Co-crystallizations of some 3, 5 and 6 mono-substituted salicylic acids with 4-aminopyridine, using a variety of solvents, have yielded a number of new complex solid forms, mainly with the 5-halide-substituted acid, including some hydrates and solvates. In all cases, proton transfer occurs from the carboxyl group of the acid to the pyridine nitrogen of the base, with the COO−⋯H+NPy synthon being found in 12/14 cases. The prime exception is 4-aminopyridinium:5-aminosalicylate:pyridine solvate, where the carboxylate group forms a 2-point synthon with one hydrogen of the 4-amino group on the aminopyridinium supplemented by a C–H⋯O interaction involving an ortho hydrogen. This synthon is also found as one component of a disordered structure of the 4-aminopyridinium:5-chlorosalicylate. The other component adopts the normal pyridinium⋯carboxylate synthon. The adoption of 2-point or 1-point synthons, and the geometry of the former, is influenced by the presence of other hydrogen-bonding interactions involving hydrate water molecules or the amine of the 4-aminopyridinium group. A detailed packing analysis shows a number of similarities, partly linked to the synthon geometries. The structures generally fall into two groups, one derived from a simple zero-dimensional pyridinium–carboxylate monomer construct, the other from a zero-dimensional pyridinium–carboxylate centrosymmetric dimer construct. The former group contains most of the hydrates and the latter all of the 5-halide and methyl anhydrates, plus the 5-I pyridine solvate. The diversity of structures found confirm the frequent unpredictability in the structures adopted by products of co-crystallizations when ionic forms are produced.
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