Propane-1,2-diaminium bis(4-methoxybenzoate)

The asymmetric unit of the title salt, C3H12N2 2+·2C8H7O3 −, contains two 4-methoxybenzoate anions and one propane-1,2-diaminium cation. All the amino H atoms of the cation are involved in N—H⋯O hydrogen bonds with the carboxylate O atoms of the anions.


Related literature
Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL. This work was supported by the start-up fund of Anyang Institute of Technology, China.
Propane-1,2-diaminium bis(4-methoxybenzoate) Z.-L. Ru Comment Molecular cocrystals are becoming increasingly important within the pharmaceutical industry as an alternative source of new solid crystalline materials with the potential to provide optimal physical properties whilst retaining the chemical properties of the cocrystal components (Almarsson & Zaworotko, 2004;Blagden et al., 2008;Vishweshwar et al., 2006). Physicochemical properties such as the melting point, stability and solubility of an active pharmaceutical ingredient can be tuned through cocrystal formulation (Kapildev et al., 2011;Schultheiss & Newman, 2009). Cocrystal synthesis often relies on the acid-amide H-bonds interactions. Herein, we report the crystal structure of the title compound, propane-1,2-diaminium di-4-methoxybenzoate.
Both the amine N atoms were protonated. And the carboxyl groups were deprotonated. The geometric parameters of the title compound are in the normal range.
In the crystal structure, all the amino H atoms are involved in N-H···O hydrogen bonds with the carboxyl O atoms (Table 1 and Fig. 2).

Experimental
A mixture of R-propane-1,2-diamine (1.0 mmol), 4-methoxybenzoic acid (2.0 mmol) and 20 ml ethanol were added into a 50 ml flask and refluxed for 5 h, then cooled and filtrated. The solution was evaporated slowly in the air. Colorless block crystals suitable for X-ray analysis were obtained after one week. The R-propane-1,2-diamine turned into racemic amine in the heating process.

Refinement
All H atoms attached to C atoms were fixed geometrically and treated as riding with C-H = 0.98 (methine), 0.97 (methylene), 0.96 (methyl) and 0.93 Å (aromatic), U iso (H) = 1.2U eq (C except methyl) and U iso (H) = 1.5U eq (C of methyl). The amino H atoms were placed in calculated positions and refined in riding mode with U iso (H) = 1.5U eq (N).

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 > 2sigma(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.