Crystal structure of {(R)-N 2-[(benzo[h]quinolin-2-yl)methyl]-N 2′-[(benzo[h]quinolin-2-yl)methylidene]-1,1′-binaphthyl-2,2′-diamine-κ4 N,N′,N′′,N′′′}(trifluoromethanesulfonato-κO)zinc(II)} trifluoromethanesulfonate dichloromethane 1.5-solvate

In the title compound, the zinc(II) atom exhibits a a distorted five-coordinate square-pyramidal geometry and is coordinated by one trifluoromethanesulfonate ligand and four N-donor atoms. The resulting complex shows a single-stranded P-helimer structure incorporating π–π and/or σ–π interactions.


Chemical context
Stereochemistry plays a very important role in the chemical interactions that dominate several fields of chemistry (North, 1998). For example, in pharmacology enantiomers of chiral drugs exhibit marked differences in toxicology, metabolism, immune response, and pharmacokinetics (Nguyen et al., 2006). As a result, there is increased demand to design practical methods to synthesize monohelical chiral compounds for use as catalysts (Aspinall, 2002). Many factors contribute to the efficiency of a catalyst such as the type of metal employed, the presence of electron-donating or withdrawing functional groups, the number of chiral centers present, and regeneration capabilities (Amendola et al., 1999). In addition, substrate accessibility to the metal atom plays an important role in catalytic reactions (French, 2007). Using bulky ligands in catalyst design may result in steric hindrance of the active site, a reduction in enantiomeric excess values, and lower yields (French, 2007). Studies of catalytic mechanisms show that substrates generally approach the active site through the least hindered quadrant during a reaction (French, 2007). ISSN 2056-9890 Designing catalysts with increased flexibility which undergo slight conformation changes as substrates approach should result in increased efficiency. This concept can be observed in nature where some enzymes can adopt flexible active sites, unlike the typical 'lock and key' model commonly used, allowing them to shape those active sites to accommodate bulkier substrates leading to improving efficiency (Tsou, 1993). Given the significance and application of flexible singlestranded monohelical complexes in asymmetric catalysis, we report on the synthesis and crystal structure of the solvated title compound, [Zn(C 48 H 32 N 4 )(CF 3 O 3 S)](CF 3 O 3 S)Á-1.5CH 2 Cl 2 (1).

Structural commentary
X-ray analysis revealed a monohelical structure ( Fig. 1) with and/orinteractions between the locked side-arms of complex (1). The Zn II cation is coordinated by four N-donor atoms from the N 2 - [(benzo[h]quinolin-2-yl)methyl]-N 2 0 -[(benzo [h]quinolin-2-yl)methylidene]-1,1 0 -binaphthyl-2,2 0 -diamine (BQMB) ligand and one triflate anion in a distorted square-pyramidal geometry ( 5 = 0.49; Addison et al., 1984). We observed the reduction of one imine double bond as the C-N bond length of the unreduced imine is 1.281 (6) Å while the C-N bond length of the reduced imine is 1.433 (6) Å . We also observed the effect of the reduction in the torsion angle as the amine side (C33-C34-N3-C36) is À22.6 (5) while the torsion angle for the imine side (C16-C15-N2-C13) is 33.0 (7) . The reduction of the imine bond also affects the bond lengths of the zinc metal center with the N-donor atoms on the imine bond. As a result of the flexibility of the amine side, we observe a longer bond length for the Zn-N bond [2.253 (4) Å ] compared to a shorter Zn-N bond length with the more rigid imine nitrogen atom [2.056 (4) Å ]. The binaphthalene backbone displays a twist to a degree of 76.54 (6) .

Supramolecular features
The molecules of the crystal structure are related only by a twofold screw axis running along the b-axis direction (Fig. 2).
The resulting space group P2 1 is chiral. The Flack x and the Hooft y parameters were determined to be À0.008 (4) and 0.003 (4), respectively, indicating that the absolute structure was unequivocally established. Anomalous dispersion was used to determine the absolute structure. There are two molecules of complex (1) in the asymmetric unit. As seen in Fig. 3, the difference in the two molecules arises due to the orientation of the coordinating triflate. In addition, one of the molecules exhibits positional disorder within the coordinating triflate ion. As a result, the two molecules are not symmetry equivalent. Minimal intramolecular interactions are observed between the molecules of (1). The two molecules of (1) in the asymmetric unit propagate along the b-axis direction via the twofold screw axis. The counter-ions and the solvent molecules fill the void spaces between symmetry-related asymmetric units.

Database survey
The survey of Cambridge Structural Database (Groom et al., 2016) revealed five instances of five-coordinate Zn complexes bonding through four amine groups and one triflate. Of the five complexes, two assume a trigonal-bipyramidal geometry (with 5 values of 0.86 and 0.93), two structures have a squarepyramidal geometry ( 5 values of 0.02 and 0.11), and the last structure assumes a distorted square-pyramidal geometry as evidenced by the 5 value of 0.48. The Zn-O bond length for (1) falls on the shorter end of the distance spectrum. Mean-while the Zn-N distances for three of the contacts agree well with those in the previously reported structures. The fourth contact at a distance of 2.253 (4) Å falls above the average Zn-N distance by 0.176 Å , presumably due to the greater flexibility within the ligand framework resulting from the imine reduction.

Synthesis and crystallization
The synthetic scheme for (1) is given in Fig. 4.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1 Overlay of the two molecules of complex (1) in the asymmetric unit. Atomic displacement ellipsoids are depicted at the 50% probability and H atoms shown as spheres of arbitrary radius. All hydrogen atoms, counter-ions, solvent molecules, and minor-disorder components have been omitted for clarity.

Figure 2
Packing diagram for complex (1), viewed along the a-axis direction. Minimal interactions are observed between the packed molecules. procedure of alternating rounds between least-squares cycles and difference-Fourier maps, located the missing nonhydrogen atoms. All hydrogen atoms, except for the amine hydrogens bonded to N3 and N3A were refined at idealized positions and allowed to ride on neighboring atoms with relative isotropic displacement parameters. The amine hydrogen atoms were refined as riding freely. The asymmetric unit contains two molecules of (1), two triflate counter-ions, and three molecules of dichloromethane solvent. One of the molecules of (1) exhibited positional disorder within the coordinating triflate ion. The positional disorder was modeled over two positions with the major component contributing 88.1 (4)%. Due to the low occupancy of the minor component, idealized geometry was used to stabilize the refinement and the component was refined isotropically. Additional disorder was observed in one of the solvent molecules of dichloromethane. Two positions were used to model the positional disorder and the major component refined to occupancy of 50 (4)%. The bond lengths C54A-Cl5A and C54A-Cl6A were restrained to be similar.

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.