Diethyl 2,5-bis[(E)-2-furylmethyleneamino]thiophene-3,4-dicarboxylate

The title compound, C20H18N2O6S, crystallizes as two independent molecules that are disposed about a pseudo-inversion center (1/2, 1/4, 1/8). The mean planes of the two terminal furyl rings are twisted with respect to the central thiophene ring by 7.33 (4) and 21.74 (5)° in one molecule, and by 6.91 (4) and 39.80 (6)° in the other.


S1. Comment
The molecule (I) was prepared as a result of our ongoing research of conjugated azomethines for electronic devices. The crystal structure of (I) confirmed that the compound consisted of a central thiophene capped by two terminal furans that are connected by two azomethine bonds. Even though two isomers are possible, only the more stable E isomer was confirmed by the resolved structure. The chemical structure occurs eight times in the Pca2 1 lattice as seen in Figure 2 with two different molecules of (I) per cell disposed near a false inversion center at 1/2, 1/4, 1/8. (Marsh et al., 1998) Neither solvent nor counter-ions were found in the closed-packed stacking.
The mean plane angles described by all three heterocycles of (I) are not entirely coplanar. The mean plane angles of the terminal furans are twisted 7.33 (4)° and 21.74 (5)° for one molecule of (I) with respect to the central thiohene. Similarly, the mean planes are twisted by 6.91 (4)° and 39.80 (6)° for the second molecule found in the lattice. Meanwhile, the average mean plane angles for the analogous all thiophene azomethine are 9.04 (4)° and 25.07 (6)°.
Interestingly, the three-dimensional network of (I) is very different than for its all thiophene analogue in which all the molecules are linear and aligned in one direction. Since no traditional hydrogen bonding occurs, the furans and thiophene adopt a mix of parallel and perpendicular π-stacking, according to Figure 3. One such π-stacking occurs between the O21 and the O21 ii rings with a distance of 3.674 (3) Å between the planes. Other interactions involve the oxygen or sulfur acting as electron donors while the heterocycles act as electron acceptors. For example, O11î^ interacts with O11-C11 -C12-C13-C14, S1 with S1 i -C16 i -C17 i -C18 i -C19 i , S2 with S2 ii -C26 ii -C27 ii -C28 ii -C29 ii and O26 ii with O26-C211-C212-C213-C214. The centre-to-centre distances for these interactions are 3.517 (3), 3.659 (3), 3.680 (3) and 3.541 (3) Å, respectively.

S2. Experimental
In 25 ml of anhydrous toluene was added 2-furaldehyde to which was subsequently added DABCO, TiCl 4 in toluene at 0 °C and then diethyl 2,5-diaminothiophene-3,4-dicarboxylate. The mixture was then refluxed for two hours after which the solvent was removed. Purification by flash chromatography yielded the title product as a red solid. Single crystals of (I) were obtained by slow evaporation of a acetone.  ORTEP representation of the two different molecules of (I) with the numbering scheme adopted (Farrugia 1997). Ellipsoids drawn at 30% probability level.

Figure 2
The three-dimensional network demonstrating the closed packing in the lattice.

Special details
Experimental. X-ray crystallographic data for I were collected from a single-crystal sample, which was mounted on a loop fiber. Data were collected using a Bruker Platform diffractometer, equipped with a Bruker SMART 2 K Charged-Coupled Device (CCD) Area Detector using the program SMART and normal focus sealed tube source graphite monochromated Cu-Kα radiation. The crystal-to-detector distance was 4.908 cm, and the data collection was carried out in 512 x 512 pixel mode, utilizing 4 x 4 pixel binning. The initial unit-cell parameters were determined by a leastsquares fit of the angular setting of strong reflections, collected by a 9.0 degree scan in 30 frames over four different parts of the reciprocal space (120 frames total). One complete sphere of data was collected, to better than 0.8Å resolution. Upon completion of the data collection, the first 101 frames were recollected in order to improve the decay correction analysis. 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.