Cytosinium hydrogen selenite

In the crystal structure of the title salt, C4H6N3O+·HSeO3 −, systematic name 6-amino-2-methylidene-2,3-dihydropyrimidin-1-ium hydrogen selenite, the hydrogenselenite anions and the cytosinium cations are linked via N—H⋯O, N—H⋯Se, O—H⋯O, O—H··Se and C—H⋯O hydrogen bonds, forming a three-dimensional framework.

We are grateful to Dr M. Giorgi, Faculté des Sciences et Techniques de Saint Jeŕome, Marseille, France, for providing access to the X-ray diffraction facilities. We also thank Abbes Laghrour Khenchela University, le Ministé re de l'Enseignement Supé rieur et de la Recherche Scientifique-Algeria and the Direction Gé né rale de la Recherche Scientifique et du Dé veloppement Technologique-Algeria for financial support.
Supporting information for this paper is available from the IUCr electronic archives (Reference: SU2689).

Comment
The crystal structure of cytosine (Barker & Marsh, 1964) and cytosine monohydrate (Jeffrey & Kinoshita, 1963) were determined many years ago. Many inorganic cytosinium salts have been synthesized, including the hydrochloride (Mandel, 1977) and the dihydrogenmonophosphate (Bagieu-Beucher, 1990) salts. Cytosinium salts of organic acids are also common, these include for example, cytosinium trichloroacetate (Gdaniec et al., 1989) and cytosinium 3,5-dinitrosalicylate (Smith et al., 2005). We report herein on the molecular structure of a new cytosinium salt formed by the reaction of cytosine with selenious acid.
The structure of the title salt is illustrated in Fig. 1. The HSeO 3ion is pyramidal with two short Se-O bonds, Se1-O3 = 1.634 (8) A° and Se1-O4 = 1.686 (6) A°, and a longer Se-OH bond, Se1-O2 = 1.738 (7) A°. These values are very similar to those described in the literature (Richie & Harrison, 2003;Wang et al., 2006;Chomnilpan et al., 1981). The geometry of this inorganic moiety clearly implies that one proton was transferred from selenious acid to cytosine.
In the crystal, the anions and cations are linked via N-H···O/Se, O-H···O/Se and C-H···O hydrogen bonds forming a three-dimensional framework (Table 1 and Fig. 2).

Experimental
Selenious acid (H 2 SeO 3 ) was added to an aqueous solution of cytosine in the stoichiometric ratio 1:1, at room temperature. After four weeks colourless prismatic crystals of the title salt were obtained.

Refinement
All the H atoms could be located in difference Fourier maps and this was confirmed by plotting difference Fourier maps using the ContourDif routine in PLATON (Spek, 2009). In the final cycles of refinement the NH 2 distances were restrained to N-H = 0.86 (2) and H···H = 1.33 (2) Å with U iso (H) = 1.2U eq (N). The OH distance was restrained to O-H = 0.82 (2) Å with U iso (H) = 1.5U eq (O). The C bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.93 Å with U iso (H) = 1.2U eq (C).

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.