Protonation-assisted spontaneous resolution: Formation of a homochiral 2D interpenetrated hydrogen-bonded network from 4,4′-binicotinic acid under highly acidic conditions

The conformationally chiral 4,4′-bipyridine-2,2′-dicarboxylic acid (herein called 4,4′-binicotinic acid, BNA) forms racemic crystals either from neutral water as the free base, or from dilute hydrochloric acid (1 M) as the monohydrochloride salt, as determined by single crystal X-ray analysis. Crystallization from more concentrated hydrochloric acid (6 M), on the other hand, yields the dihydrochloride salt of BNA, which was found to segregate into conglomerates with the chiral space group C2. In this structure, the BNA·2HCl forms a homochiral (4,4) net, with BNA molecules of the same chirality linked via COOH⋯Cl⋯HN hydrogen bonds. The resulting low-density framework self-penetrates to generate a two-fold 2D interwoven homochiral layer. Stacking of layers of the same handedness generates an enantiomorphous crystal. The increased contribution of ionic and hydrogen bonding in this crystal appears to be responsible for the observed spontaneous resolution.