Diammonium 1,1′,3,3′-tetramethyl-2,2′,4,4′,6,6′-hexaoxoperhydro-5,5′-bipyrimidine-5,5′-diide monohydrate

In the title hydrated salt, 2NH4 +·C12H12N4O6 2−·H2O, the two hexahydropyrimidine rings in the dianion are inclined to one another at a dihedral angle of 62.76 (5)°. In the crystal structure, the anions and water molecules are linked into sheets parallel to the bc plane by intermolecular O—H⋯O hydrogen bonds and sustained by C—H⋯O contacts. The linking of the anions and water molecules with the cations by N—H⋯O hydrogen bonds creates a three-dimensional extended network. The crystal structure is further stabilized by very weak C—H⋯π interactions.

In the title hydrated salt, 2NH 4 + ÁC 12 H 12 N 4 O 6 2À ÁH 2 O, the two hexahydropyrimidine rings in the dianion are inclined to one another at a dihedral angle of 62.76 (5) . In the crystal structure, the anions and water molecules are linked into sheets parallel to the bc plane by intermolecular O-HÁ Á ÁO hydrogen bonds and sustained by C-HÁ Á ÁO contacts. The linking of the anions and water molecules with the cations by N-HÁ Á ÁO hydrogen bonds creates a three-dimensional extended network. The crystal structure is further stabilized by very weak C-HÁ Á Á interactions.
The crystal structure of (I) is mainly stabilized by a network of N-H···O, and O-H···O hydrogen bonds as well as C-H···O and C-H···π contacts. Each ammonium H-atom participates in intermolecular hydrogen bonds. In the crystal structure ( Fig. 2), the anions and water molecules are linked into sheets parallel to the bc plane by O-H···O hydrogen bonds and sustained by C-H···O contacts ( Table 1). The ammonium cations act as bridges between the anions and water molecules via N-H···O hydrogen bonds (Table 1) to create a three-dimensional extended network. The crystal structure is further stabilized by weak intermolecular C10-H10C···Cg1 interactions (Table 1).

Experimental
A solution of 1,3-dimethylbarbituric acid was refluxed in acetonitrile at 363 K for 2 h (monitored by TLC). After completion of the reaction, excess of solvent was distilled off. The solid product obtained was washed with mixture of ether and acetone, and dried. The purity of the crude product was checked through TLC and recrystallized using chloroform and benzene mixture. M.p. 515-517 K.

Refinement
The H-atoms bound to atoms N5, N6 and O1W were located from the difference Fourier map and allowed to refine freely.
The other H-atoms were placed in calculated positions, with C-H = 0.96 Å, U iso = 1.5U eq (C). Rotating models were used for the methyl groups. In the absence of significant anomalous dispersion, 2042 Friedel pairs were merged for the final refinement.
Figures Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids for non-H atoms and the atom-numbering scheme.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1)K.
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.