The crystal structures of three pyrazine-2,5-dicarboxamides: three-dimensional supramolecular structures

The complete molecules of the title compounds, N 2,N 5-bis(pyridin-2-ylmethyl)pyrazine-2,5-dicarboxamide, C18H16N6O2 (I), 3,6-dimethyl-N 2,N 5-bis(pyridin-2-ylmethyl)pyrazine-2,5-dicarboxamide, C20H20N6O2 (II), and N 2,N 5-bis(pyridin-4-ylmethyl)pyrazine-2,5-dicarboxamide, C18H16N6O2 (III), are generated by inversion symmetry, with the pyrazine rings being located about centres of inversion. Each molecule has an extended conformation with the pyridine rings inclined to the pyrazine ring by 89.17 (7)° in (I), 75.83 (8)° in (II) and by 82.71 (6)° in (III). In the crystal of (I), molecules are linked by N—H...N hydrogen bonds, forming layers lying parallel to the bc plane. The layers are linked by C—H...O hydrogen bonds, forming a three-dimensional supramolecular structure. In the crystal of (II), molecules are also linked by N—H...N hydrogen bonds, forming layers lying parallel to the (10-1) plane. As in (I), the layers are linked by C—H...O hydrogen bonds, forming a three-dimensional supramolecular structure. In the crystal of (III), molecules are again linked by N—H...N hydrogen bonds, but here form corrugated sheets lying parallel to the bc plane. Within the sheets, neighbouring pyridine rings are linked by offset π–π interactions [intercentroid distance = 3.739 (1) Å]. The sheets are linked by C—H...O hydrogen bonds, forming a three-dimensional supramolecular structure. Compound (I) crystallizes in the monoclinic space group P21/c. Another monoclinic polymorph, space group C2/c, has been reported on by Cockriel et al. [Inorg. Chem. Commun. (2008), 11, 1–4]. The molecular structures of the two polymorphs are compared.