6,6′-Dihydroxy-2,2′-[(butane-1,4-diyldioxy)bis(nitrilomethylidyne)]diphenol

The molecule of the centrosymmetric title compound, C18H20N2O6, assumes an E configuration with respect to the azomethine C=N bond. The imino group is coplanar with the aromatic ring. Intramolecular O—H⋯O and O—H⋯N bonds are found between the hydroxyl groups and adjacent O (or N) atoms. In the crystal structure, intermolecular O—H⋯O bonds link each molecule to two others, forming a layered network.

The molecule of the centrosymmetric title compound, C 18 H 20 N 2 O 6 , assumes an E configuration with respect to the azomethine C N bond. The imino group is coplanar with the aromatic ring. Intramolecular O-HÁ Á ÁO and O-HÁ Á ÁN bonds are found between the hydroxyl groups and adjacent O (or N) atoms. In the crystal structure, intermolecular O-HÁ Á ÁO bonds link each molecule to two others, forming a layered network.

Comment
The design of supramolecular structures involves molecules for which intermolecular hydrogen bonds act as driving, directional and cohesive forces (Sharma, 2002). Although many stable and well documented structures have been reported (Fan et al., 2006;Wang et al., 2003), the supramolecular structures of salen-type bisoxime compounds have rarely been determined. The first reported supramolecular salen-type bisoxime compound, 6,6'-dihydroxy-2,2'-[ethylenedioxybis(nitrilomethylidyne)]diphenol has an intermolecular hydrogen bond network involving four hydroxyl groups and cocrystallized water molecules (Akine et al., 2006). In this article, we report the synthesis and structure of the title compound (I) (Fig. 1).
(I) lies across a crystallographic inversion centre to give 1/2 molecule per asymmetric unit. It assumes an E configuration with respect to the azomethine C═N bond. The imino group is coplanar with the aromatic ring. The planar units are parallel to one another but extend in opposite directions from the tetramethylene bridge. Strong intramolecular O(3)-H(3)···O (2) and O(2)-H(2)···N(1) bonds are found between the hydroxyl groups and adjacent O (or N) atoms (Table 1). This is similar to what was observed in our previously reported salen-type bisoxime compounds Dong et al., 2007;Dong et al., 2008a).
Block-shaped crystals of (I) suitable for X-ray crystal analysis were grown from a mixture of tetrahydrofuran/ethanol (1:1) solution by slow evaporation at room temperature, which afforded pale-brown crystals.

Figures
Fig . 1. The molecular structure of the title compound with atom numbering scheme [Symmetry codes: -x,-y + 2,-z + 1]. Displacement ellipsoids for non-hydrogen atoms are drawn at the 30% probability level.    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.