Tetraaquabis(2-methylbenzimidazolium-1,3-diacetato-κO)zinc(II) tetrahydrate

The asymmetric unit of the title compound, [Zn(C12H11N2O4)2(H2O)4]·4H2O, contains one-half of the complex molecule and two uncoordinated water molecules. The four water O atoms in the equatorial plane around the ZnII centre ( symmetry) form a distorted square-planar arrangement, while the distorted octahedral coordination geometry is completed by the O atoms of the zwitterionic 2-methylbenzimidazolium-1,3-diacetate ligands in the axial positions. The benzimidazole ring system is planar, with a maximum deviation of 0.041 (3) Å. Intramolecular O—H⋯O hydrogen bonding results in the formation of a non-planar six-membered ring. In the crystal structure, strong intra- and intermolecular O—H⋯O hydrogen bonds link the molecules into a three-dimensional network. π–π contacts between benzimidazole rings [centroid–centroid distance = 3.899 (1) Å] may further stabilize the structure.

The asymmetric unit of the title compound, [Zn(C 12 H 11 N 2 O 4 ) 2 (H 2 O) 4 ]Á4H 2 O, contains one-half of the complex molecule and two uncoordinated water molecules. The four water O atoms in the equatorial plane around the Zn II centre (1 symmetry) form a distorted square-planar arrangement, while the distorted octahedral coordination geometry is completed by the O atoms of the zwitterionic 2methylbenzimidazolium-1,3-diacetate ligands in the axial positions. The benzimidazole ring system is planar, with a maximum deviation of 0.041 (3) Å . Intramolecular O-HÁ Á ÁO hydrogen bonding results in the formation of a non-planar sixmembered ring. In the crystal structure, strong intra-and intermolecular O-HÁ Á ÁO hydrogen bonds link the molecules into a three-dimensional network.contacts between benzimidazole rings [centroid-centroid distance = 3.899 (1) Å ] may further stabilize the structure.

Comment
The quest to rational design and construct metal-organic frameworks (MOF) is highly topical, for their intriguing topologies and potential applications as functional materials in many areas such as catalysis, molecular adsorption, magnetism properties, non-linear optics and molecular sensing (Robson, 2000;Kitagawa et al., 2004). In order to explore further the influence of novel polycarboxylate ligand which is a good candidate as building block on MOFs, we developed a flexible ligand 1-acetoxy-2-methylbenzimidazole-3-acetate acid [HL] (Ni et al., 2007), to prepare the title mononuclear complex.
We report herein its crystal structure.
The asymmetric unit of the title compound, (Fig. 1), contains one-half molecule, two coordinated and two uncoordinated water molecules. The Zn atom is surrounded by two 2-methylbenzimidazolium-1,3-diacetate and four water molecules. The four O atoms (O5, O6, O5A and O6A atoms) in the equatorial plane around the Zn atom form a distorted square-planar arrangement, while the distorted octahedral coordination is completed by the O atoms of the 2-methylbenzimidazolium-1,3diacetate ligands (O4 and O4A) in the axial positions [symmetry code: (A) -x, -y, -z] ( Table 1). The benzimidazole ring system is planar with a maximum deviation of 0.041 (3) Å for atom C7. Intramolecular O-H···O hydrogen bond results in the formation of a six-membered ring (Zn1/O3/O4/C6/C10/H6A) having twisted conformation.

Experimental
After the pH of a mixture containing ZnCl 2 .2H 2 O (0.0408 g, 0.3 mmol) and ligand HL (0.0498 g, 0.2 mmol) was adjusted by ammonia to 7, the resulting solution was sealed in a Teflon-lined steel liner (25 ml) and then heated at 423 K for 3 d.

Refinement
H atoms of water molecules were located in difference Fourier maps and refined isotropically. The remaining H atoms were positioned geometrically with C-H = 0.93, 0.97 and 0.96 Å, for aromatic, methylene and methyl H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = xU eq (C), where x = 1.5 for methyl H and x = 1.2 for all other H atoms. Fig. 1. The molecular structure of the title molecule with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level [symmetry code: (A) -x, -y, -z].