in the crystal structure. The reason for this very unusual occurrence is the crowded environment of the H-bond donors and acceptors that leads to less than optimal H-bonding. A centrosymmetric supramolecular synthon constituted with four Cl-atoms in a planar array occupies an i site in the crystal, and this appears to provide a satisfactory alternative packing. Based on the hypothesis that H-bonding is less than optimal in the crystal structure of DDDA, pseudopolymorphs were prepared with strongly H-bond-accepting solvents. The crystal structures of five of these solvates are described, wherein the DDDA molecule is able to occupy an i site and form strong and linear O - H (...) O H-bonds with the solvent molecules. Competition experiments show that a smaller solvent molecule with a greater H-bond-accepting ability is included more readily and that the H-bonds formed are correspondingly better.