catena-Poly[[diaquabis(4-formylbenzoato-κO 1)copper(II)]-μ-pyrazine-κ2 N:N′]

In the title polymeric compound, [Cu(C8H5O3)2(C4H4N2)(H2O)2]n, the CuII atom is located on a twofold rotation axis and has a slightly distorted octahedral coordination sphere. In the equatorial plane, it is coordinated by two carboxylate O atoms of two symmetry-related monodentate formylbenzoate anions and by two N atoms of the bridging pyrazine ligand, which is bisected by the twofold rotation axis. The axial positions are occupied by two O atoms of the coordinating water molecules. In the formylbenzoate anion, the carboxylate group is twisted away from the attached benzene ring by 6.2 (2)°, while the benzene and pyrazine rings are oriented at a dihedral angle of 68.91 (8)°. The pyrazine ligands bridge the CuII cations, forming polymeric chains running along the b-axis direction. Strong intramolecular O—H⋯O hydrogen bonds link the water molecules to the carboxylate O atoms. In the crystal, O—Hwater⋯Owater hydrogen bonds link adjacent chains into layers parallel to the bc plane. The layers are linked via C—Hpyrazine⋯Oformyl hydrogen bonds, forming a three-dimensional network. There are also weak C—H⋯π interactions present.

In the title polymeric compound, [Cu(C 8 H 5 O 3 ) 2 (C 4 H 4 N 2 )-(H 2 O) 2 ] n , the Cu II atom is located on a twofold rotation axis and has a slightly distorted octahedral coordination sphere. In the equatorial plane, it is coordinated by two carboxylate O atoms of two symmetry-related monodentate formylbenzoate anions and by two N atoms of the bridging pyrazine ligand, which is bisected by the twofold rotation axis. The axial positions are occupied by two O atoms of the coordinating water molecules. In the formylbenzoate anion, the carboxylate group is twisted away from the attached benzene ring by 6.2 (2) , while the benzene and pyrazine rings are oriented at a dihedral angle of 68.91 (8) . The pyrazine ligands bridge the Cu II cations, forming polymeric chains running along the baxis direction. Strong intramolecular O-HÁ Á ÁO hydrogen bonds link the water molecules to the carboxylate O atoms. In the crystal, O-H water Á Á ÁO water hydrogen bonds link adjacent chains into layers parallel to the bc plane. The layers are linked via C-H pyrazine Á Á ÁO formyl hydrogen bonds, forming a three-dimensional network. There are also weak C-HÁ Á Á interactions present.

Related literature
For structural functions and coordination relationships of arylcarboxylate ions in transition metal complexes of benzoic acid derivatives, see ; Shnulin et al. (1981). For applications of transition metal complexes with biochemical molecules in biological systems, see: Antolini et al. (1982). Some benzoic acid derivatives, such as 4-aminobenzoic acid, have been extensively reported in coordination chemistry, as bifunctional organic ligands, due to the variety of their coordination modes, see: Chen & Chen (2002); Amir-aslanov et al. (1979);Hauptmann et al. (2000). For a related structure involving 4-formylbenzoate, see: Hö kelek et al. (2009). For bond-length data, see: Allen et al. (1987).

Comment
The structural functions and coordination relationships of the arylcarboxylate ion in transition metal complexes of benzoic acid derivatives change depending on the nature and position of the substituent groups on the benzene ring, the nature of the additional ligand molecule or solvent, and the medium of the synthesis . Transition metal complexes with biochemically active ligands frequently show interesting physical and/or chemical properties, as a result they may find applications in biological systems (Antolini et al., 1982). Some benzoic acid derivatives, such as 4-aminobenzoic acid, have been extensively reported in coordination chemistry, as bifunctional organic ligands, due to the varieties of their coordination modes (Chen & Chen, 2002;Amiraslanov et al., 1979;Hauptmann et al., 2000). The title compound was synthesized and its crystal structure is reported herein.
The asymmetric unit of the title compound contains half a Cu II ion, one formylbenzoate (FB) anion, one water molecule and half of a pyrazine molecule. Atoms Cu1, and N1 and N2 of the pyrazine ligand, are located on a two-fold rotation axis (Fig. 1). The pyrazine ligands bridge adjacent Cu II ions forming polymeric chains running along the b-axis direction (Fig. 2). The distances between the symmetry related Cu II ions [Cu1···Cu1 i ; symmetry code (i) = x, y-1, z] is 6.879 (3) Å.
In the equatorial plane of the Cu II coordination sphere is compossed of two carboxylate O atoms (O1 and O1 ii ; symmetry code: (ii) -x+1, y, -z+0.5) of two symmtery related monodentate formylbenzoate anions and two N atoms (N1 and N2 i ) of the bridging pyrazine ligand, which is bisected by the two-fold rotation axis. The axial positions are occupied by two O atoms (O4 and O4 ii ) of the coordinated water molecules.

Experimental
The title compound was prepared by the reaction of CuSO 4 .5H 2 O (1.25 g, 5 mmol) in H 2 O (250 ml) and pyrazine (0.40 g, 5 mmol) in H 2 O (30 ml) with sodium 4-formylbenzoate (1.72 g, 10 mmol) in H 2 O (100 ml) at room temperature. The mixture was filtered and set aside to crystallize at ambient temperature for several days, giving blue block-like crystals.

Refinement
The water H atoms (H41 and H42) were located in a difference Fourier map and freely refined. The C-bound H-atoms were positioned geometrically and constrained to ride on their parent atoms: C-H = 0.93 Å with U iso (H) = 1.2U eq (C).

Figure 1
A view of the coordination geometry around the Cu II atom of the title molecule, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The two-fold rotation axis bisects atoms Cu1, N1 and N2.  A partial view along the c axis of the crystal packing of the title compound. Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles 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 > 2sigma(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.