N···I halogen bonding interactions: influence of Lewis bases on their strength and characters

Halogen bonding (XB) as an emerging noncovalent interaction, due to its highly directional and devisable properties, has given rise to considerable interest for constructing supramolecular assemblies. In this work, the newly developed density functional M06-2X calculations and the quantum theory of "atoms in molecules" (QTAIM) studies were carried out on a series of N···I halogen bonding to investigate the influence of Lewis bases (XB acceptors) on the XB. For the Lewis base C6-nH6-nNn (n = 1, 2, 3), with the increasing number of nitrogen atom in the aromatic ring, the most negative electrostatic potentials (VS, min) outside the nitrogen atom becomes less negative and the XB becomes weaker. The positive cooperativity exists in the Y(-)-C6H5N···C6F5I, Y(-)-C4H4N2···C6F5I, and Y(-)-C3H3N3···C6F5I (Y(-) = Cl(-), Br(-), I(-)) termolecular complexes: the H bond or anion-π interactions have the ability to enhance the N···I halogen bond and vice versa. With the addition of halogen anions to the XB acceptor, the XB become more covalent, more electronic charge transfer from the XB acceptors to donors, the XB acceptors become more energetically stabilized and XB donors become more destabilized, and the atomic volume attraction of both the nitrogen and iodine atoms become more obvious. From the view of the Laplacian of electron density function, for the XB acceptor, the reactivity zone is the region of valence shell charge concentration (VSCC), where it is a (3, -3) critical point (CP) and referred to as a lump, thus the XB interaction can be classified as a lump-hole interaction. The more negative VS,min outside the nitrogen atom, the stronger the XB, resulting in the greater distance between the (3, -3) CP and the nitrogen nucleus.