Crystal structure of ({(1R,2R)-N,N′-bis[(quinolin-2-yl)methyl]cyclohexane-1,2-diamine}chloridoiron(III))-μ-oxido-[trichloridoferrate(III)] chloroform monosolvate

The bimetallic title compound contains an FeIII center coordinated by one chloride ligand, four N atoms and a bridging oxo ligand in a distorted octahedral geometry. The bridging oxo ligand is connected to a second FeIII atom with three coordinating chloride ligands.


Chemical context
Developing small-molecule complexes incorporating iron is an area of growing interest since the discovery of non-heme iron enzymes such as methane monooxygenase and Rieske dioxygenases to be efficient catalysts in the selective oxidation of hydrocarbons under mild reaction conditions (Company et al., 2007). Studies show that highly active non-heme iron catalysts that facilitate efficient stereo-specific alkane hydroxylation using H 2 O 2 as oxidant can be synthesized by employing tetradentate N 4 -donor ligands such as N,N 0 -dimethyl-N,N 0 -bis(2-pyridylmethyl)ethane-1,2-diamine (BPMEN) or tris(2-pyridylmethyl)amine (TPMA) (Costas et al., 2000). These catalysts have provided key insights into possible mechanisms used by enzymes to oxidize alkanes in nature (Meunier et al., 2004). In addition to the application of fourcoordinate iron complexes as catalysts in hydroxylation reactions, studies also show that these complexes can be utilized in epoxidation reactions of terminal and electrondeficient alkenes (Dubois et al., 2003). Iron oxido-bridging complexes are reported to play an important role in oxygen transport (hemerythrin), phosphate ester hydrolysis (purple acid phosphates), or DNA synthesis (ribonucleotide reductase) (Dutta et al., 1996). These oxido complexes exhibit redox and magnetic properties making them excellent candidates for future investigations into the mechanisms behind important ISSN 2056-9890 biological and chemical processes (Feig & Lippard, 1994). Given the significance and application of iron complexes made from tetradentate ligands, herein we report on the synthesis and crystal structure of the solvated title compound [Fe 2 (C 26 H 28 N 4 )(Cl)(-O)Fe(Cl) 3 ]ÁCHCl 3 (1), incorporating (1R,2R)-N,N 0 -bis[(quinolin-2-yl)methyl]cyclohexane-1,2-diamine (Fig. 1).

Structural commentary
There is one coordination complex and one molecule of chloroform solvent in the asymmetric unit. The coordination complex features two Fe(III) metal cations. One of the metal cations, Fe1, assumes a distorted octahedral coordination (Table 1). The tetradentate ligand, (1R,2R)-N,N 0 -bis[(quinolin-2-yl)methyl]cyclohexane-1,2-diamine, interacts with the Fe III cation in the equatorial plane through the four amine groups. A chloride ion and a bridging oxido ligand, which connects the two metal cations, complete the axial coordination. The distortions from the ideal octahedral geometry occur both in the equatorial and the axial positions. The equatorial angles vary widely from 74.96 (9) , as in the case of the N1-Fe1-N2 angle, to 133.98 (9) for the untethered N1-Fe1-N4 angle. The axial ligands exhibit a bent conformation with a Cl1-Fe1-O1 angle of 166.06 (7) . In contrast, the second Fe metal cation, Fe2, exhibits a near ideal tetrahedral coordination geometry composed of one O atom and three Cl atoms. As expected based on the difference in the saturation of the coordination sphere, the Fe-Cl and the Fe-O distances for Fe1 are longer than that for Fe2. The single Fe-Cl distance for Fe1 is 2.3560 (8) Å , whereas the average Fe2-Cl distance of 2.232 (9) Å is more than 0.1 Å shorter, a statistically significant variation. Similarly, the Fe-O distances for Fe1 and Fe2 are also statistically significantly different at 1.808 (2) Å and 1.756 (2) Å , respectively. The bond lengths in the title compound are comparable to the mean Fe-Cl distances from the CSD for Fe complexes in an octahedral coordination [2.33 (7) Å ] and a tetrahedral coordination [2.23 (3) Å ]. In contrast, the Fe-O distances for both the octahedral and tetrahedral configurations in the title compound are shorter than the mean distances from CSD [2.01 (9) and 1.87 (13) Å , respectively]. A very weak intra-molecular N3-H3Á Á ÁCl4 hydrogen bond (Table 2) occurs. Finally, we observe that complex (1) is present only as the M (left-handed) conformer.

Supramolecular features
The molecules in the crystal structure are related by twofold screw axes running along the a-, b-, and c-axis directions. As there are no additional symmetry elements present, the resulting space group, P2 1 2 1 2 1 , is chiral. The absolute structure was unequivocally established, as evidenced by a Hooft y parameter of 0.003 (6), using anomalous dispersion. Apart from a weak C-HÁ Á ÁCl bond from the chloroform molecule to one of the chloride ions bonded to Fe2 (Table 2), the molecules of the coordination complexes display minimal interatomic interactions. They assemble into columns that run parallel to the a axis. A circular arrangement of six columns of coordination complex molecules creates a channel. The The molecular structure of complex (1), shown with 50% probability displacement ellipsoids. All H atoms and the minor-disorder components of the solvent molecule have been omitted for clarity.  channel is filled with solvent chloroform molecules that exhibit extensive positional disorder. For two of the columns that frame the chloroform channels, the oxido-trichloride groups of the coordination complexes point into the channels, while the other four columns face the void with the (1R,2R)-N,N 0 -bis[(quinolin-2-yl)methyl]cyclohexane-1,2-diamine ligand. The packing is illustrated in Fig. 2.

Database survey
In our survey of the Cambridge Structural Database (Groom et al., 2016), we found five reported structures incorporating the (1R,2R)-N,N 0 -bis[(quinolin-2-yl)methyl]cyclohexane-1,2diamine ligand motif. Of the five, only one structure showed coordination to iron (Dengler et al., 2011). In that structure, the distorted octahedral coordination of the Fe III metal atom is completed by two chloride ligands in the axial positions. The two Fe-Cl distances are comparable (2.495 and 2.509 Å ) and the Cl1-Fe-N angles show a narrow distribution from 92-94 , except for Cl1-Fe-N1, which is 84 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms, except for the amine hydrogen atom bonded to N3, were added at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. The amine hydrogen bonded to N3 was allowed to refine freely. In , there is one molecule of chloroform solvent in the asymmetric unit. The solvent molecule exhibits extensive positional disorder over three positions. Initially, the disorder was modeled with chloroform molecule in an idealized geometry, where the 1,2 and the 1,3 bond lengths were constrained. As the refinement converged, the geometry constraints were lifted. The chlorine atoms Cl6 and Cl7 were modeled over two positions, with the major component contributing 54.4 (3)%. The carbon atom C27 required modeling over three positions with the major component contribution of 54.4 (3)% and the two minor components contributing 24.1 (4)% and 21.5 (4)%. The C-Cl distances for all of the disorder components were restrained to be similar. In addition, Cl6A-Cl7A and Cl7A-Cl5 were restrained to be similar. The absolute structure was unequivocally determined by anomalous dispersion. Synthetic scheme for complex (1).   Δρ min = −0.52 e Å −3 Absolute structure: Refined as an inversion twin Absolute structure parameter: 0.000 (14) 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. Refinement. Refined as a 2-component inversion twin.

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