catena-Poly[copper(II)-{μ3-4,4′-dibromo-2,2′-[butane-1,4-diylbis(nitrilomethanylylidene)]diphenolato-κ4 N,O:N′,O′:O′}]

The asymmetric unit of the title coordination polymer, [Cu(C18H16Br2N2O2)]n, consists of a Schiff base complex in which a crystallographic twofold rotation axis bisects the central C—C bonds of the n-butyl spacers of the designated Schiff base ligands, making symmetry-related dimer units, which are twisted around CuII atoms in a bis-bidentate coordination mode. In the crystal, these dimeric units are connected through Cu—O bonds, forming one-dimensional coordination polymers, which propagate along [001]. The CuII atom adopts a square-based pyramidal coordination geometry, being coordinated by two N and two O atoms of symmetry-related ligands and by a third O atom of a neighboring complex. Furthermore, intermolecular π–π interactions [centroid–centroid distance = 3.786 (2) Å] and C—H⋯O interactions stabilize the crystal packing.

The asymmetric unit of the title coordination polymer, [Cu(C 18 H 16 Br 2 N 2 O 2 )] n , consists of a Schiff base complex in which a crystallographic twofold rotation axis bisects the central C-C bonds of the n-butyl spacers of the designated Schiff base ligands, making symmetry-related dimer units, which are twisted around Cu II atoms in a bis-bidentate coordination mode. In the crystal, these dimeric units are connected through Cu-O bonds, forming one-dimensional coordination polymers, which propagate along [001]. The Cu II atom adopts a square-based pyramidal coordination geometry, being coordinated by two N and two O atoms of symmetryrelated ligands and by a third O atom of a neighboring complex. Furthermore, intermolecularinteractions [centroid-centroid distance = 3.786 (2) Å ] and C-HÁ Á ÁO interactions stabilize the crystal packing.

Comment
The design and construction of metal-organic coordination polymers (MOCPs) have attracted considerable attention, not only for their novel topologies but also for their potential in the area of magnetic applications and functional materials (Kido & Okamoto, 2002;Li et al., 2006;Eddaoudi et al., 2001;Dietzel et al., 2005). One of the key strategies in the construction of metal-organic coordination polymers is to select suitable bi-or multi-dentate bridging ligands. Among these, bis-bidentate NN-or NO-donor Schiff base ligands with aliphatic and aromatic spacers (Hannon et al., 1999;Lavalette et al., 2003) have attracted much attention because of the flexibility in their coordination modes and the resulting intermolecular interactions. The long chain aliphatic spacers or rigid aromatic spacers with large bite angles in these ligands favour the bis-bidentate coordination mode and allow the ligands to accomodate metal centers in one unit of the ligand. On the other hand, Schiff bases are one of the most prevalent ligands in coordination chemistry and their complexes are some of the most important stereochemical models in transition metal-organic chemistry, with their ease of preparation and structural variations (Granovski et al., 1993;Elmali et al., 2000).
The molecular structure of the title complex ( Fig. 1) consists of symmetry-related dimers in which the Schiff base ligands are twisted around Cu II centers in a bis-bidentate coordin n ation mode, having a crystallographic twofold rotation axis which passes through the central C-C bonds of the n-butyl spacers [C9-C9A i and C18-C18A i ; symmetry code: (i) -x + 1, In the crystal the dimer units are connected through Cu-O bonds, forming one-diensional coordination polymer running along the c axis ( Fig. 2), in which the Cu II atom adopts a square-based pyramidal coordination geometry. The Cu II atoms are supported by the two nitrogen and oxygen atoms of the symmetry-related ligands and a third oxygen atom of neighboring complexes. The lengths of the intermolecular Cu1-O1 i bonds [2.394 (3) Å Å; symetry code (i) -x, -y + 1, -z] is significantly shorter than the sum of the van der Waals (vdW) radii of these atoms [Cu, 1.43Å and O, 1.52 Å; Bondi, 1964]. There are different non-bonded internuclear Cu···Cu distances. The longer one is separated by the butyl spacers [4.718 Å], and the shorter one is in the centrosymmetric Cu 2 O 2 rectangular unit [3.314 Å]. Furthermore, intermolecular π-π interactions stabilize the crystal packings with centroid to centroid distances of 3.786 (2)Å [Cg1 and Cg2 are the centroids of the rings (C1-C6) and (C10-C15)]. There are also C-H···O interactions present (Table 1).
The title complex was synthesized by the template method of mixing an ethanolic solution (50 ml) of 5-bromosalicylaldeyde (4 mmol), 1,4-butanediamine (2 mmol), and CuCl 2 .4H 2 O (2.1 mmol). After stirring at reflux conditions for 2 h, the solution was filtered and the resulting green solid was crystallized from ethanol, giving single crystals suitable for X-ray diffraction.

Refinement
All H-atoms were positioned geometrically and constrained to ride on the parent atoms using the riding-model approximation: C-H = 0.93 -0.97 Å with U iso (H) = 1.2U eq (C). In case of the large maximum residual density, located < 1 Å from the Br atoms, it was not possible to find any sign of twinning or missed atoms. Fig. 1. The molecular structure of the title complex, showing 40% probability displacement ellipsoids and the atomic numbering [H-atoms have been omitted for clarity; symmetry code for A suffix: -x + 1, -y, -z + 1]. Crystal data [Cu(C 18 H 16