Dimethyl and diethyl esters of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid: a comparison

In compound (I), the dimethyl ester of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid, pyridine ring B is inclined to pyrazine ring A by 44.8 (2)°. The Npyrazine—C—C—Npyridine torsion angle is −133.7 (4)°, with the N atoms trans to each other. Pyridine ring C is inclined to pyrazine ring A by 50.3 (2)°. Here the Npyrazine—C—C—Npyridine torsion angle is 50.7 (5)° and the N atoms are cis to one another. In compound (II), the diethyl ester, which possesses twofold rotation symmetry, the pyridine rings are inclined to the pyrazine ring by 40.7 (1)°, with the N atoms cis to one another.

1. Chemical context 5,6-Bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (L1H 2 ) was synthesized to study its coordination behaviour with first row transitions metals (Alfonso, 1999). It exists as a zwitterion, with the adjacent pyridine and pyridinium rings almost coplanar due to the presence of an intramolecular N-HÁ Á ÁN hydrogen bond. The crystal structures of the zwitterion and different charged forms of L1H 2 , viz. the HCl, HClO 4 and HPF 6 salts, and details of the hydrogen bonding have been reported (Alfonso et al., 2001).
Metal-catalysed hydrolysis of amino acid esters is a well documented phenomenon (Dugas, 1989). It has been shown previously that the reaction of copper(II) salts with the dimethyl esters of pyrazine-2,3-dicarboxylic acid (Neels et al.,

Supramolecular features
In the crystal of (I), molecules are linked by C-HÁ Á ÁN hydrogen bonds, forming chains along [001]; see Table 1 and Fig. 3. The chains are linked via C-HÁ Á Á interactions (Table 1), forming a three-dimensional structure.
In the crystal of (II), molecules are linked via C-HÁ Á ÁO hydrogen bonds, forming a three-dimensional framework; see Table 2 and Fig. 4. Within the framework there are a number of C-HÁ Á Á interactions present (Table 2). A view of the molecular structure of compound (I), showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A view of the molecular structure of compound (II), showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Unlabelled atoms are related to labelled atoms by the symmetry code (Àx + 2, Ày + 3 2 , z). Table 1 Hydrogen-bond geometry (Å , ) for (I).

Database survey
Besides the structures of the zwitterion and different charged forms of L1H 2 , viz. the HCl, HClO 4 and HPF 6 salts (Alfonso et al., 2001) (Neels et al., 2003). The structure of the isoelectronic compound 3,6-bis-(pyridin-2-yl)pyrazine-2,5-dicarboxylic acid (L2H 2 ), Fig. 5, has also been reported (Wang & Stoeckli-Evans, 2012a). It too exists as a zwitterion and the structures of its dihydrochloride salt and the dimethyl sulfonate disolvate have also been reported (Wang & Stoeckli-Evans, 2012a). The crystal structures of the dimethyl (III) and diethyl (IV) esters of L2H 2 have been deposited as private communications (Wang & Stoeckli-Evans, 2012b,c) with the Cambridge Structural Database (CSD; Groom & Allen, 2014). Both compounds crystallize in the triclinic space group P1 and possess inversion symmetry. The pyridine rings lie almost in the plane of the pyrazine ring and the N atoms are trans with respect to each other and to the nearest pyrazine N atom (as illustrated in Fig. 5). The ester groups are planar and in both compounds lie almost normal to the pyrazine ring. In the crystals of both compounds, inversion-related molecules are linked via pairs of C-HÁ Á ÁO hydrogen bonds, enclosing R 2 2 (10) ring motifs, forming chains propagating along [101].

Synthesis and crystallization
The synthesis of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (L1H 2 ) has been reported (Alfonso et al., 2001). The dimethyl and diethyl esters, compounds (I) and (II), respectively, were obtained by the usual esterification procedure in acidic medium from the diacid and an excess of the corresponding alcohol.
Synthesis of compound (I): dimethyl-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate L1H 2 (1.00 g, 3.11 mmol) was heated under reflux in freshly distilled MeOH (40ml) containing H 2 SO 4 conc. (98%, 1 ml) during 16 h. After stopping the reaction, the temperature of the solution was allowed to cool to room temperature and then poured into an aqueous solution of NaOAc (6 g in 150 ml deionized water). The resulting solution was stirred in an ice bath containing NaCl to afford a white solid which was removed by filtration, washed with cold water and dried under vacuum. Single crystals suitable for X-ray analysis were obtained by the slow diffusion technique from CH 2 Cl 2 and MeOH  Table 2 Hydrogen-bond geometry (Å , ) for (II).

Figure 3
A view along the a axis of the crystal packing of compound (I). The hydrogen bonds are shown as dashed lines (see Table 1; only H atom H11 has been included).

Figure 4
A view along the a axis of the crystal packing of compound (II). The hydrogen bonds are shown as dashed lines (see Table 2; only H atom H7 has been included).

Figure 5
The chemical scheme for compound L2H 2 .

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 0.8419 (5