Tetrakis(μ-2-chloro-4-nitrobenzoato-κ2 O:O′)bis[aquacopper(II)]

In the title binuclear copper(II) complex, [Cu2(C7H3ClNO4)4(H2O)2], each of the two independent CuII center is five-coordinated by four O atoms of the carboxylate groups in the basal plane and one O atom of a water molecule in the apical position, in a distorted square-pyramidal geometry. The Cu—Cu distance is 2.6458 (4) Å. In the crystal structure, the dinuclear units are linked into a three-dimensional network by O—H⋯O, C—H⋯O and C—H⋯Cl hydrogen bonds. One of the Cl atoms is disordered over two positions with occupancies of 0.650 (2) and 0.350 (2).

In the title binuclear copper(II) complex, [Cu 2 (C 7 H 3 ClNO 4 ) 4 -(H 2 O) 2 ], each of the two independent Cu II center is fivecoordinated by four O atoms of the carboxylate groups in the basal plane and one O atom of a water molecule in the apical position, in a distorted square-pyramidal geometry. The Cu-Cu distance is 2.6458 (4) Å . In the crystal structure, the dinuclear units are linked into a three-dimensional network by O-HÁ Á ÁO, C-HÁ Á ÁO and C-HÁ Á ÁCl hydrogen bonds. One of the Cl atoms is disordered over two positions with occupancies of 0.650 (2) and 0.350 (2).

S1. Comment
In our quest to study the biological properties of Cu II complexes, we have managed to prepare several water soluble Cu II complexes. Cu II complexes have been known to exhibit DNA cleavage activity in vitro (Balaraman et al., 2006). Herein, we report the preparation and crystal structure of the title compound.
The coordination geometry around each Cu II atom is square-pyramidal. Four O atoms, one each from the carboxylate groups of four organic ligands, form the basal plane with an average Cu-O bond distance of 1.968 (2) Å. The O atom of the water molecules lie in the apical position with an average Cu-O distance of 2.148 (2) Å. The Cu1 and Cu2 atoms are slightly displaced from the corresponding basal plane by 0.2037 (2) and 0.1959 (2) Å, respectively. The Cu1-Cu2 distance is 2.6458 (4) Å. Similar characteristics of the copper atom were also reported by Kabbani et al. (2004) and Stachová et al. (2004).
Bond lengths in the ligand show normal values (Allen et al., 1987). Dihedral angles between nitro groups and benzene In the crystal structure, O-H···O, C-H···O and C-H···Cl intermolecular interactions (Table 2) form a threedimensional network (Fig.2).

S2. Experimental
An ethanol solution (50 ml) of 2-chloro-4-nitrobenzoic acid (4.84 g, 0.024 mol) was added to a solution of copper(II) sulfate pentahydrate (3.00 g, 0.012 mol) in ethanol (50 ml). This mixture was then stirred and refluxed and left to cool down to room temperature. After a few days of slow evaporation, blue crystals which are suitable for X-ray analysis were collected.

S3. Refinement
Water H atoms were located in a difference Fourier map and were allowed to ride on the O atom, with U iso (H) = 1.5U eq (O). All other H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93 Å and U iso (H) = 1.5U eq (C). Atom Cl3 attached to one of the phenyl rings is disordered over two positions with occupancies of 0.650 (2) and 0.350 (2). The structure shows a pseudo centre of symmetry. It can be solved and refined in the space group P2 1 /c but the final R value (0.098) is large. The highest residual electron density peak is located at 1.02 Å from Cl3A and the deepest hole is located at 0.63Å from Cu1. The crystal is a twin with BASF = 0.526 (8).  The molecular structure of the title compound, showing 50% probability displacement ellipsoids and the atom-numbering scheme. Both disorder components are given.

Figure 2
The crystal packing of the title compound, viewed down the a axis. Hydrogen bonds are shown as dotted lines. Only the major disorder component is shown.

Tetrakis(µ-2-chloro-4-nitrobenzoato-κ 2 O:O′)bis[aquacopper(II)]
Crystal data  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.75 e Å −3 Δρ min = −0.85 e Å −3 Absolute structure: Flack (1983), 8242 Friedel pairs Absolute structure parameter: 0.526 (8) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.