Crystal structure and Hirshfeld surface analysis of (E)-4-{[2-(4-hydroxybenzoyl)hydrazin-1-ylidene]methyl}pyridin-1-ium nitrate

The title aroyl hydrazone Schiff base salt, consists of one molecular cation in the keto tautomeric form, adopting an E configuration with respect to the azomethine bond, and one nitrate anion.


Chemical context
Hydrazone Schiff bases and their coordination compounds have gained importance recently because of their application as models in biological, analytical and antimicrobial systems, and also due to their anticancer, antibacterial as well as antifungal activities (Ruben et al., 2003). Aroyl hydrazones are a class of versatile ligands, capable of generating various molecular architectures and coordination polyhedra (Ruben et al., 2003;Uppadine Gisselbrecht & Lehn, 2004;Wood et al., 2004). Aroyl hydrazones are obtainable through hydrazide-ketone/aldehyde condensation, and they exhibit flexible metal-chelating capabilities through their keto-enol tautomerism and possible reversible deprotonation. The empty N,O-donor chelating pockets of aroyl hydrazones that are incorporated into frameworks can potentially make them amenable to post-synthetic metalation (Evans et al., 2014). The structure determination of the title aroyl hydrazone Schiff base salt was undertaken in order to compare the results obtained with those reported previously. In this context, we synthesized the title compound, (E)-4-{[2-(4-hydroxybenzoyl)hydrazin-1-ylidene]methyl}pyridin-1-ium nitrate, and report herein on its crystal and molecular structures along with the Hirshfeld surface analysis. ISSN 2056-9890

Figure 1
The molecular structure of the title aroyl hydrazone Schiff base salt, with the atom-numbering scheme. The N-H Pym Á Á ÁO N (Pym = pyridinium and N = nitrate) hydrogen bond (see Table 1) is shown as a dashed line. Displacement ellipsoids are drawn at the 50% probability level.

Hirshfeld surface analysis
In order to visualize the intermolecular interactions in the crystal of the title aroyl hydrazone Schiff base salt, a Hirshfeld surface (HS) analysis (Hirshfeld, 1977;Spackman & Jayatilaka, 2009) was carried out by using Crystal Explorer 17.5 (Turner et al., 2017). In the HS plotted over d norm (Fig. 4), the white surfaces indicate contacts with distances equal to the sum of van der Waals radii, and the red and blue colours indicate distances shorter (in close contact) or longer (distinct contact) than the van der Waals radii, respectively (Venkatesan et al., 2016). The bright-red spots appearing near N-O4, N-O5 and hydrogen atoms H1A, H1B and H3A indicate their role as the respective donors and acceptors in the dominant O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds (Spackman et al., 2008;Jayatilaka et al., 2005). The shapeindex of the HS is a tool to visualize thestacking by the presence of adjacent red and blue triangles; if there are no adjacent red and/or blue triangles, then there are nointeractions. Fig. 5  View of the three-dimensional Hirshfeld surface of the title aroyl hydrazone Schiff base salt plotted over d norm in the range À0.6521 to 1.7041 a.u.

Figure 5
Hirshfeld surface of the title aroyl hydrazone Schiff base salt plotted over shape-index.

Figure 3
A view along the b axis of the crystal packing of the title compound. The hydrogen bonds are shown as dashed lines (see Table 1), and only H atoms involved in these interactions have been included.

Figure 2
Part of the crystal structure, viewed normal to (101). The O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds (see Table 1) are shown as dashed lines, and C-bound H atoms have been omitted for clarity.
HÁ Á ÁH contacts (Fig. 6c), the 19.3% contribution to the overall crystal packing is reflected as widely scattered points of high density due to the large hydrogen content of the molecule. The single spike in the centre at d e = d i = 1.2 Å in Fig. 5c is due to the short interatomic H Á Á Á H contacts (Table 2). In the absence of C-HÁ Á Á interactions in the crystal, the pair of characteristic wings resulting in the fingerprint plot delineated into HÁ Á ÁC/CÁ Á ÁH contacts with 14.5% contribution to the HS, Fig. 6d, and the pair of thin edges at d e + d i $1.93 Å result from short interatomic HÁ Á ÁC/CÁ Á ÁH contacts ( Table 2). The HÁ Á ÁN/NÁ Á ÁH contacts in the structure with 7.9% contribution to the HS has a symmetrical distribution of points, Fig. 5e, with the tips at d e + d i $1.52 Å arising from the short interatomic HÁ Á ÁN/NÁ Á ÁH contacts listed in Table 2. The CÁ Á ÁC contacts assigned to short interatomic CÁ Á ÁC contacts with 6.0% contribution to the HS appear as an arrow-shaped distribution of points in Fig. 6f, with the vertex at d e = d i $1.65 Å . Finally, the CÁ Á ÁN/NÁ Á ÁC (Fig. 6g) and CÁ Á ÁO/OÁ Á ÁC (Fig. 6h) contacts in the structure with 3.4% and 1.9% contributions to the HS have nearly symmetrical distributions of points, with the scattered points of low densities.
The Hirshfeld surface analysis confirms the importance of H-atom contacts in establishing the packing. The large number of HÁ Á ÁO/OÁ Á ÁH, HÁ Á ÁH and HÁ Á ÁC/CÁ Á ÁH interactions suggest that van der Waals interactions and hydrogen bonding play the major roles in the crystal packing (Hathwar et al., 2015).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms of the OH and NH groups were located in a difference-Fourier map and refined freely. The C-bound H atoms were positioned geometrically with C-H = 0.93 Å , and refined as riding with U iso (H) = 1.2U eq (C). The highest residual electron density was found 2.48 Å from atom H1. Computer programs: APEX2 and SAINT (Bruker, 2012), SHELXS97 and SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows and WinGX (Farrugia, 2012), Mercury (Macrae et al., 2008) and PLATON (Spek, 2015