Crystal structure of bis[trans-(1,4,8,11-tetraazacyclotetradecane-κ4 N)bis(thiocyanato-κN)chromium(III)] tetrachloridozincate from synchrotron data

The CrIII atoms in the title compound show a distorted octahedral coordination with four N atoms of the cyclam ligand in the equatorial plane and two N-coordinated NCS− groups in axial positions. The macrocyclic ligands adopt trans-III configurations. The crystal packing is stabilized by N—H⋯S and N—H⋯Cl hydrogen bonds.

The structure of the title compound, [Cr(NCS) 2 (cyclam)] 2 [ZnCl 4 ] (cyclam = 1,4,8,11-tetraazacyclotetradecane, C 10 H 24 N 4 ), has been determined from synchrotron data. The asymmetric unit contains two independent halves of the Cr III complex cations and half of a tetrachloridozincate anion. In each complex cation, the Cr III atom is coordinated by the four N atoms of the cyclam ligand in the equatorial plane and by two N-bound NCS À anions in a trans axial arrangement, displaying a distorted octahedral geometry with crystallographic inversion symmetry. The mean Cr-N(cyclam) and Cr-N(NCS) bond lengths are 2.065 (4) and 1.995 (6) Å , respectively. The macrocyclic cyclam moieties adopt centrosymmetric trans-III configurations with six-and five-membered chelate rings in chair and gauche configurations, respectively. The [ZnCl 4 ] 2À anion, which lies about a twofold rotation axis, has a slightly distorted tetrahedral geometry. The crystal packing is stabilized by hydrogen-bonding interactions between the N-H groups of the cyclam ligands, the S atoms of the NCS À groups and the Cl À ligands of the anion.

Chemical context
In recent years, it has been found that cyclam (1,4,8,11-tetraazacyclotetradecane, C 10 H 24 N 4 ) derivatives and their metal complexes exhibit anti-HIV activity (Ronconi & Sadler, 2007;De Clercq, 2010;Ross et al., 2012). The cyclam derivatives inhibit the entry of the virus into white cells by binding to CXCR4, a chemokine receptor in the outer membrane. The strength of binding to the CXCR4 receptor correlates with the anti-HIV activity. The cyclam ligand has a moderately flexible Possible configurations for trans-cyclam complexes with the trans-III configuration adopted by the title compound highlighted in blue. structure, and can adopt both planar (trans) and folded (cis) configurations (Poon & Pun, 1980). There are five configurational trans isomers for this type of macrocycle, Fig. 1, that differ in the chirality of the sec-NH groups ). The trans-V configuration can also fold to form the cis-V isomer (Subhan et al., 2011). In addition, the thiocyanate anion can be present in complexes as either a ligand or a non-coordinating anion (Moon et al., 2013). Furthermore it can coordinate to metals as a terminal ligand through either the nitrogen or the sulfur atoms, or can use both donor atoms and function as a bridging ligand.
Counter-anionic species play a very important role in the coordination chemistry, pharmacy and biology (Fabbrizzi & Poggi, 2013)

Structural commentary
Each of the two trans-[Cr(NCS) 2 (cyclam)] + cations in the structure of the title compound are generated by inversion symmetry, hence the configurations of the cyclam ligands can be described as trans-III, Fig. 1. The Cr III cations, which are located on discrete inversion centres, are coordinated by the nitrogen atoms of the cyclam ligands that occupy equatorial sites. Two thiocyanate anions complete the distorted octahedral coordination sphere binding through their N atoms in a trans configuration. The single [ZnCl 4 ] 2À anion, which lies about a twofold rotation axis, has slightly distorted tetrahedral geometry and completes the complex salt. Fig. 2 shows an ellipsoid plot of (I), with the atom-numbering scheme. This is a second example of the structure of a trans-[Cr(NCS)] 2 -(cyclam)] + salt, but the previous example had a perchlorate counter-anion (Friesen et al., 1997).

Figure 3
The molecular packing in (I), viewed along the a axis. Dashed lines represent hydrogen-bonding interactions N-HÁ Á ÁCl (cyan) and N-HÁ Á ÁS (purple), respectively. H atoms bound to C have been omitted. chelate rings adopt gauche configurations while the sixmembered rings are in chair configurations. The average bite angles of the five-and six-membered chelate rings around the chromium(III) atoms are 85.51 (4) and 94.49 (4) , respectively. The N-coordinated NCS ligands are almost linear, with N-C-S angles of 177.42 (12) in cation A and 178.66 (12) in cation B. The C6A-S1A bond length [1.6126 (12) Å ] in the Cr1A complex cation is slightly longer than the C6B--S1B bond length [1.6056 (12) Å ] in the Cr2B complex cation. This elongation may be attributed to the weak hydrogen bond formed by S1A with the N2A-H2A group of the cyclam ligand.

Supramolecular features
Each complex molecule forms three classical N-HÁ Á ÁCl hydrogen bonds between the amine groups of the cyclam ligand in each complex cation and the Cl atoms of the tetrachloridozincate anion,

Synthesis and crystallization
The free ligand cyclam was purchased from Strem Chemicals and used as provided. All chemicals were reagent-grade materials and were used without further purification. The starting material, trans-[Cr(NCS) 2 (cyclam)]ClO 4 , was prepared according to the literature (Friesen et al., 1997). The perchlorate salt (0.33 g) was dissolved in 10 mL of 0.1 M HCl at 333 K and added to 7.5 mL of 6 M HCl containing 0.75 g of solid ZnCl 2 . The resulting solution was filtered, and allowed to stand at room temperature for two days to give pale-yellow crystals of (I) suitable for X-ray structural analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in