Crystal structure of tris(trans-1,2-cyclohexanediamine-κ2 N,N′)chromium(III) tetrachloridozincate chloride trihydrate from synchrotron data

The CrIII ion in the title compound is coordinated by six N atoms of three chelating 1,2-cyclohexanediamine (chxn) ligands, displaying a distorted octahedral environment. The crystal packing is stabilized by extensive hydrogen-bonding interactions between the N—H groups of the chxn ligands, O—H groups or O atoms of the water molecules, chloride ions and Cl atoms of the disordered [ZnCl4]2− anions.

The structure of the title double salt, [Cr(rac-chxn) 3 ][ZnCl 4 ]ClÁ3H 2 O (chxn is trans-1,2-cyclohexanediamine; C 6 H 14 N 2 ), has been determined from synchrotron data. The Cr III ion is coordinated by six N atoms of three chelating chxn ligands, displaying a slightly distorted octahedral coordination environment. The distorted tetrahedral [ZnCl 4 ] 2À anion, the isolated Cl À anion and three lattice water molecules remain outside the coordination sphere. The Cr-N(chxn) bond lengths are in a narrow range between 2.0737 (12) and 2.0928 (12) Å ; the mean N-Cr-N bite angle is 82.1 (4) . The crystal packing is stabilized by hydrogen-bonding interactions between the amino groups of the chxn ligands and the water molecules as donor groups, and O atoms of the water molecules, chloride anions and Cl atoms of the [ZnCl 4 ] 2À anions as acceptor groups, leading to the formation of a three-dimensional network. The [ZnCl 4 ] 2À anion is disordered over two sets of sites with an occupancy ratio of 0.94:0.06.

Chemical context
trans-1,2-Cyclohexanediamine (chxn) can coordinate to a central metal ion as a bidentate ligand via the two nitrogen atoms, forming a five-membered chelate ring. The synthetic procedures, crystal structures and detailed spectroscopic properties of such [Cr(chxn) 3 ] 3+ complexes with chloride or nitrate anions have been reported previously (Morooka et al., 1992;Choi, 1994;Kalf et al., 2002). Since counter-anionic species play a very important role in coordination chemistry and supramolecular chemistry (Fabbrizzi & Poggi, 2013;Santos-Figueroa et al., 2013), changing the type of anion can also result in different structural properties. With respect to the tetrachloridozincate anion, [ZnCl 4 ] 2À , the crystal structures of complexes with trivalent chromium have been determined for [Cr(NH 3 ) 6 ][ZnCl 4 ]Cl (Clegg, 1976), [Cr(en) (7) to 82.69 (10) . In comparison with the bond lengths and angles of the structure of this complex determined with 223 K data (Kalf et al., 2002), there are no remarkable differences, and also the the crystal packing has virtually identical features. Fig. 1 shows the molecular components of the title compound, (I), which consists of a discrete complex cation [Cr(rac-chxn) 3 ] 3+ , three lattice water molecules, together with one tetrahedral [ZnCl 4 ] 2À and one isolated Cl À counter-ion. The nitrogen atoms of the three 1,2-cyclohexanediamine ligands define a distorted octahedral coordination environment around the Cr(III) ion with a mean N-Cr-N bite angle of 82.1 (4) . The resulting five-membered chelate rings of chxn ligands have the expected stable gauche conformation. The Cr-N(chxn) bond lengths are in the range 2.0737 (12) to 2.0928 (12) Å , in good agreement with those determined in [Cr(RR-chxn) 3 ](NO 3 ) 3 Á3H 2 O (Morooka et al., 1992) and [Cr(rac-chxn) 3 ]Cl 3 Á2H 2 O (Kalf et al., 2002). The disordered tetrahedral [ZnCl 4 ] 2À anion, the isolated Cl À anion and the three water molecules remain outside the coordination sphere of Cr III . The complex [ZnCl 4 ] 2À anion is distorted due to its involvement in hydrogen-bonding interactions. The [ZnCl 4 ] 2À and Cl À anions are well separated by van der Waals contacts and consequently there is no basis for describing the Zn II species as a distorted [ZnCl 5 ] 3À anion.

Supramolecular features
Extensive hydrogen-bonding interactions occur in the crystal structure (Table 1), involving the N-H groups of the chxn ligands and the O-H groups of the lattice water molecules as donors, and the chloride ions and Cl atoms of the disordered [ZnCl 4 ] 2À anions and water O atoms as acceptors. The supramolecular architecture gives rise to a three-dimensional network structure (Fig. 2).

Database survey
A search of the Cambridge Structural Database (Version 5.36, May 2015 with last update; Groom et al., 2016) shows that there are three previous reports for Cr III  The structures of the molecular components of the title double salt, drawn with displacement parameters at the 50% probability level. Dashed lines represent hydrogen-bonding interactions. Table 1 Hydrogen-bond geometry (Å , ).  (1) 3.0878 (17) 179 (2)

Synthesis and crystallization
Commercially available (Aldrich) racemic trans-1,2-cyclohexanediamine was used as provided. All other chemicals with the best analytical grade available were used. The starting material, [Cr(rac-chxn) 3 ]Cl 3 Á2H 2 O was prepared according to the literature (Pedersen, 1970). The crude trichloride salt (0.22 g) was dissolved in 10 mL of 1 M HCl at 313 K and 5 mL of 1 M HCl containing 0.5 g of solid ZnCl 2 were added to this solution. The resulting solution was filtered and allowed to stand at room temperature for one week to give block-like yellow crystals of the tetrachloridozincate(II) chloride salt suitable for X-ray structural analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were found from difference maps but were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H = 0.99-1.00 Å and N-H = 0.91 Å , and with U iso (H) values of 1.2 or 1.5U eq of the parent atoms. The hydrogen atoms of water molecules were restrained using DFIX and DANG commands during the least-squares refinement (Sheldrick, 2015b). The [ZnCl 4 ] 2À anion was refined as positionally disordered over two sets of sites with a refined occupancy ratio constrained to 0.94:0.06 in the last refinement cycles.  Data collection: PAL BL2D-SMDC (Shin et al., 2016); cell refinement: HKL3000sm (Otwinowski & Minor, 1997); data reduction: HKL3000sm (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Putz & Brandenburg, 2014); software used to prepare material for publication: publCIF (Westrip, 2010). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (