Crystal structures of N 2,N 3,N 5,N 6-tetrakis(pyridin-2-ylmethyl)pyrazine-2,3,5,6-tetracarboxamide and N 2,N 3,N 5,N 6-tetrakis(pyridin-4-ylmethyl)pyrazine-2,3,5,6-tetracarboxamide

The two title compounds are pyrazine-2,3,5,6-tetracarboxamide derivatives. In the first crystal, molecules are linked by N—H⋯O and N—H⋯O hydrogen bonds, forming a two-dimensional network structure. In the second crystal, an N—H⋯N hydrogen bond links the molecules into a chain structure, while a further weak N—H⋯N hydrogen bond links the chains, forming a two-dimensional network structure.


Figure 1
A view of the molecular structure of compound (I), with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular C-HÁ Á ÁO hydrogen bond is shown as a blue dashed line (see Table 1).

Figure 2
A view of the molecular structure of compound (II), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Unlabelled atoms are related to the labelled atoms by the symmetry operation (Àx, Ày + 1, Àz + 2) and the intramolecular C-HÁ Á ÁO hydrogen bonds are shown as blue dashed lines (see Table 2). The minor component of the disordered pyridine ring, involving atom N3, is shown with black dashed lines.
(II). The whole molecule of (II) is generated by inversion symmetry; the pyrazine ring being situated about a center of inversion.
In (II), the amide group N2-C3 O1, in position 2-(and 5by symmetry), is inclined to the pyrazine ring by 81.0 (3) , while the amide group N4-C10 O2, in position 3-(and 6-by symmetry), lies in the plane of the pyrazine ring [dihedral angle = 1.91 (2) ]. Hence, from the various dihedral angles commented on above it can be seen that the conformations of the two molecules are significantly different (cf. Fig. 1 and Fig. 2).

Figure 4
A view along the b axis, of the crystal packing of compound (I). The hydrogen bonds are shown as dashed lines (see Table 1).
In the crystal of (II), molecules are linked by N-HÁ Á ÁN hydrogen bonds (

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.38, first update November 2016; Groom et al., 2016) for tetrakis-substituted pyrazines, excluding tetramethylpyrazine or pyrazine-2,3,5,6-tetracarbonitrile, gave over 550 hits. 255 of these structures concern the ligand tppz, while 88 concern the ligand H4pztc. As noted above, only one example of a pyrazine-2,3,5,6-tetracarboxamide compound has been reported, viz. N,N 0 ,N 00 ,N 000 -tetraethylpyrazine-2,3,5,6-tetracarboxamide (CSD refcode: OSUTIH; Lohrman et al., 2016). It crystallizes in the triclinic space group P1, with eleven independent molecules in the asymmetric unit. It is interesting to note that the general orientation of the amide groups resembles that observed in compound (I). Those in positions 2-and 6-are inclined to the pyrazine ring by more than ca 60 , while those at positions 3-and 5-lie close to the plane of the pyrazine ring.

Figure 6
A view normal to plane (101), of the crystal packing of compound (II). The hydrogen bonds are shown as dashed lines (see Table 2).

Figure 7
A view along the a axis of the crystal packing of compound (II). The hydrogen bonds are shown as dashed lines (see Table 2). yield 90%). 1 H NMR (400 MHz, DMSO-d 6 ): 9.50 (t, 1H, J hg = 6.2, Hh); 8.50 (dd, 2H, J ba = 4.5, J be = 1.6, Hb = Hd); 7.41 (dd, 2H, J ab = 4.5, J ad = J eb = 1.6, Ha = He); 4.59 (d, 2H, J gh = 6.2, Hg). 13  Colourless block-like crystals of both compounds were obtained by slow evaporation of methanol solutions of the respective compounds. The elemental analysis for compound (I) required the addition of a water molecule, which possibly explains the region of disordered electron density in the crystal, and half a molecule of methanol for (II), which was not detected in the final difference Fourier map of the crystal used for the X-ray diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For both molecules the NH H atoms were located in difference-Fourier maps and freely refined. The C-bound H atoms were included in calculated positions and refined as riding: C-H = 0.95-0.99 Å with U iso (H) = 1.2U eq (C). In the crystal of compound (I), a region of disordered electron density was treated with the SQUEEZE routine in PLATON (Spek, 2015). Their contribution (93 electrons for a solvent-accessible volume of 268 Å 3 ) was not taken into account during refinement. The crystal of (I) did not diffract significantly beyond 20 in and hence the R int value is high (> 0.2), and only 35% of the data can be considered to be observed [I > 2(I)]. In compound (II), pyridine ring (N3/C5-C9) is positionally disordered (see Fig. 2), and the refined occupancy ratio for the disordered atoms, N3A:N3B, C8A:C8B, C9A:C9B is 0.58 (3):0.42 (3).   ; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2016 (Sheldrick, 2015), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

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 )
x y z U iso */U eq O1 0.17252 (16)    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 )