Halogen bond symmetry: the N-X-N bond

Halogen bonding is the attractive interaction of an electrophilic region of a halogen with a nucleophile. Its geometry, energy, and dominantly electrostatic nature resemble that of the hydrogen bond. Halogen bond strength critically depends on the electron deficiency of the halogen. Accordingly, halonium ions that bear a formal positive charge are the strongest halogen bond donors known so far. Halonium ions, similar to H+, are capable of simultaneously interacting with two Lewis bases. The structure and properties of the resulting three-center halogen bonds are reviewed herein, based on their spectroscopic and computational data, with the emphasis being placed on one representative example, the [N-X-N](+) interaction. The iodine and bromine-centered systems form symmetric three-center-four-electron halogen bonds that have comparable orbital and electrostatic contributions and a large extent of charge delocalization. Their formation yields significant stabilization. The [N-Cl-N](+) bond is computationally predicted to resemble the symmetry of the three-center bond of the heavier halogens, whereas the fluorine-centered halogen bond is predicted to be asymmetric, and thus behave more similar to the analogous, asymmetric three-center [N-H-N](+) hydrogen bond. Experimental confirmation of the latter predictions remains to be accomplished, along with studies of the possible influence of the disorder of the local environment on halogen bond symmetry, and investigations in the gaseous phase. The electronic structure and the geometry of three-center halogen bonds are of fundamental interest, and also of practical importance, for example due to the occurrence of three-centered halogen bonds in synthetic reagents. Copyright (c) 2014 John Wiley & Sons, Ltd.