Preparation and Structure of Chiral Two-component Molecular Crystals from 3-(3-Indolyl)propanoic Acid and Aza Aromatic Compounds

Despite 3-(3-indolyl)propanoic acid (1) and aza aromatic compounds such as phenanthridine (2), phenazine (3), and 3-bromoquinoline (4) being achiral molecules, chiral two-component molecular crystals 1 * 2, 1 * 3, and 1 * 4 were obtained by crystallization from solutions of both components. The crystal chirality of 1 * 2 is generated through the formation of a unidirectional two-fold hydrogen bonded helix between the two components. Two enantiomorphous crystals of M-1 * 2 and P-1 * 2 were obtained by spontaneous resolution and easily discriminated by the measurement of solid state CD spectra (powder) as Nujol mulls. The absolute configuration of M-1 * 2 was determined very carefully by the Bijvoet method based on the X-ray anomalous dispersion due to the oxygen atom. In the case of 1 * 3 and 1 * 4, a hydrogen bonded helix is not formed, but the crystal chirality is generated by the non-centrosymmetric arrangement of the symmetrical pairs of 1 and 3, and the two enantiomeric pairs of 1 and 4 in the lattice, respectively.