How to Twist, Split and Warp a σ-Hole with Hypervalent Halogens

Abstract

Halogen bonds (XB) are no longer newcomers in the chemistry family. However, XB in hypervalent halogens has not been thoroughly studied. We provide a molecular orbital explanation of the shape and strength of XBs in hypervalent halogens and other species, focusing on the charge transfer and electrostatic aspects of these bonds. Our results show that σ-holes (and subsequently the XBs associated with them) can be easily divided and bent by the influence of equatorial substituents. The inductive effect of both the equatorial and axial groups can affect these distortions, but also the angle between the equatorial ligands has a large influence on the shape of the σ-holes and the molecular orbitals acting as electron acceptor. Although the observation of these warped XB can be hindered by other noncovalent interactions, they may be ubiquitous in crystal structures of hypervalent species, where multiple XB can appear as secondary interactions on each halogen. We propose what can be considered the archetypal hypervalent halogen donor (a pincer type iodosodilactone) and a Lewis dot structure that includes the σ-holes.

Iodine(III) Derivatives as Halogen Bonding Organocatalysts

“…[11] Most importantly,b ys terically blocking one or two of the electrophilic axes,the Lewis acidity of the iodine(III) compounds could be markedly reduced or switched off.This study thus establishes another class of organoiodine derivatives as XB organocatalysts.E ven though the compounds in these experiments were monodentate,they exhibited comparably strong activity. Fort he first time,strong indications for the crucial importance of halogen bonding for the observed activities were obtained by multiple comparative experiments.T hese findings are in line with previous computational studies on the importance of XB in complexes of l 3 -iodanes with nucleophiles.…”

“…In view of our interest to apply XB in organocatalysis, [10] we intended to extent this concept towards the use of iodine(III) compounds as noncovalent Lewis acids.T ot he best of our knowledge,there is only one prior example of such an application, [11] which lacked any mechanistic studies. [12] Herein, we present two proof-of-principle reactions in which the action of XB is clearly demonstrated by as eries of comparative experiments.…”

Hexacoordinated Tetrel‐Bonded Complexes between TF4 (T=Si, Ge, Sn, Pb) and NCH: Competition between σ‐ and π‐Holes

“…But the question arises as to whether a tetrel atom, within a tetravalent covalent bonding situation, is limited to only a single such bond. [25,[55][56][57][58][59][60] There has been a certain amount of consideration of the general topic of hypervalent pnicogen, halogen, and even aerogen atoms [9,[61][62][63][64][65][66][67][68][69][70][71][72][73][74] but not in the context of tetrel atoms, which have their own unique electronic and spatial issues. The theoretical literature to date has little to say on this issue.…”

Halogen, Chalcogen, and Pnicogen Bonding Involving Hypervalent Atoms

“…The issue of the interplay between noncovalent bonding and hypervalency has seen only limited prior study in the literature. Of the various sorts of interactions, much of the previous work concerned halogen bonds (XBs) . Moreover the bulk of the studies have been further limited to trivalent halogen atoms which would not present steric crowding as an important effect.…”