The crystal structures of three pyrazine-2,5-dicarb-oxamides: three-dimensional supra-molecular structures.

The complete mol-ecules of the title compounds, N(2),N(5)-bis-(pyridin-2-ylmeth-yl)pyrazine-2,5-dicarboxamide, C(18)H(16)N(6)O(2) (I), 3,6-dimethyl-N(2),N(5)-bis-(pyridin-2-yl-meth-yl)pyrazine-2,5-dicarboxamide, C(20)H(20)N(6)O(2) (II), and N(2),N(5)-bis-(pyridin-4-ylmeth-yl)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 mol-ecule 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), mol-ecules 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 supra-molecular structure. In the crystal of (II), mol-ecules 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 supra-molecular structure. In the crystal of (III), mol-ecules 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 inter-actions [inter-centroid distance = 3.739†(1)†à ]. The sheets are linked by C-H⋯O hydrogen bonds, forming a three-dimensional supra-molecular 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 mol-ecular structures of the two polymorphs are compared.