2-[(E)-(2-Morpholinoethyl)iminiomethyl]-4-nitro-1-oxocyclohexadienide

The molecule of the title compound, C13H17N3O4, exists as a zwitterion, with the H atom of the phenol group being transferred to the imine N atom. The C=O, CAr—CAr and C—N bond lengths are in agreement with the oxocyclohexadienide–iminium zwitterionic form. A strong intramolecular N+—H⋯O hydrogen bond generates an S(6) ring motif. The morpholine ring adopts a chair conformation. In the crystal, molecules are linked into centrosymmetric dimers by intermolecular N—H⋯O hydrogen bonds. In addition, C—H⋯O hydrogen bonds and very weak C—H⋯π interactions are observed.


Comment
Schiff bases have been extensively used as ligands in the field of coordination chemistry (Calligaris et al., 1972). Schiff base compounds can be classified by their photochromic and thermochromic characteristics (Cohen et al. 1964). These properties result from proton transfer from the hydroxyl O atom to the imine N atom (Hadjoudis et al., 1987). Schiff bases exhibit two well-known tautomeric forms viz. OH and NH tautomers, and they also exist in zwitterionic form (Karabıyık et al., 2008). Our investigations show that compund (I) exists in a zwitterionic form.
The molecular structure of (I) is shown in Fig.1 Petek et al., 2006]. The bonds lengths in the C1-C6 benzene ring show clear alternation in the delocalized C2-C5 portion. The nitro group is tilted out of the mean plane of adjacent ring by 7.02 (3)°, whereas the C3-N1 distance of 1.4398 (15) Å is in the characteristic range suggesting limited conjugation with the ring. Thus, the whole geometry is in the agreement with the predominace of the oxocyclohexadienide-iminum zwitterion bonding scheme (see scheme) (Krygowski & Stepien, 2005;Santos-Contreras et al., 2009), in close agreement with the reported configurations of p-nitrophenolates of alkali metal cations (Butt et al., 1987). The morpholine ring adopts a chair conformation. An intramolecular N-H···O hydrogen bond generates an S(6) ring motif (Bernstein et al., 1995).

Refinement
Atom H1 was located in a difference map and refined freely. All other H atoms were placed in calculated positions and constrained to ride on their parent atoms, with C-H = 0.93-0.97 Å and U iso (H) = 1.2 U eq (C). Fig. 1. The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.

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.
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 > 2sigma(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.