Crystal structure and DFT study of (E)-2,6-di-tert-butyl-4-{[2-(pyridin-2-yl)hydrazin-1-ylidene)methyl}phenol

The title Schiff base was synthesized via the condensation reaction of 3,5-di-tert-butyl-4-hydroxybenzaldehyde and 2-hydrazinylpyridine and crystallized with a single molecule in the asymmetric unit. The conformation about the C=N bond is E. In the crystal, N—H⋯N hydrogen bonds connect pairs of molecules into dimers. In addition, weak C—H⋯O hydrogen bonds and C—H⋯π interactions are observed.


Chemical context
Sterically hindered phenol anti-oxidants are widely used in polymers and lubricants. They can protect polymers by increasing both their process stability and their long-term stability against oxidative degradation (Yamazaki & Seguchi, 1997;Silin et al., 1999). Hydrazones and Schiff bases have attracted much attention for their excellent biological properties, especially for their potential pharmacological and antitumor properties (Kü çü kgü zel et al., 2006;Khattab, 2005;Karthikeyan et al., 2006;Okabe et al., 1993). Furthermore, 3,5-di-tert-butyl-2-hydroxybenzaldehyde-derived Schiff bases shows proton tautomerization, which plays an important role in many fields of chemistry and biochemistry. The tautomerization in salicylideneanilines has been the subject of particular interest because it is closely related to thermochromism and photochromism. While salicylideneanilines are widely used as precursor compounds for the design of various type new metal complexes, they are also convenient model compounds for studying theoretical aspects of coordination chemistry and photochemistry, as well as for designing molecular architectures by means of molecular motifs capable of hydrogen-bond formation. The present work is a part of an ongoing structural study of Schiff bases and their utilization in the synthesis of quinoxaline derivatives (Faizi et al., 2016a), fluorescence sensors (Faizi et al., 2016b) and azoimine compounds (Faizi et al., 2015(Faizi et al., , 2017. We report herein on the synthesis and crystal structure and DFT computational calculation of the new title Schiff base compound with a sterically hindered phenol, (I). The results of calculations by density functional theory (DFT) on (I) carried out at the B3LYP/6-311 G(d,p) level are compared with the experimentally determined molecular structure in the solid state.

Structural commentary
The molecular structure of (I), shown in Fig. 1, is not planar, with the dihedral angle between the pyridyl and tert-butyl substituted benzene rings being 18.19 (3) . The N-N and N-C bond lengths are of 1.396 (7) and 1.253 (7) Å , respectively, indicate single-and double-bond character for these bonds. The C1-O1 bond length of 1.370 (6) Å indicates single-bond character. The conformation about the C15 N1 bond is E with an N2-N1-C15-C4 torsion angle of 177.9 (5) . Bond distances for (I) are comparable to those found in closely related structures (Fun et al., 2013). It appears that the hydroxy group is prevented from forming a hydrogen bond because of steric hindrance by the tert-butyl groups.

DFT study
The DFT quantum-chemical calculations were performed at the B3LYP/6-311 G(d,p) level (Becke, 1993) as implemented in GAUSSIAN09 (Frisch et al., 2009). DFT structure optimization of (I) was performed starting from the X-ray geometry and the values compared with experimental values (see Table 2). From these results we can conclude that basis set 6-311 G(d,p) is well suited in its approach to the experimental data.
The DFT study of (I) shows that the HOMO and LUMO are localized in the plane extending from the whole pyridine ring to the phenol ring. The electron distribution of the HOMO-1, HOMO, LUMO and the LUMO+1 energy levels are shown in Fig. 4. The HOMO molecular orbital exhibits both and character, whereas HOMO-1 is dominated byorbital density. The LUMO is mainly composed of -density while LUMO+1 has both and electronic density. The HOMO-LUMO gap was found to be 0.1562 a.u. and the frontier molecular orbital energies, E HOMO and E LUMO are À0.201 and À0.045 a.u., respectively.  Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
Part of the structure exhibiting weak C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions (shown as dashed lines) along a axis.

Figure 1
The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 40% probability level.

Database survey
There are very few examples of similar compounds in the literature. To the best of our knowledge, the similar compound synthesized by (Cuadro et al., 1998) for biological evaluation of 5-lipoxygenase inhibitors has not been structurally characterized. Two very similar compounds have been reported, one synthesized from 2-hydrazinylpyridine and 4-tert-butyl-2,6-diformylphenol (Li et al., 2013) as a fluorescent chemosensor for Zn II and applications in live cell imaging. The other compound is the Schiff base 2,4-di-tert-butyl-6-{[2-(pyridin-2-yl)hydrazono]methyl}phenol used for stabilization of oxidovanadium(IV) (Kundu et al., 2013).A search of the Cambridge Structural Database (CSD, Version 5.37, update May 2016; Groom et al., 2016) shows that these compounds have not been characterized by X-ray diffraction.

Synthesis and crystallization
A mixture of 3,5-di-tert-butyl-4-hydroxybenzaldehyde 0.100 g (0.427 mmol) and 2-hydrazinylpyridine 0.046 g (0.427 mmol) in methanol was refluxed for 3 h in the presence of a catalytic amount of glacial acetic acid. After cooling, the red-coloured precipitate was washed with hot methanol several times, and then dried, giving a red-coloured shiny crystalline compound in 86% yield (0.120 g). Red block-like crystals of the title compound were obtained by slow evaporation of a solution in dichloromethane and ethanol (5:1 v/v). Electron distribution of the HOMO-1, HOMO, LUMO and LUMO+1 energy levels for the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All C-bound hydrogen atoms were included in calculated positions with C-H = 0.93 (aromatic) or 0.96 Å (methylene) and allowed to ride, with U iso (H) = 1.2U eq (C). The N-bound H atom was located in a difference-Fourier map but was also allowed to ride in the refinement with N-H = 0.86 Å and U iso (H) = 1.2U eq (N). SHELXL2016 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009).

(E)-2,6-Di-tert-butyl-4-{[2-(pyridin-2-yl)hydrazin-1-ylidene)methyl}phenol
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.