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

The reaction of 4-aminopyridine (4-AP) with ICI in a 1:1 molar ratio in CH2Cl2 produced the expected charge-transfer complex [4-NH2-1 lambda(4)-C5H4N-1-ICl] (1.ICl) and the ionic species [(4-NH2-1 lambda(4)-C5H4N)(2)-1 mu-I+][Cl-] (2.Cl-) in a 2:1 relation, as indicated by H-1 NMR spectroscopy in solution. In contrast, only the ionic compound [(4-NH2-1 lambda(4)-C5H4N)(2)-1 mu-I+][IBr2-] (2.IBr2-) was observed in the analogous reaction with IBr. The reaction between 4-AP and I-2 in a 1:1 molar ratio also afforded two components, one of which was identified as the congeneric cation in [(4-NH2-1 lambda(4)-C5H4N)(2)-1 mu-I+[2.I-7(-)] 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 center dot center dot center dot I+center dot center dot center dot N contact. Surprisingly, the reaction of 4-AP with Br-2 in CH2Cl2 resulted in an immediate protonation of the 4-aminopyridine (H-1 NMR) and [4-NH2-1 lambda(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'-Br-3-1 lambda(4-)[1,2'-((CH4N)-H-5)(2)]-4,4'-(NH2)(2)} [X-] (4.Br-, 4-Br-3(-)) and {3',5'-Br-2-1 lambda(4)-[1,2'-(C5H4N)(2)]-4,4'-(NH2)(2)}(+)[X-] (5.Br-, 5.Br-3(-)). Compounds 1-5 as well as two protonated species, [4-NH2-1 lambda(4)-C5H4N-1-H+](2)[Cl-][I-3(-)] (3(2)-Cl-.I-3(-)) and [(4-NH2-1 lambda(4)-C5H4N)(2)-1 mu-H+ [I-] (6.I-), all display extended 3D networks supported by halogen and hydrogen bonding in the solid state.