An octanuclear zinc(II) complex with 6,6′-dihydroxy-2,2′-[1,2-phenylenebis(nitrilomethylidyne)]diphenol

The asymmetric unit of the title compound, tetraaquatetrakis{μ3-6,6′-dioxido-2,2′-[1,2-phenylenebis(nitrilomethylidyne)]diphenolato}octazinc(II) dimethyl sulfoxide tetrasolvate dihydrate, [Zn8(C20H12N2O4)4(H2O)4]·4C2H6OS·2H2O, contains one quarter of a ZnII octanuclear complex with symmetry, one dimethyl sulfoxide molecule and one half of a water molecule which lies on a twofold rotation axis. The ZnII atoms of the octanuclear complex have two different five-coordinate environments, viz. ZnN2O3 and ZnO5. All eight ZnII centers adopt a distorted square-pyramidal coordination; four ZnII ions have the N2O2 tetradentate Schiff base ligand bound in a basal plane and the coordinated water molecule occupying the apical site, while the remaing four ZnII ions are bound by five O atoms from three Schiff base ligands. In the crystal structure, ZnII complex molecules, coordinated and uncoordinated water molecules and dimethyl sulfoxide molecules are linked via O—H⋯O and C—H⋯O hydrogen bonds and C—H⋯π interactions, forming a three-dimensional framework.

The asymmetric unit of the title compound, tetraaqua-tetrakis{ 3 -6,6 0 -dioxido-2,2 0 -[1,2-phenylenebis(nitrilomethylidyne)]diphenolato}octazinc(II) dimethyl sulfoxide tetrasolvate dihydrate, [Zn 8 (C 20 H 12 N 2 O 4 ) 4 (H 2 O) 4 ]Á4C 2 H 6 OSÁ-2H 2 O, contains one quarter of a Zn II octanuclear complex with 4 symmetry, one dimethyl sulfoxide molecule and one half of a water molecule which lies on a twofold rotation axis. The Zn II atoms of the octanuclear complex have two different five-coordinate environments, viz. ZnN 2 O 3 and ZnO 5 . All eight Zn II centers adopt a distorted square-pyramidal coordination; four Zn II ions have the N 2 O 2 tetradentate Schiff base ligand bound in a basal plane and the coordinated water molecule occupying the apical site, while the remaing four Zn II ions are bound by five O atoms from three Schiff base ligands. In the crystal structure, Zn II complex molecules, coordinated and uncoordinated water molecules and dimethyl sulfoxide molecules are linked via O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions, forming a threedimensional framework.

Comment
There has been considerable interest in the synthesis of metal Schiff base complexes due to their coordination chemistry and applications (Basak et al., 2007;Eltayeb et al., 2007a,b,c;Pal et al., 2005;Tarafder et al., 2002). Zinc complexes with Schiff bases are important in biological systems and coordination chemistry (Collinson & Fenton, 1996;Tarafder et al., 2002). Previously, we have reported crystal structures of Zn II complexes with Schiff base ligands (Eltayeb et al., 2007a,b,c).
As a continuation of our research on Schiff base complexes, we report here the crystal structure of the title octanuclear Zn II complex.
The asymmetric unit of the title compound ( base ligand in the present complex is in an umbrella conformation with the dihedral angle between the two outer rings (C1-C6 and C15-C20) being 48.1 (2) °. In the octanuclear complex (Fig. 2), the four Schiff bases have their concave sides alternating up and down. The coordination geometry of the five-coordinate atoms Zn1 and Zn2 (and their equivalents) can be viewed as that of a slightly distorted square antiprism. Bond lengths and angles observed in the structure are in normal ranges (Allen et al., 1987) and comparable with the related structures (Eltayeb et al., 2007a,b,c).
In the crystal packing (Fig. 3), the Zn II complex molecules, coordinated and free water molecules and dimethyl sulfoxide molecules are linked via O-H···O and C-H···O hydrogen bonds and C-H···π interactions involving the C1-C6 (centroid Cg1) ring (Table 1) forming a three-dimensional framework.

Experimental
The title compound was synthesized by adding 2,3-dihydroxybenzaldehyde (0.552 g, 4 mmol) to a solution of o-phenylenediamine (0.216 g, 2 mmol) in ethanol 95% (20 ml). The mixture was refluxed with stirring for 30 min. Zinc chloride (0.544 g, 4 mmol) in ethanol (10 ml) was then added, followed by triethylamine (1.0 ml, 7.2 mmol). The mixture was stirred at room temperature for 3 h. The yellow precipitate obtained was washed with about 5 ml e thanol, dried, and then washed with copious amounts of diethylether. Orange single crystals of the title compound suitable for X-ray diffraction were formed after recrystallization in the dimethyl sulfoxide/ethanol (3:5 v/v) at room temperature after several days.

Refinement
Water H atoms were found in the difference map and their positions were refined with a restrained geometry, with O-H = 0.84 (2) Å and H···H = 1.37 (2) Å. The remaining H atoms are placed in calculated positions with d(C-H) = 0.93 Å, U iso = 1.2U eq (C) for CH and aromatic and 0.96 Å, U iso = 1.5U eq (C) for CH 3 atoms. A rotating group model was used for the methyl groups. The highest residual electron density peak is located at 0.36 Å from H22C and the deepest hole is located at 0.47 Å from S1. Fig. 1. The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level.