Halogen Bonding Between Anions: Association of Anion Radicals of Tetraiodo-p-benzoquinone with Iodide Anions.

Halogen bonding between two negatively charged species, tetraiodo-p-benzoquinone anion radicals (I Q ) and iodide anions, was observed and characterized for the first time. X-ray structural and EPR/UV-Vis spectral studies revealed that the anion-anion bonding led to the formation of crystals comprising 2D layers of I Q anion radicals linked by iodides and separated by Et N counter-ions. Computational analysis suggested that the seemingly antielectrostatic halogen bonds in these systems were formed via a combination of several factors. First, an attenuation of the interionic repulsion by the solvent facilitated close approach of the anions leading to their mutual polarization. This resulted in the appearance of positively charged areas (σ-holes) on the surface of the iodine substituents in I Q responsible for the attractive interaction. Finally, the solid-state associations were also stabilized by multicenter (4:4) halogen bonding between I Q and iodide.