Aquabis(4-chloro-2-hydroxybenzoato-κO)(1,10-phenanthroline-κ2 N,N′)zinc(II)

In the title compound, [Zn(C7H4ClO3)2(C12H8N2)(H2O)], the ZnII cation is coordinated by two 4-chloro-2-salicylate anions, one 1,10-phenanthroline ligand and one water molecule in a square-pyramidal coordination geometry; the Zn cation lies 0.4591 (11) Å from the basal plane. The benzene rings of the anions are involved in π–π stacking. The centroid–centroid distance between parallel benzene rings of adjacent molecules is 3.9017 (17) Å, and the centroid–centroid distance between benzene and pyridine rings of adjacent molecules is 3.584 (2) Å. Intramolecular O—H⋯O hydrogen bonding is present.


Comment
As π-π stacking between aromatic rings plays an important role in the electron transfer process in some biological system (Deisenhofer & Michel, 1989), the π-π stacking has attracted our much attention in past years. In order to understand the nature of π-π stacking between aromatic rings, we have determined crystal structures of metal complexes with aromatic ligands to investigate the factors controlling aromatic stacking.
Our previous studies on dihydroxybenzoate complexes has revealed that hydroxy-substitution of the aromatic ring may be an effective factor for π-π stacking (Yang et al., 2006;Zhang et al., 2008). As a continued investigation, the title chlorine-substituted salicylate complex has been prepared in the laboratory and its crystal structure is presented here to show the effect of chlorine-substitution on π-π stacking between benzene rings of chlorine-substituted salicylates.
The molecular structure of the title compound is shown in Fig. 1. The Zn(II) cation is coordinated by one phenanthroline (phen) ligand, two chloro-salicylate (chls) anions and one water molecule in a distorted square-pyramidal coordination geometry ( Table 1). The Zn atom is 0.4591 (12) Å deviated from the basal plane towards the apical O1 atom. Uncoordinated carboxyl oxygen atoms, O2 and O5, are simultaneously hydrogen bonded to the coordinated water molecule and hydroxyl group of the same chls anion (Table 2).
It is notable π-π stacking between benzene rings of chls anions in the crystal structure. A partially overlapped arrangement is observed between parallel chls anions of neighboring complexes (Fig. 2). The face-to-face separation between C14-benzene ring C14 i -benzene ring is 3.449 (3) Å, and the centroid-to-centroid distance is 3.9003 (17) Å [symmetry code: (i) 1 -x, 1 -y, 2 -z]. These facts clearly indicate the existence of aromatic stacking between benzene rings of chls anions.
As π-π stacking interaction does not occur between benzene ring in salicylate complexes (Allen, 2002), but occurs in the chloro-salicylate complex. This reveals the effect of chloro-substitution on aromatic π-π stacking.

supplementary materials sup-2 Refinement
In the final cycles of refinement, a reflection (001) was omitted. Water and hydroxyl H atoms were located in a difference Fourier map and refined as riding in their as-found relative positions with U iso (H) = 1.5U eq (O). Aromatic H atoms were placed in calculated positions with C-H = 0.93 Å and refined in riding mode with U iso (H) = 1.2U eq (C).