catena-Poly[[[cis-aquadibromidocobalt(II)]-μ-(pyrazine-2-carboxylic acid)-κ3 N 1,O:N 4] monohydrate]

The title compound, {[CoBr2(C5H4N2O2)(H2O)]·H2O}n, is a one-dimensional coordination polymer which crystallizes as a monohydrate. The asymmetric unit contains one CoII atom in a distorted octahedral geometry, forming a chain parallel to [010] with the pyrazine carboxylic acid ligands coordinating on one side in a bidentate fashion through one N and one O atom, and in a monodentate fashion through a N atom, with N atoms trans, and with both ligands lying in the same plane. The bromide atoms are cis to each other, while a water molecule occupies the final octahedral coordination site. The chains are linked together though an O—H⋯Br hydrogen bonding network, and are further stabilized by an O—H⋯Br and O—H⋯O hydrogen-bonding framework with the solvent water molecule.

The title compound, {[CoBr 2 (C 5 H 4 N 2 O 2 )(H 2 O)]ÁH 2 O} n , is a one-dimensional coordination polymer which crystallizes as a monohydrate. The asymmetric unit contains one Co II atom in a distorted octahedral geometry, forming a chain parallel to [010] with the pyrazine carboxylic acid ligands coordinating on one side in a bidentate fashion through one N and one O atom, and in a monodentate fashion through a N atom, with N atoms trans, and with both ligands lying in the same plane. The bromide atoms are cis to each other, while a water molecule occupies the final octahedral coordination site. The chains are linked together though an O-HÁ Á ÁBr hydrogen bonding network, and are further stabilized by an O-HÁ Á ÁBr and O-HÁ Á ÁO hydrogen-bonding framework with the solvent water molecule.

Related literature
For the synthesis of related compounds, see: Gao et al. (2007) and references therein. For other examples of linear coordination polymers utilizing pyrazine derivatives, see: Mao et al. (1996).

Comment
The title compound (I) forms a linear 1-D coordination polymer aligned along b, with pyrazine carboxylic acid ligands linking Co II metal centres together in a bidentate fashion to one cobalt through N and O atoms, and in a monodentate fashion through the remaining N atom, with N atoms trans to each other, and neighboring pyrazine rings within the same plane.
The two bromide anions are coordinated in a cis arrangement, with a water molecule completing the distorted octahedral geometry about the Co II . The asymmetric unit includes only a single monomer, with the 2 1 screw axis generating the neighboring 'inverted' linked monomer.The Co-N bonds average 2.16 Å, while the Co-O pz bond length is 2.18 Å. The Co-Br bonds are essentially identical at 2.55 Å.
Linear chains directly interact with each other through hydrogen bonding between the coordinated water, and bromide ligands. The single water solvate is involved heavily in the hydrogen bonding network interacting with both bromide anions, as well as the carboxylic acid group further stabilizing the crystal structure.

Experimental
In a synthesis designed to form mer-tris(pyrazine carboxylato)cobalt(III), CoBr 2 . 6(H 2 O) was dissolved in methanol at room temperature to which three equivalents of pyrazine carboxylic acid was added. The initial red precipitate that formed almost immediately and was identified as mer-tris(pyrazine carboxylato)cobalt(III) bromide was removed by filtration. To the mother liquor was added an equal volume of water. Subsequently, the blue solution was allowed to stand for 2 months at room temperature allowing (I) to crystallize by slow evaporation yielding bright pink prismatic crystals suitable for X-ray diffraction. Attempts to remake (I) via more rational routes using CoBr 2 . 6(H 2 O) and one equivalent of pyrazine carboxylic acid were not successful.

Refinement
All H atoms attached to C atoms were added in ideal locations, and constrained to ride on the parent atoms with U iso = 1.2U eq (C). The H atoms attached to O atoms were located in the electron density difference map, and, with the exception of H1B were allowed to refine spatially and thermally. H1B was restrained to be 0.82 ± 0.02 Å from O1.

Figures
Fig . 1. A view of (I) with atom numbering scheme showing the molecular structure and intraand intermolecular H bonding present. Displacement ellipsoids are drawn at the 50% probability level.

Special details
Experimental. multi-scan from symmetry-related measurements Sortav (Blessing 1995) Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(F 2 ) is used only for calculating Rfactors(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.