Crystal structure of 4-aminopyridinium 5-(5-chloro-2,4-dinitrophenyl)-1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate hemihydrate

In the title molecular salt, the two rings of the barbiturate anion are inclined to one another by 43.40 (3)°. In the crystal, the cations and anions are linked via N—H⋯O hydrogen bonds, forming zigzag chains along [10], which in turn are linked by O—H⋯O and C—H⋯O hydrogen bonds, forming slabs lying parallel to (10).


Chemical context
Barbiturates occupy an important place in pharmacopoeia due to their central nervous system (CNS) depressing nature (Nogrady, 1988;Ashutoshkar, 1993;Hardman et al., 2001;Yadav, 2004;Nadkarni et al., 2005). Many barbiturates are ideal drugs for treating major epilepsy (Olsen et al., 1986;Dhiman, 2013). In a continuation of our previous work on the synthesis of crystalline barbiturates and, in particular, similar nitro-substituted aromatic compounds (Babykala et al., 2014), we report herein on the synthesis and crystal structure of the title molecular salt.

Structural commentary
The molecular structure of the title salt is depicted in Fig. 1. The two rings in the barbiturate anion (N3/N4/C7-C10 and C1-C6) are not coplanar but are inclined to one another by 43.17 (16) . The two nitro groups on the benzene ring (N1/O1/ O2 and N2/O3/O4) deviate to different extents from the plane of the ring. The dihedral angle for the former group, adjacent to the ring junction, is 39.3 (4) while for the later it is 4.2 (5) . ISSN 1600-5368 as a result of this, the latter nitro group is more involved in delocalizing the negative charge of the anion than the former. The cation is protonated at the pyridine N atom, as has been observed previously (Babykala et al., 2014).

Supramolecular features
In the crystal, the cations and anions are linked via N-HÁ Á ÁO hydrogen bonds ( Fig. 2
The dihedral angle between the benzene ring and the barbiturate ring varies from ca 42.64 in YAVSOF to ca 51.88 in OCEWUQ, compared to only 43.17 (16) in the title salt. This difference is surprising considering that the barbiturate anion is the same in both OCEWUQ and the title salt.

Figure 1
A view of the molecular structure of the title salt, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level.
(0.02 mol, 1.88 g) dissolved in 20 ml of absolute ethanol was added. The mixture was shaken well for 2-3 h and kept as such at 298 K. After 24 h, the excess of solvent was removed by distillation under reduced pressure and to the resulting slurry was added to 50 ml of dry ether and the mixture was refrigerated for 5 h. The maroon-red-coloured solid obtained was filtered, powdered well and washed with 50 ml of dry ether. The dry solid was recrystallized from absolute ethanol and slow evaporation of this solvent at 293 K yielded good quality single crystals (yield 75%; m.p. 488 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The cation NH and NH 2 H atoms were located in a difference Fourier map and freely refined.
The water H atoms were also located in a difference Fourier map and refined with U iso (H) = 1.2U eq (O). The C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.93-0.98 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and = 1.2U eq (C) for other H atoms.

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