Crystal structures of 2-amino­pyridine citric acid salts: C5H7N2+·C6H7O7− and 3C5H7N2+·C6H5O73−

摘要

2-Amino­pyridine and citric acid mixed in 1:1 and 3:1 ratios in ethanol yielded crystals of two 2-amino­pyridinium citrate salts, viz. C5H7N2 ⁺·C6H7O7 ⁻ (>I) (systematic name: 2-amino­pyridin-1-ium 3-carb­oxy-2-carb­oxy­methyl-2-hy­droxy­propano­ate), and 3C5H7N2 ⁺·C6H5O7 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 Namino—H⋯Oacid, Oacid⋯H—Oacid, 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 (CA³⁻) 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.