2-{(E)-[(2-Methyl-3-nitrophenyl)imino]methyl}-4-nitrophenol

The title compound, C14H11N3O5, is a Schiff base that adopts the enol–imine tautomeric form in the solid state. The dihedral angle between the aromatic rings is 37.4 (3)° and the dihedral angles between the nitro groups and their attached rings are 4.0 (6) and 46.2 (8)°. The molecular structure is stabilized by an intramolecular O—H⋯N hydrogen bond, which generates an S(6) ring motif. In the crystal, molecules are linked by C—H⋯O interactions, forming a two-dimensional network parallel to the bc plane.

Depending on the tautomers, two types of intramolecular hydrogen bonds are observed in Schiff bases: O-H···N in enolimine and N-H···O in keto-amine tautomers (Tanak et al., 2010).
In the title crystal structure (Fig. 1), the molecules of the title compound are not planar. The dihedral angle between the aromatic ring systems is 37.4 (3)°. The imino group is coplanar with the hydroxyphenyl ring as it can be shown by the C2 -C1-C7-N2 torsion angle [-1.6 (8)°]. The C-O and C=N bond lengths confirm the enol-imine form of the title compound. The length of the C7=N2 double bond is 1.269 (7) Å It is consistent with the related structure (Tanak, 2011).
It is also known that Schiff bases may exhibit thermochromism depending on the planarity or non-planarity of the molecule, respectively (Moustakali-Mavridis et al., 1978).
The molecular structure is stabilized by an intramolecular hydrogen bond. The phenol H atom forms a strong intramolecular hydrogen bond with the imine N atom ( Fig. 1) generating an S(6) ring motif (Bernstein et al., 1995). In the crystal structure, molecules are linked together by intermolecular C-H···O interactions (Fig. 2).

Refinement
All H atoms were positioned geometrically and treated using a riding model, fixing the bond lengths at 0.82 Å for OH, at 0.93 Å for aromatic CH, at 0.96 Å for CH 3 . The displacement parameters of the H atoms were constrained as U iso (H)= 1.2U eq (1.5U eq for methyl) of the parent atom.   The crystal packing of the title compound. Hydrogen bonds are shown as dashed lines. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.23 e Å −3 Δρ min = −0.25 e Å −3

Special details
Experimental. 215 frames, detector distance = 130 mm Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.