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

The asymmetric unit of the title molecular salt, C3H10N+·C12H9N4O7 − [alternative name: trimethylammonium 5-(2,4-dinitrophenyl)-1,3-dimethyl barbiturate], contains one anion and two half-occupancy cations. The cations are disordered about inversion centres. The tetrahydropyrimidine ring is essentially planar [maximum deviation = 0.007 (2) Å] and forms a dihedral angle of 41.12 (6)° with the plane of the benzene ring. In the crystal, N—H⋯O hydrogen bonds link the cations to the anions.


Related literature
H atoms treated by a mixture of independent and constrained refinement Á max = 0.23 e Å À3 Á min = À0.20 e Å À3 Table 1 Hydrogen-bond geometry (Å , ). (Mangaiyarkarasi & Kalaivani, 2013) comprises of two cations and two anions. However, in the present investigation, the asymmetric unit of the molecular salt is comprised of one 5-(2,4-dinitrophenyl)-1,3-dimethylbarbiturate anion and two half occupancy trimethylammonium cations. The molecular structure of the title compound is shown in Fig. 1. The cations lie on inversion centres and hence are disordered. In the crystal, N-H···O hydrogen bonds exist between the protonated nitrogen atoms of the cations and oxygen atoms of the carbonyl groups of anions. The tetrahydropyrimidine ring is essentially planar (maximum deviation = 0.007 (2) Å for N3) and forms a dihedral angle of 41.12 (6)Å with the benzene ring. The nitro group which para with respect to the junction of two rings forms a dihedral of -5.7 (2)° with the benzene ring and may be involved to a greater extent in electron delocalization with the benzene than the nitro group ortho to the junction of the two rings which forms a dihedral angle of 38.8 (2)° with the benzene ring.

S2. Experimental
Analytical grade 1-chloro-2,4-dinitrobenzene (2.02 g,0.01 mol) was dissolved in 20 ml of absolute alcohol. 1,3-Dimethylbarbituric acid (1.56 g,0.01 mol) was also dissolved in 30 ml of absolute alcohol separately. These two solutions were then mixed. To this mixture, 4 ml of trimethylamine (0.03 mol) was added and shaken well for 5-6 hrs. The slightly turbid solution obtained was filtered and kept as such at 298K. After a period of three weeks, dark shiny maroon red coloured crystals of the title salt crystallized out from the solution. The crystals were filtered, powdered well using agate mortar and washed with 30 ml of dry ether. The dry solid of the title compound obtained was quickly washed with 1 ml of absolute alcohol to remove unreacted reactants and finally with 25 ml of dry ether. The pure powder was recrystllized from hot etanol (Yield: 80%; m.pt; 513 K). Good quality single crystals, suitable for X-ray diffraction studies, were obtained by slow evaporation of a solution of the title compound in ethanol at room temperature. The crystals obtained were non-hygroscopic and extraordinarily stable at room temperature. Solubility at 298 K: 2.78 g/100 mL(water); 4.58 g/100 mL(Ethanol); 17.78 g/100 mL(DMSO).

S3. Refinement
The hydrogen atoms boned to the N atoms of the cation were located in a difference Fourier map and refined isotropically with U iso (H) = 1.2U eq (N) and restrained to N-H = 0.91 (2)Å. Hydrogen atoms on methyl groups of the trimethyl ammonium cation were located in a difference Fourier map and refined with fixed isotropic displacement parameters with U iso (H) = 1.5U eq (C). The C-H and H···H distances were restrained to 0.96 (2)Å and 1.568 (2)Å respectively. All the other hydrogen atoms were geometrically constrained and allowed to ride on their parent atoms. The cations are disordered across inversion centres. There are two cation moieties in the asymmetric unit each with 0.5 site occupancy.
The anisotropic displacement parameters of the disordered atoms were restrained with an effective standard deviation of 0.02. The N-C and C···C distances were restrained to 1.45 (2) Å and 2.36 (2) Å respectively.

Figure 1
The asymmetric unit of title compound showing 30% probability displacement ellipsoids. The cations are half occupancy.

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