σ‐Hole Interactions in Catalysis

Abstract

Noncovalent interactions like halogen, chalcogen, and pnictogen bonding are known for a very long time. During the last decade, these interactions have found different applications in catalysis. These forces are often called σ‐hole interactions which can be explained by the anisotropic distribution of the electron density around these atoms. In this MiniReview, we will present recent applications of halogen, chalcogen, and pnictogen bonding in catalysis and discuss experimental and computational investigations to gain more insights into the underlying mechanisms.

Halogen Complexes of Anionic N‐Heterocyclic Carbenes

“…The transition from hypervalency to secondary, noncovalent bonding certainly takes place for the significantly less stable complexes 2 a⋅ L (L=C 6 H 5 Cl, C 6 H 5 Me, CH 3 CN, THF, ONMe 3 ), [9] in which the 2‐iodoimidazolium borate 2 a acts as a halogen bond donor. As noncovalent halogen bonding is often described with the so‐called σ‐hole model, [15, 18, 20, 22] we also calculated the electrostatic potential (ESP) surface of 2 a (Figure 7). Examination of the ESP reveals an area of significant positive potential (in blue) associated with the iodine atom that is available for the directed, linear interaction with the nucleophiles L (C 6 H 5 Cl, C 6 H 5 Me, CH 3 CN, THF, ONMe 3 , IDipp, IMes, WCA‐IDipp, WCA‐IMes) studied in this contribution.…”

“…Halogen bonding is also an important issue in azolium‐based compounds such as 2‐iodo‐ and 2‐bromoimidazolium salts, which serve as important entities in organic synthesis and catalysis, [17, 18] anion recognition, [19, 20] and supramolecular chemistry [21] . In these systems, the electrophilicity of the polarized halogen atom is exploited, which formally acts as a halogen bond (XB) donor towards nucleophilic substrates or anions.…”

Tuning the Halogen Bonding Strength of Cyclic Diaryliodonium Salts

“…DAI salts show a T‐shaped geometry, where the counter anion is bound to the iodine through a secondary interaction, [12] which can be classified as a halogen bond (XB) [13,14] . This non‐covalent interaction, formed between a Lewis base (XB acceptor) and a species featuring an electrophilic halogen substituent (XB donor), [15–17] has found various use in fields such as crystal engineering, [18,19] recognition, [20] as well as organocatalysis [21–23] . Typical strong XB donors feature multidentate neutral perfluorinated or cationic heterocyclic arene backbones.…”

Nature of the Hydrogen Bond Enhanced Halogen Bond

“…Noncovalent interactions are ubiquitous in practically all fields of chemistry [1]. In particular, halogen bonding [2][3][4], defined as the interaction between an electrophilic halogen substituent (called "XB donor") and a nucleophilic Lewis base (called "XB acceptor") [5], has found applications in different fields such as crystal engineering [6], molecular and anion recognition [7], peptide chemistry [8], and even organocatalysis [9,10]. The so-called σ-hole [11,12] is typically considered as the factor responsible for the ability of halogen atoms to accept electron density from the Lewis base [13].…”