Crystal structure of bis[bis(1,4,7-triazacyclononane-κ3 N,N′,N′′)chromium(III)] tris(tetrachloridozincate) monohydrate from synchrotron X-ray data

Each CrIII cation in the title compound is coordinated by the six N atoms from two 1,4,7-triazacyclononane (tacn) ligands, displaying a distorted octahedral environment. The crystal packing is stabilized by extensive hydrogen-bonding interactions involving the N—H groups of the tacn ligands, O—H groups or O atoms of the water molecules and Cl atoms of the [ZnCl4]2− anions.

The title compound is another example of a [Cr(tacn) 2 ] 3+ complex but with a different counter-anion. In order to confirm that the crystal is a salt of the [ZnCl 4 ] 2À anion, we report here the molecular and crystal structure of the new ISSN 2056-9890 complex [Cr(tacn) 2 ] 2 [ZnCl 4 ] 3 ÁH 2 O, (I) determined from synchrotron X-ray data.

Structural commentary
The X-ray structural determination of (I) was carried out at 100 (2) K with synchrotron radiation to confirm its exact geometry and composition. The structure consists of two independent [Cr(tacn) 2 ] 3+ cations, three [ZnCl 4 ] 2À anions and one lattice water molecule. Fig. 1 shows an ellipsoid plot of the asymmetric unit of compound (I) with the atomic labelling scheme. The Cr III cation in both [Cr1A(tacn) 2 ] 3+ and [Cr2B(tacn) 2 ] 3+ is coordinated by the six N atoms from the two tacn ligands, displaying a distorted octahedral geometry. The Cr-N(tacn) bond distances for [Cr1A(tacn) 2 ] 3+ and [Cr2B(tacn) 2 ] 3+ are in the ranges 2.0709 (11) to 2.0828 (11) Å and 2.0621 (11) to 2.0851 (11) Å , respectively, in good agreement with the observed values in [Cr(tacn)  The angles are N-M-N intra for the intraligand angles, and N-M-N trans and N-M-N inter for trans and cis interligand angles, respectively (Lord et al., 2009  The structures of the molecular components in the asymmetric unit of the title complex (I), drawn with displacement ellipsoids at the 70% probability level. Dashed lines represent hydrogen-bonding interactions. The H atoms on the C atoms have been omitted for clarity.
Each ZnCl 4 2À anion has a slightly distorted tetrahedral coordination geometry because of the influence of hydrogen bonding on the Zn-Cl lengths and the Cl-Zn-Cl angles. The Zn-Cl bond lengths involved in hydrogen bonds were all found to have longer bonds than those not involved.

Supramolecular features
Extensive hydrogen-bonding interactions occur in the crystal structure (Table 1). The supramolecular architecture involves hydrogen-bonding interactions with the N-H groups from each of the tacn ligands, the O-H groups of the lattice water molecules acting as donors, and Cl atoms of the [ZnCl 4 ] 2À anions and the O atoms of the water molecules acting as acceptors, giving rise to a three-dimensional network structure. The network comprises columns of molecules that form along the a-axis direction (Fig. 2). These hydrogen-bonded networks help to stabilize the crystal structure.  (Zompa & Margulis, 1978), [Fe(tacn) 2 ]Cl 3 Á5H 2 O (Boeyens et al., 1985), [Pd(tacn) 2 ](PF 6 ) 3 (Blake et al., 1988) and [Co(tacn) 2 ](ClO 4 ) 3 (Wang et al., 2002) have been published previously. However, only one structure containing the [Cr(tacn) 3 ] 3+ form is present (Scarborough et al., 2011). Each metal ion in all of these complexes is sandwiched between two tridentate tacn macrocycles. Until now, no structure of any salt of [Cr(tacn) 2 ] 3+ with the [ZnCl 4 ] 2À anion has been deposited.

Synthesis and crystallization
Commercially available (Sigma-Aldrich) 1,4,7-triazacyclononane was used as provided. All other chemicals were the best AR grade available. The starting material [Cr(tacn) 2 ]Cl 3 was prepared according to the literature (Wieghardt et al., 1983). The crude trichloride salt (0.10 g) was dissolved in 7 mL of 0.5 M HCl at 313 K. 5 mL of a 1 M HCl solution containing 0.25 g of solid ZnCl 2 were added to this solution. The resulting mixture was filtered, and allowed to stand at room temperature for two days to give plate-like yellow crystals of the title tetrachloridozincate(II) salt suitable for single-crystal X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Non-hydrogen atoms were refined anisotropically. All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H = 0.99 Å and N-H = 1.00 Å , and with U iso (H) values of 1.2U eq of the parent atoms. The O-bound H atoms of the water molecules were assigned based on a difference-Fourier map, and were refined with distance restraints of 0.95 (10) Å (using the DFIX and DANG commands), and U iso (H) values of 1.5U eq of the oxygen atom.     Table 1 Hydrogen-bond geometry (Å , ).  (1) 3.2520 (15) 166 (2) Symmetry codes:

Bis[bis(1,4,7-triazacyclononane-κ 3 N,N′,N′′)chromium(III)] tris(tetrachloridozincate) monohydrate
Crystal data 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 )