Crystal structure, Hirshfeld surface analysis and HOMO–LUMO analysis of (E)-N′-(3-hydroxy-4-methoxybenzylidene)nicotinohydrazide monohydrate

The title Schiff base compound displays an E configuration with respect to the C=N double bond. The pyridine and benzene rings subtend a dihedral angle of 29.63 (7)°. In the crystal, the molecules are linked by N—H⋯O, C—H⋯O, O—H⋯O and O—H⋯N hydrogen-bonding interactions.


Chemical context
Schiff bases are nitrogen-containing compounds that were first obtained by the condensation reaction of aromatic amines and aldehydes (Schiff, 1864). A wide range of these compounds, with the general formula RHC NR 1 (R and R 1 can be alkyl, aryl, cycloalkyl or heterocyclic groups) have been synthesized. Schiff bases are of great importance in the field of coordination chemistry because they are able to form stable complexes with metal ions (Souza et al., 1985). The chemical and biological significance of Schiff bases can be attributed to the presence of a lone electron pair in the sp 2 -hybridized orbital of the nitrogen atom of the azomethine group (Singh et al., 1975). These compounds are used in the fields of organic synthesis, chemical catalysis, medicine and pharmacy, as well as other new technologies (Tanaka et al., 2010). Schiff bases are also used as probes for investigating the structure of DNA (Tiwari et al., 2011) and have gained special attention in pharmacophore research and in the development of several bioactive lead molecules (Muralisankar et al., 2016). Schiff bases showing photochromic and thermochromic properties have been used in information storage, electronic display systems, optical switching devices and ophthalmic glasses (Amimoto et al., 2005). As a further contribution to this field of research, we report herein the crystal structure of the title compound, (E)-N 0 -(3-hydroxy-4-methoxybenzylidene)nicotinohydrazide monohydrate. ISSN 2056-9890

Structural commentary
The asymmetric unit of the title compound ( Fig. 1) consists of one independent Schiff base molecule displaying a trans configuration with respect to the C N bond and a water molecule. All the bond lengths are within the normal ranges (Allen et al., 1987). The C7 N3 bond length of 1.274 (2) Å is consistent with a double-bond character. The C6-N2 and N2-N3 bond lengths of 1.343 (2) and 1.3866 (16) Å , respectively, are comparable to those observed in related compounds (Sivajeyanthi et al., 2017;Balasubramani et al., 2018). The O1/ C6/N2/N3/C7 core is almost planar (r.m.s. deviation = 0.022 Å ) and forms dihedral angles of 20.75 (7) and 8.93 (5) , respectively, with the pyridine and benzene rings.

Supramolecular features
In the crystal of the title compound (Fig. 2), the water molecule interacts with three neighbouring nicotinohydrazide molecules with the O4 water oxygen atom acting as a hydrogen acceptor through N2-H2NÁ Á ÁO4 and C2-H2Á Á ÁO4 hydrogen bonds (Table 1), and both water H atoms acting as bifurcated donors to form rings of R 1 2 (5) graph-set motif. The nicotinohydrazide molecules are further linked by O-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds to form a threedimensional network.

Frontier molecular orbitals
The HOMO (highest occupied molecular orbital) acts as an electron donor and LUMO (lowest occupied molecular orbital) acts as an electron acceptor. If the HOMO-LUMO energy gap is small, then the molecule is highly polarizable and has high chemical reactivity. The energy levels for the title compound were computed by DFT-B3LYP/6-311G++(d,p) method (Sivajeyanthi et al., 2017). The energy levels, energy gaps, chemical hardness, chemical potential, electronegativity and electrophilicity index are given in Table 2. As shown in Fig. 5, the frontier molecular orbital LUMO is located over the whole of the molecule. The energy gap of the molecule clearly shows the charge-transfer interaction involving donor and acceptor groups. If the HOMO-LUMO energy gap is small, then the molecule is defined as soft, i.e. it is highly polarizable and has high chemical reactivity, whereas if the energy gap is large the molecule can be defined as hard. Therefore from Two-dimensional fingerprint plots for the title compound and relative contributions of the atom pairs to the Hirshfeld surface.

Figure 5
Molecular orbital energy levels of the title compound.

Figure 3
Hirshfeld surfaces of the title compound mapped over d norm .

Synthesis and crystallization
The title compound was synthesized by the reaction of a 1:1 molar ratio mixture of a hot ethanolic solution (20 ml) of nicotinohydrazide (0.137 mg) and a hot ethanolic solution of 3-hydroxy-4-methoxy benzaldehyde (0.152 mg). After refluxing for 8 h, the solution was then cooled and kept at room temperature to precipitate. Colourless block-shaped crystals suitable for X-ray analysis were obtained by slow evaporation of a 10 ml dimethyl sulfoxide/water (1:1 v/v) solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3

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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.