The nature of [N–Cl–N]+ and [N–F–N]+ halogen bonds in solution
Halonium ions are synthetically useful, transient species that may be stabilized by attachment to two electron donors. Whereas studies of [C–X–C]+-type ions have greatly contributed to the fundamental understanding of chemical bonding and reaction mechanisms, investigations of the corresponding [N–X–N]+ halogen bond complexes are only at an early stage. Herein we present solution NMR spectroscopic and theoretical evidence for the nature of [N–Cl–N]+ and [N–F–N]+ complexes, and we discuss their geometries and stabilities in comparison to their iodine and bromine-centered analogues as well as the corresponding three-center [N–H–N]+ hydrogen bond. We show the chlorine-centered halogen bond to be weaker but yet to resemble the symmetric geometry of the three-center bond of heavier halogens. In contrast, the [N–F–N]+ bond is demonstrated to prefer asymmetric geometry analogous to the [N–H–N]+ hydrogen bond. However, the [N–F–N]+ system has a high energy barrier for interconversion, and due to entropy loss, its formation is slightly endothermic.