Crystal structure of bis[(oxalato-κ2 O 1,O 2)(1,4,8,11-tetraazacyclotetradecane-κ4 N)chromium(III)] dichromate octahydrate from synchrotron X-ray data

The asymmetric unit of the title compound comprises of one complex cation, one half of a [Cr2O7]2− anion and four water molecules. The CrIII ion has a distorted octahedral coordination by four N atoms of the cyclam ligand and one bidentate oxalate ligand in the cis positions; the conformation of the dichromate anion is staggered.

The asymmetric unit of the title compound, [Cr(C 2 O 4 )(C 10 H 24 N 4 )] 2 [Cr 2 O 7 ]Á-8H 2 O (C 10 H 24 N 4 = 1,4,8,11-tetraazacyclotetradecane, cyclam; C 2 O 4 = oxalate, ox) contains one [Cr(ox)(cyclam)] + cation, one half of a dichromate anion that lies about an inversion centre so that the bridging O atom is equally disordered over two positions, and four water molecules. The terminal O atoms of the dichromate anion are also disordered over two positions with a refined occupancy ratio 0.586 (6):0.414 (6). The Cr III ion is coordinated by the four N atoms of the cyclam ligand and one bidentate oxalato ligand in a cis arrangement, resulting in a distorted octahedral geometry. The Cr-N(cyclam) bond lengths are in the range 2.069 (2)-2.086 (2) Å , while the average Cr-O(ox) bond length is 1.936 Å . The macrocyclic cyclam moiety adopts the cis-V conformation. The dichromate anion has a staggered conformation. The crystal structure is stabilized by intermolecular hydrogen bonds involving the cyclam N-H groups and water O-H groups as donors, and the O atoms of oxalate ligand, water molecules and the Cr 2 O 7 2À anion as acceptors, giving rise to a three-dimensional network.

Chemical context
Chromium (Cr) is considered a trace element essential for the proper functioning of living organisms and is also a highly toxic material (Yusof & Malek, 2009). Cr can exist in all oxidation states from 0 to VI, the most common oxidation states in water being Cr III and Cr VI . In an aqueous environment, the toxicity of Cr VI has been shown to be greater than that of Cr III (Guzel et al., 2016). Transition metal complexes of the cyclam (1,4,8,11-tetraazacyclotetradecane, C 10 H 24 N 4 ) ligand have been the subject of numerous investigations because of their particular conformational stereochemistry (Poon & Pun, 1980;Choi, 2009;Subhan et al., 2011). Recently, it has been found that cyclam derivatives and their metal complexes exhibit anti-HIV activity (Ronconi & Sadler, 2007;De Clercq, 2010;Ross et al., 2012). The conformation of the macrocyclic ligand is a very important factor for co-receptor recognition. Therefore, knowledge of the conformation and hydrogen-bonding interactions in Cr III -Cr VI complex systems containing the cyclam ligand has become important in the development of new anti-HIV drugs (De Clercq, 2010). The use of such complexes for the more effective removal of toxic metals is also important (Guzel et al., 2016). As part of a study of the conformation and structure of (cyclam)chromium(III) complexes with auxiliary ligand(s) and various anions, we

Structural commentary
An ellipsoid plot of the molecular components in (I) is shown in Fig. 1 along with the atom-numbering scheme. The structure is another example of a [Cr(ox)(cyclam)] + cation (Choi et al., 2004b;Moon & Choi, 2016b), but with a different counteranion. The asymmetric unit contains one [Cr(ox)(cyclam)] + cation, one half of a Cr 2 O 7 2À anion (completed by inversion symmetry with the bridging O atom disordered about the inversion centre) and four non-coordinating water molecules. The three terminal O atoms of the dichromate anion are also disordered over two positions with occupancy ratio of the atom pairs O2B1/O2B2, O3B1/O3B2 and O4B1/O4B2 converging at 0.586 (6):0.414 (6). The conformation of the cyclam ligand can be described as cis-V (anti-anti) (Subhan et al., 2011). In the complex cation, the Cr III ion is coordinated by the four nitrogen atoms of the cyclam ligand in a folded conformation. Two oxygen atoms of the oxalato ligand complete the distorted octahedral coordination sphere. The Cr-N bond lengths from the donor atoms of cyclam ligand lie in the range 2.069 (2) to 2.086 (2) Å , in good agreement with those determined in cis-[Cr(N 3 ) 2 (cyclam)]ClO 4 [2.069 (3)-2.103 (3) Å ] (Meyer et al., 1998) (Taylor, 1978), respectively. The five-and six-membered chelate rings of the cyclam ligand adopt gauche and stable chair conformations, respectively. As expected for a bidentate ox ligand, the O1A-Cr1A-O3A bite angle 82.34 (7) is considerably less than 90 , while the folding angle of the cyclam in the [Cr(ox)(cyclam)] + cation is 98.97 (8) . The significant distortion of the octahedron and the larger folding angle in the [Cr(ox)(cyclam)] + cation seem to arise from the small bite angle of the bidentate oxalato ligand.
It is of interest to compare the conformation of the Cr 2 O 7 2À anion with that found in other ionic crystals. In (I), the Cr 2 O 7 2À anion exhibits a staggered conformation whereas a nearly eclipsed conformation is observed for (C 9 H 14 N) 2 [Cr 2 O 7 ] and (C 10 H 22 N 2 )[Cr 2 O 7 ], when viewed along the backbone of the dichromate anion (Trabelsi et al., 2015;Chebbi et al., 2016). This structural conformation of dichromate seems to depend on the size of the associated countercation (Moon et al., 2015(Moon et al., , 2017.

Supramolecular features
In the asymmetric unit, O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds link the water molecules to the Cr 2 O 7 2À anion, A perspective view of the asymmetric unit of the title of compound, (I), with the dichromate anion, which lies about an inversion centre, drawn in full. Displacement ellipsoids are drawn at the 30% probability level and primed atoms are related by the symmetry operation (2 À x, Ày, 1 À z).
For clarity, only the major disorder components are shown for the disordered dichromate anion.

Synthesis and crystallization
The free ligand cyclam (98%) was purchased from Sigma-Aldrich and used without further purification. All chemicals were reagent grade materials, and were used as received. The starting material, [Cr(ox)(cyclam)]ClO 4 was prepared according to the literature method (House & McKee, 1984). The perchlorate salt of the complex (0.03 g) was dissolved in 10 mL of distilled water at 347 K. The solution was filtered and the filtrate was added to 5 mL of water containing solid K 2 Cr 2 O 7 (0.02 g). Orange block-like crystals of (I) suitable for X-ray structural analysis were obtained after one week of slow evaporation at room temperature.

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.