Tetraaquabis(3-fluoropyridine-4-carboxylato-κN)zinc(II) dihydrate

In the title compound, [Zn(C6H3FNO2)2(H2O)4]·2H2O, the ZnII atom is octahedrally coordinated in a ZnO4N2 environment by two 3-fluoropyridine-4-carboxylate (3-fpy4-cbx) ligands and four water molecules. The [Zn(3-fpy4-cbx)2(H2O)4] molecules form a three-dimensional network through strong O—H⋯O and weak O—H⋯F hydrogen bonds between 3-fpy4-cbx and water molecules. The crystal used for data collection was a twin, with the twin law corresponding to a 180° rotation about the real-space [001] axis. The major twin fraction refined to 0.795 (1).

Financial support from the National Science Foundation, awards CHE-0714555 and CHE-0714439, is gratefully acknowledged.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: JJ2018). Metal-organic compounds based on multifunctional ligands that contain both pyridyl and carboxylate donor atoms have been under study in part because of their diverse coordination modes and because they may exhibit useful properties (Ellsworth et al., 2008;Erxleben, 2003). Within this context, the 3-fluoropyridine-4-carboxylate ligand (3-fpy4-cbx), has attracted our interest as a potential component for the construction of these novel materials. A further motivation is that its nonfluorinated analogue, the isonicotinate ligand (ina), has been successfully utilized to generate metal-organic frameworks having the desirable properties alluded to above (Chen et al., 2009;Evans et al., 1999;Wang et al., 2006;Xie et al.,2008). Hence, we have deemed it worthwhile to also explore the coordinatingproperties of the related 3-fpy4-cbx ligand. This ligand has the additional possibility of C-F···H interactions, in contrast to ina. Herein, we wish to report the crystal structure of the title compound (I), which is a hydrogen bonded, three-dimensional framework.
The asymmetric unit of (I) consists of one-half of the [Zn(3-fpy4-cbx) 2 (H 2 O) 4 ] complex and a lattice water. The Zn(II) atom is located on an inversion center through which the other half of the molecular complex and another lattice water are generated from the asymmetric unit, thus completing the formula unit of (I) (Fig. 1).
The Zn(II) atom resides in a distorted ZnO 4 N 2 octahedral environment. The equatorial positions are occupied by four O atoms from water molecules and the axial positions are occupied by N atoms from two 3-fpy4-cbx ligands. The Zn-O bond distances fall within the normal range of 2.0953 (14) -2.1504 (13) Å (Wang et al., 2006), while the Zn-N distances are also normal at 2.1356 (13) Å (Wang et al., 2006). The 3-fpy4-cbx ligand is noticeably noncoplanar, with a dihedral angle of 34.2 (1)° between the mean planes of its carboxylate group and its pyridyl ring.
While the carboxylate group of 3-fpy4-cbx is not coordinated to Zn(II), it does assist in the assembly of the crystal structure by acting as hydrogen bond acceptors for both coordinated and lattice waters. The lattice waters are also involved in weak C-F···H 2 O hydrogen bonding with the 3-fpy4-cbx ligand. These interactions create a three-dimensional, hydrogen bonded network (Table 1, Fig. 2).

Experimental
An aqueous solution of sodium 3-fluoropyridine-4-carboxylate (25 ml, 2.0 mmol) was slowly added to an aqueous solution of zinc nitrate hexahydrate (25 ml, 1.0 mmol). Colorless crystals of the title compound were obtained after slow evaporation of the resulting solution under ambient conditions.

Refinement
All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon were placed in geometrically idealized positions and included as riding atoms: C-H = 0.93Å and with U iso (H) = 1.20-supplementary materials sup-2 2.12 U eq (C). Oxygen-bound hydrogen atoms were located in difference Fourier maps and refined isotropically: O-H = 0.74 (2)-0.83 (3) Å and with U iso (H) = 0.94-1.69 U eq (O).

Figures
Fig . 1. The molecular complex plus lattice waters with atom-labeling scheme of (I) showing 50% probability ellipsoids for nonhydrogen atoms. All H atoms except for those of water are omitted for clarity. Hydrogen bonds are represented by dashed lines. Primed atoms are generated by the inversion symmetry operation about Zn(II), with symmetry code: 1 -x, 1 -y, 1 -z.

1.000
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.