Crystal structure of 2-(azaniumylmethyl)pyridinium bis(hydrogen squarate)

The structure of the title squarate salt is reported. Classical N—H⋯O and O—H⋯O hydrogen bonds combine with weak C—O⋯π(ring) and π–π contacts to stabilize the crystal packing.


Chemical context
Hydrogen bonding is the most common way of generating supramolecular organic systems in crystal engineering and molecular recognition. Hydrogen-bonded systems generated from organic cations and anions are of special interest as they would be expected to form stronger hydrogen bonds than those in neutral molecules (Reetz et al.,1994;Bertolasi et al., 2001). Squaric acid (H 2 C 4 O 4 , H 2 sq), has been of interest because of its cyclic structure and potential aromaticity and is used as a building block in crystal engineering due to the simplicity of the cyclic units. It can be found in three forms: uncharged H 2 sq, the Hsq À monoanion and the sq 2À dianion. The mono-and dianions are often produced following deprotonation by amines (Lam & Mak, 2000;Mathew et al., 2002). The squarate derivatives are almost flat because of the -conjugation of their C-C and C-O bonds, and therefore their four oxygen atoms behave as planar (sp 2 ) electron donors of one or two lone pairs of electrons. Recently, we reported the synthesis and characterization of the same organoammonium squarate as the title compound but as a hydrate in the triclinic space group P1 (Korkmaz & Bulut, 2013). We report here the unsolvated form of this salt, which crystallizes in the monoclinic space group P2 1 /c.

Structural commentary
2-(Aminomethyl) pyridine forms a salt with two squaric acid molecules and each molecule of the acid loses one proton. One of these is transferred to the N atom of the pyridine ring, generating the 4-(aminomethyl)morpholinium mono-cation. The other from the second acid molecule is engaged in the formation of a homo-conjugated hydrogen squarate anion via a short, symmetric O5-H5AÁ Á ÁO3 [2.4583 (14) Å ] hydrogen bond (Fig. 1). The electron density associated with this H atom is shared by the O5 and O3 atoms, indicating a large degree of ionic character (Gilli & Gilli, 2000). Considering the range (2.38-2.50 Å ) of Gilli's classification for such an interaction, this hydrogen bonding can be referred to as negative chargeassisted hydrogen bonding [(À) CAHB] (Gilli & Gilli, 2009;Gilli et al., 2001;Becke, 1993, Lee et al., 1988 and can be represented as [-OÁ Á ÁHÁ Á ÁO-] À . N1/C1-C5, C7-C10 and C11-C14 are defined as rings 1, 2 and 3, respectively, with centroids Cg1, Cg2 and Cg3. The dihedral angles between the mean plane of ring 1 and those of rings 2 and 3 are 18.818 (8) and 31.564 (6) , respectively. The dihedral angle between the two squarate anions is 29.19 (1) . The angles between the C-C bonds in the Hsq À anions are close to 90 , with the oxygen atoms directed almost along the diagonals.

Figure 1
A view of the asymmetric unit of (I), showing the atom-numbering scheme and 30% probability displacement ellipsoids. Dashed lines indicate hydrogen bonds.
properties and that they can be used as electron acceptors in photo-sensitive devices (Korkmaz et al., 2016).

Synthesis and crystallization
All chemical reagents were analytical grade commercial products. The solvent was purified by conventional methods. Squaric acid (H 2 Sq; 0,46 g, 4 mmol) and 2-(aminomethyl)pyridine (0,24 g; 2 mmol) were dissolved in water (25 cm 3 ) to obtain a mixture in the molar ratio 2:1 and the solution was heated to 323 K in a temperature-controlled bath and stirred for one h. The reaction mixture was then slowly cooled to room temperature. The crystals formed were filtered and washed with 10 cm 3 of methanol, and dried in air.

2-(Azaniumylmethyl)pyridinium bis(2-hydroxy-3,4-dioxocyclobutanolate)
Crystal data 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.