Crystal structure and Hirshfeld surface analysis of (Z)-6-[(2-hydroxy-4-methylanilino)methylidene]-4-methylcyclohexa-2,4-dien-1-one

In the crystal structure of the title compound, the molecules are linked by pairs of O—H⋯O hydrogen bonds, forming inversion dimers with an (18) ring motif.

The title compound, C 15 H 15 NO 2 , is a Schiff base that exists in the keto-enamine tautomeric form and adopts a Z configuration. The molecule is almost planar, with the two phenyl rings twisted relative to each other by 9.60 (18) . There is an intramolecular N-HÁ Á ÁO hydrogen bond present forming an S(6) ring motif. In the crystal, pairs of O-HÁ Á ÁO hydrogen bonds link adjacent molecules into inversion dimers with an R 2 2 (18) ring motif. The dimers are linked by very weak interactions, forming layers parallel to (201). Hirshfeld surface analysis, two-dimensional fingerprint plots and the molecular electrostatic potential surfaces were used to analyse the intermolecular interactions, indicating that the most important contributions for the crystal packing are from HÁ Á ÁH (55.2%), CÁ Á ÁH/HÁ Á ÁC (22.3%) and OÁ Á ÁH/HÁ Á ÁO (13.6%) interactions.

Chemical context
Schiff bases contain the azomethine moiety (-RCH N-R 0 ) and are prepared by condensation reactions between amines and active carbonyl compounds (Schiff, 1864). In the majority of cases, the synthesis involves an aromatic amine and an aldehyde (Schiff et al., 1881). Schiff bases are very important for production of chemical specialties such as pharmaceuticals including antibiotics, and of antiallergic, antitumor, antifungal, antibacterial, antimalarial or antiviral drugs. Schiff bases are also employed as catalyst carriers (Grigoras et al., 2001), thermo-stable materials (Vančo et al., 2004), metal-cation complexing agents or in biological systems (Taggi et al., 2002). Schiff bases containing phenol indicate two possible tautomeric forms, viz. phenol-imine and keto-enamine.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The asymmetric unit comprises one molecule that adopts the keto-enamine tautomeric form, i.e. the H atom is located at the amine functionality (N1). The molecule is almost planar, with an r.m.s. deviation of 0.1061 Å for the complete molecule except the H atoms [largest deviation 0.176 (3) Å for C8]. The two phenyl rings (C1-C6 and C9-C14) are inclined by 9.60 (18) . The C1-O1 bond length [1.356 (3) Å ] to the hydroxy group is in the normal range, while the C14 O2 bond length is comparatively elongated [1.302 (4) Å ] due to the involvement of the carbonyl O atom in an intramolecular N-HÁ Á ÁO hydrogen bond, forming an S(6) ring motif. The C6-N1 and C8 N1 bond lengths are 1.404 (4) and 1.310 (4) Å , respectively. Overall, the bond lengths in the title structure compare well with those of other keto-enamine tautomers known from the literature (see: Database Survey).   Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view of the crystal packing of the title compound. Dashed lines denote the intermolecular O-HÁ Á ÁO hydrogen bonds (Table 1) forming an inversion dimer with an R 2 2 (18) ring motif.

Figure 3
The crystal packing of the title compound.

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 40% probability level. Dashed lines denote the intramolecular N-HÁ Á ÁO hydrogen bond (Table 1) forming an S(6) ring motif.

Hirshfeld surface analysis
A Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed with Crystal-Explorer17 (Turner et al., 2017) for specifying the intermolecular interactions in the title compound. Fig. 4a illustrates the Hirshfeld surface mapped over d norm . The red spots highlight the interatomic contacts included in O-HÁ Á ÁO hydrogen bonding. The three-dimensional d norm surfaces were plotted with a colour scale of À0.7370 to 1.3366 Å with a standard (high) surface resolution. Fig. 4b shows the molecular electrostatic potential plotted over the three-dimensional Hirshfeld surface using the STO-3G basis set in the range À0.0975 to 0.2197 a.u. within the Hartree-Fock level of theory. The O-HÁ Á ÁO hydrogen-bond donors and acceptors are shown as blue and red areas around the atoms related with positive (hydrogen-bond donors) and negative (hydrogenbond acceptors) electrostatic potentials, respectively. Fig. 5a shows the two-dimensional fingerprint of the sum of all contacts contributing to the Hirshfeld surface indicated in normal mode. Fig. 5b illustrates the two-dimensional fingerprint of (d i , d e ) points related to HÁ Á ÁH contacts that represent a 55.2% contribution in the title structure. In Fig. 5c, two symmetrical wings on the left and right sides indicate CÁ Á ÁH/ HÁ Á ÁC interactions with a contribution of 22.3%. Furthermore, there are OÁ Á ÁH/HÁ Á ÁO (13.6%; Fig. 5d), CÁ Á ÁC (4.9%) and CÁ Á ÁN/NÁ Á ÁC (2.6%) contacts. Fig. 6 shows the molecular electrostatic potential surface generated using the STO-3G basis set in the range À0.050 to 0.050 a.u. within the Hartree-Fock level of theory. The blue and red regions are associated with positive and negative molecular electrostatic potentials and represent the donor and acceptor groups, respectively, in hydrogen bonding.   Two-dimensional fingerprint plots for the title compound giving the relative contribution of atom pairs to the Hirshfeld surface.

Figure 6
A view of the molecular electrostatic potential, in the range À0.0500 to 0.0500 a.u. calculated using the STO-3 G basis set in the range À0.050 to 0.050 a.u. within the Hartree-Fock level of theory.