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In the title compounds, 4-aminopyridinium 4-aminobenzoate dihydrate, C(7)H(6)NO(2)(-).C(5)H(7)N(2)(+).2H(2)O, (I), and 4-aminopyridinium nicotinate, C(5)H(7)N(2)(+).C(6)H(4)NO(2)(-), (II), the aromatic N atoms of the 4-aminopyridinium cations are protonated. In (I), the asymmetric unit is composed of two 4-aminopyridinium cations, two 4-aminobenzoate anions and four water molecules, and the compound crystallizes in a noncentrosymmetric space group. The two sets of independent molecules of (I) are related by a centre of symmetry which is not part of the space group. In (I), the protonated pyridinium ring H atoms are involved in bifurcated hydrogen bonding with carboxylate O atoms to form an R(1)(2)(4) ring motif. The water molecules link the ions to form a two-dimensional network along the (101) plane. In (II), an intramolecular bifurcated hydrogen bond generates an R(1)(2)(4) ring motif and inter-ion hydrogen bonding generates an R(4)(2)(16) ring motif. The packing of adduct (II) is consolidated via N-H...O and N-H...N hydrogen bonds to form a two-dimensional network along the (102) plane.

In the structure of the title compound, 2C5H7N2+·SO42−, the S atom of the sulfate anion lies on a twofold axis. The structure is stabilized by an extensive network of N—H⋯O hydrogen bonds. [ABSTRACT FROM AUTHOR]

In the title compound, C5H7N2+·C7H4NO4−, the asymmetric unit consists of an aminopyridinium cation and a p-nitro­benzoate anion, with proton transfer occurring to the aromatic N atom of the 2-amino­pyridine molecule. In the crystal structure, the two ions are inter­connected by N—H⋯O hydrogen bonds. [ABSTRACT FROM AUTHOR]