Piperazine-1,4-diium 2-(carboxymethyl)-2-hydroxybutanedioate monohydrate

In the crystal structure of the title compound, C4H12N2 2+·C6H6O7 2−·H2O, the cations, anions and water molecules are linked by intermolecular N—H⋯O, O—H⋯O and weak C—H⋯O hydrogen bonds into a three-dimensional network. An intramolecular O—H⋯O interaction occurs in the dianion.

In the crystal structure of the title compound, C 4 H 12 N 2 2+ Á-C 6 H 6 O 7 2À ÁH 2 O, the cations, anions and water molecules are linked by intermolecular N-HÁ Á ÁO, O-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds into a three-dimensional network. An intramolecular O-HÁ Á ÁO interaction occurs in the dianion.
Renmin Hospital of Wuhan University is thanked for for financial support of this work.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: LH5096). Bond, 2009, Du et al., 2009. Herein, the crystal structure of the title compound (I) is reported.
The asymmetic unit of (I) composed of one piperazinium divalent cation, one citrate divalent anion and one solvent water molecule (see Fig.1). In the crystal structure, the piperazinium cations, citrate anions and water molecules are linked by intermolecular N-H···O, O-H..O and weak C-H···O hydrogen bonds (Table 1) into a three-dimensional network (Fig.2).

Experimental
All the reagents and solvents were used as obtained without further purification. Equivalent molar amount of piperazine (0.2 mmol, 17.2 mg) and citric acid (0.2 mmol, 42.1 mg) were dissolved in 10 ml 95% methanol. The mixture was stirred for ten minutes at ambient temperature and then filtered. The resulting colorless solution was kept in air for three week.
Block-shaped crystals of (I) suitable for single-crystal X-ray diffraction analysis were grown by slow evaporation of the solution at the bottom of the vessel.

Refinement
In the absence of anomalous dispersion effects the Fridel pairs were merged. H atoms bonded to C atoms were positioned geometrically with C-H = 0.97Å and refined in a riding-model approximation with U iso (H) = 1.2U eq (C)]. H atoms bonded to N and O atoms were found from the difference maps and the N-H and O-H distances were refined using commands of 'SADI' and 'DFIX' in SHELXL (Sheldrick, 2008). The U iso (H) values were set to 1.2 and 1.5 times of 1.2U eq (N) and 1.5U eq (O), respectively. Fig. 1. The asymmetric unit of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.