Diaquabis(4-carboxy-2-ethyl-1H-imidazole-5-carboxylato-κ2 N 3,O 4)cadmium dihydrate

The asymmetric unit of the title compound, [Cd(C7H7N2O4)2(H2O)2]·2H2O, consists of one CdII ion, one 4-carboxy-2-ethyl-1H-imidazole-5-carboxylate anion, one coordinated water molecule and one lattice water molecule. The CdII ion lies on a twofold axis, and is hexacoordinated by four O atoms from water molecules and carboxylate groups and two N atoms from two imidazole rings, in a distorted octahedral arrangement. An extensive framework of N—H⋯O and O—H⋯O hydrogen bonds with the participation of coordinated and free water molecules is found in the crystal structure, which contributes to the formation of a three-dimensional structure.

The asymmetric unit of the title compound, [Cd(C 7 H 7 N 2 O 4 ) 2 -(H 2 O) 2 ]Á2H 2 O, consists of one Cd II ion, one 4-carboxy-2-ethyl-1H-imidazole-5-carboxylate anion, one coordinated water molecule and one lattice water molecule. The Cd II ion lies on a twofold axis, and is hexacoordinated by four O atoms from water molecules and carboxylate groups and two N atoms from two imidazole rings, in a distorted octahedral arrangement. An extensive framework of N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds with the participation of coordinated and free water molecules is found in the crystal structure, which contributes to the formation of a three-dimensional structure.

Comment
Self-assembly of supramolecular architectures based on imidazole carboxylate ligands has draw much attention during recent decades. To the best of our knowledge, coordination polymers based on 2-ethyl-4,5-imidazolecarboxylate have been rarely reported so far (Wang et al., 2008;Zhang et al., 2010;Li et al., 2011;Tian et al., 2010). Herein we report the synthesis of the title compound by the reaction of cadmium nitrate with 2-ethyl-4,5-imidazoledicarboxylic acid (H 3 EIDC) in an aqueous solution under hydrothermal conditions, and its crystal structure.
The title compound, [Cd(C 7 H 7 N 2 O 4 ) 2 (H 2 O) 2 ].2H 2 O, differs from the Cd(II) complex based on the similar ligand 5carboxy-2-methyl-1H-imidazole-4-carboxylate, where the Cd(II) ion is six-coordinated in a centrosymmetric arrangement (Nie et al., 2007). As depicted in Fig. 1, the title complex has two symmetrical coordination water molecules, two interstitial water molecules and two 4-carboxy-2-ethyl-1H-imidazole-5-carboxylate ligands (H 2 EIDC). The Cd(II), placed on a 2-fold axis, is surrounded by two terminal water molecules, two N atoms and two O atoms from two different H 2 EIDC ligands, forming a distorted octahedral coordination environment.
One solvent water molecule completes the asymmetric unit, and forms hydrogen bonds with the imidazole N atom (N1), the carboxylic O atom (O4) and the O atom from the coordinated water molecule (O1W), whose distances and angles are shown in Table 1. Each H 2 EIDC ligand is bonded to Cd(II) ion in a chelating mode. A three-dimensional supramolecular structure is consolidated by intermolecular hydrogen-bonding (N-H···O and O-H···O) and intramolecular hydrogenbonding O-H···O.

Experimental
A mixture of Cd(NO 3 ) 2 (0.5 mmol, 0.120 g) and 2-ethyl-1H-imidazole-4,5-dicarboxylic acid (0.5 mmol, 0.95 g) in 15 ml of H 2 O solution was placed in a 23 ml Teflon-lined reactor, which was heated to 423 K for 2 days, and then cooled to room temperature at a rate of 10 K h -1 . Crystals of the title compound were obtained by slow evaporation of the solvent at room temperature.

Refinement
The carboxyl H atom H3 was located in a difference map but refined as riding on the parent O atom with O3-H3 = 0.81 Å and U iso (H3) = 1.5 U eq (O3). Carbon and nitrogen bound H atoms were placed at calculated positions and were treated as  Diaquabis(4-carboxy-2-ethyl-1H-imidazole-5-carboxylatoκ 2 N 3 ,O 4 )cadmium dihydrate Crystal data [Cd(C 7