On the Importance of σ–Hole Interactions in Crystal Structures

Antonio Frontera,Antonio Bauzá

doi:10.3390/cryst11101205

Abstract

Elements from groups 14–18 and periods 3–6 commonly behave as Lewis acids, which are involved in directional noncovalent interactions (NCI) with electron-rich species (lone pair donors), π systems (aromatic rings, triple and double bonds) as well as nonnucleophilic anions (BF4−, PF6−, ClO4−, etc.). Moreover, elements of groups 15 to 17 are also able to act as Lewis bases (from one to three available lone pairs, respectively), thus presenting a dual character. These emerging NCIs where the main group element behaves as Lewis base, belong to the σ–hole family of interactions. Particularly (i) tetrel bonding for elements belonging to group 14, (ii) pnictogen bonding for group 15, (iii) chalcogen bonding for group 16, (iv) halogen bonding for group 17, and (v) noble gas bondings for group 18. In general, σ–hole interactions exhibit different features when moving along the same group (offering larger and more positive σ–holes) or the same row (presenting a different number of available σ–holes and directionality) of the periodic table. This is illustrated in this review by using several examples retrieved from the Cambridge Structural Database (CSD), especially focused on σ–hole interactions, complemented with molecular electrostatic potential surfaces of model systems.

Highlights

Atoms from elements of groups 13–18 covalently bound to electron-withdrawing groups (EWGs) possess a strong ability to interact with Lewis bases [1,2,3,4,5,6,7,8,9,10,11,12,13]
Elements from groups 14–18 and periods 3–6 commonly behave as Lewis acids, which are involved in directional noncovalent interactions (NCI) with electron-rich species, π systems as well as nonnucleophilic anions (BF4−, PF6−, ClO4−, etc.)
The molecular electrostatic potential (MEP) values at the Tt σ–holes of the four fluoride derivatives included are given in Table 1, and the MEP surface of SnF4 is shown in Figure 2 as a representative case

Summary

Introduction

Atoms from elements of groups 13–18 covalently bound to electron-withdrawing groups (EWGs) possess a strong ability to interact with Lewis bases (e.g., lone pair donors, anions, and π systems) [1,2,3,4,5,6,7,8,9,10,11,12,13]. Matere bonding (MaB, group 7) [21], osme bonding (OmB, group 8) [22], spodium bonding (SpB, group 12) [23], and regium or coinage bonding (CiB, group 11) have been used to refer to NCIs in which elements from groups 10 and 11 act as Lewis acids and to differentiate these types of interactions from classical coordination bonds [24,25,26,27] In this regard, several studies have demonstrated that σ–holes (regions of positive electrostatic potential located on the extension of covalent bonds) can be used in a broad spectrum of fields, such as host–guest chemistry, catalysis, supramolecular chemistry, membrane transport, crystal engineering, etc. It is important to mention that this review is focused on the σ–hole family of interactions, and other NCIs present in the X-ray crystal structures (e.g., hydrogen bonding) have not been described

Tetrel Bonding

Pnictogen Bonding

Chalcogen Bonding

Halogen Bonding

Noble Gas Bonding

Concluding Remarks