A series of nine complexes and 109 literature examples containing a carboxylic acid functional group and a pyridine functional group on separate molecules follow the ΔpK a rule such that proton transfer occurs at values above 3 to form a molecular salt and none at values below 0 to form a co-crystal. In the intermediate range, there is a predominance of molecular salt over co-crystal formation. The complexes discussed show that calculated pK a 's are good predictors of the outcome.

The title compound is essentially planar, with an r.m.s. deviation of 0.054 Å. The mol­ecular structure is fixed in the azo tautomer by intra­molecular C—H⋯N inter­actions, with O—H⋯O hydrogen bonds creating linked dimers. Charge-transfer inter­actions are observed, with the segregated stacks linked by Br⋯Br inter­actions.

In the title hydrated mol­ecular salt, C5H7N2 +·C7H3ClNO4 −·H2O, the ions and water mol­ecules assemble into ribbons of R 6 5(22) rings along the c axis via O(water)—H⋯O−, N+—H⋯O(water) and N—H⋯O− hydrogen bonds. N—H⋯O− hydrogen bonds connect adjacent ribbons along the c-axis direction via R 4 4(12) rings, forming hydrogen-bonded layers. The CO2 and NO2 groups make dihedral angles of 81.8 (2) and 1.4 (2)°, respectively, with the ring in the anion.