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

Abstract

Halogen bonding between two negatively charged species, tetraiodo‐p‐benzoquinone anion radicals (I4Q−.) 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 I4Q−. anion radicals linked by iodides and separated by Et4N+ 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 I4Q−. responsible for the attractive interaction. Finally, the solid‐state associations were also stabilized by multicenter (4:4) halogen bonding between I4Q−. and iodide.

“Anti-electrostatic” halogen bonding in solution

“…halide anions and the anionic iodinated bis(dicyanomethylene)cyclopropanide derivatives 1 (see Scheme 1) or the anionic tetraiodo-p-benzoquinone radical, were characterized recently in the solid state. 13,14 The identication of such complexes substantially extends the range of feasible XB systems, and it provides vital information for the discussion of the nature of this interaction. Computational results, however, signicantly depend on the used methods and applied media (gas phase vs. polar environment and solvation models) and the solid state arrangements of the XB species might be affected by crystal forces and/or counterions.…”

Anion–Anion Interactions in Aerogen-Bonded Complexes. Influence of Solvent Environment

“…Despite the Coulombic repulsions that keep these ions apart in the gas phase, charge dispersal effects accompanying solvation can allow them to approach close enough together so as to overcome the electrostatic repulsion and engage in a stable complex. After initial findings of this effect in the case of H-bonds [ 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 ], more recent work has shown these ideas can be extended to halogen [ 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 ], triel [ 78 ], pnicogen [ 79 ], and related types of noncovalent bonds [ 80 , 81 , 82 , 83 , 84 ]. There is an important question as to whether aerogen bonds, which are generally much weaker than most of the other related interactions, can likewise occur between pairs of anions, and if so, how strongly polarizing a solvent is needed.…”