Short is strong: experimental electron density in a very short N•••I halogen bond

2,3,5,6-Tetrafluoro-1,4-diiodobenzene and 4-(dimethylamino)pyridine co-crystallize in 1:2 stoichiometry. A diffraction experiment at standard resolution was already conducted in 2010 and revealed one of the shortest N center dot center dot center dot I contacts ever reported. We collected X-ray intensities at 100 K up to a very high resolution of 1.23 angstrom(-1). These experimental data allowed to refine a structure model based on atom-centered multipoles according to the Hansen-Coppens approach and provided an experimental electron density. A subsequent analysis with the help of Bader's atoms in molecules theory showed a strong interaction between the pyridine N atom and the sigma hole of its closest iodine neighbor on the halogenated benzene. This contact is characterized by a distance of 2.6622(4) angstrom and associated with a remarkably large electron density of 0.359(5) e.angstrom(-3) in the (3, -1) critical point, unprecedented for a secondary interaction. This bona fide shortest halogen bond ever investigated by an experimental charge density study is associated with a significantly negative total energy density in the bond critical point and thus can reliably be classified as strong. Both the electron density and the position of the bond critical point suggest to compare the short N center dot center dot center dot I contact to coordinative or covalent bonds rather than to sigma hole interactions.