The crystal structure of the 1:2 dihydrate compound of chloranilic acid (systematic name: 2,5-di-chloro-3,6-dihy-droxy-1,4-benzo-quinone) with 2-carb-oxy-pyridine (another common name: picolinic acid; systematic name: pyridine-2-carb-oxy-lic acid), namely, 2C6H5.5NO20.5+·C6HCl2O4-·2H2O, (I), has been determined at 180 K, and the structure of the 1:2 dihydrate compound of chloranilic acid with 2-carb-oxy-quinoline (another common name: quinaldic acid; systematic name: quinoline-2-carb-oxy-lic acid), namely, 2C10H7NO2·C6H2Cl2O4·2H2O, (II), has been redetermined at 200 K. This determination presents a higher precision crystal structure than the previously published structure [Marfo-Owusu & Thompson (2014 ▸). X-ray Struct. Anal. Online, 30, 55-56]. Compound (I) was analysed as a disordered structure over two states, viz. salt and co-crystal. The salt is bis-(2-carb-oxy-pyridinium) chloranilate dihydrate, 2C6H6NO2+·C6Cl2O42-·2H2O, and the co-crystal is bis-(pyridinium-2-carboxyl-ate) chloranilic acid dihydrate, 2C6H5NO2·C6H2Cl2O4·2H2O, including zwitterionic 2-carb-oxy-pyridine. In both salt and co-crystal, the water mol-ecule links the chloranilic acid and 2-carb-oxy-pyridine mol-ecules through O-H⋯O and N-H⋯O hydrogen bonds. The 2-carb-oxy-pyridine mol-ecules are connected into a head-to-head inversion dimer by a short O-H⋯O hydrogen bond, in which the H atom is disordered over two positions. Compound (II) is a 1:2 dihydrate co-crystal of chloranilic acid and zwitterionic 2-carb-oxy-quinoline. The water mol-ecule links the chloranilic acid and 2-carb-oxy-quinoline mol-ecules through O-H⋯O hydrogen bonds. The 2-carb-oxy-quinoline mol-ecules are connected into a head-to-tail inversion dimer by a pair of N-H⋯O hydrogen bonds.
Keywords: 2-carboxypyridine; 2-carboxyquinoline; chloranilic acid; crystal structure; disorder; hydrogen bond; twitterion.