Poly[[octaaqua-μ4-(benzene-1,2,4,5-tetracarboxylato)-dicobalt(II)] octahydrate]

The title polymeric coordination compound, {[Co2(C10H2O8)(H2O)8]·8H2O}n, was obtained by slow diffusion of a dilute aqueous solution of CoCl2 and the sodium salt of benzene-1,2,4,5-tetracarboxylic acid (H4btec) through an agar–agar gel bridge in a U-shaped tube. The two independent Co2+ ions are each situated on an inversion centre and are coordinated in a slightly distorted octahedral geometry by four water O atoms and two carboxylate O atoms from two btec4− ligands (-1> symmetry), forming a layer parallel to (11-1). This layer can be described as a molecular two-dimensional square grid with the benzene rings at the nodes and the CoII atoms connecting the nodes. O—H⋯O hydrogen-bonding interactions involving the coordinating water molecules, the carboxylate O atoms and lattice water molecules lead to the formation of a three-dimensional network.

The title polymeric coordination compound, {[Co 2 (C 10 H 2 O 8 )-(H 2 O) 8 ]Á8H 2 O} n , was obtained by slow diffusion of a dilute aqueous solution of CoCl 2 and the sodium salt of benzene-1,2,4,5-tetracarboxylic acid (H 4 btec) through an agar-agar gel bridge in a U-shaped tube. The two independent Co 2+ ions are each situated on an inversion centre and are coordinated in a slightly distorted octahedral geometry by four water O atoms and two carboxylate O atoms from two btec 4À ligands (1 symmetry), forming a layer parallel to (111). This layer can be described as a molecular two-dimensional square grid with the benzene rings at the nodes and the Co II atoms connecting the nodes. O-HÁ Á ÁO hydrogen-bonding interactions involving the coordinating water molecules, the carboxylate O atoms and lattice water molecules lead to the formation of a three-dimensional network.

Experimental
All carboxylic groups of the organic ligand in the title compound are deprotonated and each of them adopts a monodentate coordination mode. There are two crystallographically independent Co II atoms in the structure. In both cases the Co II atoms are coordinated by two carboxylate oxygen atoms from two btec ligands and four water oxygen atoms ( Fig. 1). There is no significant difference in the coordination distance between carboxyl and water oxygen atoms. The coordination Co···O distances in the title polymeric compound range from 2.067 (3) to 2.129 (3) Å. Each btec 4ligand links four Co II atoms and each Co II atom is bond to two btec 4ligands to form a two-dimensional layer. These layers can be described as a molecular two-dimensional square grid in which the phenyl rings are at the nodes and the Co II atoms connecting the nodes (Fig. 2). The area of the square cells of the grids is larger than 120 Å 2 (11.3 Å x 11.5 Å). The crystal packing is enforced by a complex hydrogen bonds network that involves the crystallization water molecules located in the inter-layer space.

Experimental
All reagents were used as obtained without further purification. Cobalt chloride was purchased from STREM Chemicals.
benzene-1,2,4,5-tetracarboxylic acid was purchased from Acros Organics. Its sodium salt was prepared by addition of four equivalents of sodium hydroxide to a suspension of benzene-1,2,4,5-tetracaboxylic acid in de-ionized water until complete dissolution. Then, the solution was evaporated to dryness. The solid phase was then put in suspension in ethanol, stirred and refluxed during 1 h. After filtration and drying in a desiccator, a white powder of tetra-sodium benzene-1,2,4,5-tetracarboxylate was obtained. The yield of this synthesis is 90%.
Single crystals of the coordination polymer were obtained by slow diffusion of dilute aqueous solutions of Co(II) chloride (0.25 mmol in 20 ml) and of sodium salt of benzene-1,2,4,5-tetracarboxylic acid (0.25 mmol in 20 mL) through an agar-agar gel bridge in a U-shaped tube. The gel was purchased from Acros Organics and jellified according to established procedure. For a related procedure, see: Henisch & Rustum, 1970;Henisch, 1988;Daiguebonne et al., 2003. After several weeks, very light pink single crystals were obtained.

Refinement
H-atoms from crystallization water molecules could not be assigned reliably and were thus not included in the refinement, but they were taken into account for the chemical formula sum, moiety, weight, as well as for the absorption coefficient and the number of electrons in the unit cell. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).  View along the a axis of the molecular square grid layer of the title compound.