5,5′-Dimethoxy-2,2′-[(pentane-1,5-diyldioxy)bis(nitrilomethylidyne)]diphenol

The molecule of the title compound, C21H26N2O6, which lies across a crystallographic inversion centre, crystallizes with two unique half-molecules in the symmetric unit and adopts a linear configuration and the imino group is coplanar with the aromatic ring, making a dihedral angle of 3.26 (3)°. Strong intramolecular O—H⋯N and weak intermolecular O—H⋯O and C—H⋯O hydrogen bonds and weak intermolecular π–π stacking interactions [centroid–centroid distance = 4.419 (2) Å]establish an infinite three-dimensional supramolecular structure.

The molecule of the title compound, C 21 H 26 N 2 O 6 , which lies across a crystallographic inversion centre, crystallizes with two unique half-molecules in the symmetric unit and adopts a linear configuration and the imino group is coplanar with the aromatic ring, making a dihedral angle of 3.26 (3) . Strong intramolecular O-HÁ Á ÁN and weak intermolecular O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds and weak intermolecularstacking interactions [centroid-centroid distance = 4.419 (2) Å ]establish an infinite three-dimensional supramolecular structure.

Related literature
For the properties and uses of salen-type compounds, see: Lacroix (2001);Nishijo et al. (2006); Onda et al. (2007); Sun et al. (2004). For the structures of free salen-type compounds, see: Akine et al. (2005). For related structures, see: Dong et al. (2008aDong et al. ( ,b, 2009 Table 1 Hydrogen-bond geometry (Å , ).  properties (Nishijo et al., 2006). They can also be used as elemental building blocks for construction of supramolecular structures via intermolecular hydrogen bonding or short contact interaction (Sun et al., 2004). Many salen-type complexes have been structurally characterized, but only a relatively small number of free salen-type compounds have had their X-ray structures reported (Akine et al., 2005). In order to extent our work (Dong et al., 2008a) on structural characterization of salen-type bisoxime compounds, we reported the synthesis and structure of the title compound in this paper in Fig. 1.
The molecule of the title salen-type bisoxime compound lies across a crystallographic inversion centre and adopts a linear configuration with respect to the azomethine C=N bonds. The dihedral angle formed by the two benzene rings is 23.4 (2) °, and the imino group is coplanar with the aromatic ring. This structure is different from our previous work reported in which the molecules assume W-shaped configuration (Dong et al., 2008a) or E configuration (Dong et al., 2008b).
There are two strong intramolecular O-H···N hydrogen bonds involving the hydroxy group and oxime N atoms in each molecule. In the crystal structure, intermolecular C-H···O and O-H···O hydrogen bonds link the each molecule to three others, and weak intermolecular π-π stacking interaction between the neighbouring benzene rings (the inter-molecular plane-to-plane dihedral angle along b axis is 0.48 (4) °). Thus, an infinite three-dimensional supramolecular structure is established (Fig. 2).
Colorless block-like single crystals suitable for X-ray diffraction studies were obtained after about two months by slow evaporation from an ethanol solution of the title compound.