Role of Charge Transfer in Halogen Bonding

Abstract

Halogen bonding has received intensive attention recently for its applications in the construction of supramolecular assemblies and crystal engineering and its implications and potentials in chemical and biological processes and rational drug design. Peculiarly, in intermolecular interactions, halogen atoms are known as electron-donating groups carrying partial negative charges in molecules due to its high electronegativity, but they can counterintuitively act as Lewis acids and bind with Lewis bases in the form of a halogen bond. The unsettling issue regarding the nature of the halogen bonding is whether the electrostatics or charge transfer interaction dominates. The recently proposed σ-hole concept nicely reinforces the role of electrostatic attraction. Also, good correlations between the halogen bonding strength and the interaction energy from the simple point-charge model have been found. This leads to the claim that there is no need to invoke the charge transfer concept in the halogen bond. But there is alternative evidence supporting the importance of charge transfer interaction. Here, we visited a series of prominent halogen bonded complexes of the types Y 3 C−X•••Z (X = Br, I; Y = F, Cl, Br; Z = F − , Cl − , Br − , I − , NMe 3 ) with the block-localized wave function (BLW) method at the M06-2X-D3/6-311+G(d,p) (def2-SVP for iodine) level of theory. As the simplest variant of ab initio valence bond (VB) theory, the BLW method is unique in the strict localization of electrons within interacting moieties, allowing for quantitative evaluation of the charge transfer effect on geometries, spectral properties, and energetics in halogen bonding complexes. By comparing the halogen bonding complexes with and without the charge transfer interaction, we proved that the charge transfer interaction significantly shortens the X•••Z bonding distance and stretches the C−X bonds. But the shortening of the halogen bonding results in the less favorable steric effect, which is composed of Pauli repulsion, electrostatics, and electron correlation. There are approximate linear correlations between the charge transfer effect and binding energy and between bonding distance and binding energy. These correlations may lead to the illusion that the charge transfer interaction is unimportant or irrelevant, but further analyses showed that the inclusion of charge transfer is critical for the proper description of the halogen bonding, as considering only electrostatics and polarization leads to only about 45−60% of the binding strengths and much elongated bonding distances.

The mechanism and impact of mono/bis(iodoimidazolium) halogen bond donor catalysts on Michael addition of indole with trans-crotonophenone: DFT calculations

“…The more the charge transfer, the larger the second order perturbation energy (Δ 2 E ), the lower the reaction energy barrier, and the better the catalytic activity of the halogen bond donor. This illustrates that charge transfer 82 is significant for the activation of the reactant with catalysts, which is consistent with the study by Kozuch and Huber. 75 The catalysis of halogen bonds activates the carbonyl oxygen atom in the Michael addition reaction, which is used to accelerate the reaction by promoting the charge transfer from indole to the Michael acceptor, thereby accelerating the formation of the carbon–carbon bond.…”

Dissection of the Origin of π-Holes and the Noncovalent Bonds in Which They Engage

“…However, it must be underscored that the shift patterns in Figure include not only the intermolecular transfer but also the internal reallocations within each monomer. And as recently demonstrated by Inscoe et al, who separated these two types of shifts from one another for halogen bonds, these two phenomena oppose one another in the intermolecular region, where it is the polarization that is the stronger of the two. The pattern of charge shifts documented in Figure , especially the internal polarizations, is not entirely unlike that observed in σ-hole interactions.…”

“…The intensive scrutiny of σ-hole bonds has led to a solid understanding of the fundamental phenomena from which they draw their strength. The chain of processes, beginning with the distortion of the σ­(R–X) covalent bond arising from the R–X electronegativity difference, and the resulting σ-hole in the density, which in turn leads to the positive potential, have been characterized. ,,,− Also studied extensively has been the relationship between the magnitude of the σ-hole potential and the strength of the ensuing interaction with a nucleophile and how other factors enter into the equation such as charge transfer and dispersion. ,− Research has also built a body of understanding of the effects of the σ-hole bond on the intermolecular distance and perturbations within the individual monomers, both geometric and spectroscopic. − …”

Proximity Effects of Substituents on Halogen Bond Strength

“…In each of the chalcogen, pnicogen, tetrel, and triel bonds, an atom of that same particular family of the periodic table replaces the bridging proton of the H-bond. − Among these interactions, the halogen bond (XB) is arguably the one that has been acknowledged for the longest time − and has engendered the greatest amount of scrutiny. Extensive study has demonstrated that the XB owes its stability to several factors. − In the first place, the electron-density cloud surrounding the X atom is quite anisotropic; while the overall charge on the X atom is partially negative, there is a pocket of positive potential that lies along the extension of the covalent C–X bond, which is commonly referred to as a σ-hole. This positive region attracts the negative potential of an approaching nucleophile, for example, its lone pair, leading to a Coulombic attraction.…”