Crystal structure of cis-dichlorido(1,4,8,11-tetraazacyclotetradecane-κ4 N)chromium(III) (oxalato-κ2 O 1,O 2)(1,4,8,11-tetraazacyclotetradecane-κ4 N)chromium(III) bis(perchlorate) from synchrotron data

Two CrIII ions (each with site symmetry 2..) in the title compound have a distorted octahedral coordination environment with four N atoms of a cyclam ligands and two chloride ions or one oxalate bidentate ligand in a cis position whereby the cyclam ligands adopt a cis-V conformation. The crystal packing is stabilized by extensive hydrogen-bonding interactions among the cyclam N–H groups, the Cl ligands, and O atoms of the oxalate and ClO4 − anions.


Chemical context
Transition metal complexes with cyclam (1,4,8,11-tetraazacyclotetradecane, C 10 H 24 N 4 ) ligands can adopt both planar (trans) and folded (cis) configurations (Poon & Pun, 1980). The possible conformers of the trans isomer are trans-I (+ + + +), trans-II (+ -+ +), trans-III (+ --+) and trans-V (+ + --), which differ in the chirality of the sec-NH groups (Choi, 2009) and where + indicates if the H atom of the NH group is above the plane of the macrocycle and -indicates if it is below. The trans-I, trans-II and trans-V conformations can fold to form cis-I, cis-II and cis-V conformers, as shown in Fig. 1. The trans-III conformation gives the most thermodynamically stable complex with two six-membered rings in chair and two five- Possible conformers of cis-[CrL 2 (cyclam)] n+ complexes. membered rings in gauche conformations (Choi, 2009). However, the most stable conformation cannot fold to give the cis-III complex as this requires the diagonal NH groups to both lie above or below the plane of the macrocycle.
Recently, it has been shown 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 and the orientations of the N-H bonds are very important factors for co-receptor recognition. Therefore, knowledge of the conformation and crystal packing of transition metal complexes containing the cyclam ligand has become important in the development of new highly effective anti-HIV drugs that specially target alternative events in the HIV replicative cycle (De Clercq, 2010).

Figure 2
A perspective view of the two chromium(III) complex cations and two perchlorate anions in compound (I), drawn at the 30% probability level. The primed atoms are related by symmetry code (Àx + 1 2 , Ày + 3 2 , Àz).
significant distortion of the octahedral coordination sphere and the larger folded angle in the [Cr(ox)(cyclam)] + cation seem to arise from the small bite angle of the oxalato ligand.
The tetrahedral ClO 4 À anion remains outside the coordination sphere of two Cr III ions. It is distorted due to its involvement in hydrogen-bonding interactions. Cl-O bond lengths range from 1.426 (5) to 1.443 (5) Å and the O-Cl-O angles from 107.8 (4)-111.0 (3) .

Synthesis and crystallization
The free ligand cyclam was purchased from Fluka and used as provided. All chemicals were reagent grade materials and were used without further purification. The starting materials, cis-[CrCl 2 (cyclam)]ClO 4 and [Cr(ox)(cyclam)]ClO 4 , were prepared according to literature methods (House & McKee, 1984). The double complex, cis-[CrCl 2 (cyclam)][Cr(ox)(cyclam)](ClO 4 ) 2 , was prepared by mixing concentrated equimolar aqueous solutions of the two starting compounds. A saturated solution of NaClO 4 was added to the resulting solution for crystallization, and allowed to stand at room temperature for two days to give needle-like orange crystals of (I) suitable for X-ray structural analysis.

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 = 0.98 Å and N-H = 0.99 Å , and with U iso (H) values of 1.2U eq of the parent atoms. The crystal packing in compound (I), viewed perpendicular to the bc plane. Dashed lines represent N-HÁ Á ÁO (pink) and N-HÁ Á ÁCl (cyan) hydrogen-bonding interactions, respectively.  ; 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).

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