Bis(2-hydroxybenzaldehyde oxime) O,O′-butane-1,4-diyldicarbonyl ether

The molecule of the title compound, C20H20N2O6, lies across a crystallographic inversion centre, the asymmetric unit comprising one half-molecule. An intramolecular O—H⋯N hydrogen bond generates a six-membered ring, producing an S(6) ring motif. Pairs of intermolecular C—H⋯O hydrogen bonds link neighbouring molecules into a layer with R 2 2(38) ring motif. The crystal structure is further stabilized by the intermolecular C—H⋯π interactions.

The molecule of the title compound, C 20 H 20 N 2 O 6 , lies across a crystallographic inversion centre, the asymmetric unit comprising one half-molecule. An intramolecular O-HÁ Á ÁN hydrogen bond generates a six-membered ring, producing an S(6) ring motif. Pairs of intermolecular C-HÁ Á ÁO hydrogen bonds link neighbouring molecules into a layer with R 2 2 (38) ring motif. The crystal structure is further stabilized by the intermolecular C-HÁ Á Á interactions.
Data collection: X-AREA (Stoe & Cie, 2005); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 Schiff base compounds are some of the most important stereochemical models in transition metal coordination chemistry, with their ease of preparation and structural variations (Granovski et al., 1993). In continuation of our works on the synthesis and structural characterization of Schiff base ligands here we report the structure of the title compound.
The asymmetric unit of the title compound, Fig. 1, lies across a crystallographic inversion centre. Intramolecular O-H···N hydrogen bond generates a six-membered ring, producing an S(6) ring motif (Bernstein et al., 1995). Pairs of intermolecular C-H···O hydrogen bonds link neighbouring molecules into a layer with R 2 2 (38) ring motif (Fig. 2). The crystal structure is further stabilized by the intermolecular C-H···π interactions [Cg1 is the centroid of the C1-C6 benzene ring] (Table 1).

S2. Experimental
An ethyl acetate solution (40 ml) of salicylaldoxime (2 mmol, 768 mg) was treated with butanedicarboxylic acid chloride (1 mmol, 183 mg) at -5 0 C. The mixture was stirred for 30 min and then filtered and the resulting white powder dried under air (Hosseini Sarvari, 2003). Single crystals suitable for X-ray diffraction were obtained from an ethanol solution.

S3. Refinement
The O-bound and C7 bound hydrogen atoms were located from the difference Fourier map and refined freely. The rest of the hydrogen atoms were positioned geometrically [C-H = 0.93-97 Å] and refined using a riding model approximation with U iso (H) = 1.2U eq (C).

Figure 1
The molecular structure of the title compound, showing 50% probability displacement ellipsoids and the atomic numbering. Intramolecular hydrogen bonds are shown as dashed lines. Symmetry code for A suffix: -x + 1/2, -y + 1/2, -z  The crystal packing of the title compound, showing linking of the molecules into a layer through R 2 2 (38) motifs. Intermolecular interactions are drawn as dashed lines.

Special details
Experimental. The low-temperature data was collected with the Oxford Cyrosystem Cobra low-temperature attachment. 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.