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2-Amino-pyridine and citric acid mixed in 1:1 and 3:1 ratios in ethanol yielded crystals of two 2-amino-pyridinium citrate salts, viz . C 5 H 7 N 2  + ·C 6 H 7 O 7  - ( I ) (systematic name: 2-amino-pyridin-1-ium 3-carb-oxy-2-carb-oxy-methyl-2-hy-droxy-propano-ate), and 3C 5 H 7 N 2  + ·C 6 H 5 O 7  3- ( II ) [systematic name: tris-(2-amino-pyridin-1-ium) 2-hy-droxy-propane-1,2,3-tri-carboxyl-ate]. The supra-molecular 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 carb-oxy-lic group of citric acid, while in II all three carb-oxy-lic groups of the acid are deprotonated and the charges are compensated for by three 2-amino-pyridinium cations. In both structures, a complex supra-molecular three-dimensional architecture is formed. In I , the supra-molecular 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 inter-actions. The mol-ecular conformation of the citrate ion (CA 3- ) in II is stabilized by an intra-molecular 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-amino-pyridinium-carboxyl-ate heterosynthon exhibited in many 2-amino-pyridinium carboxyl-ates 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-amino-pyridinium-carboxyl-ate 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.

acta crystallographica. section e, crystallographic communications

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