Crystal structure of (E)-2,6-di-tert-butyl-4-{[2-(2,4-dinitrophenyl)hydrazinylidene]methyl}phenol

The title compound was obtained from the condensation reaction of 3,5-di-tert-butyl-4-hydroxybenzaldehyde and 2,4-dinitrophenylhydrazine. The essential part (including all the non-hydrogen atoms except two methyl carbons) of the molecule lies on a mirror plane, which bisects the t-butyl groups.


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). 2,4-Dinitrophenylhydrazine is frequently used as a reagent for the characterization of aldehydes and ketones (Furniss et al., 1999). Its derivatives are widely used as dyes (Guillaumont & Nakamura, 2000). They are also found to have versatile coordinating abilities towards different metal ions (Raj & Kurup, 2007). 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 coordination compounds (Faizi & Prisyazhnaya, 2015). We report herein on the synthesis and crystal structure of the title Schiff base compound with a sterically hindered phenol group.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. All the non-hydrogen atoms except C16 and C19 lie on a crystallographic mirror plane at y = 1 4 : the complete tert-butyl groups are generated by mirror symmetry. The conformation of the C7 N1 bond of this Schiff base compound is E, and there is an intramolecular N2-H2Á Á ÁO1 hydrogen bond present, forming an S(6) ring motif ( Fig. 1 and Table 1). The N1-N2 bond length is 1.385 (6) Å and the N1 C7 bond length is 1.278 (7) Å . The bond distances and angles in the title compound are comparable to those found in a closely related structure (Fun et al., 2013).

Supramolecular features
In the crystal, molecules are linked by O-HÁ Á ÁO hydrogen bonds, forming zigzag chains propagating along the a-axis direction ( Fig. 2 and Table 1). There are no other significant intermolecular contacts present.

Database survey
There are very few examples of similar compounds in the literature. To the best of our knowledge, the recent report (Bhardwaj & Singh, 2015) of a similar compound with an hydroxy group in the ortho position, capable of visual and reversible sensing of cyanide in DMSO solution, has not been characterized crystallographically. A search of the Cambridge Structural Database (CSD, Version 5.37, update May 2016; Groom et al., 2016) revealed the structure of one very similar compound, viz. 1-(2,4-dinitrophenyl)-2-[(E)-2,4,5-trimethoxybenzylidene]hydrazine (II) (Fun et al., 2013), in which the 4-phenol group in the title compound is replaced by a trimethoxy group. In (II), the dihedral angle between the two benzene rings is 3.15 (11) , compared to 0 in the title compound, owing to the mirror symmetry.

Synthesis and crystallization
A mixture of 3,5-di-tert-butyl-4-hydroxybenzaldehyde 0.100 g (0.427 mmol) and 2,4-dinitrophenylhydrazine (0.085 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 high yield 170 g (96%). Yellow blocklike crystals of the title compound (m.p. 372-373 K) were obtained by slow evaporation of a solution in dichloromethane and ethanol (5:1 v/v).

Figure 1
The molecular structure of the title compound, with atom labelling [symmetry code: (i) x, Ày + 1 2 , z]. Displacement ellipsoids are drawn at the 30% probability level. The intramolecular N-HÁ Á ÁO hydrogen bond is shown as a dashed line (see Table 1) Table 1 Hydrogen-bond geometry (Å , ).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The OH H atom was located in a difference Fourier map and refined with a distance restraint of 0.82 (2) Å with U iso (H) = 1.5U eq (O). The NH and C-bound H atoms were included in calculated positions and allowed to ride on the parent atoms: N-H = 0.86 Å , C-H = 0.93-0.96 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (N,C) for other H atoms.   SHELXL2016/4 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009).