Categorizing Chalcogen, Pnictogen, and Tetrel Bonds, and Other Interactions Involving Groups 14-16 Elements

The objective of this two-year project is to develop a non-ambiguous terminology for interactions formed by chalcogens, pnictogens, and tetrels, namely the elements of Groups 16, 15, and 14, respectively. Group 16-14 elements can form attractive interactions with both nucleophiles and electrophiles. All of these interactions will be examined systematically in order to identify and to classify cases where the Group 16-14 elements work as electrophilic species. A consensus is emerging in the chemical community that these latter interactions be named chalcogen bond, pnictogen bond, and tetrel bond. [1]

This project will assess and register this consensus by involving the whole community of researchers dealing with the study and use of these intermolecular interactions. Thanks to this involvement, modern definitions of chalcogen, pnictogen, and tetrel bonds will be issued; they will be as general as possible, and will take into account all current experimental and theoretical pieces of information on gaseous and condensed systems in chemical and biological systems. Consistent with the use of the terms hydrogen and halogen bond only for interactions where hydrogen and halogens are the electrophiles, it will be proposed that the terms chalcogen bond, pnictogen bond, and tetrel bond are used exclusively for interactions wherein the respective elements are the electrophile. It is intended by the Task Group of this project that interactions where Group 16-14 elements work as the nucleophile are named after the name of the Group the electrophilic atom belongs to. For instance, it is expected that, in keeping with the common terminology, interactions wherein a sulfur atom shares electron density with an electron deficient hydrogen are named hydrogen bonds.

The Project will contribute to the development of an unambiguous, systematic, and periodic naming of most interactions where it is possible to identify an element or moiety working as the electrophile. [1] The proposed terminology will balance wide applicability (i.e., generality) and robust descriptive power (i.e., specificity) and convey specific information on the interactions, such as their polar characters and geometric features. This nomenclature will be helpful in systemizing the field of interactions and in clarifying intrinsic or extrinsic relationships between concepts. [2]

A brief description of the state of the art of the topics addressed in this Project is given below with the aim to involve all those interested in related fields. The ability of Group 16-14 elements to form attractive interactions with both nucleophiles and electrophiles is a consequence of the highly anisotropic distribution of the electron density around these elements. In elements of Group 16-14 forming σ-bonds, areas of lower electron density, with an often positive electrostatic potential named the σ-hole, are present at the extension of covalent σ-bonds formed by these elements. Areas of higher electron density exist on the element’s surface, where the electrostatic potential is negative. [3] Nucleophiles preferentially enter the region(s) of highest electrons density and electrophiles the region(s) of lowest density, and the resulting interactions present different and complementary directionalities. The electron density is also anisotropically distributed around Group 16-14 elements forming covalent π-bonds. [4] Electron density can be remarkably thinned out above, and below, the π-bond plane; a region of positive electrostatic potential, sometimes denoted as the π-hole, is associated with these areas, and nucleophilic sites attractively interact with them This behavior mirrors the well-established ability of unsaturated atoms to work as nucleophiles.

Figure 1. Partial ball and stick representation of the two-dimensional network that chalcogen bonds (black dashed lines) form in the crystal of tellurium dicyanide (Refcode FEGBID). Color code: grey, carbon; blue, nitrogen; orange, tellurium.

Figure 2. Partial representation of the infinite chains formed by pnictogen bonds in crystalline cyanodimethylarsine (A, Refcode CNMARS) and by tetrel bonds in crystalline dimethylsilicon dicyanide (B, Refcode DMCYSI) and dimethylgermanium dicyanide (C, Refcode DMCYGE). Color code: grey, carbon; blue, nitrogen; violet, arsenic; yellow, silicon; dark green, germanium.

Experimental results have given forceful indications of the ability of Group 16-14 elements to work as electrophiles much earlier than the above-described model was developed. NMR techniques have afforded useful information, [5] but X-ray crystallography has been particularly effective in identifying the studied interactions. The solid-state structures of mono- and poly-cyanides of elements of Groups 16-14 give extensive and consistent examples of chalcogen, pnictogen, and tetrel bonds. Chalcogen bonds are present in the dicyanide of sulfur, selenium, and tellurium: any chalcogen atom forms two such interactions with two distinct cyano nitrogen atoms, and two-dimensional networks are formed (Figure 1). [6]

In P(CN)₃, all three cyano groups participate in short contact with phosphorous. Analogous interactions are present in all three As(CH₃)_n(CN)_3-n (n = 0-2) derivatives. [7] Tetrel(CH₃)₂(CN)₂ (Tetrel = Si, Ge, Sn) show quite directional tetrel bonds (Figure 2) and a similar behavior is presented by trimethylcyano analogues. [8]

The project will be advertised in major symposia and conferences relevant to related fields (e.g., the International Conference on the Chemistry of Selenium and Tellurium, the International Conference on Phosphorous Chemistry, and the International Symposium on Organic Chemistry of Sulfur). A kick-off meeting of the Project will be organized in Milan (Italy) early in 2017.

For more information and comments, contact Task Group Chairs Pierangelo Metrangolo <pierangelo.metrangolo@polimi.it> and Giuseppe Resnati <giuseppe.resnati@polimi.it>.

www.iupac.org/project/2016-001-2-300