1,3-Bis[(−)-(S)-(1-phenylethyl)iminomethyl]benzene

The title compound, C24H24N2, is an enantiomerically pure bis-aldimine, which displays twofold crystallographic symmetry, with two C atoms of the central benzene ring lying on the symmetry axis. The imine group is slightly twisted from the benzene core, with a dihedral angle of 12.72 (16)° between the benzene ring and the C=N—C* plane. The terminal phenyl rings make an angle of 66.44 (4)° and are oriented in opposite directions with respect to the benzene ring. In the crystal, molecules interact weakly through a C—H⋯π interaction involving the phenyl rings, and form chains along the 21 screw-axis in the [100] direction.

The title compound, C 24 H 24 N 2 , is an enantiomerically pure bis-aldimine, which displays twofold crystallographic symmetry, with two C atoms of the central benzene ring lying on the symmetry axis. The imine group is slightly twisted from the benzene core, with a dihedral angle of 12.72 (16) between the benzene ring and the C N-C * plane. The terminal phenyl rings make an angle of 66.44 (4) and are oriented in opposite directions with respect to the benzene ring. In the crystal, molecules interact weakly through a C-HÁ Á Á interaction involving the phenyl rings, and form chains along the 2 1 screw-axis in the [100] direction.

Related literature
For the structure of the analogous molecule with naphthyl in place of phenyl, see: Espinosa Leija et al. (2009). For the structure of the isoformular molecule with a 1,4-disubstituted benzene ring, see: García et al. (2010). For the Pd(II) and Pt(II) coordination complexes formed using the title ligand, see: Fossey et al. (2007). For background to the synthesis carried out in solvent-free conditions, see: Tanaka & Toda (2000); Jeon et al. (2005).  Table 1 Hydrogen-bond geometry (Å , ).

Comment
The title compound was synthesized in an almost quantitative yield, using a one-step solvent-free route. Such procedures are becoming primordial in organic synthetic methods, in order to minimize the quantity of toxic waste and byproducts and to decrease the amount of solvents in the reaction media and/or during the following workups. Indeed, solvent-free reactions or solid-state reactions have been particularly developed these last years. (Jeon et al., 2005;Tanaka & Toda, 2000).
The molecular structure of the title compound is as expected. The imine groups N1═C8 are found in the E configuration, which is known to be more stable than Z. The molecule is placed on a crystallographic twofold axis, passing through benzene atoms C10 and C11 ( Fig. 1). Imine groups are not fully conjugated with the benzene core: the dihedral angle between C7-N1═ C8 and benzene mean-planes is 12.72 (16)°. The benzene ring makes an angle of 69.35 (5)° with the phenyl group, and terminal phenyl rings make an angle of 66.44 (4)°. Although the analogous bis-imine bearing a naphthyl group in place of phenyl crystallizes in the same space group, P2 1 2 1 2, and with identical molecular symmetry, it is stabilized in a different conformation compared to the title molecule. For instance, imine groups are almost perfectly conjugated with the benzene ring (dihedral angle between benzene and C * -N═C planes less than 0.6°; Espinosa Leija et al., 2009). The title molecule and the naphthyl analogue are also differentiated by the fact that the latter crystallized with lattice solvent, CH 2 Cl 2 . The title molecule also shows a different conformation to that of the isoformular compound with a central 1,4-disubsituted benzene ring (García et al., 2010): in that case, the molecule crystallizes in P2 1 2 1 2 1 and is placed in general position (C 1 point group).
Interestingly, (Fossey et al. 2007) reported on the synthesis of chiral bis-aldimine NCN-pincer complexes, where the NCN ligand is derived from the title compound by deprotonation at C10. These authors probed the catalytic activity of Pd(II) and Pt(II) complexes, where the ancillary ligand is an halide ion, Bror Cl -, for Pd and Pt complexes, respectively.
The studied reaction, a classical Michael addition between methyl 2-cyanopropanoate and methyl vinyl ketone, showed that addition was not stereocontrolled. This poor selectivity was related to conformational flexibility of the chiral phenylethyl moiety of the ligand. Indeed, that point is confirmed by our structure, since a poor overlay is observed for this part of the molecule, when attempting to fit the title molecule and the main ligand in the complexes. However, differences in point symmetry also deserve to be considered regarding the catalytic activity: the title molecule belongs to C 2 point-group, while complexes prepared by Fossey et al. crystallize in space group P2 1 2 1 2 1 , the whole complexes being placed in general positions. The complexes used for the Michael addition thus actually displayed the non-crystallographic C 2 symmetry.

Refinement
All H atoms were found in a difference map and refined with free coordinates and isotropic displacement parameters fixed to U iso = 1.2U eq (carrier C atom). C-H bond lengths are in the range 0.92 (2)-1.023 (18) Å. The absolute configuration at C7 was assigned from the known configuration of the chiral amine used as starting material, and measured Friedel pairs (775) were merged.
Figures Fig. 1. The title molecule with displacement ellipsoids for non-H atoms shown at the 60% probability level. Non-labeled atoms are generated by symmetry operation -x, 1 -y, z.