1,3-Bis[(4-nitrobenzylidene)aminooxy]propane

The complete molecule of title compound, C17H16N4O6, is generated by a crystallographic twofold axis. Within the molecule, the two benzene units are approximately perpendicular, making a dihedral angle of 85.91 (4)°. In the crystal, molecules are linked into a three-dimensional network by C—H⋯O hydrogen bonds and short O⋯O and N⋯O interactions, with distances of 2.998 (2) and 2.968 (3) Å, respectively.

The complete molecule of title compound, C 17 H 16 N 4 O 6 , is generated by a crystallographic twofold axis. Within the molecule, the two benzene units are approximately perpendicular, making a dihedral angle of 85.91 (4) . In the crystal, molecules are linked into a three-dimensional network by C-HÁ Á ÁO hydrogen bonds and short OÁ Á ÁO and NÁ Á ÁO interactions, with distances of 2.998 (2) and 2.968 (3) Å , respectively.

Comment
Schiff bases are among the most prevalent mixed-donor ligands in the field of coordination chemistry in which there has been growing interest, mainly because of their wide application in areas such as biochemistry (Niederhoffer et al., 1984), et al., 1990), medical imaging (Tisato et al., 1994), optical materials (Lacroix, 2001) and thin films (Sundari et al., 1997). Although most Schiff bases are stable in both solution and the solid state, C=N bonds often suffer exchange reactions (Koehler et al., 1964) as well as hydrolysis (Cordes & Jencks, 1962). Rate constants of oxime formation are smaller than those of imine formation and the equilibrium constants are larger by several orders (Akine et al., 2006). Hence, the title compound should be stable enough to resist the metathesis of the C=N bonds. Many bisdentate Schiff base compounds have been structurally characterized (Fun et al., 2008a;Fun et al., 2008b;Kia et al., 2009), but only a relatively small number of bisoxime compounds have had their X-ray structures reported (Shi et al., 2007;Ren et al., 2008). As an extension of our work (Ding et al., 2009;Dong et al., 2008a) on the structural characterization of bisoxime compounds, the title compound, is reported here (Fig. 1).
In the title compound all bond lengths are in normal ranges. The molecule sits on a crystallographic twofold passing through the central CH 2 group (symmetry code: -x, y, -z) such that there is 1/2 molecule per asymmetric unit. Within the molecule, the dihedral angle between the plane of oxime functional group and benzene ring is about 0.54 (3)° for O1-N1-C3 and the C4-C9 ring, and the two benzene rings are approximately perpendicular with a dihedral angle of 85.91 (4)°. In the crystal intermolecular C-H···O hydrogen bonds link the molecules into an infinite three-dimensional supramolecular network. The molecules are held together by intermolecular hydrogen bonds (Table 1) to form infinite zigzag chains along the a axis and wave-like layers parallel to the ac plane (Fig. 2). In addition, the interesting features of the crystal structure are short intermolecular O···O and N···O interactions that form infinite helical chains along the b axis as depicted in Fig. 3. The O···O and N···O distances of 2.998 (2) and 2.968 (3) Å, respectively, are significantly shorter than the sum of the van der Waals radii of the relevant atoms. Thus, the zigzag and helical chains form a three-dimensional supramolecular structure through the crosslinked hydrogen-bonded and short intermolecular O···O and N···O interactions (Fig. 4).

Experimental
The title compound was synthesized according to an analogous method reported earlier (Dong et al., 2008b Non-H atoms were refined anisotropically. H atoms were treated as riding atoms with distances C-H = 0.97 (CH 2 ) and 0.93 Å (CH), and U iso (H) = 1.2 U eq (C) and 1.5 U eq (O).Since it was not possible to determine the absolute configuration of the molecule from the experimental data the Friedel equivalents were merged prior to final refinement cycles.
Figures Fig. 1. The molecular structure of the title compound with atom numbering scheme. Displacement ellipsoids for non-hydrogen atoms are drawn at the 30% probability level.

Special details
Geometry. All e.s. 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.