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, C(18)H(16)N(6)O(2) (I), 3,6-dimethyl-N (2),N (5)-bis­(pyridin-2-yl­meth­yl)pyrazine-2,5-dicarboxamide, C(20)H(20)N(6)O(2) (II), and N (2),N (5)-bis­(pyridin-4-ylmeth­yl)pyrazine-2,5-dicarboxamide, C(18)H(16)N(6)O(2) (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 P2(1)/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.