2-({[(Pyridin-1-ium-2-ylmethyl)carbamoyl]formamido}methyl)pyridin-1-ium bis(3,5-dicarboxybenzoate): crystal structure and Hirshfeld surface analysis

The crystal structure of the title salt comprises supramolecular tapes of dications arising from amide-N—H⋯O(amide) hydrogen bonds which thread through supramolecular layers of anions connected via hydroxy-O—H⋯O(carbonyl) and charge-assisted hydroxy-O—H⋯O(carboxylate) hydrogen bonds.


Chemical context
Of the isomeric N,N 0 -bis(pyridin-n-ylmethyl)ethanediamides, n = 2, 3 or 4, the molecule with n = 2 appears to have attracted the least attention in co-crystallization studies; for the chemical structure of the diprotonated form of the n = 2 isomer see Scheme 1. By contrast, the n = 3 and 4 molecules have attracted interest from the crystal engineering community in terms of their ability to form co-crystals with iodocontaining species leading to aggregates featuring NÁ Á ÁI halogen bonding (Goroff et al., 2005;Jin et al., 2013) as well as carboxylic acids (Nguyen et al., 2001). It is the latter that has formed the focus of our interest in co-crystallization experiments of these molecules which has led to the characterization of both co-crystals (Arman, Kaulgud et al., 2012; and salts (Arman et al., 2013). It was during the course of recent studies in this area (Syed et al., 2016) that the title salt was isolated from the 1:1 co-crystallization experiment between the n = 2 isomer and trimesic acid. The ISSN 2056-9890 crystal and molecular structures as well as a Hirshfeld surface analysis of this salt is described herein.

Structural commentary
The title salt, Fig. 1, was prepared from the 1:1 reaction of trimesic acid and N,N 0 -bis(pyridin-2-ylmethyl)ethanediamide conducted in ethanol. The harvested crystals were shown by crystallography to comprise (2-pyridinium)CH 2 N(H)C( O)-C( O)CH 2 N(H)(2-pyridinium) dications and 3,5-dicarboxybenzoate anions in the ratio 1:2; as the dication is located about a centre of inversion, one anion is found in the asymmetric unit. The confirmation for the transfer of protons during the co-crystallization experiment is found in (i) the pattern of hydrogen-bonding interactions as discussed in Supramolecular features, and (ii) the geometric characteristics of the ions. Thus, the C-N-C angle in the pyridyl ring has expanded by over 3 cf. that found in the only neutral form of N,N 0 -bis(pyridin-2-ylmethyl)ethanediamide characterized crystallographically in an all-organic molecule, i.e. in a 1:2 cocrystal with 2-aminobenzoic acid , Table 1. The observed angle is in agreement with the sole example of a diprotonated form of the molecule, i.e. in a 1:2 salt with 2,6-dinitrobenzoate (Arman et al., 2013), Table 1. Further, the experimental equivalence of the C14-O2, O3 bond lengths, i.e. 1.259 (2) and 1.250 (2) Å is consistent with deprotonation and the formation of a carboxylate group, and contrasts the great disparity in the C15-O4, O5 [1.206 (2) and 1.320 (2) Å ] and C16-O6, O7 [1.229 (2) and 1.315 (2) Å ] bond lengths.
In the dication, the central C 4 N 2 O 2 chromophore is almost planar, having an r.m.s. deviation of 0.009 Å and, from symmetry, the carbonyl groups are anti. An intramolecular amide-N-HÁ Á ÁO(carbonyl) hydrogen bond is noted, Table 2. The pyridinium-N1 and amide-N2 atoms are approximately syn as seen in the value of the N1-C1-C6-N2 torsion angle of 34.8 (2) . This planarity does not extend to the terminal pyridinium rings which are approximately perpendicular to and lying to either side of the central chromophore, forming dihedral angles of 68.21 (8) . The central C7-C7 i bond length of 1.538 (4) Å is considered long for a C-C bond involving sp 2 -hybridized atoms (Spek, 2009). Geometric data for the two previously characterized molecules Arman et al., 2013)  The molecular structures of the ions comprising the title salt, showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level: (a) 2-({[(pyridin-1-ium-2-ylmethyl)carbamoyl]formamido}methyl)pyridin-1-ium, and (b) 3,5-dicarboxybenzoate; unlabelled atoms are related by the symmetry operation Àx, 1 À y, 1 À z. Table 1 Selected geometric details (Å , ) for an N,N 0 -bis(pyridin-2-ylmethyl)ethanediamide molecule and protonated forms a .
exception of the relative disposition of the pyridinium-N1 and amide-N2 atoms. Thus, in the neutral form of the molecule, these are anti, the N1-C1-C6-N2 torsion angle being 165.01 (10) Å , and almost perpendicular in the salt, with N1-C1-C6-N2 being 73.84 (15) . These differences are highlighted in the overlay diagram shown in Fig. 2.
In the anion, the C13-C8-C14-O2 and C9-C10-C15-O4 torsion angles of 15.3 (3) and 16.4 (3) , respectively, indicate twisted conformations between these residues and the ring to which they are attached whereas the C11-C12-C16-O6 torsion angle of 2.0 (3) shows this carboxylic acid group to be co-planar with the ring. The conformational flexibility in 3,5-dicarboxybenzoate anions is well illustrated in arguably the four most closely related structures in the crystallographic literature (Groom & Allen, 2014), identified from approximately 35 organic salts containing this anion. Referring to Scheme 2, the most closely related structure features the dication C_I with two protonated pyridyl N atoms (Santra et al., 2009). Here, with two crystallographically independent anions, twists are noted from the mean-plane data collated in Table 3. For one anion, all groups are twisted out of the leastsquares plane through the benzene ring but, in the second anion, the carboxylate group is effectively co-planar with the ring with up to a large twist noted for one of the carboxylic acid groups. In the other example with a diprotonated cation, C_II (Singh et al., 2015), both independent anions exhibit twists of less than 8 with all three residues effectively coplanar in one of the anions. In the example with a single protonated pyridyl residue, C_III (Ferguson et al., 1998), twists are evident for one of the carboxylic acid groups and for the carboxylate but, the second carboxylic acid residue is effectively co-planar. Finally, in the mono-protonated species related to C_I, i.e. C_IV (Basu et al., 2009), twists are evident for all groups with the maximum twists observed in the series for the carboxylate residue, i.e. 25.13 (10) , and for one of the carboxylic acid groups, i.e. 22.50 (10) .

Supramolecular features
The molecular packing may be conveniently described in terms of O-HÁ Á ÁO hydrogen bonding to define an anionic network which is connected into a three-dimensional architecture by N-HÁ Á ÁO hydrogen bonds; Table 2  Overlay diagram of the dication in the title compound (red image), the neutral molecule in its co-crystal (green), and dication in the literature salt (blue). The molecules have been overlapped so that the O C-C O residues are coincident. The ring N atoms are indicated by an asterisk. Table 3 Dihedral angles ( ) for the 3,5-dicarboxybenzoate anion in the title salt and in selected literature precedents a .  (104) and having an undulating topology, Fig. 3a. The dications also self-associate to form supramolecular tapes via C(4) chains featuring pairs of amide-N-HÁ Á ÁO(amide) hydrogen bonds and 10-membered {Á Á ÁHNC 2 O} 2 synthons, Fig. 3b. The tapes are aligned along the a axis and, in essence, thread through the voids in the anionic layers to form a three-dimensional architecture, Fig. 3c. The links between the anionic layers and cationic tapes are hydrogen bonds of the type charge-assisted pyridinium-N-O(carboxylate). In this scheme, no apparent role for the carbonyl-O4 atom is evident. However, this atoms accepts two C-HÁ Á ÁO interactions from pyridyl-and methylene-H to consolidate the molecular packing. Additional stabilization is afforded by pyridyl-C-HÁ Á ÁO(carboxylate, carbonyl) interactions, Table 2.   Views of the Hirshfeld surface mapped over d norm in the title salt: (a) dication, (b) and (c) anion.

Figure 5
View of the Hirshfeld surface mapped over the calculated electrostatic potential the tri-ion aggregate in the title salt.
Crystal Explorer, and mapped on the Hirshfeld surfaces using the STO-3G basis set at the Hartree-Fock level theory over the range AE0.25 au. The contact distances d i and d e from the Hirshfeld surface to the nearest atom inside and outside, respectively, enable the analysis of the intermolecular interactions through the mapping of d norm . The combination of d e and d i in the form of two-dimensional fingerprint plots provides a summary of intermolecular contacts in the crystal (Rohl et al., 2008). Views of the Hirshfeld surface mapped over d norm in the title salt are given in Fig. 4. The formation of charge-assisted hydroxyl-O-HÁ Á ÁO(carboxylate) and pyridinium-N-HÁ Á ÁO(carboxylate) hydrogen bonds in the crystal appear as distinct dark-red spots near the respective donor and acceptor atoms. In Fig. 5, the blue and red colouration are the corresponding regions on the surface mapped over the electrostatic potential. The dark-red spots on the Hirshfeld surface of the dication corresponds to a pair of amide-N-HÁ Á ÁO(amide) hydrogen bonds leading to the supramolecular tape. Intermolecular C-HÁ Á ÁO and N-HÁ Á ÁO interactions, representing weak hydrogen bonds over and above those discussed above in Supramolecular features, result in light-red spots near some of the carbon, nitrogen and oxygen atoms, Fig. 4. Hence, the contribution to the surface from these interactions involve not only OÁ Á ÁH/HÁ Á ÁO contacts but also CÁ Á ÁO/OÁ Á ÁC and NÁ Á ÁO/OÁ Á ÁN contacts, Table 4. The relative contributions of the different contacts to the Hirshfeld surfaces are collated in Table 5 for the entire structure and also delineated for the dication and anion. The linkage of ions through the formation of hydrogen bonds is illustrated in Fig. 6.
The overall two-dimensional fingerprint plot (FP) of the salt together with those of the dication and anion, and FP's delineated into HÁ Á ÁH, OÁ Á ÁH/HÁ Á ÁO, CÁ Á ÁH/HÁ Á ÁC and CÁ Á ÁO/OÁ Á ÁC contacts are illustrated in Fig. 7. The OÁ Á ÁH/ HÁ Á ÁO contacts have the largest overall contribution to the Hirshfeld surface, i.e. 43.2%, and these interactions dominate in the crystal structure. The prominent spike with green points appearing in the lower left region in the FP for the anion at d e + d i $ 1.7 Å has a major contribution, i.e. 47.2%, from OÁ Á ÁH contacts; the spike at the same d e + d i distance is due to a small contribution, 10.0%, from HÁ Á ÁO contacts. The different contributions from OÁ Á ÁH and HÁ Á ÁO contacts to the Hirshfeld surface of the dication, i.e. 6.8 and 34.8%, respectively, lead to asymmetric peaks at d e + d i $ 1.8 and 2.0 Å , respectively, indicating the varying strength of these interactions. However, the overall FP of the salt delineated into OÁ Á ÁH/  Table 4 Short interatomic contacts (Å ) in the title salt.
2.70 (2) -x, 1 À y, Àz  HÁ Á ÁO contacts shows a symmetric pair of spikes at d e + d i $ 1.7 Å with nearly equal contributions from OÁ Á ÁH and HÁ Á ÁO contacts. A smaller contribution is made by the HÁ Á ÁH contacts, Table 1, and these appear as the scattered points without a distinct peak, Fig. 7. The presence of short interatomic CÁ Á ÁH/HÁ Á ÁC contacts, Table 4, result in a 17.3% overall contribution to the surface, although there are no C-HÁ Á Á contacts within the acceptance distance criteria for such interactions (Spek, 2009). These are represented by a pair of symmetrical wings at d e + d i $ 2.9 Å in the FP plot, Fig. 7. The contribution from CÁ Á ÁO/OÁ Á ÁC contacts to the Hirshfeld surface is also evident from the presence of intermolecular C-HÁ Á ÁO interactions as well as short interatomic CÁ Á ÁO/ OÁ Á ÁC contact, Table 4. These appear as cross-over wings in the (d e , d i ) region between 1.7 and 2.7 Å . A small but significant contribution to the Hirshfeld surface of the dication due to NÁ Á ÁO/OÁ Á ÁN contacts is the result of intermolecular amide-N-HÁ Á ÁO(amide) interactions. The intermolecular interactions were further analysed using a recently reported descriptor, the enrichment ratio, ER (Jelsch et al., 2014), which is based on Hirshfeld surface analysis and gives an indication of the relative likelihood of specific intermolecular interactions to form; the calculated ratios are given in Table 6. The relatively poor content of hydrogen atoms in the salt and the involvements of many hydrogen atoms in the intermolecular interactions, as discussed above, reduces the ER value of non-bonded HÁ Á ÁH contacts to a value less unity, i.e. 0.8, due to a 23.7% contribution from the 54.5% available Hirshfeld surface and anticipated 29.7% random contacts. The ER value of 1.4 corresponding to OÁ Á ÁH/HÁ Á ÁO contacts results from a relatively high 43.2% contribution by O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO interactions. The carbon and oxygen atoms involved in the intermolecular C-HÁ Á ÁO interactions and short inter CÁ Á ÁO/OÁ Á ÁC contacts are at distances shorter than the sum of their respective van der Waals radii, hence they also have a high formation propensity, so the ER value is > 1. The CÁ Á ÁH/HÁ Á ÁC contacts in the crystal are enriched due to the poor nitrogen content and the presence of short interatomic CÁ Á ÁH/HÁ Á ÁC contacts so the ratio is close to unity, i.e. 0.99. Finally, the ER value of 1.68 corresponding to NÁ Á ÁO/OÁ Á ÁN contacts for the surface of dication is the result of the chargeassisted N-HÁ Á ÁO interactions consistent with their high propensity to form.

Database survey
As mentioned in the Chemical context, N,N 0 -bis(pyridin-2ylmethyl)ethanediamide (LH 2 ), has not been as well studied as the n = 3 and 4 isomers. This notwithstanding, the coordination chemistry of LH 2 is more advanced and diverse. Thus, co-crystals have been reported with a metal complex, i.e.

Synthesis and crystallization
The diamide (0.25 g), prepared in accord with the literature procedure (Schauer et al., 1997), in ethanol (10 ml) was added to a ethanol solution (10 ml) of trimesic acid (Acros Organic, 0.18 g). The mixture was stirred for 2 h at room temperature. After standing for a few minutes, a white precipitate formed which was filtered off by vacuum suction. The filtrate was then left to stand under ambient conditions, yielding pale-yellow crystals after 2 weeks.

2-({[(Pyridin-1-ium-2-ylmethyl)carbamoyl]formamido}methyl)pyridin-1-ium bis(3,5-dicarboxybenzoate)
Crystal data 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.