Hydrogen-bonding versus pi-pi stacking interactions in dipyrido[f,h]quinoxaline-6,7-dicarbonitrile and 6,7-dicyanodipyrido[f,h]quinoxalin-1-ium chloride dihydrate.

The solvent-free title compound, C(16)H(6)N(6), is an aromatic derivative of phenanthroline with an extended pi system. It exhibits a remarkable pi-pi columnar stacking in the crystal structure, with interplanar distances of 3.229 (3) and 3.380 (3) A, the shorter spacing being between the two molecules within the asymmetric unit. Adjacent units along the stacked arrays are rotated in-plane with respect to one another by approximately 120 degrees . The hydrochloride derivative, C(16)H(7)N(6)(+).Cl(-).2H(2)O, in which one of the phenanthroline N atoms has been protonated, crystallized as a dihydrate. The supramolecular organization in this compound is characterized by continuous hydrogen bonding between the component species, yielding two-dimensional hydrogen-bonded networks. This study demonstrates the high significance of the pi-pi stacking interactions in the solvent-free aromatic system and how they can be undermined by introducing hydrogen-bonding capacity into the ligand.