Trimethylammonium 5-(2,4-dinitrophenyl)-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate 0.125-hydrate

The asymmetric unit of the title salt C3H10N+·C10H5N4O7 −·0.125H2O [trivial name: trimethylammonium 5-(2,4-dinitrophenyl)barbiturate 0.125-hydrate], contains two independent cations, two independent anions and a 0.25-occupancy solvent water molecule. In one of the cations, the C atoms are disordered over two sets of sites with refined occupancies of 0.538 (8) and 0.462 (8). In the anions, the dihedral angles between the pyrimidine and benzene rings are 42.77 (6) and 46.55 (7)°. In the crystal, N—H⋯O hydrogen bonds connect anions and cations into chains along [010]. Within these chains, R 2 2(8) ring motifs are formed by inversion-related barbiturate anions. The H atoms of the partial occupancy water molecule were not located nor included in the refinement.


Govindan Mangaiyarkarasi and Doraisamyraja Kalaivani Comment
Many different types of anionic sigma complexes such as carbon bonded, nitrogen bonded, oxygen bonded and spiro Meisenheimer complexes have been reported by different group of scientists (Terrier, 1982;Al-Kaysi et al., 2005).
The asymmetric unit of the title compund ( Fig. 1) comprises of two cations and two anions. In addition, the asymmetric unit contains a 0.25 occupancy water molecule. The bond lengths and bond angles of the title compound are comparible with those of related barbiturates (Kalaivani & Malarvizhi, 2009;Kalaivani et al., 2012). In one the cations the C atoms C24, C25 and C26 are disordered over two sets of sites with refined occupancies of 0.538 (8) and 0.462 (8). Since there are two nitro groups attached to the benzene rings in the anions a steric effect may operate which is reflected in the dihedral angles between the benzene and pyrimidine rings (42.77 (6) and 46.55 (7)°). The nitro group para with respect to barbiturate ring are approximately planar (8.2 (4) and 6.0 (5)°) with benzene ring and are effectively involved in the delocalisation of electrons. The nitro groups ortho with respect to barbiturate ring form dihedral angles of 40.7 (3) and 33.8 (3)° with the benzene rings. In the crystal, N-H···O hydrogen bonds connect anions and cations into onedimensional chains along [010]. Within these chains R 2 2 (8)ring motifs (Bernstein et al., 1995) are formed by inversionrelated barbiturate anions (Fig. 2).
Experimental 1-Chloro-2,4-dinitrobenzene (2.02 g, 0.01 mol) was dissolved in 20 ml of absolute alcohol. Barbituric acid (1.28 g, 0.01 mol) was also dissolved in 30 ml of absolute alcohol. After mixing these two solutions, 3 ml of trimethylamine (0.03 mol) was added and shaken well for 6 hrs. The solution was filtered and the clear solution was kept as such at room temperature. After a period of four weeks, dark shiny maroon red coloured crystals formed from the solution. The crystals were filtered and washed with 30 ml of dry ether and recrystallized from absolute ethanol (M.p: 545K; yield: 80%). Good quality crystals for single crystal X-ray studies were obtained by slow evaporation of ethanol at room temperature.

Refinement
The N-bound H atoms were located in a difference electron density map and refind with a N-H distance restraint of 0.90 (2) Å. The C-bound hydrogen atoms were placed in calculated positions and refined as riding atoms: C-H = 0.93 and 0.96 Å for CH and CH 3 H atoms, respectively, with U iso (H) = k × U eq (C), where k = 1.5 for methyl H atoms and = 1.2 for other H atoms. The H atoms of the partial occupancy water molecule were not located nor are they included in the refinement. They are however included in the molecular formula.

Figure 1
The asymmetric unit of the title compound, with displacement ellipsoids drawn at the 30% probability level.  Part of the crystal atructure of the title compound with hydrogen bonds shown as dashed lines.
Trimethylammonium 5-(2,4-dinitrophenyl)-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate 0.125-hydrate  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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )