Cytosinium orotate dihydrate

The title compound, C4H6N3O+·C5H3N2O4 −·2H2O or Cyt+·Or−·2H2O, was synthesized by a reaction between cytosine (4-amino-2-hydroxypyrimidine, Cyt) and orotic acid (2,4-dihydroxy-6-carboxypyrimidine, Or) in aqueous solution. The two ions are joined by two N+—H⋯O− (±)-(CAHB) hydrogen bonds, forming a dimer with graph-set motif R 2 2(8). In the crystal, the ion pairs of the asymmetric unit are joined by four N—H⋯O interactions to adjacent dimers, forming hydrogen-bonded rings with R 2 2(8) graph-set motif in a two-dimensional network. The formation of the three-dimensional array is facilitated by water molecules, which act as bridges between structural sub-units linked in R 3 2(8) and R 3 2(7) hydrogen-bonded rings. The orotate anion is essentially planar, as the dihedral angle between the planes defined by the carboxylate group and the uracil fragment is 4.0 (4)°.

The title compound, C 4 H 6 N 3 O + ÁC 5 H 3 N 2 O 4 À Á2H 2 O or Cyt + ÁOr À Á2H 2 O, was synthesized by a reaction between cytosine (4-amino-2-hydroxypyrimidine, Cyt) and orotic acid (2,4-dihydroxy-6-carboxypyrimidine, Or) in aqueous solution. The two ions are joined by two N + -HÁ Á ÁO À (AE)-(CAHB) hydrogen bonds, forming a dimer with graph-set motif R 2 2 (8). In the crystal, the ion pairs of the asymmetric unit are joined by four N-HÁ Á ÁO interactions to adjacent dimers, forming hydrogen-bonded rings with R 2 2 (8) graph-set motif in a twodimensional network. The formation of the three-dimensional array is facilitated by water molecules, which act as bridges between structural sub-units linked in R 3 2 (8) and R 3 2 (7) hydrogen-bonded rings. The orotate anion is essentially planar, as the dihedral angle between the planes defined by the carboxylate group and the uracil fragment is 4.0 (4) .
In Oranion the pyrimidine ring is essentially planar and the carboxylate group forms dihedral angle of 4.0 (4)° with the mean plane of the uracil fragment, which is close to the value observed in orotic acid (2.2 (2)°, Portalone, 2008). In this anion bond lengths and bond angles of the heteroaromatic ring are in accord with those obtained for other similar structures of ammonium orotate monohydrate (Solbakk, 1971), lithium orotate monohydrate (Lutz, 2001) and benzamidinium orotate hemihydrate (Portalone, 2010). Bond distances around atom C7 indicate a carboxylate group with delocalization of the negative charge between atoms O3 and O4.
The two ions are joined by two N + -H···O -(±)CAHB hydrogen bonds to form a dimer with graph-set motif R 2 2 (8) (Bernstein et al., 1995). In the crystal, the ion pairs of the asymmetric unit are joined by four N-H···O interactions with D···A distances ranging from 2.809 (2) to 3.051 (2) Å. Adjacent ion pairs form hydrogen-bonded rings with R 2 2 (8) graphset motif in a bi-dimensional network ( Table 2). The formation of the three-dimensional array is facilitated by water molecules, which act as bridges between structural subunits linked in R 2 3 (8) and R 2 3 (7) hydrogen-bonded rings. Water molecules play an important role in the cohesion and the stability of the crystal structure: they are involved in four O-H···O hydrogen bonds, three connecting two Oranions and one Cyt + cation as donor (O6W-H61···O2, O7W-H71···O4 and O7W-H72···O5), and one between two water molecules (O6W-H62···O7W).

Experimental
A water solution (6 ml) of cytosine (0.01 mmol, Fluka at 96% purity) was mixed with an aqueous solution (5 ml for 3 h. After cooling the solution to an ambient temperature, colourless crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of the solvent after two weeks.

Refinement
All H atoms were identified in difference Fourier maps, but for refinement all C-bound H atoms were placed in calculated positions, with C-H = 0.93 Å and refined as riding on their carrier atoms. The U iso values were kept equal to 1.2U eq (C).
Positional and thermal parameters of H atoms attached to N atoms were refined freely, giving N-H distances in the range 0.76 (2) -0.97 (3) Å. For the water molecules, the O-H distances of the H atoms attached to O6W and O7W were restrained to 0.85 (2) -0.90 (2) Å, and the H atoms were refined with U iso equal to 1.5U eq (O).     Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s 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 > 2σ(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.