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 [Susse & 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. sigma for C-C bonds = 0.002-0.003 angstrom). Each molecule features two intramolecular N-H center dot center dot center dot O hydrogen bonds. In the crystal, molecules are linked by strong face-to-face pi-pi stacking interactions involving both the six-and five-membered rings [centroid-centroid separations = 3.6290 (14) and 3.6506 (14) angstrom] and intermolecular N-H center dot center dot center dot O hydrogen bonds.