(E)-2-[(2-Hydroxy-5-nitrophenyl)iminiomethyl]phenolate

In the title molecule, C13H10N2O4, the dihedral angle between the mean planes of the benzene and phenolate rings is 21.6 (4)°. The nitro O atoms are twisted slightly out of the plane of the ring to which the nitro group is attached [dihedral angle 8.4 (3)°]. The amine group forms an intramolecular hydrogen bond with both nearby O atoms. An extended π delocalization throughout the entire molecule exists producing a zwitterionic effect in this region of the molecule. The shortened C—O bond [1.2997 (15) Å] in concert with the slightly longer C—OH bond [1.3310 (16) Å] provide evidence for this effect. The crystal packing is influenced by strong intermolecular O—H⋯O hydrogen bonding. As a result, molecules are linked into an infinite zigzag chain running along the b axis. A MOPAC PM3 calculation provides support to these observations.

In the title molecule, C 13 H 10 N 2 O 4 , the dihedral angle between the mean planes of the benzene and phenolate rings is 21.6 (4) . The nitro O atoms are twisted slightly out of the plane of the ring to which the nitro group is attached [dihedral angle 8.4 (3) ]. The amine group forms an intramolecular hydrogen bond with both nearby O atoms. An extended delocalization throughout the entire molecule exists producing a zwitterionic effect in this region of the molecule. The shortened C-O bond [1.2997 (15) Å ] in concert with the slightly longer C-OH bond [1.3310 (16) Å ] provide evidence for this effect. The crystal packing is influenced by strong intermolecular O-HÁ Á ÁO hydrogen bonding. As a result, molecules are linked into an infinite zigzag chain running along the b axis. A MOPAC PM3 calculation provides support to these observations.

Comment
Schiff bases have extensive application in industry. Much interest in these compounds and their complexes is due to their anti-tumor activities. The boronate complex derivative of the title compound (Barba et al. 2001) displays it in the phenolimine form, which can undergo an imine Diels-Alder reaction to produce a 3,4-dihydroquinoline (Rodriguez et al., 2005).
Compounds of this type can also be used as anion sensors in acetonitrile (Hijji et al., 2008) and tend to exist in the keto-amine form, which is generally favored over the phenol-imine form in the solid state. While the keto-amine tautomer is commonly produced in these complexes (Ersanlı et al., 2003;Odabaşoğlu et al., 2006;Elerman et al., 1995;Jasinski et al., 2007), the title compound adapts a phenol-iminio tautomer as the stable form which is confirmed by the X-ray data and in agreement with that found via NMR data in DMSO solution. The stability of this form may be enhanced by the electronic effect of the nitro group. The dark color in the solid state is generally an indication of increased conjugation, while the yellow colored solutions, as in acetonitrile or anhydrous DMSO, may be due to conversion to the phenol-iminio tautomer.
The title molecule, C 13 H 10 N 2 O 4 , consists of a 2-hydroxy-5-nitrophenyliminio group and a phenolate group bonded to a methylene carbon atom with both of the planar six-membered rings twisted from the plane of the molecule. forms an intramolecular hydrogen bond with each of the nearby oxygen atoms (O1 and O4) (see Fig. 1 and Table 1). There appears to be an extended π delocalization effect throughout the entire molecule producing a zwitterionic effect in this region of the molecule similar to that seen in a close structurally related dinitro compound (Hijji et al., 2009). The shortened C9-O4 bond (1.2997 (15) Å in concert with the slightly longer C1-O1 bond (1.3310 (16) Å) provide structural evidence of this effect in a similar fashion.
After a MOPAC PM3 calculation [Parameterized Model 3 approximation together with the Hartree-Fock closed-shell (restricted) wavefunction was used and minimizations were teminnated at an r.m.s. gradient of less than 0.01 kJ mol -1 Å -1 ] of the zwitterionic form with WebMO Pro (Schmidt & Polik, 2007), the mean planes between the phenyl and phenolate supplementary materials sup-2 rings changes to 3.7 (7)°, producing a significantly less twisted, nearly planar, molecule than that observed in the crystalline environment. It is apparent that the extensive hydrogen bonding, π-π stacking and π-ring intermolecular interactions significantly influence crystal packing with this molecule.
Experimental 2-Amino-4-nitrophenol (0.15 g, 1 mmol) and salicylaldehyde (0.12 g, 1 mmol) were mixed neat in a loosely capped vial, forming a light orange solid. The mixture was heated at full power in a conventional spacemaker II microwave oven for two minutes forming a deep orange product. The reaction mixture was recrystallized from a 1:1 mixture (v:v) of methanol and diethyl ether affording 0.18 g, 67% yield of the title compound as dark purple crystals. The physical data is in agreement with literature (Barba et al., 2001)     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 Rfactors(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.  (7) 0.0050 (7) −0.0038 (6) C12 0.0551 (9) 0.0401 (9) 0.0390 (7) 0.0018 (7) 0.0113 (6) −0.0043 (6) C13 0.0434 (7) 0.0347 (8) 0.0382 (7) 0.0007 (6) 0.0156 (6) 0.0002 (5) Geometric parameters (Å, °)