The 1:2 co-crystal formed between N,N′-bis(pyridin-4-ylmethyl)ethanediamide and benzoic acid: crystal structure, Hirshfeld surface analysis and computational study

The 4-pyridyl residues lie to either side of the central, planar C2N2O2 chromophore of the oxalamide molecule which has a + anti-periplanar conformation. Conventional hydrogen-bonding interactions lead to supramolecular tapes in the crystal.


Supramolecular features
As anticipated from the chemical composition, significant conventional hydrogen bonding is noted in the crystal of (I) over and above the intramolecular amide-N-HÁ Á ÁO(carbonyl) hydrogen bonds already noted. The geometric parameters characterizing the specified intermolecular contacts are listed in Table 1. The most prominent feature in the crystal is the formation of the expected threemolecule aggregate sustained by hydroxy-O-HÁ Á ÁN(pyridyl) hydrogen bonding. This is connected into a six-molecule aggregate via amide-N-HÁ Á ÁO(amide) hydrogen bonding, which leads to a centrosymmetric ten-membered {Á Á ÁHNC 2 O} 2 synthon. The second amide forms an amide-N-HÁ Á ÁO(carbonyl) bond with the result of that adjacent sixmolecule aggregates are connected into a supramolecular tape via 22-membered {Á Á ÁHOCOÁ Á ÁNC 4 NH} 2 synthons, Fig. 2(a). The other notable contact within the tape is a pyridyl-C-HÁ Á ÁO(carbonyl) interaction, which cooperates with a hydroxy-O-HÁ Á ÁN(pyridyl) hydrogen bond to form a sevenmembered {Á Á ÁOCOHÁ Á ÁNCH} pseudo-heterosynthon; no analogous interaction is noted for the O5-benzoic acid. The supramolecular tapes are aligned along the c-axis direction and have a linear topology.

Hirshfeld surface analysis
The program Crystal Explorer 17 (Turner et al., 2017) was used for the calculation of the Hirshfeld surfaces and two-dimensional fingerprint plots based on the procedures described previously (Tan, Jotani et al., 2019). The three-molecule aggregate whereby the two benzoic acid (BA) molecules are connected to 4 LH 2 via the hydroxy-O-HÁ Á ÁN(pyridyl) hydrogen bonds was used as the input for calculations. A list of the short interatomic contacts discussed below is given in Table 2. Through this analysis, several red spots were identified on the d norm surfaces, Fig. 3, of the individual 4 LH 2 and BA molecules, hereafter BA-I for the O3-containing molecule and BA-II for the O5-molecule, which indicate the presence of close contacts with distances shorter than the sum of the respective van der Waals radii (Spackman & Jayatilaka, 2009). Among all contacts, the terminal benzoic acid-O4-H4OÁ Á ÁN1(pyridyl), benzoic acid-O6-H6OÁ Á ÁN4(pyridyl), amide-N2-H2NÁ Á ÁO2(amide) and amide-N3-H3NÁ Á Á O5(carbonyl) hydrogen-bonding interactions exhibit the most intense red spots on the d norm surfaces, suggestive of strong interactions.
Other, relatively less intense red spots in Fig. 3(a) and (b) [in the order of moderate intensity (m) to weak intensity (w)] were identified for C6-H6BÁ Á ÁO1 (m), C12-H12Á Á ÁO3 (m), C2-H2Á Á ÁO5 (m) and C1-H1Á Á ÁO3 (w), Table 1, and C20-   intense red spot observed for C20-H20Á Á ÁC8 as well as those with relatively weak intensity, the other contacts are consistent with the interactions detected through an analysis with PLATON (Spek, 2020). As for the two benzoic acid molecules in the asymmetric unit, the contacts between them are established through C25-H25Á Á ÁC21, C27-H27Á Á ÁC18 as well as C24-H24Á Á ÁO4 interactions with diminutive intensity on the d norm maps shown in Fig. 3(c) and (d).
The electrostatic potential mapping was performed on the individual 4 LH 2 , BA-I and BA-II molecules through DFT-B3LYP/6-31G(d,p) to further study the nature of the close contacts, Fig. 4. The results are consistent with the above in that the O--HÁ Á ÁN and N-HÁ Á ÁO hydrogen-bonding contacts that exhibited the most intense red spots on the d norm map are highly electrostatic in nature, as evidenced from the intense electronegative (red) and electropositive (blue) regions on the Hirshfeld surfaces of the individual molecules. Other regions are relatively pale, indicating the complementary role of the remaining contacts in sustaining the molecular network in the crystal.
The two-dimensional fingerprint plots were generated in order to quantify the close contacts for compound (I) overall, i.e. the three-molecule aggregate specified above, as well as its individual 4 LH 2 , BA-I and BA-II components, Fig. 5. The overall fingerprint plot of (I) exhibits a shield-like profile with a pair of symmetric spikes and contrasts those for the individual components with asymmetric spikes, indicating the interdependency between 4 LH 2 and benzoic acid in constructing the molecular packing of the system, in contrast to the previously reported benzene monosolvate of 4 LH 2 (Tan, Halcovitch et al., 2019).

Contact
Distance Symmetry operation The interatomic distances are calculated in Crystal Explorer 17 (Turner et al., 2017) whereby the X-H bond lengths are adjusted to their neutron values; (b) these interactions correspond to conventional hydrogen bonds.

Figure 4
The electrostatic potential mapped onto the Hirshfeld surface within the isosurface range of À0.0562 to 0.0861 atomic units for (   counterparts at 11.7 and 11.8%, respectively. A detailed analysis of the d i + d e distances shows that the closest HÁ Á ÁO/ OÁ Á ÁH and HÁ Á ÁC/CÁ Á ÁH contacts of $1.95 Å and $2.62 Å , respectively, occur at distances shorter than the sum of the respective van der Waals radii of 2.61 and 2.79 Å , while the HÁ Á ÁH ($2.20 Å ) and NÁ Á ÁH/ HÁ Á ÁN (2.80 Å ) contacts are longer than the sum of van der Waals radii of 2.18 and 2.64 Å , respectively. The 4 LH 2 molecule also displays a shield-like profile with asymmetric spikes which upon further decomposition could be delineated into HÁ Á ÁH (38.0%), HÁ Á ÁO/OÁ Á ÁH (25.6%), HÁ Á ÁC/CÁ Á ÁH (21.4%) and HÁ Á ÁN/NÁ Á ÁH (9.9%) contacts. The HÁ Á ÁO/OÁ Á ÁH contact exhibits a forceps-like profile with the distribution inclined towards internal-HÁ Á ÁO-external (15.2%) as compared to internal-OÁ Á ÁH-external (10.4%), and both with tips at d i + d e $1.94 Å which is indicative of significant hydrogen bonding. Similarly, the asymmetric, needle-like profile for the HÁ Á ÁN/NÁ Á ÁH contact is inclined towards the internal-NÁ Á ÁH-external (9.0%) with the tip at d i + d e = $1.6 Å , while the remaining 0.9% is attributed to the internal-HÁ Á ÁN-external contact with d i + d e of $2.94 Å (> sum of van der Waals radii). The HÁ Á ÁC/CÁ Á ÁH contacts are evenly distributed on both sides of the contacts with the d i + d e of $2.64 Å which is slightly shorter than the sum of van der Waals radii. On the other hand, the HÁ Á ÁH contacts have little direct influence in sustaining the molecular packing as shown from the shortest d i + d e value of $2.2 Å , which is longer than the sum of the van der Waals radii despite the prominent contributions these make to the overall surface As for the pair of BA molecules, both BA-I and BA-II possess similar, claw-like profiles which differ in the diffuse region, with the former being the characteristic of HÁ Á ÁH contacts while the latter is due to HÁ Á ÁC/CÁ Á ÁH interactions. Quantitatively, differences mainly relate to the percentage contribution by HÁ Á ÁH contacts, i.e. 31.9% for BA-I cf. 38.7% for BA-II. The discrepancy in the distribution for BA-I is compensated by the increase in OÁ Á ÁC/CÁ Á ÁO and CÁ Á ÁC contacts with the distribution being 4.8 and 2.9%, respectively. The distribution for HÁ Á ÁC/CÁ Á ÁH (29.0 vs 29.1%), HÁ Á ÁO/ OÁ Á ÁH (23.8 vs 24.2%) and HÁ Á ÁN (6.7 vs 5.7%) contacts is approximately the same in both BA-I and BA-II, except that the HÁ Á ÁC/CÁ Á ÁH distribution for BA-II is significantly more inclined towards internal-CÁ Á ÁH-external (20.5%) than the internal-HÁ Á ÁC-external (8.6%) in contrast to the relatively balanced distribution for BA-I 15.5% for internal-CÁ Á ÁHexternal vs 13.5% for internal-HÁ Á ÁC-external. In BA-I, the d i + d e values for HÁ Á ÁO/OÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH and HÁ Á ÁN/NÁ Á ÁH at the tips are $2.26-2.70, 2.62 and 1.64 Å , respectively, while the equivalent values for the analogous contacts for BA-II have tips at 2.02-2.56, 2.62-2.86 and 1.58 Å , respectively. Among these contact distances, the OÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH and HÁ Á ÁN for BA-I as well as HÁ Á ÁO/OÁ Á ÁH, HÁ Á ÁC and HÁ Á ÁN for BA-II are shorter than the sum of van der Waals radii. As expected, the minimum d i + d e value for the HÁ Á ÁH contacts is longer than the sum of van der Waals radii, even if it is the most dominant contact for each molecule. The aforementioned data for BA-I and BA-II clearly distinguishes the independent molecules.
Overall, the crystal of (I) is mainly sustained by electrostatic forces owing to the strong ten-membered {Á Á ÁHNC 2 O} 2 synthon as well as the terminal interactions between 4 LN 2 and BA molecules, through hydroxy-O-H4Á Á ÁN(pyridyl) hydrogen bonds, lead to a zigzag electrostatic energy framework, Fig. 6(a). The packing system is further stabilized by the dispersion forces contributed by the ten-membered {Á Á ÁHNC 2 O} 2 synthon complemented by other peripheral interactions such the pairwise C20-H20Á Á ÁC8/C12Á Á ÁC21, C8Á Á ÁC26 and C6-H6BÁ Á ÁO1 interactions, which result in a dispersion energy framework resembling a spider web, Fig. 6(b). The combination of the electrostatic and dispersion forces leads to an overall energy framework that resembles a ladder, Fig. 6(c).

Database survey
As indicated in the Chemical context, 4 LH 2 molecules have long been known to form co-crystals with carboxylic acids. A list of 4 LH 2 /carboxylic acid co-crystals is given in Table 4, highlighting the symmetry of 4 LH 2 , the length of the central C-C bond, recognized as being long (Tiekink, 2017;Tan & Tiekink, 2020) Table 4 Selected geometric data, i.e. central C-C bond length, O-HÁ Á ÁN and NC-HÁ Á ÁO(carbonyl) separations (Å ) for 4 LH 2 in its co-crystals with carboxylic acids and salt with a carboxylate anion.

Figure 6
Perspective views of the energy frameworks of (I), showing the (a) electrostatic force, (b) dispersion force and (c) total energy. The cylindrical radius is proportional to the relative strength of the corresponding energies and they were adjusted to the same scale factor of 100 with a cut-off value of 8 kJ mol À1 within a 2 Â 2 Â 2 unit cells.
the C-H atom adjacent to the pyridyl-nitrogen atom) separations associated with the hydroxy-O-HÁ Á ÁN(pyridyl) hydrogen bond. The data are separated into 1:1 and 1:2 4 LH 2 :carboxylic acid species. In all cases, 4 LH 2 adopts an antiperiplanar disposition of the pyridyl rings whereby the pyridyl rings lie to either side of the central C 2 N 2 O 2 chromophore; often this is crystallographically imposed. This matches the situation in the two known polymorphs of 4 LH 2 (Lee & Wang, 2007;Lee, 2010), but contrasts with the conformational diversity found in the isomeric 3 LH 2 molecules, i.e. in the polymorphs  and multi-component crystals (Tan & Tiekink, 2020 , 2008). In this structure, the carboxylic acid is able to form an intramolecular hydroxy-O-HÁ Á ÁN(azo) hydrogen bond to close an S(6) loop, in accord with Etter's rules, i.e. 'sixmembered ring intramolecular hydrogen bonds form in preference to intermolecular hydrogen bonds' (Etter, 1990). Finally, and for completeness, details for a salt are included in Table 4. Here, proton transfer has occurred, leading to a pyridinium-N-HÁ Á ÁO(carboxylate) hydrogen bond.

Synthesis and crystallization
The precursor, N,N 0 -bis(pyridin-4-ylmethyl)oxalamide ( 4 LH 2 ), was prepared according to the literature; M.p.: 486.3-487.6 K; lit. 486-487 K (Nguyen et al., 1998). Reagent-grade benzoic acid (Merck) was used as received without further purification. Solid 4 LH 2 (0.271 g, 0.001 mol) was mixed with benzoic acid (0.122 g, 0.001 mol) and the physical mixture was then ground for 15 min in the presence of a few drops of methanol. The procedures were repeated three times. Similar experiments with 4 LH 2 :benzoic acid in molar ratios of 1:2, 1:3 and 1:4 were also attempted but only the 2:1 cocrystal (I) was isolated after recrystallization of the powders.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The carbon-bound H atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C). The oxygen-and nitrogenbound H atoms were located from a difference Fourier map and refined with O-H = 0.84AE0.01 Å and N-H = 0.88AE0.01 Å , respectively, and with U iso (H) set to 1.5U eq (O) or 1.2U eq (N).

N,N′-bis(pyridin-4-ylmethyl)ethanediamide; bis(benzoic acid)
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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )