The reaction of 4-aminopyridine (4-AP) with ICl in a 1:1 molar ratio in CH2Cl2 produced the expected charge-transfer complex [4-NH2-1λ 4-C5H4N-1-ICl] (1·ICl) and the ionic species [(4-NH2-1λ 4-C5H4N)2-1μ-I+]­[Cl–] (2·Cl–) in a 2:1 relation, as indicated by 1H NMR spectroscopy in solution. In contrast, only the ionic compound [(4-NH2-1λ 4-C5H4N)2-1μ-I+]­[IBr2 –] (2·IBr2 –) was observed in the analogous reaction with IBr. The reaction between 4-AP and I2 in a 1:1 molar ratio also afforded two components, one of which was identified as the congeneric cation in [(4-NH2-1λ 4-C5H4N)2-1μ-I+]­[I7 –] (2·I7 –) that contains a polyiodide anion as a result of transformation in a 1:2 molar ratio between the starting materials. In all of these ionic products, the crystal structures feature an iodonium ion, I+, trapped between two 4-AP rings through N···I+···N contact. Surprisingly, the reaction of 4-AP with Br2 in CH2Cl2 resulted in an immediate protonation of the 4-aminopyridine (1H NMR) and [4-NH2-1λ 4-C5H4N-1-H+]­[Br–] (3·Br–) was characterized as the main product. A subsequent peculiar bromination–dimerization process afforded the novel pyridyl-pyridinium cations {3,3′,5′-Br3-1λ 4-[1,2′-(C5H4N)2]-4,4′-(NH2)2}+[X–] (4·Br–, 4·Br3 –) and {3′,5′-Br2-1λ 4-[1,2′-(C5H4N)2]-4,4′-(NH2)2}+[X–] (5·Br–, 5·Br3 –). Compounds 1–5 as well as two protonated species, [4-NH2-1λ 4-C5H4N-1-H+]2[Cl–]­[I3 –] (3 2·Cl–·I3 –) and [(4-NH2-1λ 4-C5H4N)2-1μ-H+]­[I–] (6·I–), all display extended 3D networks supported by halogen and hydrogen bonding in the solid state.

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