Supramolecular patterns and Hirshfeld surface analysis in the crystal structure of bis(2-amino-4-methoxy-6-methylpyrimidinium) isophthalate

The structure of the substituted 2-aminopyrimidine salt of isophthalic acid comprises an almost planar unit in which the two pyrimidinium cations are cyclically hydrogen-bonded to the succinate dianion and further extended through hydrogen bonds into a one-dimensional supramolecular structure.

In the title molecular salt, 2C 6 H 10 N 3 O + ÁC 8 H 4 O 4 2À , the N atom of each of the two 2-amino-4-methoxy-6-methylpyrimidine molecules lying between the amine and methyl groups has been protonated. The dihedral angles between the pyrimidine rings of the cations and the benzene ring of the succinate dianion are 5.04 (8) and 7.95 (8) . Each of the cations is linked to the anion through a pair of N-HÁ Á ÁO(carboxylate) hydrogen bonds, forming cyclic R 2 2 (8) ring motifs which are then linked through inversion-related N-HÁ Á ÁO hydrogen bonds, giving a central R 2 4 (8) motif. Peripheral amine N-HÁ Á ÁO hydrogen-bonding interactions on either side of the succinate anion, also through centrosymmetric R 2 2 (8) extensions, form one-dimensional ribbons extending along [211]. The crystal structure also featuresstacking interactions between the aromatic rings of the pyrimidine cations [minimum ring centroid separation = 3.6337 (9) Å ]. The intermolecular interactions were also investigated using Hirshfeld surface studies and two-dimensional fingerprint images.

Chemical context
Pyrimidine and aminopyrimidine derivatives have useful applications in many fields, for example as pesticides and pharmaceutical agents (Condon et al., 1993), while imazosulfuron, ethirmol and mepanipyrim have been commercialized as agrochemicals (Maeno et al., 1990). Pyrimidine derivatives have also been developed as antiviral agents, such as AZT, which is the most widely used anti-AIDS drug (Gilchrist, 1997). Hydrogen bonding plays a vital role in molecular recognition. It is significant to know the types of hydrogen bonds present to design new materials with highly specific features. Supramolecular chemistry plays a pivotal role in many biological systems and is involved in artificial systems. It refers to the specific relation between two or more molecules through non-covalent interactions such as hydrogen bonding, hydrophobic forces, van der Waals forces andinteractions. The origin of supramolecular architectures is correlated to the positions and properties of the active groups in molecules (Desiraju, 1989;Steiner, 2002). As part of our recent studies in this field, the synthesis, crystal structure and Hirshfeld surface analysis of the title salt have been undertaken and are presented herein.

Figure 1
The atom numbering for the two cations and the dianion in the asymmetric unit of the title salt, with probability displacement ellipsoids drawn at the 50% probability level. Hydrogen bonds (Table 1) are shown as dashed lines.

Hirshfeld surface analysis
The d norm parameter takes negative or positive values depending upon whether the intermolecular close contact is shorter or longer than the van der Waals radii, respectively (Spackman & Jayatilaka, 2009;McKinnon et al., 2007). The 3D d norm surface of the title salt is shown in Fig. 4

Synthesis and crystallization
The title compound was synthesized in a reaction involving a hot methanolic solution (20 ml) of 2-amino-4-methoxy-6methylpyrimidine (139 mg, 1.0 mmol) and a hot methanolic solution (20 ml) of isophthalic acid (166 mg Two-dimensional fingerprint plots of the crystal and relative contribution of the atom pairs to the Hirshfeld surface.
stirrer for few minutes. The colorless solution was cooled and kept at room temperature for slow evaporation. After a few days, the crystals of the title compound suitable for the X-ray analysis appeared, yield 65%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms were positioned geometrically (N-H = 0.86 Å and C-H = 0.96 or 0.93 Å ) and were refined using a riding model with U iso (H) = 1.2 U eq (N or C) or 1.5U eq (methyl C).

Bis(2-amino-4-methoxy-6-methylpyrimidinium) benzene-1,3-dicarboxylate
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.