5,5-Dihydroxybarbituric acid 1,4-dioxane hemisolvate

The asymmetric unit of the title compound,, C4H4N2O5·0.5C4H8O2, contains one molecule of 5,5-dihydroxybarbituric acid with a nearly planar barbiturate ring and half a molecule of 1,4-dioxane. The geometry of the centrosymmetric dioxane molecule is close to an ideal chair conformation. The crystal structure exhibits a complex three-dimensional hydrogen-bonded network. Barbiturate molecules are connected to one another via N—H⋯O=C, O—H⋯O=C and N—H⋯O(hydroxy) interactions, while the barbituric acid molecule is linked to dioxane by an O—H⋯O contact.

The asymmetric unit of the title compound,, C 4 H 4 N 2 O 5 Á-0.5C 4 H 8 O 2 , contains one molecule of 5,5-dihydroxybarbituric acid with a nearly planar barbiturate ring and half a molecule of 1,4-dioxane. The geometry of the centrosymmetric dioxane molecule is close to an ideal chair conformation. The crystal structure exhibits a complex three-dimensional hydrogenbonded network. Barbiturate molecules are connected to one another via N-HÁ Á ÁO C, O-HÁ Á ÁO C and N-HÁ Á ÁO(hydroxy) interactions, while the barbituric acid molecule is linked to dioxane by an O-HÁ Á ÁO contact.
TG acknowledges financial support from the Lise Meitner Program of the Austrian Science Fund (FWF, project LM 1135-N17).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: JH2150).

Comment
The crystal structures of an unsolvated form (Singh, 1965;Harrowfield et al., 1989), a monohydrate (Lewis & Tocher, 2004a) and a trihydrate (Mootz & Jeffrey, 1965;Lewis & Tocher, 2004b) of 5,5-dihydroxybarbituric acid have been reported previously. The asymmetric unit of the title structure consists of one molecule of the barbituric acid derivative and one half of a dioxane moiety (Fig. 1). The six-membered C 4 N 2 ring of the former is essentially planar, and its bond distances and angles are in agreement with the parameters observed for the unsolvated and hydrate forms.
This crystal structure is characterized by extensive hydrogen bonding. Each dihydroxybarbituric acid molecule is linked to two molecules of the same kind via two centrosymmetric N-H···O=C double bridges and a double bridge O-H···O=C connects it to a third molecule. Joining these R 2 2 (8) and R 2 2 (10) motifs (Bernstein et al., 1995) gives a larger ring of six dihydroxybarbituric acid molecules. Two molecules of each such ring are additionally O-H···O bonded to a dioxane molecule which lies in the centre of the ring. Fig. 2 shows the 2-dimensional H-bonded net parallel to (121) which is obtained from these interactions. Additionally, one NH and one OH group of each dihydroxybarbituric acid molecule are engaged as H-bond donor and acceptor, respectively, in N-H···O(hydroxy) interactions. These particular contacts, indicated by arrows in Fig. 2, connect adjacent H-bonded 2D units of the kind discussed above to one another, and an overall three-dimensional hydrogen bonded network is therefore formed. As expected, the two hydrogen bonds in which the N1-H group is involved exhibit a much less favourable geometry than the single hydrogen bond in which the N3-H group is employed.

Experimental
A solution of 5,5-dibromobarbituric acid (Sigma-Aldrich) in dioxane was filled into an NMR tube for a crystallisation experiment by slow evaporation of the solvent. After four months, an amber-coloured syrup had formed, indicating decomposition of the original compound. This liquid contained a large colourless crystal that prooved to be composed of the title compound.  Fig. 1. The molecular structures of (I) with displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms are shown as spheres of arbitrary size. Symmetry code: (i) -x+1, -y+2, -z+2.

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 > σ(F 2 ) is used only for calculating Rfactors(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.