Reactivity of 4-Aminopyridine with Halogens and Interhalogens : Weak Interactions Supported Networks of 4-Aminopyridine and 4-Aminopyridinium

Abstract

The reaction of 4-aminopyridine (4-AP) with ICl in a 1:1 molar ratio in CH2Cl2 produced the expected charge-transfer complex [4-NH2-1λ4-C5H4N-1-ICl] (1·ICl) and the ionic species [(4-NH2-1λ4-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-NH2-1λ4-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-NH2-1λ4-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-NH2-1λ4-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′-Br3-1λ4-[1,2′-(C5H4N)2]-4,4′-(NH2)2}+[X–] (4·Br–, 4·Br3–) and {3′,5′-Br2-1λ4-[1,2′-(C5H4N)2]-4,4′-(NH2)2}+[X–] (5·Br–, 5·Br3–). Compounds 1–5 as well as two protonated species, [4-NH2-1λ4-C5H4N-1-H+]2[Cl–][I3–] (32·Cl–·I3–) and [(4-NH2-1λ4-C5H4N)2-1μ-H+][I–] (6·I–), all display extended 3D networks supported by halogen and hydrogen bonding in the solid state.