Tetraaquabis(2-methyl-1H-imidazole-κN 3)cobalt(II) naphthalene-1,5-disulfonate

In the title complex, [Co(C4H6N2)2(H2O)4](C10H6O6S2), the cation and anion both reside on crystallographic inversion centers, such that the asymmetric unit comprises one half cation and one half anion. The central CoII ion is coordinated by four water molecules and two 2-methylimidazole ligands, resulting in a trans-octahedral coordination geometry. The existence of strong N—H⋯O and O—H⋯O hydrogen-bonding interactions gives rise to a three-dimensional structure.

In the title complex, [Co(C 4 H 6 N 2 ) 2 (H 2 O) 4 ](C 10 H 6 O 6 S 2 ), the cation and anion both reside on crystallographic inversion centers, such that the asymmetric unit comprises one half cation and one half anion. The central Co II ion is coordinated by four water molecules and two 2-methylimidazole ligands, resulting in a trans-octahedral coordination geometry. The existence of strong N-HÁ Á ÁO and O-HÁ Á ÁO hydrogenbonding interactions gives rise to a three-dimensional structure.
The author thanks the Ordered Matter Science Research Center, Southeast University, for its generous financial support.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: FJ2459).

Comment
In recent years, simple molecular-ionic compounds containing inorganic cations and organic anions have attracted great interest owing to the tunability of their special structural features and their potential ferroelectrics property. Ferroelectric materials that exhibit reversible electric polarization in response to an external electric field have found many applications such as nonvolatile memory storage, electronics and optics. The freezing of a certain functional group at low temperature forces significant orientational motions of the guest molecules and thus induces the formation of the ferroelectric phase. (Fu et al., 2009;Zhang et al., 2010;Zhang et al., 2008;Ye et al., 2006).
The asymmetric unit of the title compound is shown in Fig1, which consists of one (C 10 H 6 O 6 S 2 ) 2anion and one 2C 4 H 6 N 2 .4H2O.Co(II) molecule-based cation.The title complex crystallizes in monoclinic P 21/n space group, the whole compound shall be stable thanks to the numerous hydrogen bonds formed in molecules, such as the N-H···O and the O-H···O bonds. The length of N-H···O is 2.06 Å, while the length of O-H···O hydrogen bonds ranges from 1.878 to 2.205 Å. Further details about the hydrogen bonds are listed in Table 1.

Experimental
Co(CH 3 COO -) 2 .4H 2 O (4.96 g, 20 mmol) mixed with K 2 CO 3 (2.76, 20 mmol) were dissolved into 15 ml distilled water under stirring for 5 minutes, and turbid liquid was filtered,and then CoCO 3 was obtained in about 90% yield. CoCO 3 (1.19, 10 mmol) were dissolved into solution containing 2.88 g 1,5-naphthalene disulfonic acid under stirring for 5 minutes, and then 2-methylimidazole (3.28 g, 40 mmol) were added to the solution.At last, the solution was filtered, then tansparent solution was located in a quiet and clean place, block pink crystals suitable for X-ray diffraction were obtained in about 78% yield after two days and filtered and washed with distilled water.