Bis(μ-3-carboxy-2-hydroxypropane-1,2-dicarboxylato)bis(diaquazinc)–1,2-bis(pyridin-4-yl)ethene–water (1/1/2)

The asymmetric unit of the title compound, [Zn2(C6H6O7)2(H2O)4]·C12H10N2·2H2O, comprises half of a centrosymmetric complex dimer, half of a 1,2-bis(pyridin-4-yl)ethene molecule, which lies across an inversion centre, and one lattice water molecule. Carboxylate groups of two dianionic citrate ligands bridge two ZnII ions to give the cyclic dimer, with each ZnII ion coordinated by four O atoms from the chelating citrate ligand (one hydroxy and three carboxylate, with one bridging) and two water O atoms, forming a distorted octahedral environment [Zn—O = 2.040 (3)–2.244 (3) Å]. In the crystal, O—H⋯O and O—H⋯N hydrogen bonds involving hydroxy groups and both coordinating and lattice water molecules link the dimers to give a three-dimensional framework structure.

The asymmetric unit of the title compound, [Zn 2 (C 6 H 6 O 7 ) 2 -(H 2 O) 4 ]ÁC 12 H 10 N 2 Á2H 2 O, comprises half of a centrosymmetric complex dimer, half of a 1,2-bis(pyridin-4-yl)ethene molecule, which lies across an inversion centre, and one lattice water molecule. Carboxylate groups of two dianionic citrate ligands bridge two Zn II ions to give the cyclic dimer, with each Zn II ion coordinated by four O atoms from the chelating citrate ligand (one hydroxy and three carboxylate, with one bridging) and two water O atoms, forming a distorted octahedral environment [Zn-O = 2.040 (3)-2.244 (3) Å ]. In the crystal, O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds involving hydroxy groups and both coordinating and lattice water molecules link the dimers to give a three-dimensional framework structure.

In Hong Hwang, Pan-Gi Kim, Cheal Kim and Youngmee Kim Comment
Citric acid has often been used as a model ligand to examine the interaction between transition metal ions with biologically active molecules (Daniele et al., 2008;Parkin, 2004;Tshuva & Lippard, 2004;Stoumpos et al., 2009).
Recently, our group has also reported a novel compound from the reaction of manganese(II) nitrate as a building block and citric acid as a ligand (Hwang et al., 2012). In order to study the effects of secondary metal ions on the interaction between transition metal ions and citric acid Yu et al., 2009;Kim et al., 2011), we have employed zinc as a metal ion source. We report here the structure of [Zn 2 (H 2 O) 4 (C 6 H 8 O 7 ) 2 ].C 12 H 10 N 2 .2H 2 O.

Refinement
H atoms bonded to carbon were placed in calculated positions with C-H = 0.95 Å (aromatic C) and 0.99 Å (methylene C) and were included in the refinement in a riding-motion approximation with U iso (H) = 1.2U eq (C). The H atom bonded to

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.