2-Carboxyl­atopyridinium–4-nitro­phenol (1/1)

In the title 1:1 adduct, C₆H₅NO₃·C₆H₅NO₂, both mol­ecules are almost planar (r.m.s. deviations for the non-H atoms = 0.027 and 0.023 Å for 4-nitro­phenol and 2-carboxyl­atopyridinium, respectively). The pyridine mol­ecule crystallizes as a zwitterion (nominal proton transfer from the carb­oxy­lic acid group to the N atom in the ring). In the crystal, inversion dimers of the zwitterions linked by pairs of N—H...O hydrogen bonds generate R₂²(10) loops; two 4-nitro­phenol mol­ecules link to the dimer by O—H...O hydrogen bonds, generating a four-molecule aggregate. These are linked by C—H...O inter­actions, forming a three-dimensional network.

1-Hy­droxy­isoquinolin-2-ium hydrogen succinate

In the title salt, C₉H₈NO⁺·C₄H₅O₄⁻, the isoquinolinium ring system is approximately planar [r.m.s deviation = 0.011 (2) Å]. In the crystal, adjacent cations and anions are linked by O—H...O and N—H...O hydrogen bonds, forming columns along the b axis. The columns are connected by weak C—H...O inter­actions into a three-dimensional network.