Crystal structures of 2-aminopyridine citric acid salts: C₅H₇N₂ ⁺·C₆H₇O₇ ⁻ and 3C₅H₇N₂ ⁺·C₆H₅O₇ ³⁻

Abstract

2-Aminopyridine and citric acid mixed in 1:1 and 3:1 ratios in ethanol yielded crystals of two 2-aminopyridinium citrate salts, viz . C 5 H 7 N 2 + ·C 6 H 7 O 7 − ( I ) (systematic name: 2-aminopyridin-1-ium 3-carboxy-2-carboxymethyl-2-hydroxypropanoate), and 3C 5 H 7 N 2 + ·C 6 H 5 O 7 3− ( II ) [systematic name: tris(2-aminopyridin-1-ium) 2-hydroxypropane-1,2,3-tricarboxylate]. The supramolecular synthons present are analysed and their effect upon the crystal packing is presented in the context of crystal engineering. Salt I is formed by the protonation of the pyridine N atom and deprotonation of the central carboxylic group of citric acid, while in II all three carboxylic groups of the acid are deprotonated and the charges are compensated for by three 2-aminopyridinium cations. In both structures, a complex supramolecular three-dimensional architecture is formed. In I , the supramolecular aggregation results from N amino —H...O acid , O acid ...H—O acid , O alcohol —H...O acid , N amino —H...O alcohol , N py —H...O alcohol and C ar —H...O acid interactions. The molecular conformation of the citrate ion (CA 3− ) in II is stabilized by an intramolecular O alcohol —H...O acid hydrogen bond that encloses an S (6) ring motif. The complex three-dimensional structure of II features N amino —H...O acid , N py —H...O acid and several C ar —H...O acid hydrogen bonds. In the crystal of I , the common charge-assisted 2-aminopyridinium–carboxylate heterosynthon exhibited in many 2-aminopyridinium carboxylates is not observed, instead chains of N—H...O hydrogen bonds and hetero O—H...O dimers are formed. In the crystal of II , the 2-aminopyridinium–carboxylate heterosynthon is sustained, while hetero O—H...O dimers are not observed. The crystal structures of both salts display a variety of hydrogen bonds as almost all of the hydrogen-bond donors and acceptors present are involved in hydrogen bonding.