C(sp3) atoms as tetrel bond donors: A crystallographic survey

The σ-hole and π-hole interactions allow for a systematic understanding of some features of the attractive interactions involving elements of groups 13–18 of the periodic table and of some other groups. Areas of depleted electron density, where the electrostatic potential can be positive, exist on these atoms and these areas can form attractive interactions with electron rich sites (nucleophiles). When the electrophilic atom belongs to groups 14, 15, 16, or 17, the resulting interactions are named tetrel, pnictogen, chalcogen, and halogen bond, respectively. Here we discuss the tetrel bonds (TtBs) formed in crystalline solids on interaction of sp3 hybridized carbon atoms with lone pair possessing atoms and anions. A mapping of the specific short contacts formed in the solid by C(sp3) atoms is realized by discussing selected structures from the Cambridge Structural Database. This mapping led to the identification of some functional groups particularly tailored to form TtBs which can affect or control the packing in crystalline solids. Specifically, it is shown that methyl and methylene groups bound to ammonium, pyridinium, and sulfonium residues can give rise to particularly short and directional TtBs. Topologically, the formed adducts can exist as discrete species or one, two, or three dimensional networks. Fluorine atoms and perfluorinated residues as well as nitro and cyano substituents can also lead to the formation of TtBs which can control molecular conformation and packing in the solid.