Isoquinolinium 5-(2,4-dinitrophenyl)-1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate: crystal structure, Hirshfeld surface analysis and pharmacological evaluation

The title compound, which comprises an isoquinolinium cation and 5-(2,4-dinitrophenyl)-1,3-dimethylbabriturate anion, exhibits anticonvulsant and hypnotic activities. 3D Hirshfeld surface analysis establishes the predominant O⋯H/H⋯O intermolecular contacts in the crystal lattice.


Structural commentary
In the title compound, (I) (Fig. 1), all the bond lengths and bond angles are normal and comparable with those observed in the related barbiturates (Sridevi & Kalaivani, 2012;Gunaseelan & Doraisamyraja, 2014). The plane of the dinitroaromatic ring C1-C6 and that of the barbiturate ring C7/C8/ N4/C9/N3/C10 form a dihedral angle of 42.78 (9) . The nitro groups in the 2,4-dinitrophenyl fragment attached to the aromatic ring in the para and ortho positions are twisted from its plane by 3.1 (2) and 45.5 (2) , respectively. Thus the para nitro group is more involved in delocalizing the negative charge than the ortho nitro group in the anionic part. This sort of delocalization of the charge over a large area imparts a maroon red colour to the title compound.

Supramolecular features
The aminium group is involved in formation of an N-HÁ Á ÁO hydrogen bond (Table 1) between the isoquinolinium cation (N5-H5A) and the deprotonated enol oxygen atom O7. In the crystal, weak C-HÁ Á ÁO hydrogen bonds (Table 1) consolidate the crystal packing (Fig. 2). An R 1 2 (6) motif is generated by the C-H groups [C13-H13 and C20-H20] of the isoquinolinium cation and oxygen atom O5 of the carbonyl group of the barbiturate ring of the anion. Although there are three rings with cyclically delocalized electron clouds, no stacking interactions are observed between them.

3D Hirshfeld Surface Analysis and 2D Fingerprint Analysis
Hirshfeld surfaces (Spackman & Jayatilaka, 2009) and the associated 2D-fingerprint plots (McKinnon et al., 2007) of the title molecular salt have been generated using Crystal Explorer 3.1 (Wolff et al., 2013). Hirshfeld surfaces mapped with different properties, e.g. d e , d norm , d i , shapeindex, curvedness, have proven to be a useful visualization tool for the analysis of intermolecular interactions. The 2D-fingerprint plots of Hirshfeld surfaces have been used to pinpoint and scrutinize the percentage of hydrogen-bonding interactions present in the crystal structure. The presented graphical plots use the same red-white-blue color scheme, wherein red highlights the shortest intermolecular atomic contacts (negative d norm values), white is used for contacts around the van der Waals separation, and blue corresponds to longer ones (positive d norm values). Hirshfeld surface analysis of the new barbiturate of present interest has d norm values ranging from À0.723 (red) to 1.464 (blue), as specified in Fig. 3. The globularity value (a measure of the degree to which the surface area differs from that of the shape) is less than 1 The asymmetric unit of (I) showing the atom numbering and 40% probability displacement ellipsoids. The doubled-dashed line denotes the N-HÁ Á ÁO hydrogen bond between the cation and anion. Symmetry codes: (i) x þ 1 2 ; Ày þ 1 2 ; Àz þ 1; (ii) Àx þ 3; y À 1 2 ; Àz þ 1 2 ; (iii) Àx þ 5 2 ; Ày; z À 1 2 .

Figure 2
Crystal packing of (I) viewed approximately down the a axis. Hydrogen bonds are shown as purple dotted lines.

Pharmacological activity
Epilepsy affects about 0.5% of the world's population. A seizure is caused by an asynchronous high-frequency discharge of a group of neurons, starting locally and spreading to a varying extent to affect other parts of the brain.  with a heterocyclic structure and exists in two tautomeric forms (keto and enol) due to the mobility of active methylene group hydrogen atoms in its molecule. Barbiturates are drugs that act as central nervous system depressants and can therefore produce a wide spectrum of effects from mild sedation to total anaesthesia. They are also effective as anxiolytics, hypnotics and anticonvulsants. As the molecular salt of the present investigation is a derivative of 1,3-dimethylbarbituric acid, it has been subjected to the Maximal Electro Shock method to evaluate its anticonvulsant activity (Misra et al.,1973;Kulkarni, 1999). It reduces all phases of convulsion (tonic-flexor, tonic-extensor, clonic-convulsion and stupor) even at low dosage (25 mg kg À1 ) and the animals recovered after the experiment.

Synthesis and crystallization
1-Chloro-2,4-dinitrobenzene (2.02 g, 0.01 mol) in 40 mL of absolute alcohol was mixed with 1,3-dimethylbarbituric acid (1.56 g, 0.01 mol) in 30 mL ethanol. To this mixture, 0.02 mol of isoquinoline was added and the mixture was shaken well for 5 h and kept as such for 24 h. Excess ethanol was removed through evaporation. A maroon-red pasty mass was obtained. This paste was digested with hot ethanol to obtain a maroonred solid. The solid deposited at the bottom of the flask was filtered, powdered well using an agate mortar, washed again with 20 mL of dry ether and recrystallized from absolute alcohol. Good quality single crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of ethanol at room temperature (yield: 80%; m.p. 413 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The N-bound H atom was located in a difference Fourier map and refined isotropically. C-bound H atoms were positioned geometrically and refined as riding, with C-H = 0.93-0.96 Å and U iso (H) = 1.2-1.5 U eq (C).    (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).

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. Refinement. The elements in the sample do not have sufficient anomalous scattering power for Mo(kα) radiation. Hence the Flack parameter and its standard deviation obtained from refinement have no physical significance.