Crystal structure of zwitterionic 4-(ammoniomethyl)benzoate: a simple molecule giving rise to a complex supramolecular structure

The asymmetric unit consists of an isolated 4-(ammoniomethyl)benzoate zwitterion derived from 4-aminomethylbenzoic acid through the migration of the acidic proton, together with a solvate water that is disordered over three sites. In the crystal structure, N—H⋯O hydrogen bonds together with π–π stacking of the benzene rings result in a strongly linked, compact three-dimensional structure.

The asymmetric unit of the title compound, C 8 H 9 NO 2 ÁH 2 O consists of an isolated 4-(ammoniomethyl)benzoate zwitterion derived from 4-aminomethylbenzoic acid through the migration of the acidic proton, together with a water molecule of crystallization that is disordered over three sites with occupancy ratios (0.50:0.35:0.15). In the crystal structure, N-HÁ Á ÁO hydrogen bonds together withstacking of the benzene rings [centroid-centroid distance = 3.8602 (18) Å ] result in a strongly linked, compact three-dimensional structure.

Chemical context
As part of a long-range project to find new transition-metal complexes of simple ligands such as carboxylates and amines, we have screened a number of derivatives of benzoic acid, in particular those that a search of the Cambridge Structural Database (CSD, Version 5.35, updated to May 2014; Groom & Allen, 2014) reveals to have formed few coordination complexes whose structures have been reported. The title compound was the unexpected product of an attempt to form a Co II complex with 4-aminomethylbenzoic acid [HAMBA, (a) in scheme below], which has no entries in the CSD, and diaminopurine (DAP).
No coordination complex resulted, but the reaction provided, as an unexpected bonus, a crystalline phase of the monohydrate of the zwitterion of HAMBA (see scheme below), in which the acidic proton has migrated to the amino group resulting in COO À and CH 2 NH 3 + substituents on the aromatic ring and forming 4-(ammoniomethyl)benzoate [(b) in scheme above]. In contrast to the utmost simplicity of its molecular structure, the zwitterion displays an extremely complex hydrogen-bonding scheme and concomitant supramolecular structure as reported herein.  The C-C 6 -C backbone is essentially planar [maximum deviation of 0.005 (3) Å for C8], and subtends dihedral angles of 6.8 (2) and 83.9 (2) with the O 2 C-C (major disorder component) and C-CN planes, respectively. Bond lengths and angles are normal, with the C-O bond lengths of the carboxylate group close to equal, indicating extensive electron delocalization over the unit [C7-O1: 1.266 (4), C7-O2: 1.262 (4) Å ].

Supramolecular features
As indicated previously, the most interesting features in the structure are those derived from the intermolecular interactions, presented in Table 1 (hydrogen bonds) and Table 2 (-contacts). Each ammonium group is bound through N-HÁ Á ÁO hydrogen bonds to three different molecules of (I), with the carboxylato oxygen atoms as acceptors (Fig. 2a). In addition, the benzene rings stack almost parallel to each other in slanted columns (Fig. 2b). N1-H1AÁ Á ÁO2 and N1-H1CÁ Á ÁO1 hydrogen bonds link four molecules together, generating R 4 4 (24) ring motifs, Fig. 3a, while a second synthon with an R 3 4 (10) graph set motif is generated through contacts involving all three hydrogens of the ammonium cation, Fig. 3b (for graph-set notation see, for example, Bernstein et al., 1995). The R 4 4 (24) synthons combine with thestacking interactions to generate layers of molecules in the ac plane. The contacts are inclined parallel to either the (101) plane for one set of contacts ( Fig. 4a) or the (101) plane for the other (Fig. 4b).
Cg1 is the centroid of atoms C1-C6. ccd is the centroid-centroid distance, da is the dihedral angle between rings and ipd is the interplanar distance, or (mean) distance from one plane to the neighbouring centroid. For details, see Janiak (2000).

Figure 4
Sheets of molecules of (I) in the ac plane linked by N-HÁ Á ÁO hydrogen bonds (single dashed lines) andinteractions (double dashed lines).
Finally, Fig. 6 presents a view approximately along the ac diagonal displaying the two hydrogen-bonding synthons, A and B, together with theinteractions and demonstrates how they combine to generate the three-dimensional network.

Database survey
Neither 4-(ammoniomethyl)benzoate nor its zwitterionic form described here appear in the CSD (Version 5.35, updated to May 2014). The most closely related structures are those of a zwitterionic form of 4-ammoniomethylcyclohexane-1-carboxylic acid (IIa) (Shahzadi et al., 2007; CSD refcode AMMCHC11) and its hemihydrated analogue (IIb) (Yamazaki et al., 1981; CSD refcode AMCHCA), in which the phenyl ring is replaced by cyclohexane. This introduces some obvious differences with (I), forcontacts are clearly precluded and there are different relative orientations of the hydrogenbonding donors and acceptors. In spite of this, the hydrogenbonding schemes do show some striking similarities, leading to similar (though differently connected) two-dimensional substructures. In particular, the same R 4 4 (24) and R 3 4 (10) synthons are present in both cases as in (I), and play predominant roles in the crystal packing. This is despite the presence of the water solvate in (IIb), which is not involved in classical hydrogen bonding to the zwitterion.

Synthesis and crystallization
To an aqueous solution of HAMBA (1 mmol, 0.15116g) were added an aqueous solution of Co(Ac) 2 Á4H 2 O (2 mmol, 0.49816g) and an ethanolic solution of DAP (1 mmol, 0.15009 g). The resulting mixture was heated at reflux for 4 h and left to cool down and evaporate at room temperature. After a few days, crystals suitable for X-ray diffraction of the (uncomplexed) zwitterion (I) appeared. These were used as grown.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.
There are two disorder features in this structure. The oxygen atoms of the carboxylate group were disordered over Overall packing for (I) showing how the A and B ring motifs combine withstacking interactions to generate a three-dimensional network.

Figure 5
Chains of molecules of (I) linked by N-HÁ Á ÁO hydrogen bonds to form a three-dimensional network. two positions that were refined with similarity restraints with occupancy factors 0.912 (13)/0.088 (13). Disorder involving the water molecule was more pronounced, with the oxygen atoms disordered over three distinct sites. When refined, the occupancies appeared to be strongly correlated with their displacement factors, showing an oscillating behaviour. In the final refinement cycles, occupancies were fixed to the mean values of these oscillation ranges with occupancy ratios 0.50:0.35:0.15. All the H atoms (except for those of the disordered water molecules) were recognizable in an early difference Fourier map. Hydrogen atoms of the NH 3 group were refined with N-H distances restrained to be equal to within 0.01Å [final d(N-H) = 1.07 (3) Å ]. All H atoms bound to carbon were refined using a riding model with d(C-H) = 0.93 Å and U iso = 1.2U eq (C) for aromatic and 0.98 Å , U iso = 1.2U eq (C) for methylene H atoms. The hydrogen atoms on the disordered water solvate were not identified.
When trying to calculate the Flack parameter of the inverted structure, it was recognised that the space group was one of the few (seven, in fact) where the structure cannot be inverted by simple inversion of the atomic coordinates. In the case of Fdd2, the 'inversion rule' to be applied is Inv(x, y, z) = 1 4 À x, 1 4 À y, Àz, After this, the refinement proceeded smoothly without any change in the symmetry operators. Even so, the resulting Flack Parameters were both disparate and high [À1.2 (4) vs 2.2 (4) for the reported/inverted structures, respectively]. Hence, the absolute configuration could not be determined reliably. Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).