Crystal structure of a new 2,6-bis(imino)pyridine derivative: (1E,1′E)-1,1′-(pyridine-2,6-diyl)bis[N-(4-chlorophenyl)ethan-1-imine]

This new 2,6-bis(imino)pyridine derivative with terminal 4-chlorophenyl rings crystallizes with two independent molecules in the asymmetric unit.

1. Chemical context 2,6-Bis(imino)pyridines have acquired widespread interest because of their potential application as ligands in olefin polymerization reactions: see, for example, the work of Antonov et al. (2012) or Kawakami et al. (2015). Metal complexes of such ligands have been applied to aryl C-H activation (Dayan et al., 2010;Sigen et al., 2013) and transfer hydrogenation reactions (Dayan & Ç etinkaya, 2007). As a result of the redox activity of the ligand (Noss et al., 2018), electrochemical and luminescent properties of its complexes have been reported (Fan et al., 2004). Recently, the biomimetic reactivity of Zn-alkyl complexes has also been revealed (Sandoval et al., 2018). We report herein on the crystal structure of a new 2,6-bis(imino)pyridine derivative with terminal 4-chlorophenyl rings.

Structural commentary
The asymmetric unit of the title compound contains two crystallographically independent molecules (A and B), illustrated in Fig. 1. Both molecules have E-configurations for both imine double bonds with regard to the aryl and pyridine groups. The C N bond lengths of the imine groups are in a narrow range, 1.2675 (15) to 1.2808 (14) Å (Table 1). These values are similar to the C N bond lengths found in the ISSN 2056-9890 crystal structures of other 2,6-bis(imino)pyridyl ligands; for example 1.266 (4) Å in the 'parent' compound 2,6-bis[1-(phenylimino)ethyl]pyridine (Mentes et al., 2001).

Supramolecular features
In the crystal, molecules are linked by a series of C-HÁ Á Á interactions, forming layers lying parallel to the bc plane (Table 2 and Fig. 3). There are no other significant intermolecular interactions present in the crystal structure. All HÁ Á ÁN and HÁ Á ÁCl intermolecular distances exceed the sum of their van der Waals radii. View of the molecular overlay of the two independent molecules. Table 2 Hydrogen-bond geometry (Å , ).

Figure 1
Molecular structure of the title compound showing the two crystallographically independent molecules (A and B), with the atom labelling. Displacement ellipsoids drawn at the 30% probability level.

Figure 3
A view along the a axis of the crystal packing of the title compound, showing the C-HÁ Á Á interactions as dashed lines (

Synthesis and crystallization
To a solution of 2,6-diacetylpyridine (0.5 g, 3.06 mmol) and p-chloroaniline (0.977 g, 7.66 mmol) in 20 ml of toluene was added 20 mg of p-toluenesulfonic acid (Gö rl et al., 2011). The reaction mixture was refluxed for 24 h using a Dean-Stark trap, then cooled to room temperature and 50 ml of saturated sodium bicarbonate solution was added. The organic layer was separated and filtered over sodium sulfate. The solvent was removed in a rotary evaporator giving a light-brown-coloured mass. Ethanol (ca 25 ml) was added to this solid mass followed by the addition of hexane (ca 10 ml). The solution was then kept in the deep-freezer at 253 K. The title compound was obtained as a yellow solid in 31% yield (0.363 g, 0.95 mmol). A very dilute solution of the compound was prepared in a 1:1 mixture of ethanol and hexane. On slow evaporation of the solvents at room temperature, pale-yellow crystals were obtained over a period of two weeks.
An alternate method for the synthesis is as follows: To a solution of 2,6-diacetylpyridine (0.5 g, 3.06 mmol) and pchloroaniline (0.782 g, 6.13 mmol) in 5 mL of absolute ethanol was added three drops of acetic acid. The reaction mixture was refluxed for 24 h, cooled to room temperature and then approximately 15 mL of hexane were added. The mixture was heated on a water bath and filtered hot using filter paper. The solution was kept in a deep freezer at 253 K. The title compound was obtained as a yellow solid in 26% yield (0.305 g, 0.80 mmol).

(1E,1′E)-1,1′-(Pyridine-2,6-diyl)bis[N-(4-chlorophenyl)ethan-1-imine]
Crystal data 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.