Bis(2-aminopyridinium) 2,5-dicarboxybenzene-1,4-dicarboxylate

In the title compound, 2C5H7N2 +·C10H4O8 2−, the 2-aminopyridinium (2-apyH) cation and 2,5-dicarboxybenzene-1,4-dicarboxylate (btcH2) anion are both nearly planar, with r.m.s. deviations of 0.015 and 0.050 Å, respectively. The angle between the latter least-squares planes is 17.68 (9)°. The overall crystal structure results from the packing of two-dimensional networks, formed by alternating 2-apyH and btcH2 linked by hydrogen bonds, parallel to (100).


Related literature
For similar and most common conformations of 2-aminopyridinium, see: Guelmami & Jouini (2011); Chitra et al. (2008); Quah et al. (2008); Bis & Zaworotko (2005); Bü yü kgü ngö r & Odabasoǧ lu (2002); Odabasoǧ lu et al. (2003); Acheson (1967). For similar and most common conformations of 2,5-dicarboxybenzene-1,4-dicarboxylate, see: Dong et al. (2011); Wang & Tang (2010). For graph-set analysis of hydrogen-bond patterns in organic crystals, see: Etter et al. (1990).  Table 1 Hydrogen-bond geometry (Å , ). Data collection: APEX2 (Bruker-Nonius, 2004); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL97. This work is a further contribution to the broad family of structural studies of 2-aminopyridinium (2-apyH) systems with hydrogen-bond donors. A considerable number of analogous materials formed from 2-aminopyridine and a given carboxylic acid has already been reported (Guelmami & Jouini, 2011;Chitra et al., 2008;Quah et al., 2008;Bis & Zaworotko, 2005;Büyükgüngör & Odabas&ogon;og&caron;lu, 2002;Odabas&ogon;og&caron;lu et al., 2003;etc). This is due to the fact that 2-aminopyridine is protonated in acidic solutions. It is well known that the bonding of the H atom to the ring N atom of 2-aminopyridine, and not to the amino N atom, produces an ion for which an additional resonance structure must be considered (Acheson, 1967). We have inferred the positive charge in the 2-apyH ion lies on the amino group based on a difference fourier map, a common practice when allowed by the quality of the collected intensities. The charge state, related to the hydrogen loss, in each of the two candidate carboxylic acid groups belonging to the assymetric unit was also inferred from a difference map and further reinforced by analysis of the C-O bond lengths.

Experimental
Ellucidation of the numbering scheme and a view of the H-bonds giving rise to two-dimensional networks parallel to (100) are shown in Figs. 1 and 2, respectively. Both the 2-apyH cation and 2,5-dicarboxybenzene-1,4-dicarboxylate (btcH2) anion are nearly planar, with r.m.s. deviations of 0.015 and 0.050 A, respectively. The angle between the latter idealized planes is 17.68 (9)°. The two-dimensional networks are formed by alternating 2-apyH and btcH2 linked by Hbonds and include all H-bonds found. The first order network describing the H-bonding in the title compound is N 1 =4DS (7), as established by applying the rules of graph-set analysis of hydrogen-bond patterns in organic crystals (Etter et al., 1990).
After slow evaporation of the solvent at room-temperature colorless prisms of the compound were obtained.

Refinement
The structure was solved by direct methods using SHELXS97 (Sheldrick, 2008). H atoms bound to aromatic C were placed at idealized positions and refined as riding, with C-H=0.93 (Sheldrick, 2008); amine and carboxyl H atoms were found from a difference fourier map and their coordinates refined freely. U iso (H) was fixed to 1.2 times U eq of the heavy atom they are bonded to, for all hydrogen atoms.
Examination of the crystal structure with PLATON (Spek, 2009)

Figure 1
ORTEPII plot of the title compound. Displacement ellipsoids are drawn at the 50% level.  Representation of the two-dimensional networks, paralell to the (100) planes, of H-bonded molecules.

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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.