Crystal structure of trans-difluoridotetrakis(pyridine-κN)chromium(III) trichlorido(pyridine-κN)zincate monohydrate from synchrotron data

The CrIII atoms in the title compound show a distorted octahedral coordination with four pyridine N atoms in the equatorial plane and two F atoms in axial positions. The [ZnCl3(C5H5N)]− anion has a distorted tetrahedral geometry.

In the asymmetric unit of the title compound, [CrF 2 (C 5 H 5 N) 4 ]-[ZnCl 3 (C 5 H 5 N)]ÁH 2 O, there are two independent complex cations, one trichlorido(pyridine-N)zincate anion and one solvent water molecule. The cations lie on inversion centers. The Cr III ions are coordinated by four pyridine (py) N atoms in the equatorial plane and two F atoms in a trans axial arrangement, displaying a slightly distorted octahedral geometry. The Cr-N(py) bond lengths are in the range 2.0873 (14) to 2.0926 (17) Å while the Cr-F bond lengths are 1.8609 (10) and 1.8645 (10) Å . The [ZnCl 3 (C 5 H 5 N)] À anion has a distorted tetrahedral geometry. The Cl atoms of the anion were refined as disordered over two sets of sites in a 0.631 (9):0.369 (9) ratio. In the crystal, two anions and two water molecules are linked via O-HÁ Á ÁCl hydrogen bonds, forming centrosymmetric aggregates. In addition, weak C-HÁ Á ÁCl, C-HÁ Á Á andstacking interactions [centroid-centroid distances = 3.712 (2) and 3.780 (2)Å ] link the components of the structure into a three-dimensional network.

Chemical context
Anionic species play very important roles in chemistry, medicine, catalysis, molecular assembly, biology and environmental processes, yet their binding characteristics have not received much recognition (Martínez-Má ñ ez & Sancenó n, 2003;Fabbrizzi & Poggi, 2013). The study of the effect of anions and geometric isomers in octahedral metal complexes may be expected to yield a great variety of new structures and properties of both chemical and biological significance. Octahedral Cr III complexes and their 3d-4f clusters containing lanthanides revealing paramagnetic features are of great importance for the development of new molecule-based magnets and solid-state laser materials (Powell, 1998;Dreiser et al., 2012;Singh et al., 2013). We are therefore interested in the preparation, crystal structures and spectroscopic properties of chromium(III) complexes containing mixed various ligands (Choi, 2000a,b;Choi et al., 2004Choi et al., , 2006).
The [ZnCl 3 (py)] À anion and uncoordinating water molecule remain outside the coordination sphere. In the counter-anion, the Zn II ion is in a distorted tetrahedral environment, coordinated by one N atom of the py ligand and by three Cl atoms. The Cl atoms of the anion were refined as disordered over two sets of sites in a 0.631 (9):0.369 (9) ratio (Fig. 2). The Zn-Cl distances, ranging from 2.126 (14) to 2.360 (2) Å , and the Zn-N(py) distance of 2.075 (2) Å are in agreement with those found in the anion of [Cr(acacen)(py) 2 ][ZnCl 3 (py)] [acacen = N,N 0 -ethylenebis(acetylacetoneiminato)] (Toscano et al., 1994). The mean Cl-Zn-Cl angle of 115.22 is larger than the corresponding tetrahedral angle and the mean Cl-Zn-N angle of 105.45 (10) . The charge of the trichlorido(pyridine)zincate anion is counter-balanced by two half trans-[CrF 2 (py) 4 ] + cations. The complex cations lie on inversion centers and therefore the cations have exact molecular C i symmetry.

Supramolecular features
In the crystal, two anions and two water molecules are linked via O-HÁ Á ÁCl hydrogen bonds, forming centrosymmetric aggregates with R 4 4 (12) rings (Fig. 3). In addition, weak C-HÁ Á ÁCl (Table 2), C-HÁ Á Á (Table 3) andstacking The molecular structure of the title compound showing 50% probability displacement ellipsoids. Only one of the independent cations is shown. The minor disorder component of the anion is not shown. The primed atoms are related by the symmetry code (Àx, Ày + 1, Àz).

Figure 2
The molecular structure of the anion. The minor disorder component is shown with dashed lines.

Figure 3
Part of the crystal structure with hydrogen bonds shown as dashed lines.

Synthesis and crystallization
All chemicals were reagent grade materials and used without further purification. The starting material, trans-[CrF 2 (py) 4 ]-ClO 4 was prepared according to the literature (Glerup et al., 1970). The crude trans-[CrF 2 (py) 4 ]ClO 4 (0.2 g) was dissolved in 10 mL water. The 10 mL solution of 1M HCl and 0.5 g of ZnCl 2 were added to this solution. The mixture was refluxed at 328 K for 30 min and then cooled to room temperature. The crystalline product which formed was filtered, washed with cold 2-propanol and diethyl ether. Recrystallization from a hot aqueous solution of the title compound yielded purple crystals suitable for X-ray structure analysis.