Hexaaquazinc(II) bis[tris(3-carboxypyridine-2-carboxylato)zincate(II)]

The title compound, [Zn(H2O)6][Zn(C7H4NO4)3]2, consists of two [Zn(py-2,3-dcH)3]− anions (py-2,3-dcH is 3-carboxypyridine-2-carboxylate) and one [Zn(H2O)6]2+ cation. The anion is a six-coordinate complex located on a threefold rotation axis with a slightly distorted octahedral geometry around Zn2+ ion. The cation is also six-coordinate with an octahedral geometry around the Zn atom, located at a axis. Non-covalent interactions such as π–π stacking [centroid–centroid distance = 3.828 (4)Å] and O—H⋯O hydrogen bonds play important roles in stabilizing the supramolecular structure.


Experimental
Crystal data [Zn(H 2   Metal-organic coordination complexes with one-, two-or three-dimensional structures have attracted attentions for their potential applications as photoelectric materials, catalysis, carriers, sensors, etc. (Prior & Rosseinsky, 2001;Swiegers & Malefetse, 2000). From a chemical point of view, L-lysine, a base, contains two amino groups and one carboxylic acid group, these amino groups often participate in hydrogen bonding and as a general base in catalysis. There are previously reported compounds containing pyridine-2,3-dicarboxylic acid, (py-2,3-dcH 2 ), (Goher et al., 1993;Yin & Liu 2009;Aghabozorg, Daneshvar et al., 2007, Kang et al., 2006. The title compound consists of two [Zn(py-2,3-dcH) 3 ]¯ anions and one Zn(H 2 O) 6 2+ cation. The anion is a six-coordinate complex located at a 3-fold crystallographic axis around Zn1 atom by three chelating (py-2,3-dcH)¯ ligands via one N and one O atom from carboxylate groups (Fig. 1). The cation is also six-coordinate with an octahedral geometry around Zn2 atom, located at a 3-bar axis. L-lysine, even it was included during the synthesis, is not part of this crystal structure and we were surprised that the product material contains Zn atoms in both form of cation and anion units. In anionic complex the three O-Zn1-N angles indicate that there is a distorted octahedral geometry around Zn1 atoms (for selected bond distances and angles see Table 1), but in cationic unit there are three O-Zn2-O angles exactly 180° as imposed by the crystallographic symmetry and good octahedral geometry environment around Zn2 atom (Table 1). The anionic Zn-O distances (Table 1) fall within the range of those found in related Zn complexes, 2.031-2.117 Å (Aghabozorg, Daneshvar et al., 2007;Aghabozorg, Sadr-khanlou et al., 2007;Kang, et al., 2006;Li, et al., 2006;Yin, et al., 2009).
There are three principal hydrogen bonds of O-H···O type (see Table 2) forming a complicated and extensive hydrogen bonding pattern. Graph-set analysis (Bernstein et al., 1995) on the first level is indicating chains with symbols C 2 2 (20) and C 2 2 (16) for hydrogen bond with donor atoms O3 and O5, respectively, as well as ring R 2 2 (20) for hydrogen bond with O3 as a donor. On second level graph-set, most important are ring R 1 2 (6) and chains C 2 2 (14) and C 2 2 (16) between hexaaqua zinc cations, as well as rings R 3 3 (15) formed between anions and cations.

Refinement
Aromatic hydrogen atoms were refined isotropically with U iso (H) = 1.2U eq (C) and their positions were constrained to ideal geometry using an appropriate riding model, with C-H = 0.95. The carboxylate and water H atoms were located at the difference Fourier map and refined isotropically with U iso (H) = 1.5U eq (O), and restrained to ideal geometry with O-H distances 0.84 (2)Å and H···H distance 1.34 (2) Å for the water molecule.
Figures Fig. 1. The molecular structure of the title compound, with the displacement ellipsoids drawn at 50% probability level.  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 Rfactors(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )