Synthesis, structure, and discrimination of chiral bimolecular crystals by using diphenylacetic acid and aza aromatic compounds

Although diphenylacetic acid (1) and aza aromatic compounds such as phenanthridine (2) and acridine (3) are achiral molecules, chiral bimolecular crystals 1.2 and 1.3 were obtained by recrystallization from the solutions. A 1:1 hydrogen bonding between 1 and 2 or 3 is formed in the crystal lattice. Spontaneous crystallization gave two enantiomorphous crystals of M- and P-form. The absolute configuration of P-1.2 could be determined by the Bijvoet method using anomalous dispersion of an oxygen atom during X-ray analysis. The chirality in the crystalline state is generated from the propeller-like chiral conformation of two phenyl planes and a carboxyl plane of the diphenylacetic acid molecule. The two enantiomorphous M- and P-crystals of 1.2 and 1.3 could be easily discriminated by the solid-state CD spectra by using Nujol.