Crystal structure of cis-(1,4,8,11-tetraazacyclotetradecane-κ4 N)bis(thiocyanato-κN)chromium(III) bromide from synchrotron X-ray diffraction data

The CrIII ion in the [Cr(NCS)2(cyclam)]+ cation has a distorted octahedral coordination environment with four N atoms of cyclam and two N-bonded NCS groups in a cis arrangement. The cyclam ligand adopts the cis-V conformation.

The crystal structure of the title complex, cis-[Cr(NCS) 2 (cyclam)]Br (cyclam = 1,4,8,11-tetraazacyclotetradecane, C 10 H 24 N 4 ), has been determined from synchrotron X-ray data. The asymmetric unit contains one [Cr(NCS) 2 (cyclam)] + cation and one bromide anion. The Cr III ion of the complex cation is coordinated by the four N atoms of the cyclam ligand and by two N-coordinating NCS groups in a cis arrangement, displaying a distorted octahedral coordination sphere. The Cr-N(cyclam) bond lengths are in the range 2.075 (3) to 2.081 (3) Å while the average Cr-N(NCS) bond length is 1.996 (16) Å . The macrocyclic cyclam moiety adopts the most stable cis-V conformation. The crystal structure is stabilized by intermolecular hydrogen bonds involving the cyclam N-H groups as donor groups, and the bromide anion and the S atom of one of the NCS ligands as acceptor groups.

Chemical context
Compounds containing cyclam (1,4,8,11-tetraazacyclotetradecane, C 10 H 24 N 4 ) or its derivatives have a potential inhibitory effect on the replication of the human immunodeficiency virus (HIV) and have the ability to mobilize hematopoietic progenitor stem cells from the bone marrow into the blood (Ronconi & Sadler, 2007;De Clercq, 2010;Ross et al., 2012). In order to develop new anti-HIV drugs using transition-metal complexes with the cyclam ligand, at first it is necessary to obtain accurate information about their conformations and crystal packing forces (De Clercq, 2010). Cyclam has a moderately flexible structure, and can adopt both planar (trans) and folded (cis) conformations in [CrL 2 (cyclam)] n+ (L = monodentate or bidentate/2) complexes (Poon & Pun, 1980). There are five conformational trans isomers for the macrocycle, which differ in the chirality of the sec-NH groups (Choi, 2009;Jeon et al., 2020). The trans-I, trans-II and trans-V conformations also can fold to form cis-I, cis-II and cis-V conformers, respectively (Subhan et al., 2011;Jeon et al., 2020). Knowledge of the conformation for the macrocyclic ligand including various counter-anions are important factors in developing new highly effective anti-HIV drugs (Ronconi & Sadler, 2007;De Clercq, 2010;Ross et al., 2012). Furthermore, the NCS group is interesting either as a co-ligand or a counteranion in transition-metal complexes. As an ambidentate ligand, the NCS group can coordinate either through the N or S atom, and can adopt various bridging modes (Moon & Choi, 2021).
As an extension of our investigations on the coordination chemistry and conformations of Cr III complexes containing the cyclam ligand, one auxiliary bidentate or two monodentate ligands and various anions (Choi et al., 2004a,b;Choi & Lee, 2009;Subhan et al., 2011;Moon et al., 2013Moon et al., , 2017Moon & Choi, 2021), we describe here the synthesis of a new salt complex, [Cr(NCS) 2 (cyclam)]Br, (I) and its structural characterization by synchrotron single-crystal X-ray diffraction.

Supramolecular features
The Br À counter-anion remains outside the coordination sphere of the Cr III ion. In the crystal, N-HÁ Á ÁBr and N-HÁ Á ÁS hydrogen-bonding interactions occur between the N- The molecular structure of (I), drawn with displacement ellipsoids at the 30% probability level. Only H atoms of amine groups are shown for clarity. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
The commercially available free ligand cyclam (98%), chromium(III) chloride hexahydrate (98%) and sodium bromide (99%) were obtained from Sigma-Aldrich and used as provided. All other chemicals were purchased from commercial sources and used without further purification. The starting material, cis-[Cr(NCS) 2 (cyclam)]SCN, was prepared as previously described (Ferguson & Tobe, 1970). For crystallization of (I), cis-[Cr(NCS) 2 (cyclam)]SCN (0.006 g) was dissolved in 5 mL of tetrahydrofuran at 343 K and the solution filtrated. The filtrate was added to 2 mL of water containing 0.13 g of solid NaBr. The resulting solution was evaporated slowly at room temperature until the formation of crystals suitable for X-ray structural analysis. The obtained needle-like orange crystals of (I) were washed with small amounts of 2-propanol and dried in air before collecting the synchrotron data.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. 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.      -(1,4,8,11-tetraazacyclotetradecane-κ 4 N)