(μ-2,2′,2′′,2′′′-{[Pyrazine-2,3,5,6-tetrayltetrakis(methylene)]tetrakis(sulfanediyl)}tetraacetato)bis[aquanickel(II)] heptahydrate

In the binuclear nickel(II) title compound, the ligand 2,2′,2′′,2′′′-{[pyrazine-2,3,5,6-tetrayltetrakis(methylene)]tetrakis(sulfanediyl)}tetraacetic acid coordinates two NiII cations in a bis-pentadentate manner and the sixfold coordination sphere of each nickel(II) atom is completed by a water molecule.

Reaction of H 4 L1 with NiCl 2 yielded the binuclear complex I, with the ligand coordinating two Ni II ions in a bis-pentadentate manner. Complex I was shown to exhibit a weak antiferromagnetic coupling between the Ni centres via the pyrazine ring with a J value of À1.78 cm À1 (Pacifico, 2003).
Reaction of H 4 L1 with nickel(II) chloride leads to the formation of the binuclear title compound I, which crystallizes with two half molecules in the asymmetric unit ( Fig. 1 and Table 1). The complete molecules are generated by inversion symmetry, with the centres of the pyrazine rings being located at crystallographic centres of inversion.

Figure 3
A view normal to the (101) plane of the crystal packing of the two independent molecules of complex I (atom Ni1 light-green ball; atom Ni2 dark-green ball). Hydrogen bonds (see Table 2) are shown as dashed lines. For clarity, solvent water molecules and C-bound H atoms have been omitted.
In the crystal structure of I, binuclear nickel(II) complexes are linked by O water -HÁ Á ÁO carbonyl hydrogen bonds, forming layers parallel to the (101) plane (Fig. 3, Table 2). Within the layers, weak C-HÁ Á ÁO hydrogen bonds are present (Table 2). Solvent water molecules are linked by O-H water Á Á ÁO water hydrogen bonds to form ribbons propagating along the b-axis direction that consist of eight and twenty-four membered rings of the R 4 4 (8) and R 10 12 (24) types ( Fig. 4 and Table 2). Additional O-H water Á Á ÁO carbonyl hydrogen bonds involving the binuclear complexes and solvent water molecules, together with weak C-HÁ Á ÁS hydrogen bonds, link the layers to form a supramolecular framework (Fig. 5).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For complex I, the average HKL measurement multiplicity was low at 2.6, hence an empirical absorption correction was applied.

data-1
IUCrData ( 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. H atoms of coordinated and non-coordinated water molecules were all located from difference-Fourier maps and freely refined. The C-bound H atoms were included in calculated positions and treated as riding on their parent C atom: C-H = 0.97 -0.99 Å with U iso (H) = 1.2U eq (C).