Selective crystallization of indigo B by a modified sublimation method and its redetermined structure

Good-quality single crystals of the title compound, indigo B [systematic name: 2-(3-oxoindolin-2-ylidene)indolin-3-one], C16H10N2O2, have been prepared with high selectivity by a sublimation process. The previous structure of indigo B [Süsse & Wolf (1980 ▶). Naturwissenschaften, 67, 453], which showed that the complete molecule is generated by crystallographic inversion symmetry has been confirmed, but the present study reports more realistic geometrical parameters and modern standards of precision (e.g. σ for C—C bonds = 0.002–0.003 Å). Each molecule features two intramolecular N—H⋯O hydrogen bonds. In the crystal, molecules are linked by strong face-to-face π–π stacking interactions involving both the six- and five-membered rings [centroid–centroid separations = 3.6290 (14) and 3.6506 (14) Å] and intermolecular N—H⋯O hydrogen bonds.

Good-quality single crystals of the title compound, indigo B [systematic name: 2-(3-oxoindolin-2-ylidene)indolin-3-one], C 16 H 10 N 2 O 2 , have been prepared with high selectivity by a sublimation process. The previous structure of indigo B [Sü sse & Wolf (1980). Naturwissenschaften, 67, 453], which showed that the complete molecule is generated by crystallographic inversion symmetry has been confirmed, but the present study reports more realistic geometrical parameters and modern standards of precision (e.g. for C-C bonds = 0.002-0.003 Å ). Each molecule features two intramolecular N-HÁ Á ÁO hydrogen bonds. In the crystal, molecules are linked by strong face-to-facestacking interactions involving both the six-and five-membered rings [centroid-centroid separations = 3.6290 (14) and 3.6506 (14) Å ] and intermolecular N-HÁ Á ÁO hydrogen bonds.

D-HÁ
Data collection: X-AREA (Stoe & Cie, 2005); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2004); software used to prepare material for publication: WinGX (Farrugia, 1999). Support by Universitä t Leipzig and the Fonds der Chemischen Industrie are gratefully acknowledged. The structure was solved and refined in space group P 2 1 /n, Z = 2, with a unit cell setting related to that reported by Süsse & Wolf (1980) ( Table 1). Since the quality of the former data set did not allow an interpretation of bond lengths and angles, we used the new crystals to redetermine the crystal structure with improved quality.

Structure Reports Online
The new data set allows a better description of the bonding situation in the indigo molecule. The new values of the bond lengths and angles are consistent with the values reported for indigo A. For example, the bond length between the carbon atoms C2 and its symmetry equivalent C2', previously determined as 1.30 Å in indigo B (1.34 Å in indigo A), which is too short for a delocalized π-system, could be corrected to a distance of 1.359 (2) Å, which is in agreement with a partial double bond. C-C distances within the six-membered ring are in the range of 1.38 to 1.41 Å, consistent with the aromatic character of the ring system.
Responsible for the low solubility of indigo is the intermolecular stabilization by face-to-face π-π-stacking of parallel aromatic rings with a distance of 3.41 Å. Furthermore, each NH and each carbonyl group form strong intermolecular hydrogen bonds N-H···O to one neighbouring unit per functional group with a N···O distance of 2.883 (2) Å (H···O 2.17 Å). In addition, weaker intramolecular hydrogen bonds are observed with N···O 2.925 (2) Å (Figure 1). In both modifications the intermolecular interactions result in almost identical arrangements of indigo molecules in layers, i.e. layers parallel (100) in indigo A and layers parallel (-101) in indigo B, respectively. This leads to edge-to-face π-interactions between indigo molecules of adjacent layers. The distance of 3.50 Å for C6 to the neighbouring ring system is in good agreement with literature data (Meyer et al., 2003). Viewing along [010], the difference in the crystal structures of both modifications can be observed as an offset in different directions in the stacking of these layers (Figure 2) (von Eller-Pandraud, 1958). As a consequence, the unit cell volume of modification B is 2.8% larger than that of indigo A, although the data set has been collected at lower temperature (-60 °C).
Powder diffraction data of crushed single crystals of indigo B do not show a phase transition by cooling to -60 °C; the diffraction pattern fits well to the simulated pattern based on our single crystal data. Additional peaks with low intensities are due to small amounts of indigo A (Figure 3).

supplementary materials sup-2 Experimental
Crystallization of indigo modification B was achieved by sublimation, similar to the procedure described by von Eller (1955).