2,6-Bis[1-(2-isopropylphenylimino)ethyl]pyridine

The title compound, C27H31N3, has E substitution at each imine double bond where the two N atoms adopt a trans–trans relationship. The benzene rings are twisted out of the mean plane of the pyridine ring; the mean planes of the aromatic groups are rotated by 63.0 (1) and 72.58 (8)°. The crystal structure is sustained mainly by C—H⋯π and hydrophobic methyl–methyl interactions.

Many reports have appared in the literature concerning the effects (sterically and/or electronic) of ligand modifications, to find a structure-activity relationships. The crystal structure of different 2,6-bis(arylimino)pyridine ligands and their transition metal complexes offer the possibilty to compare directly structural parameters. Here we report the synthesis and crystal structure of the title compound, (I), (Fig. 1).
The molecule adopts a nonplanar conformation in which an E configuration around each C?N imine group is observed, likewise the two N atoms display a trans-trans relationship. The conformation of the system N-N-N system is of course different in each case. In general, X-ray structures of bis(arylimino)pyridines reveal that in the solid state the imino nitrogen atoms prefer to be disposed trans with respect to the central pyridine nitrogen (Mentes, et al. 2001;Huang et al., 2006) in order to minimize the interaction between the nitrogen lone pairs. The phenyl rings in (I) are twisted out of the mean plane of the pyridine ring, the mean planes of C8-C13 and C19-C24 being rotated by 63.0 (1)° and 72.58 (8)°, respectively. This molecular conformation is determined by the formation of pairs of intramolecular C-H···N hydrogen bonds, involving methyl groups with the N of the pyridine ring and isopropyl groups with imine groups with a range of distances C···N = 2.799 (3)-2.892 (4) Å (Fig. 2). These interactions lead to the formation of five-membered rings described by graph-set simbol S(5) (Bernstein et al., 1995).
The crystal structure of (I) consists of dimers linked by self-complementary C-H···π interactions related by an inversion centre C15···Cg1 = 3.757 Å; were Cg1 is the centroid of the N1,C1-C5 ring (Fig. 2). Neighbouring dimers are connected through additional C-H···π between phenyl rings (Fig. 3), generating supramolecular sheets parallel to the c axis. Details of geometrical parameters of these hydrogen bonding interactions are summarized in Table 2. Finally, the stacking of adjacent sheets is sustained by hydrophobic methyl-methyl interactions along the a axis (Fig. 4).

S2. Experimental
The tile compound was synthetized by condensation of 2,6-diacetylpiridine (1.63 g, 10 mmol) with 2-iso-propylaniline (2.74 g, 20.3 mmol) in 25 ml dry methanol and five drops of formic acid. The solution was refluxed for 18 h. Upon slow cooling to room temperature and overnight to 273 K. Yellow prisms of (I) were obtained and filtered with a yield 75%.

S3. Refinement
All H atoms bound to carbon were included in calculated positions (C-H = 0.93-096 Å) and refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C).

Figure 1
Molecular structure of (I) with displacement ellipsoids drawn at the 30% probability level (H atoms omitted for clarity).   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.24 e Å −3 Δρ min = −0.18 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.