Crystal structure of catena-poly[[diaquabis(4-formylbenzoato-κO 1)cobalt(II)]-μ-pyrazine-κ2 N:N′]

The CoII atom in this one-dimensional coordination polymer, with aqua, 4-formylbenzoate and bridging pyrazine ligands, is located on a twofold rotation axis and has a slightly distorted octahedral coordination sphere. Strong intramolecular O—H⋯O hydrogen bonds link the water molecules to the carboxylate O atoms.

In the title polymeric compound, [Co(C 8 H 5 O 3 ) 2 (C 4 H 4 N 2 )(H 2 O) 2 ] n , the Co 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 two bridging pyrazine ligands. The latter are 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 7.50 (8) , while the benzene and pyrazine rings are oriented at a dihedral angle of 64.90 (4) . The pyrazine ligands bridge the Co II cations, forming linear chains running along the b-axis direction. Strong intramolecular O-HÁ Á ÁO hydrogen bonds link the water molecules to the carboxylate O atoms. In the crystal, weak 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.

Chemical context
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 (Adiwidjaja et al., 1978;Antsyshkina et al., 1980;Nadzhafov et al., 1981;Shnulin et al., 1981). Transition metal complexes with biochemically active ligands frequently show interesting physical and/or chemical properties and, 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).

Structural commentary
The asymmetric unit of the title compound contains a Co II ion, one formylbenzoate (FB) anion, one water molecule and half of a pyrazine molecule. Atoms N1 and N2 of the pyrazine ISSN 2056-9890 ligand and Co1 are located on a twofold rotation axis (Fig. 1). The pyrazine ligands bridge adjacent Co II ions, forming polymeric chains running along the b-axis direction (Fig. 2). The distance between symmetry-related Co II ions [Co1Á Á ÁCo1 iii ; symmetry code: (iii) x, y + 1, z] is 7.1193 (4) Å .
The equatorial plane of the Co II O 4 N 2 coordination sphere is composed of two carboxylate O atoms [O1 and O1 i ; symmetry code: (i) 2 À x, y, 3 2 À z] of two symmetry-related monodentate formylbenzoate anions and two N atoms [N1 and N2 ii ; symmetry code: (ii) x, À1 + y, z] of two bridging pyrazine ligands, which are bisected by the twofold rotation axis. The axial positions are occupied by two O atoms (O4 and O4 i ) of the coordinating water molecules.
The near equality of the C1-O1 [1.272 (2) Å ] and C1-O2 [1.245 (2) Å ] bonds in the carboxylate group indicates a delocalized bonding arrangement, rather than localized single and double bonds. The Co-N bond length is 2.165 (9) Å , while the Co-O bond lengths are 2.0551 (9) Å (for benzoate oxygen) and 2.1491 (11) Å (for water oxygen), close to standard values. The Co1 atom is displaced by 0.1034 (2) Å from the mean plane of the carboxylate group (O1/C1/O2). The dihedral angle between the carboxylate group and the adjacent benzene ring A (C2-C7) is 7.50 (8) , while the benzene and pyrazine rings are oriented at a dihedral angle of 64.90 (4) .

Supramolecular features
Strong intramolecular O-HÁ Á ÁO hydrogen bonds (Table 1) link the water molecules to the non-coordinating carboxylate oxygen atoms. In the crystal, weak O-H water Á Á ÁO water hydrogen bonds (Table 1) 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 (Fig. 3). There are also weak C-HÁ Á Á interactions present (Table 1).
Acta Cryst. (2015). E71, 339-341 research communications Figure 1 A view of the coordination environment around the Co II atom of the title molecule, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The twofold rotation axis bisects atoms Co1, N1 and N2. Non-labelled atoms are generated by the symmetry code Àx + 2, y, Àz + 3 2 . Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A partial view of the crystal packing of the title compound.

Refinement
The experimental details including the crystal data, data collection and refinement are summarized in Table 2. Atoms H41 and H42 (for H 2 O) were located in a difference Fourier map and were refined freely. The methine H atom was also located in a difference Fourier map and the C-H distance restrained to 0.984 (13) Å . The aromatic C-bound H atoms were positioned geometrically with C-H = 0.93 Å , and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 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.