Astatine Facing Janus: Halogen Bonding vs. Charge-Shift Bonding.

Gilles Montavon,Serigne Sarr,Jean-Yves Le Questel,Julien Pilmé,Nicolas Galland

doi:10.3390/molecules26154568

Abstract

The nature of halogen-bond interactions was scrutinized from the perspective of astatine, potentially the strongest halogen-bond donor atom. In addition to its remarkable electronic properties (e.g., its higher aromaticity compared to benzene), C6At6 can be involved as a halogen-bond donor and acceptor. Two-component relativistic calculations and quantum chemical topology analyses were performed on C6At6 and its complexes as well as on their iodinated analogues for comparative purposes. The relativistic spin–orbit interaction was used as a tool to disclose the bonding patterns and the mechanisms that contribute to halogen-bond interactions. Despite the stronger polarizability of astatine, halogen bonds formed by C6At6 can be comparable or weaker than those of C6I6. This unexpected finding comes from the charge-shift bonding character of the C–At bonds. Because charge-shift bonding is connected to the Pauli repulsion between the bonding σ electrons and the σ lone-pair of astatine, it weakens the astatine electrophilicity at its σ-hole (reducing the charge transfer contribution to halogen bonding). These two antinomic characters, charge-shift bonding and halogen bonding, can result in weaker At-mediated interactions than their iodinated counterparts.

Highlights

Many standard classifications in chemistry cross the group of p-elements, especially the chemical families known as pnictogens, chalcogens, and halogens
Topological Analyses of the C6X6 XB Donors Figure 1 shows the electron localization function (ELF) localization domains determined for C6At6, which is representative of the topology obtained for C6I6
Astatine is the less electronegative and the most polarizable of the halogen elements, it was shown that the strength of halogen bonds mediated by astatine (At–XBs) can be similar or weaker than one of their iodinated counterparts (I–XBs) when comparing complexes formed by C6At6 and C6I6

Summary

Introduction

Many standard classifications in chemistry cross the group of p-elements, especially the chemical families known as pnictogens, chalcogens, and halogens. Bound to an electron-withdrawing group, these elements display a region of electron deficiency, the so-called σ-hole, which can induce highly directional attractive interactions with a nucleophilic region from another, or from the same, molecular entity These interactions are better known, since the 2000s, as pnictogen, chalcogen, and halogen bonds, the latter being increasingly used in materials science, supramolecular chemistry, organocatalysis, chemical biology, and medicinal chemistry [3,4,5,6,7,8,9,10]. Among this framework, halobenzenes are an interesting class of halogen-bond donors, R–X, because halogen atoms X can be introduced into a benzene ring at different positions.

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Conclusion