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

The whole molecules of the three title pyrazine-2,5-dicarboxamide compounds are generated by inversion symmetry. Each molecule has an extended conformation with the pyridine rings being inclined to the pyrazine ring by 89.17 (7), 75.83 (8) and 82.71 (6)°.


Chemical context
The title compounds are part of a series of pyrazine mono-and di-and tetrakiscarboxamide derivatives synthesized to study their coordination chemistry with essentially first-row transition metals (Cati, 2002). Compound (I) crystallizes in the monoclinic space group P2 1 /c. Another monoclinic polymorph, space group C2/c, has been described by Cockriel et al. (2008).

Structural commentary
The molecular structures of the title compounds, (I), (II) and (III), are illustrated in Figs. 1, 2 and 3, respectively. The whole molecule of each compound is generated by inversion symmetry, with the pyrazine rings being located about centers of inversion. Each molecule has an extended conformation with the pyridine rings inclined to the pyrazine ring by 89.17 (7) in (I), by 75.83 (8) in (II) and by 82.71 (6) in (III). The methylcarboxamide units (C4-N2-C3 O1) are ISSN 2056-9890 inclined to the pyrazine ring by 4.24 (9), 3.13 (10) and 9.32 (8) in (I), (II) and (III), respectively.
In the monoclinic C2/c polymorph of (I) (Cockriel et al., 2008), the whole molecule is also generated by inversion symmetry (Fig. 4). However, here the molecule is almost planar with the pyridine rings being inclined to the pyrazine ring by only 5.70 (7) . The pyridine ring is orientated in such a manner that the NH hydrogen atom forms short contacts with both the adjacent pyrazine and pyridine N atoms, as shown in Fig. 4. The carbonyl O atom also accepts a short contact from a pyrazine H atom (Fig. 4).

Supramolecular features
In the crystal of (I), molecules are linked by N-HÁ Á ÁN hydrogen bonds, forming layers lying parallel to the bc plane A view of the molecular structure of compound (I), with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The unlabelled atoms are related to the labelled atoms by inversion symmetry (symmetry operation: Àx, Ày, Àz + 2).

Figure 2
A view of the molecular structure of compound (II), with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The unlabelled atoms are related to the labelled atoms by inversion symmetry (symmetry operation: Àx, Ày, Àz).

Figure 5
A view along the a axis of the crystal pack of compound (I). The N-HÁ Á ÁN hydrogen bonds are shown as dashed lines (see Table 1). For clarity, in this and subsequent figures, only the H atoms involved in hydrogen bonding have been included.

Figure 6
A view along the b axis of the crystal pack of compound (I). The hydrogen bonds are shown as dashed lines (see Table 1).
Compound (I): was prepared by refluxing dimethyl pyrazine-2,5-dicarboxylate (1.00 g, 5 mmol) and an excess of 2-(aminomethyl)pyridine (1.55g, 14.3 mmol) in 30 ml of methanol in a two-necked flask (100 ml). After 150 min a precipitate appeared, and after refluxing for 5 h the suspension was cooled to room temperature. A white solid was filtered off and washed with 10 ml of cold methanol. It was then recrystallized from dichloromethane solution to give colourless plate-like crystals of (I) suitable for X-ray diffraction analysis (yield 81%, m.p. 479 K).

Figure 8
A view along the b axis of the crystal pack of compound (II). The hydrogen bonds are shown as dashed lines (see Table 2).

Figure 9
A partial view, normal to plane (101), of the crystal pack of compound (III). The N-HÁ Á ÁN hydrogen bonds are shown as dashed lines (see Table 3).

Figure 10
A view along the a axis of the crystal pack of compound (III). The hydrogen bonds are shown as dashed lines (see Table 3).
Compound (II): was prepared by refluxing dimethyl 3,6-dimethylpyrazine-2,5-dicarboxylate (1.5 g, 5.92 mmol) and an excess of 2-(aminomethyl)pyridine (1.63 g, 15 mmol) in 25 ml of methanol, in a two-necked flask (100 ml) for 55 h. A colourless precipitate formed and this suspension was then cooled to room temperature. The solid that had formed was filtered off and washed with 10 ml of cold methanol. It was then recrystallized from ethyl acetate solution to give colourless rod-like crystals of (II) [yield 90%, m.p. 470 K].

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Intensity data for (I) and (III) were measured at 153 K on a one-circle image-plate diffractometer, while for (II) intensity data were measured at 293 K on a fourcircle diffractometer. For all three compounds, the NH H atoms were located in difference-Fourier maps and freely refined. The C-bound H atoms were included in calculated positions and treated as riding: C-H = 0.95-0.99 Å for (I) and (III) with U iso (H) = 1.2U eq (C); C-H = 0.93-0.96 Å for (II), with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.