Crystal structure of trans-dichlorido(1,4,8,11-tetraazaundecane-κ4 N)chromium(III) perchlorate determined from synchrotron data

In the title compound, [CrCl2(C7H20N4)]ClO4, the CrIII ion is coordinated by four N atoms from the 1,4,8,11-tetraazaundecane ligand and two chloride ions in a trans geometry, displaying a distorted octahedral arrangement. The crystal packing is stabilized by N—HCl and N—H⋯O hydrogen bonds.

The structure of the title complex, [CrCl 2 (2,3,2-tet)]ClO 4 (2,3,2-tet is 1,4,8,11tetraazaundecane, C 7 H 20 N 4 ), has been determined from synchrotron data. The Cr III ion is coordinated by the four N atoms of the 1,4,8,11-tetraazaundecane ligand in the equatorial plane and two chloride ions in an axial arrangement, displaying a slightly distorted octahedral coordination environment. The two H atoms of the secondary amines are grouped on the same side of the equatorial N 4 plane (meso-RS conformation). The Cr-N bond lengths range from 2.069 (2) to 2.084 (2) Å , while the mean Cr-Cl bond length is 2.325 (2) Å . The crystal structure is stabilized by intermolecular hydrogen-bonding interactions between the primary and secondary amine groups of the 2,3,2-tet ligands, the Cl ligands and the O atoms of the perchlorate counter-anion, forming corrugated layers parallel to (010).

Chemical context
Geometric and conformational isomerism in chromium(III) complexes of linear flexible tetradentate ligands is an interesting field because it has played an important role in extending the concept of stereochemistry. The 1,4,8,11-tetraazaundecane ligand (2,3,2-tet) is a structural isomer of 1,4,7,11-tetraazaundecane (2,2,3-tet). These two ligands have four nitrogen atoms as donor groups and can adopt three different configurations in chromium(III) complexes with two ISSN 2056-9890 Figure 1 Schematic representation of the 2,3,2-tet and 2,2,3-tet ligands, and two possible conformational isomers of trans-[CrCl 2 (2,3,2-tet)] + . additional Cl ligands (Choi et al., 2008a,b). Two conformations of meso-RS or racemic-RR/SS isomers with respect to the orientation of the secondary amine hydrogen atoms in the trans isomer are also possible (Fig. 1). The two hydrogen atoms of the conformers may be on the same side (RS) of the equatorial N 4 plane or on opposite sides (RR/SS) of this plane.
The different symmetries of transition metal complexes allow the determination of their stereochemistry from electronic absorption and infrared spectra. Indeed, infrared and electronic spectroscopic properties often are useful in determining the geometric isomers of chromium(III) complexes with linear tetradentate ligands (House & Garner;1966;Kutal & Adamson, 1973;House & Yang, 1983;Kirk & Fernando, 1994). However, it should be noted that the geometric assignments based on spectroscopic studies alone are less conclusive. Both trans and cis isomers of [CrCl 2 (2,3,2-tet)]-ClO 4 have been isolated (House & Yang, 1983;Kirk & Fernando, 1994). Whereas the crystal structure and spectroscopic properties of the cis--dichloridochromium(III) complexes containing the 2,3,2-tet ligand were reported (Choi et al., 2008b), the trans isomers with any anion have so far not been structurally characterized. The orientation of the secondary amine hydrogen atoms in the metal complexes is also highly relevant for medical application and likely to be a major factor in determining the antiviral activity (Ronconi & Sadler, 2007;Ross et al., 2012). In order to confirm the orientation of the secondary N-H hydrogen atoms of the Cr(III) complex with 2,3,2-tet and additional Cl ligands, we report the structure of the title compound, trans-[CrCl 2 (2,3,2tet)]ClO 4 , (I), in this communication. In the distorted octahedral complex chromium(III) cation, the four N atoms of the 2,3,2-tet ligand occupy the equatorial sites and the two chlorine atoms coordinate axially to the metal. The two hydrogen atoms of the secondary amine groups are grouped on the same side (meso-RS type) of the equatorial N 4 plane. Such a conformation is consistent with those of trans-[CrF 2 (2,3,2-tet)]ClO 4 (Bang & Pedersen, 1978) and trans-[Cr(NCS) 2 (2,3,2-tet)]NCS (Mä cke et al., 1982). The meso-RS conformation may be compared with rac-RR/SS types of trans-[CrF 2 (2,2,3-tet)]ClO 4 (Choi & Moon, 2014) and trans-[CrF(3,2,3-tet)(H 2 O)](ClO 4 ) 2 ÁH 2 O (Choi & Lee, 2008).

Synthesis and crystallization
The free ligand 1,4,8,11-tetraazaundecane was purchased from Strem Chemical Company, USA. All other chemicals were reagent grade materials and were used without further purification. Compound (I) was prepared by a literature method (Kirk & Fernando, 1994 The crystal packing of complex (I) viewed perpendicular to (010). Dashed lines represent N-HÁ Á ÁO (pink) and N-HÁ Á ÁCl (green) hydrogenbonding interactions, respectively.

Figure 4
The crystal packing of complex (I) viewed approximately along [100]. The colour code is as in Fig. 3.  for slow evaporation at room temperature. Green block-like crystals suitable for X-ray structural analysis were isolated after one week. The crystals were washed with small amounts of 2-propanol and dried in air before collecting the synchrotron data.