Structural similarities and diversity in a series of crystalline solids composed of 2-aminopyridines and glutaric acid

A solvent co-crystallization of three 2-aminopyridine derivatives, 2-aminopyridine (AP), 2-amino-6-methylpyridine (AMP), and 2,6-diaminopyridine (DAP) with the odd-membered propane-1,3-dicarboxylic acid (C5H12O4 = GAH(2), glutaric acid) resulted in six ionic crystalline products, (HAP)(GAH) (1), (HAMP)(GAH) (2, 3), (HDAP)(GAH) (4), (HDAP)(2)(GA) (5), and (HDAP)(2)(DAP)(GA)(EtOH) (6, EtOH = ethanol). New compounds were characterized by single-crystal and powder X-ray diffraction, melting points, and IR spectra. The proton transfer to the pyridine nitrogen atom in all compounds and the location of H-atom in the carboxylic group in the hydrogen glutarate anion in binary adducts 1-4 was determined reliably from the low-temperature X-ray experiments. All compounds adopt the recurring R (2) (2) (8) 2-aminopyridine-carboxylic acid heteromeric supramolecular synthon. The aggregation of hydrogen glutarate anions in the C(8) chain motifs in 1-4 occurs via the homomeric COOH center dot center dot center dot COO (-) robust pattern. Adducts 2 and 3 represent conformational polymorphs; adducts 4, 5 and 6 reveal the diversity in the components' forms (ionic and neutral), acid-base ratios (1:1, 1:2, and 1:3), and hydrogen-bonding systems. This work demonstrates the variety of forms of glutaric acid in the H-bonded adducts.