Halogen Bonding and/or Covalent Bond: Analogy of 3c-4e N···I···X (X = Cl, Br, I, and N) Interactions in Neutral, Cationic, and Anionic Complexes

X-ray structural measurements and computational analysis demonstrated the similarity of the geometries and electronic structures of the X-I···N (X = Cl, Br, I, and N) bonding in strong halogen-bonded (HaB) complexes and in the anionic or cationic halonium ions. In particular, I···N bond lengths in the solid-state associations formed by strong HaB donors (e.g., I₂, IBr, ICl, and N-iodosuccinimide) and acceptors (e.g., quinuclidine or pyridines) were in the same range of 2.3 ± 0.1 Å as those in the halonium ions [e.g., the bis(quinuclidine)iodonium cation or the 1,1′-iodanylbis(pyrrolidine-2,5-dione) anion]. In all cases, bond lengths were much closer to those of the N-I covalent bond than to the van der Waals separations of these atoms. The strong N···I bonding in the HaB complexes led to a substantial charge transfer, lengthening and weakening of the I···X bonds, and polarization of the HaB donors. As a result, the central iodine atoms in the strong HaB complexes bear partial positive charges akin to those in the halonium ions. The energies and Mayer bond orders for both N···I and I···X bonds in such associations are also comparable to those in the halonium ions. The similarity of the bonding in such complexes and in halonium ions was further supported by the analysis of electron densities and energies at bond critical (3, −1) points in the framework of the quantum theory of atoms in molecules and by the density overlap region indicator. Overall, all these data point out the analogy of the symmetric N···I···N bonding in the halonium ions and the asymmetric X···I···N bonding in the strong HaB complexes, as well as the weakly covalent character of these 3c-4e interactions.