catena-Poly[[zinc-bis(μ-2-sulfido-1H-benzimidazol-3-ium-5-carboxylato)-κ2 O:S;κ2 S:O] trihydrate]

In the title compound, {[Zn(C8H5N2O2S)2]·3H2O}n, the ZnII atom, lying on a twofold rotation axis, is four-coordinated by two S atoms and two O atoms from four 2-sulfido-1H-benzimidazol-3-ium-5-carboxylate (H2mbidc) ligands in a distorted tetrahedral geometry. Two H2mbidc ligands bridge two ZnII atoms, generating a double-chain along [01]. Adjacent chains are linked by N—H⋯O and O—H⋯O hydrogen bonds, forming a three-dimensional supramolecular network. One of the two water molecules also lies on a twofold rotation axis.

In the title compound, {[Zn(C 8 H 5 N 2 O 2 S) 2 ]Á3H 2 O} n , the Zn II atom, lying on a twofold rotation axis, is four-coordinated by two S atoms and two O atoms from four 2-sulfido-1Hbenzimidazol-3-ium-5-carboxylate (H 2 mbidc) ligands in a distorted tetrahedral geometry. Two H 2 mbidc ligands bridge two Zn II atoms, generating a double-chain along [101]. Adjacent chains are linked by N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds, forming a three-dimensional supramolecular network. One of the two water molecules also lies on a twofold rotation axis.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HY2396).

Comment
In recent years, the synthesis of novel coordination polymers with helical structures has attracted much attention owing to the fundamental role of helicity in biology and their potential utilization in advanced materials (Cui et al., 2003;Ngo & Lin, 2002;Yan et al., 2005). In general, the V-shaped organic ligands have already been proven to be efficient for the generation of helical complexes (Chen & Liu, 2002;Hu et al., 2008;Xiao et al., 2007). 2-Mercapto-1H-benzo[d]imidazole-5-carboxylic acid (H 3 mbidc) is a rigid V-shaped ligand, in which the S atom can coordinate to a variety of metal ions and the carboxylate group can adopt rich coordination modes, meeting the requirements of the coordination geometries of metal ions in assembly process. We selected H 3 mbidc as a bridging ligand and Zn II ion as a metal center, generating a new double-chain coordination polymer, whose structure is reported here.
In the title compound ( Fig. 1), the Zn II atom is four-coordinated by two S atoms and two carboxylate O atoms from four individual H 2 mbidc ligands in a distorted tetrahedral coordination geometry. The Cd-O and Cd-S bond lengths are 1.987 (3) and 2.3159 (12) Å. Each H 2 mbidc ligand bridges two neighboring Zn II atoms, generating a double-chain (Fig.   2). Furthermore, N-H···O and O-H···O hydrogen bonds (Table 1) link the chains together, resulting in a supramolecular structure.

Experimental
A mixture of H 3 mbidc (0.971 g, 5 mmol), NaOH (0.4 g, 10 mmol) and ZnCl 2 (1.36 g, 10 mmol) in water (50 ml) was boiled for 20 min with stirring. Then the mixture was cooled to room temperature. The resulting solution was filtered and allowed to stand. After a week, colorless crystals of the title compound were obtained.