Does a halogen bond require positive potential on the acid and negative potential on the base?

Abstract

It is usually expected that formation of a halogen bond (XB) requires that a region of positive electrostatic potential associated with a σ or π-hole on the Lewis acid will interact with the negative potential of the base, either a lone pair or π-bond region. Quantum calculations of model systems suggest this not to be necessary. The placement of electron-withdrawing substituents on the base can reverse the sign of the potential in its lone pair or π-bond region to positive, and this base can nonetheless engage in a XB with the positive σ-hole of a Lewis acid. The reverse scenario is also possible in certain circumstances, as a negatively charged σ-hole can form a XB with the negative lone pair region of a base. Despite these classical Coulombic repulsions, the overall electrostatic interaction is attractive in these XBs, albeit only weakly so. The strengths of these bonds are surprisingly insensitive to changes in the partner molecule. For example, even a wide range in the depth of the σ-hole of the approaching acid yields only a minimal change in the strength of the XB to a base with a positive potential.