Bis(4-carboxypiperidinium) 5-nitroisophthalate

Cocrystallization of 4-carboxypiperdine with 5-nitroisophthalic acid afforded the title salt, 2C6H12NO2 +·C8H3NO6 2−, in which the heterocyclic N atoms are protonated and the carboxylic acid groups are deprotonated. In the crystal, intermolecular N—H⋯O and O—H⋯O hydrogen-bonding interactions assemble the ions into a three-dimensional network.

Cocrystallization of 4-carboxypiperdine with 5-nitroisophthalic acid afforded the title salt, 2C 6 H 12N O 2 + Á-C 8 H 3 NO 6 2À , in which the heterocyclic N atoms are protonated and the carboxylic acid groups are deprotonated. In the crystal, intermolecular N-HÁ Á ÁO and O-HÁ Á ÁO hydrogenbonding interactions assemble the ions into a three-dimensional network.
The author gratefully acknowledges the financial support of the Tianjin Key Laboratory of Structure and Performance for Functional Molecule.

Comment
Recently, molecular self-assembly by non-covalent interactions has attacted considerable interest in supramolecular chemistry and crystal engineering fields due to its potential applications in materials (Zaworotko, 2001), molecular recognition Oxtoby et al., 2005), and pharmaceutical chemistry (Remenar et al., 2003). Obviously, the conguated organic components with rich carboxylate or amino groups have became good blocks for the construction of self-assembly systems, since popular hydrogen-bonding and π··· π interactions are the main driven forces of the assembly process. In this regard, bearing two functional groups (-NH-and -COOH-) capable of producing abundant hydrogen-bonding interactions as well as coordination with transitional ions, 4-piperdinecarboxylic acid (Hpipe) exists as a zwitterion with the amino group protonated and the carboxylic group deprotonated in aqueous solution (Mora et al., 2002). While, in the solid state, the zwitterionic Hpipe can either coordinate with metal ions by its deprotonated carboxylate group or form cocrystals with other compensated components by hydrogen-bonding interactions (Inomata et al. 2002;Adams et al., 2006;Podesta & Orpen, 2002;Zhang et al., 2009;Delgado et al. 2001). To continue to investigate the self-assembly behavior of Hpipe in the solid state, herein, we report the cocrystal of Hpipe and 5-nitroisophthalic acid (H 2 nip).
As shown in Figure 1, the asymmetric unit of (I) comprises one doubly deprotonated 5-nitro-isophthalate anion (nip 2-) and two chemically equal but crystallographically independent 4-piperdinecarboxylic acid cations (H 2 pipe + ). In the crystal, a pair of symmetry-related nip anions and two crystallographically equivalent Hpipe + cations are connected together in a head-to-tail manner by N-H···O and O-H···O hydrogen-bonds between the protonated amino/carboxylic groups of H 2 pipe + and the deprotonated carboxylate of nip anions (Table 1). Thus, closed four-component-based supramolecular rings are gerenated and extended in [1 -1 1] direction ( Figure 2). Then, these supramolecular rings are further non-covalently extended by pairs of the second crystallographically unique Hpipe + cation, leading to a three-dimensional (3-D) hdrogen-bonds network ( Figure 3 and Table 1). Thus, the abundant hydrogen-bonding interactions significantly dominate the formation of 3-D supramolecular network of the title cocrystal.

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.