Hydrogen-bonded co-crystal structure of benzoic acid and zwitterionic l-proline

Benzoic acid–pyrrolidin-1-ium-2-carboxylate (1/1) is an example of the application of non-centrosymmetric co-crystallization for the growth of a crystal containing a typically centrosymmetric component in a chiral space group. It co-crystallizes in the space group P212121 and contains benzoic acid and l-proline in equal proportions. The crystal structure exhibits chains of l-proline zwitterions capped by benzoic acid molecules which form a C(5)[(11)] hydrogen-bonded network along [100].


Chemical context
Non-centrosymmetric materials are of particular importance in the field of materials chemistry for the large number of symmetry-dependent properties they can possess, including circular dichroism, pyroelectricity, and non-linear optical behavior (Halasyamani & Poeppelmeier, 1998;McMillen et al., 2012;Aitken et al., 2009). While purposefully engineering these materials can be difficult, one method for eliminating centrosymmetry in crystalline materials is co-crystallization with an enantiopure chiral compound (Kwon et al., 2006). In this way, provided that the chiral compound is not capable of racemization, the potential point groups are limited only to those which are chiral, and therefore non-centrosymmetric. The amino acid proline plays an important role in determining the structure of proteins, due to its structural rigidity. Proline has also been shown to be a good candidate for synthesizing non-centrosymmetric co-crystals. In fact, Timofeeva et al. (2003) reported success co-crystallizing dicyanovinyl aromatic compounds with l-proline while the same compounds would grow neat crystals when co-crystallization with l-tartaric acid was attempted.

Structural commentary
l-proline zwitterion (LP) and benzoic acid (BA) co-crystallize in the chiral space group P2 1 2 1 2 1 with one molecule of ISSN 2056-9890 l-proline and one molecule of benzoic acid in the asymmetric unit, shown in Fig. 1. The l-proline exists in its zwitterionic form within the lattice while the carboxylic acid group of the benzoic acid molecules remain protonated. Although the Flack parameter could not be used to unambiguously assign the absolute configuration, the enantiomer was reliably assigned by reference to an unchanging chiral centre in the synthetic procedure.

Supramolecular features
In this structure, each LP hydrogen bonds with four other LP molecules and one BA. The LP hydrogen bonding forms 1D chains along [100] via (carboxylate) OÁ Á ÁH-N (pyrollium) interactions in a C(5)[R 3 3 (11)] motif ( Table 1). The BA molecules act as capping groups and hydrogen bond to each of the LP carboxylates through O-HÁ Á ÁO (carboxylate) interactions. The complete BA-LP chains, as shown in Fig. 2, propagate along [100] and are approximately contained in (021) and (021). These chains are held together by edge-facetypestacking between adjacent BA molecules approximately along [010], with a ring-centroid to ring-centroid distance of 4.8451 (16) Å .

Database survey
Recently, the co-crystal structure of LP and para-aminobenzoic acid (PABA) was reported (Athimoolam & Natarajan, 2007). While the structure of BA-LP retains some structural similarities with the PABA-LP co-crystal, due to the absence of one hydrogen-bonding moiety, the amino group, the structure of BA-LP (Fig. 3) also exhibits some important differences when compared to that of PABA-LP. The head-to-tail LP chains in PABA-LP are similar to those in BA-LP, though instead of two chains hydrogen-bonded together to form rings, the chains hydrogen bond to form a continuous 2D sheet of LP molecules. Much like BA-LP, the PABA molecules hydrogen bond to the periphery of the LP chains; however, this crystal incorporated water into the lattice and it is to these water molecules that the PABA molecules are bound. The major difference between the two structures is the presence of the hydrogen-bond donating group at the 4-position of the PABA molecules. This moiety allows the PABA molecules to bridge the LP chains in PABA--LP, a supramolecular feature absent in the title compound. The result of the lack of para-substitution and water in the lattice is that BA-LP forms a hydrogen-bonding network which extends in only one dimension, instead of the three-dimensional network of PABA-LP.  Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x À 1; y; z; (ii) x À 1 2 ; Ày þ 3 2 ; Àz þ 1; (iii) Àx þ 1 2 ; Ày þ 1; z À 1 2 ; (iv) Àx þ 3 2 ; Ày þ 1; z þ 1 2 .

Figure 2
Diagram illustrating the hydrogen-bonding interactions in BA-LP cocrystal.

Figure 1
The asymmetric unit of the title compound, showing the atom-naming scheme. Displacement ellipsoids are shown at the 50% probability level.

Figure 3
Diagram illustrating the hydrogen bonding network of LP in the previously reported PABA-LP co-crystal (left) and view of the PABA hydrogen-bonding network in the previously reported co-crystal (right).

Synthesis and crystallization
Solid BA (10.1 mg, 9.01 Â 10 À2 mmol) and LP (9.3 mg, 8.08 Â 10 À2 mmol) were added to a 25 ml scintillation vial. To this was added approximately 8 ml of ethanol followed by sonication until all solutes were fully dissolved. The loosely capped vial was then placed on an open shelf. After three weeks, colorless needle-shaped crystals of the title compound suitable for single-crystal X-ray diffraction measurements were obtained.

Refinement
The crystal, data collection, and refinement details are listed in Table 2. The positions of the carboxylate and pyrollium hydrogen atoms were determined from the Fourier difference map, and all other hydrogen atoms were placed in idealized positions with C-H bond lengths set to 0.93 and 0.97 Å for aryl and alkyl hydrogen atoms, respectively. These hydrogen atoms were refined using a riding model with U iso (H) = 1.5U eq (O) for the carboxylic acid proton on the BA molecules and U iso (H) = 1.2U eq in all other cases. No other constraints were applied to the refinement model.

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. 1. Fixed Uiso At 1.2 times of: All C(H) groups, All C(H,H) groups, All N(H,H)