2,5-Bis{[(–)-(S)-1-(4-methylphenyl)ethyl]iminomethyl}thiophene

The title chiral bis-aldimine, C24H26N2S, was synthesized using a solvent-free Schiff condensation. The molecule displays crystallographic C 2 symmetry, with the S atom lying on the twofold axis parallel to [100]. As a consequence of the (S,S) stereochemistry, the tolyl groups are oriented towards opposite faces of the thiophene core, giving a twisted conformation for the whole molecule. Molecules are arranged in the crystal in a herringbone-like pattern, without any significant intermolecular contacts.

The title chiral bis-aldimine, C 24 H 26 N 2 S, was synthesized using a solvent-free Schiff condensation. The molecule displays crystallographic C 2 symmetry, with the S atom lying on the twofold axis parallel to [100]. As a consequence of the (S,S) stereochemistry, the tolyl groups are oriented towards opposite faces of the thiophene core, giving a twisted conformation for the whole molecule. Molecules are arranged in the crystal in a herringbone-like pattern, without any significant intermolecular contacts.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: NC2315). In the last few years, our attention has been focused on the synthesis and structure of chiral bis-imines (e.g. Espinosa Leija et al., 2009), mostly due to their versatile coordination behavior and interesting properties as ligands for building of chiral metal complexes. Along this line, the title compound was synthesized through a Schiff condensation between a low-melting point dialdehyde and a liquid amine having a high boiling-point (above 200 °C), which also serves as a solvent for the reaction. No other solvents were used for the reaction (Tanaka & Toda, 2000).
The crude compound crystallized from CH 2 Cl 2 , allowing to determine its chiral purity and crystal structure. The obtained bis-aldimine is the expected (S,S) diastereoisomer, with imine bonds in the common E configuration (Fig. 1).
The molecule is placed on the 2-fold axis of space group P22 1 2 1 , with the S atom lying on the symmetry axis. The resulting molecular conformation displays the C 2 symmetry, with imine arms oriented towards opposite sides of the central thiophene core ring. Other thiophenes substituted in positions 2 and 5 by imine groups have been characterized (e.g. Skene & Dufresne, 2006;Fridman & Kaftory, 2007;de Lima et al., 2010;Kudyakova et al., 2011Kudyakova et al., , 2012. However, all were achiral compounds, and only one actually presented a crystallographic C 2 symmetry (space group C2/c, Kudyakova et al., 2011), as in the title compound.
The molecules are arranged in the crystal in such a way they form a herringbone-like structure (Fig. 2). However, no actual supramolecular pattern is formed in the solid-state, since no intermolecular contacts of significant strength are present.

Refinement
All C-bonded H atoms were placed in idealized positions and refined as riding to their carrier C atoms, with bond lengths fixed to 0.93 (aromatic CH), 0.96 (methyl CH 3 ), and 0.98 Å (methine CH). Isotropic displacement parameters were calculated as U iso (H) = 1.5U eq (C7, C14) for methyl groups and U iso (H) = 1.2U eq (carrier C) for other H atoms. The absolute configuration was assigned from the known configuration of the chiral amine used as starting material. The absolute supplementary materials sup-2 Acta Cryst. (2013). E69, o1428 structure was confirmed through the Parsons-Flack test (Parsons & Flack, 2004).

Figure 1
The title molecule with displacement ellipsoids for non-H atoms shown at the 50% probability level. The asymmetric unit consists of half of the molecule, and symmetry code to generate equivalent atoms is i = x, 1 -y, 2 -z.

Figure 2
Part of the crystal structure, viewed along the a-axis.