1,4-Bis{(+)-(S)-[1-(1-naphthyl)ethyl]iminomethyl}benzene

The title compound, C32H28N2, is a chiral bis-imine in which both imine groups display the common E configuration. The naphthyl groups present different orientations with respect to the central core, as reflected in the dihedral angles of 21.4 (2) and 78.83 (14)° between the benzene and naphthyl mean planes, thus the highest possible C 2 local molecular symmetry is not attained. This C 1 molecular conformation allows multiple C—H⋯π intermolecular contacts involving all aromatic rings, while no π–π interactions are available for the stabilization of the crystal structure. The resulting packing structure is based on molecules stacked along [100].

The title compound, C 32 H 28 N 2 , is a chiral bis-imine in which both imine groups display the common E configuration. The naphthyl groups present different orientations with respect to the central core, as reflected in the dihedral angles of 21.4 (2) and 78.83 (14) between the benzene and naphthyl mean planes, thus the highest possible C 2 local molecular symmetry is not attained. This C 1 molecular conformation allows multiple C-HÁ Á Á intermolecular contacts involving all aromatic rings, while nointeractions are available for the stabilization of the crystal structure. The resulting packing structure is based on molecules stacked along [100].   Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) x þ 1 2 ; Ày þ 3 2 ; Àz. Cg1 is the centroid of ring C27-C32, Cg2 is the centroid of ring C23-C27/C32, Cg3 is the centroid of ring C14-C19, Cg4 is the centroid of ring C1-C5/C10 and Cg5 is the centroid of ring C5-C10.

1,4-Bis{(+)-(S)-[1-(1-naphthyl)ethyl]iminomethyl}benzene
Armando Espinosa Leija, Sylvain Bernès, Guadalupe Hernández, Pankaj Sharma, Ulises Peña and René Gutiérrez S1. Comment There is an increased interest in the use of environmentally benign reagents and conditions particularly to solvent-free procedures. Thus, avoiding organic solvents during the reactions in organic synthesis leads to clean, efficient and economical technology: safety is largely increased, working is considerably simplified, cost is reduced, increased amounts of reactants can be used, etc. Also, reactivities and sometimes selectivities are enhanced (Jeon et al., 2005;Noyori, 2005;Tanaka & Toda, 2000). On the other hand, bis-imines have lately attracted much attention, mostly due to their versatile coordination behavior and the interesting properties of their metal complexes. These compounds are particularly interesting since they can potentially act in a variety of coordination modes. Continuing our work on the synthesis and characterization of this kind of compounds (Tovar et al., 2007;Espinosa Leija et al., 2009), we synthesized the title compound under solvent-free conditions and report herein its crystal structure.
The molecule (Fig. 1) is constructed of a benzene ring para-substituted by two identical chiral fragments including imine functionality. The conformation stabilized in the solid-state has both imine groups displaying E configuration, previously observed in related systems (e.g. Allouchi et al., 1994). Naphthyl groups, which are potentially free to rotate about their σ bonds C1-C11 and C21-C23, show different orientations with respect to the central benzene ring. The dihedral angles between the central benzene ring C14···C19 and the naphthyl rings C1···C10 and C23···C32 are 21.4 (2) and 78.83 (14)°, respectively. The naphthyl systems make a dihedral angle of 73.69 (10)°. As a consequence, the molecule has C 1 point symmetry rather than C 2 , and is not a good candidate for coordination to transition metals. In contrast, other related bis-imines based on a para-substituted benzene core approximate the C 2 point symmetry (e.g.

Hamaker & Oberts, 2006).
The crystal structure features a number of C-H···π intermolecular interactions of variable strength, involving all available aromatic rings (Fig. 2). Although no π-π contacts contribute to the stabilization of the crystal structure, the molecules are efficiently packed along the short [100] axis in the crystal. As a consequence, no voids are available for lattice solvent insertion, a situation contrasting with that observed for an isomeric system previously described (Espinosa Leija et al., 2009): for the meta-substituted molecule, a 1:1 solvate was crystallized with CH 2 Cl 2 , with solvent molecules filling large voids generated by the molecular conformation.
Interestingly, the enantiomer of the title compound has been registered (Watanabe & Fukuda, 2008; CAS registry number: 1021327-88-7) as a chiral dopant for nematic or cholesteric liquid crystals for generating large helical twisting power. This use is consistent with the high optical rotation measured for this molecule (see Experimental).

S3. Refinement
All H atoms were placed in idealized positions with C-H bond lengths fixed to 0.93 (aromatic), 0.96 (methyl) or 0.98 Å (methine), and with methyl groups allowed to rotate about their C-C bonds. A riding refinement was applied, and isotropic displacement parameters were computed as U iso (H) = 1.5U eq (carrier atom) for the methyl groups and U iso (H) = 1.2U eq (carrier atom) otherwise. Friedel pairs (1571) were merged and the absolute configuration inferred from that of the commercial optically pure amine used as starting material.

Figure 1
The title molecule with displacement ellipsoids for non-H atoms shown at the 30% probability level.