trans-Diaqua-bis-[5-carb-oxy-4-carboxyl-ato-2-(4-pyridinio)-1H-imidazol-1-ido-κN,O]zinc(II).

In the title complex, [Zn(C(10)H(6)N(3)O(4))(2)(H(2)O)(2)], the Zn(II) atom is located on a twofold rotation axis and is coordinated by two trans-positioned N,O-bidentate and zwitterionic 5-carb-oxy-4-carboxyl-ato-2-(4-pyridinio)-1H-imidazol-1-ide (H(2)PIDC(-)) ligands and two water mol-ecules, defining a distorted octa-hedral environment. The complete solid-state structure can be described as a three-dimensional supra-molecular framework, stabilized by extensive hydrogen-bonding inter-actions involving the coordinated water mol-ecules, uncoordin-ated imidazole N atom, protonated pyridine N and carboxyl-ate O atoms of the H(2)PIDC(-) ligands.

In the title complex, [Zn(C 10 H 6 N 3 O 4 ) 2 (H 2 O) 2 ], the Zn II atom is located on a twofold rotation axis and is coordinated by two trans-positioned N,O-bidentate and zwitterionic 5-carboxy-4carboxylato-2-(4-pyridinio)-1H-imidazol-1-ide (H 2 PIDC À ) ligands and two water molecules, defining a distorted octahedral environment. The complete solid-state structure can be described as a three-dimensional supramolecular framework, stabilized by extensive hydrogen-bonding interactions involving the coordinated water molecules, uncoordinated imidazole N atom, protonated pyridine N and carboxylate O atoms of the H 2 PIDC À ligands.
As an extension of our previous investigations, we have isolated a new Zn(II) complex, [Zn(H 2 PIDC) 2 (H 2 O) 2 ], (I), by the reaction of H 3 PIDC and Zn(II) diacetate under the hydrothermal condition. We report here the single-crystal structure of this complex.
As shown in Fig. 1, the molecule of (I) is a discrete neutral monomer, in which the Zn atom resides on a crystallographic inversion centre and the asymmetric unit contains one-half of the [Zn(H 2 PIDC) 2 (H 2 O) 2 ] formula unit. Each Zn atom is six-coordinated by N 2 O 4 with two chelating rings from two H 2 PIDC ligands arranged symmetrically in the equatorial plane and two water molecules occupying the apical sites, showing a distorted octahedral geometry (Table 1). In this complex, one carboxyl group and imidazole group are deprotonated and the pyridyl group is protonated, and the ligand bears a formal charge of -1, and the uncoordinated carboxylate atoms O3 and O4 form an intramolecular hydrogen bond (Table 2). All non-H atoms in the imidazole-4,5-dicarboxyl group are nearly coplanar [the mean deviation is 0.075 (3) Å], and the dihedral angle between imidazole group and pyridine group is 11.4 (2) °.
A three-dimensional supramolecular network is constructed via hydrogen-bonding interactions involving the coordinated water molecules, uncoordinated imidazole N atom, protonated pyridine N and carboxylate O atoms of the H 2 PIDCligands (Table 2 and Fig. 2).

Refinement
H atoms attached to N and O atoms were located in a difference Fourier maps and refined as riding in their as-found relative positions, with U iso (H) = 1.5U eq (O,N). Other H atoms were positioned geometrically with C-H = 0.95 Å and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C). Fig. 1. A view of the molecular of (I), showing the atom-labelling scheme and displacement ellipsoids at the 30% probability level.