Experimental and computational study of the crystal packing of isoelectronic 4,11-diaza[5]helicene and 4,7,8,11-tetraaza[5]helicene

The crystal structures of isoelectronic 4,11-diaza[5]helicene (DNH) and 4,7,8,11-tetraaza[5]helicene (TNH) are accurately determined at room temperature by single crystal X-ray diffraction. Analysis of crystal packing with computational chemistry tools, including the evaluation of intermolecular and cohesive energies with force field and charge density (PIXEL) methods, highlights the effect of skeletal replacement of two aromatic C-H with nitrogen atoms. Both crystals are centrosymmetric racemates, with asymmetric units (ASUs) exploiting the same conformation. The ASU of TNH bears two symmetry-independent enantiomers, and the different settings of symmetry operators mirror in different packing motifs with respect to DNH. Both structures exhibit columnar stacking arrangements, which differ in their mutual orientation, distance and strength. Stacking interactions are more frequent and stronger in the tetraaza compound, owing to the greater intrinsic polarization of its ground-state electron density that favors antiparallel arrangements of neighboring molecular dipole moments. Finally, the possible existence of thermodynamically stable chiral forms of the title compounds is investigated by symmetry-constrained Monte Carlo (SC-MC) calculations.