N,N′-Bis(pyridin-4-ylmethyl)oxalamide benzene monosolvate: crystal structure, Hirshfeld surface analysis and computational study

The asymmetric unit of the title solvate comprises a half molecule of each component as both species are disposed about a centre of inversion. In the crystal, two-dimensional arrays are formed by amide-N—H⋯N(pyridyl) hydrogen bonds, which are connected into a three-dimensional architecture by C—H⋯π(benzene and pyridyl) interactions with benzene acting as the acceptor and donor, respectively.

The asymmetric unit of the title 1:1 solvate, C 14 H 14 N 4 O 2 ÁC 6 H 6 [systematic name of the oxalamide molecule: N,N 0 -bis(pyridin-4-ylmethyl)ethanediamide], comprises a half molecule of each constituent as each is disposed about a centre of inversion. In the oxalamide molecule, the central C 2 N 2 O 2 atoms are planar (r.m.s. deviation = 0.0006 Å ). An intramolecular amide-N-HÁ Á ÁO(amide) hydrogen bond is evident, which gives rise to an S(5) loop. Overall, the molecule adopts an antiperiplanar disposition of the pyridyl rings, and an orthogonal relationship is evident between the central plane and each terminal pyridyl ring [dihedral angle = 86.89 (3) ]. In the crystal, supramolecular layers parallel to (102) are generated owing the formation of amide-N-HÁ Á ÁN(pyridyl) hydrogen bonds. The layers stack encompassing benzene molecules which provide the links between layers via methylene-C-HÁ Á Á(benzene) and benzene-C-HÁ Á Á(pyridyl) interactions. The specified contacts are indicated in an analysis of the calculated Hirshfeld surfaces. The energy of stabilization provided by the conventional hydrogen bonding (approximately 40 kJ mol À1 ; electrostatic forces) is just over double that by the C-HÁ Á Á contacts (dispersion forces).

Chemical context
With a combination of centrally located amide and terminal pyridyl functional groups, the isomeric molecules related to the title compound of the general formula (n-C 5 H 4 N)CH 2 N(H)C( O)C( O)N(H)CH 2 (C 5 H 4 N-n), for n = 2, 3 and 4, abbreviated as n LH 2 , have long attracted the attention of structural chemists and their structural chemistry has been reviewed very recently (Tiekink, 2017). Taking the 3 LH 2 species as an exemplar, its 1:1 co-crystal with N,N 0 -dicarboxymethylurea, HO 2 CCH 2 N(H)C( O)N(H)CH 2 CO 2 H, features two distinct supramolecular tapes sustained by N-HÁ Á ÁO hydrogen bonding. The first of these arises from amide-N-HÁ Á ÁO(amide) hydrogen bonding between the amide groups, on both sides of the 3 LH 2 molecule, through tenmembered amide synthons {Á Á ÁHNC 2 O} 2 (Nguyen et al., 2001). Parallel tapes comprising N,N 0 -dicarboxymethylurea molecules, sustained by six-membered {Á Á ÁOÁ Á ÁHNCNH} synthons, are also formed. The links between the tapes leading to a two-dimensional array are of the type hydroxy-O-HÁ Á ÁN(pyridyl). Molecules of n LH 2 also featured prominently in early, systematic studies of halogen bonding. An illustrative example is found in the 1:1 co-crystal formed between 3 LH 2 and 1,4-di-iodobuta-1,3-diyne, I-C C-C C-C-I (Goroff et al., 2005). A two-dimensional array is also found in ISSN 2056-9890 this co-crystal whereby supramolecular tapes between 3 LH 2 molecules are formed as for the previous example and these are connected by NÁ Á ÁI halogen bonding. In the crystals of both polymorphs of pure 3 LH 2 , similar supramolecular tapes mediated by amide hydrogen bonding are formed. However, that this mode of supramolecular association is not all pervasive in the n LH 2 systems is seen the structures of the two polymorphs of pure 4 LH 2 (Lee & Wang, 2007;Lee, 2010). In one of the polymorphs of this isomer, supramolecular dimers are formed via amide-N-HÁ Á ÁO(amide) hydrogen bonding and these are linked into a two-dimensional array via amide-N-HÁ Á ÁN(pyridyl) hydrogen bonds (Lee & Wang, 2007). In the second polymorph, all potential amide-N-H and pyridyl-N donors and acceptors associate via amide-N-HÁ Á ÁN(pyridyl) hydrogen bonds to generate a two-dimensional array. In this context, and in the context of recent work on 4 LH 2 in co-crystals (Syed et al., 2016) and adducts of zinc 1,1-dithiolates (Arman et al., 2018;Tan, Chun et al., 2019), it was thought of interest to conduct a polymorph screen for 4 LH 2 . From a series of crystallizations of 4 LH 2 taken in dimethylformamide and layered with benzene, o-xylene, m-xylene, p-xylene, toluene, pyridine and cyclohexane in separate experiments, only crystals of the title benzene solvate, (I), were isolated. Herein, the crystal and molecular structures of (I) are described along with a further evaluation of the supramolecular association via an analysis of the calculated Hirshfeld surfaces as well as a computational chemistry study.

Structural commentary
The title co-crystal (I) is the result of crystallization of 4 LH 2 , taken in dimethylformaide, with benzene. The crystallographic asymmetric unit comprises half a molecule each of 4 LH 2 and benzene, Fig. 1, each being disposed about a crystallographic centre of inversion. The central C 2 N 2 O 2 plane is strictly planar with the r.m.s. deviation of the fitted atoms being 0.0006 Å ; the C7 atoms lie 0.0020 (16) Å to either side of the plane. An intramolecular amide-N-HÁ Á ÁO(amide) i hydrogen bond, occurring between the symmetry related amide groups, gives rise to an S(5) loop, Table 1; symmetry operation (i) 1 À x, 1 À y, À z. The crystallographic symmetry also implies an antiperiplanar disposition of the pyridyl rings. The dihedral angle between the central plane and terminal pyridyl ring is 86.89 (3) , indicating an orthogonal relationship.

Supramolecular features
The geometric parameters characterizing the interatomic contacts identified in the crystal of (I) are given in Table 1. The key feature of the molecular packing is the formation of amide-N-HÁ Á ÁN(pyridyl) hydrogen bonding. This generates a two-dimensional, rectangular grid lying parallel to (102), Fig. 2(a), with dimensions defined by O10Á Á ÁO10 and N8Á Á ÁN8 separations of 9.6770 (11) and 12.3255 (11) Å , respectively. The other notable contacts in the crystal are of the type C-HÁ Á Á, Table 1. Thus, methylene-C7-HÁ Á Á(benzene) and benzene-C11-HÁ Á Á(pyridyl) interactions are formed. From symmetry, each benzene molecule forms four, i.e. two (as acceptor) and two (as donor), such interactions, Fig. 2(b). The side-on view of Fig. 2 Fig. 2(c) indicates the amide-N-H and pyridyl-N project in all directions around the five-molecule aggregate. Indeed, it is the C-HÁ Á Á interactions that connect the layers into a three-dimensional architecture, Fig. 2(d).
Upon removing the benzene molecules within a 2 Â 2 Â 2 set of unit cells, the packing was subjected to a calculation of solvent-accessible void space in Mercury (Macrae et al., 2006) with a probing radius of 1.2 Å . The results showed that the packing devoid of benzene comprises approximately 25.8% of 1134 Tan Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structures of the constituents of the asymmetric unit of (I), showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level. The molecules are each disposed about a centre of inversion with the unlabelled atoms in (a) related by the symmetry operation: 1 À x, 1 À y, Àz and those in (b) related by 1 À x, 1 À y, 1 À z.
the volume which is equivalent to 227.3 Å 3 of void space, as illustrated in Fig. 3.

Hirshfeld surface analysis and computational study
To gain a better understanding of the nature of the intermolecular interactions identified in (I), the overall structure of (I) as well as the individual 4 LH 2 and benzene molecules were subjected to a Hirshfeld surface analysis using Crystal Explorer 17 (Turner et al., 2017) based on the procedures as described in the literature . The Hirshfeld surface mapped over d norm map of 4 LH 2 displays several red spots, that range from intense to weak, which reflect the interactions identified in the crystal (Spackman & Jayatilaka, 2009). The intense red spots arise from amide-N-HÁ Á ÁN(pyridyl) hydrogen bonds while the diminutive spots originate from methylene-C7-H7BÁ Á Á(benzene) interactions, Fig. 4(a), with both indicative of contact distances shorter than the respective sum of the van der Waals radii. Reflecting the relatively long separation, the benzene-C11-H11Á Á Á(pyridyl) interaction is reflected as A plot of the solvent-accessible voids in the crystal of (I) upon removal of the solvent benzene molecules within a 2 Â 2 Â 2 set of unit cells.

Figure 4
The d norm maps within the range of À0.0567 to 0.9466 arbitrary units for the 4 LH 2 (left) and benzene (right) molecules: (a) highlighting the amide-N-HÁ Á ÁN(pyridyl) (intense red) and methylene-C7-H7BÁ Á Á(benzene) (faint red) contacts with the intensity relative to the contact distance and (b) highlighting the connections between molecules mediated by benzene-C11-H11Á Á Á(pyridyl) interactions. only a white spot as the contact distance is only just within the sum of van der Waals radii, as shown in Fig. 4(b).
The C-HÁ Á Á interactions were subjected to electrostatic potential mapping for verification purposes. The result shows that the methylene-C7-H7BÁ Á Á(benzene) contact is indeed electrostatic in nature as revealed by the distinct blue (i.e. electropositive) and red (i.e. electronegative) colour scheme on the surface of the contact points, Fig. 5(a). In contrast, the benzene-C11-H11Á Á Á(pyridyl) contact displays pale colouration around the contact zone suggesting that the interaction could be attributed to weak dispersion forces, Fig. 5(b).
The two-dimensional fingerprint plots were generated for overall (I) as well as its individual molecules to quantify the close contacts identified through the Hirshfeld surface analysis, see Fig. 6(a)-(e). As shown in the overall fingerprint plot in Fig. 6(a), (I) exhibits a bug-like profile with distinctive symmetrical spikes which are similar to those exhibited by the individual 4 LH 2 molecule, therefore indicating that the intermolecular interactions in (I) are mainly sustained by 4 LH 2 molecules. Decomposition of the overall fingerprint plots of (I) shows that the contacts are mainly dominated by HÁ Á ÁH 88 Å ) and other contacts (0.8%). Except for the HÁ Á ÁH contacts, to differing extents, the remaining major contacts are shorter than the corresponding sum of van der Waals radii for HÁ Á ÁC ($2.90 Å ), HÁ Á ÁO ($2.72 Å ) and HÁ Á ÁN ($2.75 Å ).
The individual 4 LH 2 molecule exhibits at similar distribution of the major contacts compared to overall (I). However, some distinctions are observed on the external and internal contacts upon further delineation of the corresponding decomposed fingerprint plots. While the distribution is rather symmetric in overall (I), for 4 LH 2 these are either inclined towards the external or internal contacts presumably due to interaction with the solvent benzene molecule. For instance, the HÁ Á ÁC/CÁ Á ÁH contact in the individual 4 LH 2 molecule comprises 9.9% (internal)-HÁ Á ÁC-(external) and 14.6% (internal)-CÁ Á ÁH-(external) contacts as compared to 12.0 and 14.6% for the equivalent contacts in overall (I), Fig. 6(c). Similar observations pertain for the HÁ Á ÁO/ OÁ Á ÁH and HÁ Á ÁN/ NÁ Á ÁH interactions, Fig. 6(d)-(e).

Computational chemistry study
The calculation of interaction energy was performed using Crystal Explorer 17 based on the procedures as described previously . As expected, the greatest interaction energy in the crystal of (I) is found for the amide-N-HÁ Á ÁN(pyridyl) contact having a total energy (E int ) of À38.1 kJ mol À1 , Table 2. This is followed by methylene-C7-H7BÁ Á Á(benzene) and benzene-C11-H11Á Á Á(pyridyl) contacts with a very similar E int values of À18.9 and À16.9 kJ mol À1 , respectively, despite the d norm contact distance being significantly greater for the latter. The calculation results reveal that the repulsion energy is greater in methylene-C7-H7BÁ Á Á(benzene) compared with the benzene-C11-H11Á Á Á(pyridyl) contact, which contributes to the slight variation in their E int values. In short, the N-HÁ Á ÁN interaction is stabilized largely by electrostatic forces while the C-HÁ Á Á interactions are stabilized largely by dispersion forces. Overall, the crystal of (I) is dominated by electrostatic forces that form a cross-shaped energy framework that encompasses the void space in the unit cell. This framework is further stabilized by dispersion forces that co-exist within the void owing to the weaker interactions between the solvent molecules with the host, Fig. 7

(a)-(c).
Calculations were also performed to compare the molecular packing similarity of (I) with the two polymorphic forms of 4 LH 2 available in the literature (Lee & Wang, 2007;Lee, 2010). Molecular clusters of (I), Form I and Form II containing 20 4 LH 2 molecules each were subjected to molecular packing analysis using Mercury (Macrae et al., 2006), with the geometric tolerances being set to 20% (i.e. only molecules within the 20% tolerance for both distances and angles were included in the calculation and molecules with a variation >20% were discarded); molecular inversions were enabled during calculation. The result shows that out of the 20 molecules in the cluster, only one 4 LH 2 molecule in each polymorph resembled the reference packing in (I) with an r.m.s. deviation of 0.587 and 0.403 Å , respectively, Fig. 8(a) and (b). The result clearly demonstrates the influence of solvent molecule upon the molecular packing in (I).

Figure 7
Energy framework of (I) as viewed down along the a-axis direction, showing the (a) electrostatic potential force, (b) dispersion force and (c) total energy diagrams. The cylindrical radii are proportional to the relative strength of the corresponding energies and they were adjusted to the same scale factor of 120 with a cut-off value of 5 kJ mol À1 within 2 Â 2 Â 2 unit cells.
Finally, and referring to Fig. 9, (I) and the two polymorphic forms of 4 LH 2 exhibit a close similarity in the distribution of molecular contacts as judged from the percentage contribution of the corresponding contacts on the Hirshfeld surface. The maximum variation in the distribution of HÁ Á ÁH, HÁ Á ÁC/ CÁ Á ÁH, HÁ Á ÁO/OÁ Á ÁH and HÁ Á ÁN/NÁ Á ÁH contacts ranged from 7.1, 4.9, 2.2 and 3.8%, respectively among the three crystals.

Database survey
As mentioned in the Chemical Context, there are two polymorphs available for 4 LH 2 (Lee & Wang, 2007;Lee, 2010). In Form I (Lee & Wang, 2007), two independent molecules comprise the asymmetric unit whereas in Form II (Lee, 2010), half a centrosymmetric molecule comprises the asymmetric unit. Selected geometric parameters for the polymorphs and (I) are given in Table 3. To a first approximation, the molecular structures present the same geometric features, i.e. a planar central region and an antiperiplanar relationship between the pyridyl rings. It is noted that the central C-C bond is relatively long, a consistent observation traced to the influence of electronegative carbonyl-O and amide-N substituents and confirmed by DFT calculations in the case of polymorphic 3 LH 2  and in the sulfur analogues of 3 LH 2 , i.e. (n-C 5 H 4 N)CH 2 N(H)C( S)C( S)N(H)CH 2 (C 5 H 4 N-n), for n = 2, 3 and 4 (Zukerman- Schpector et al., 2015). The similarity between the four molecules of 4 LH 2 in its polymorphs and benzene solvate are highlighted in Fig. 10.

Funding information
CrysAlis PRO (Rigaku OD, 2015); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012), OLEX2 ( Dolomanov et al., 2009), Mercury (Macrae et al., 2006), DIAMOND (Brandenburg, 2006) and QMol (Gans & Shalloway, 2001); software used to prepare material for publication: publCIF (Westrip, 2010). 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.