Crystal structures of 2-amino-pyridine citric acid salts: C(5)H(7)N(2)(+)·C(6)H(7)O(7)(-) and 3C(5)H(7)N(2)(+)·C(6)H(5)O(7)(3).

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.