l-Histidinium dipicrate dihydrate

In the title molecular salt, C6H11N3O2 2+·2C6H2N3O7 −·2H2O, the histidine molecule exists as a histidinium dication, being protonated at the N atom of the imidazole ring. The charges are balanced by two picrate anions and the compound crystallizes as a dihydrate. In the crystal, the components are linked via N—H⋯O and O—H⋯O hydrogen bonds and weak C—H⋯O interactions, forming a three-dimensional supermolecular structure.

In the title molecular salt, C 6 H 11 N 3 O 2 2+ Á2C 6 H 2 N 3 O 7 À Á2H 2 O, the histidine molecule exists as a histidinium dication, being protonated at the N atom of the imidazole ring. The charges are balanced by two picrate anions and the compound crystallizes as a dihydrate. In the crystal, the components are linked via N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á ÁO interactions, forming a three-dimensional supermolecular structure.
Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2 and SAINT (Bruker, 2004); data reduction: SAINT and XPREP (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009). MS thanks the UGC Networking Centre, School of Chemistry, University of Hyderabad, India, for the award of a Visiting Research Fellowship to use the facilities at the School, which the authors also thank for access to the X-ray diffraction equipment.
M. Sethuram, M. V. Rajasekharan, M. Dhandapani, G. Amirthaganesan and M. NizamMohideen Comment Intermolecular and inter-ionic hydrogen bonding interactions, which are not only the strongest of the noncovalent interactions but also highly directional, play an important role in constructing supramolecular structures (Braga et al., 2004). Picrate is generally used as an accompanying ion in many systems involving extraction and transport of metal ions to improve the extractability (Bibal et al., 2003). Picrate interacts as a monodentate, bidentate and tridentate ligand (Olsher et al., 1996). Furthermore, picrate is a penta-dentate ligand when it coordinates with cation by chelating pairs of oxygen atoms from p-nitro groups of adjacent picrates, and with successive cations linking the array into a two or threedimensional network (Harrowfield et al., 1995) and picrates of biologically important molecules (Harrison et al., 2007;Swamy et al., 2007). We have prepared a new picrate of L-Histidinium hydrate and its crystal structure is reported herein.
The asymmetric unit of the title compound, Fig. 1, contains an L-histidinium cation, two picrate anions and two water molecules. The histidine molecule exists as an histidinium ion due to the protonation at the N atom of the imidazole ring.
In the crystal the cation, the picrate anions and the water molecules of crystallization are involved in N-H···O and O-H···O hydrogen bonds and week C-H···O interactions, to form a three-dimensional supramolecular network (Table 1 and  water-ethanol solvent mixture and kept aside without any mechanical movement for crystal growth in a dust free environment. Bright yellowish crystals that formed in 5 days were collected carefully from the mother liquor. Several recrystallizations were done to get ultra pure crystals. The yield in the reaction was ca. 60%. Analysis calc.

Refinement
The water molecule H-atoms, the methine (CH) H atom, and the CH and one NH H atom of the imidazole ring, were located in a difference Fourier map and freely refined. The OH, the NH 3, one NH H atom of the imidazole ring, and the    Table 1 for details).  (12) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'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 > σ(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.