Tetraaquadiimidazolenickel(II) naphthalene-1,5-disulfonate

The triclinic unit cell of the title compound, [Ni(C3H4N2)2(H2O)4](C10H6O6S2), contains one centrosymmetric cation and one centrosymmetric anion. In the cation, the NiII ion is six-coordinated by two imidazole ligands [Ni—N = 2.0568 (14) Å] and four water molecules [both independent Ni—O distances are 2.098 (1) Å] in a distorted octahedral geometry. Intermolecular O—H⋯O and N—H⋯O hydrogen bonds form an extensive three-dimensional network, which consolidates the crystal packing.


Comment
The weak coordination nature of SO 3 − makes its coordination mode very flexible and sensitive to the chemical environment (Côté et al., 2003). It is known that the coordination behavior of arenesulfonates with transition metals can be tailored in the presence of amino ligands (Lian et al., 2007;Liu et al., 2006;Zhou et al., 2004;Chen et al., 2001;Cai et al., 2001;Chen et al., 2002). Herewith we present the crystal structure of the title compound, [C 6 H 16 N 4 NiO 4 ] 2+ ·[C 10 H 6 O 6 S 2 ] 2− (I) (Fig. 1).
The asymmetric unit of (I) contains a half of complex cation and a half of organic anion. Four water molecules coordinate to the nickel ion in trans position, respectively, and two imine nitrogen atoms from two imidazole ligands coordinate to nickel atom in trans position too. Thus, the nickel ion has a slightly distorted octahedral coordination geometry.
The title compound adopts the same hybrid organic-inorganic packing pattern as that reported earlier (Cai, 2004;Cai et al., 2001;Chen et al., 2002). The intermolecular O-H···O and N-H···O hydrogen bonds (Table 1) form an extensive three-dimensional network, which consolidates the crystal packing.

Experimental
Disodium naphthalene-1,5-disulfonate (0.33 g, 1 mmol) and imidazole (0.27 g, 4 mmol) were added to an aqueous solution of NiCl 2 6H 2 O (0.24 g, 1 mmol). The result solution was stirred at 60°C for four hours in a water bath. After filtration, a clear solution was set aside to crystallize. Platelike blue crystals were collected in 70% yield (base on Ni) after three days.

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