Draft:Tetrel bond

In chemistry, a tetrel bond (TtB) is a type of non-covalent interaction in the family of σ-hole interactions. This bonding is recently identified in the field of supramolecular chemistry. Tetrel bond is formed by a tetrel atom,which is an element of  Group 14 in the Periodic Table (carbon, silicon, germanium...) and an electronegative atom  (nitrogen, oxygen or fluorine).

Bonding
A tetrel bond is geometrically represented by the motif R-Tt···A, with Tt the TtB donor, a tetrel atom with an electrophilic region, R the rest of the molecule entity and A the TtB acceptor. The TtB donor entity is either electrically neutral or a molecular cation that may or may not possess a π-system. A can be a lone-pair region in a molecular entity, or a molecular anion, and must carry a nucleophile.

Behaviour of the different tetrel atoms
The σ- and π-hole interactions are used to define the attractive forces involved in tetrel bonding.

σ-Hole
Halogen, chalcogen, pnictogen, and tetrel bonds arise from regions of low electron density, known as σ holes, which interact with electron-rich acceptor sites (which are often lone pairs, including anions). Once formed, these different types of chemical bonds are directlly associated with the position and number of covalent bonds that the atoms form. Thus, each family of elements (halogens, chalcogens, pnictogens, and tetrels) possesses a distinct number of σ holes: four for tetrels,three for pnictogens,two for chalcogens and one for halogens. The more polarizable the chemical element, the deeper the σ hole, meaning it increases from top to bottom and from right to left in the periodic table (Figure 2).

For example,in contrast to halogen and chalcogen bonding, catalysis through pnictogen and tetrel bonding provides greater depth and number of σ holes within chiral binding pockets. Nevertheless, as σ holes become deeper, interactions not only intensify but also begin to approach covalent character, transitioning gradually from mere interaction to chemical reaction.

Historical aspects
In the 1960s and 70s, important studies revealed that elements like carbon, silicon, sulfur, and noble gases could act like Lewis acids in interactions similar to halogen bonds. Henry Bent, a prominent physical chemistry professor, initially explained these interactions within electron donor-acceptor complexes, emphasizing the importance of an orbital interaction model. Bent's reviews and Alcock's research on secondary bonding provided detailed insights into various interactions, including tetrel bonds (Group 14), pnictogen bonds (Group 15), chalcogen bonds (Group 16), and aerogen bonds (Group 18).

Nathaniel Alcock, an American scientist in 1971, introduced the term 'secondary bond' to describe linear intermolecular interactions where main group elements act as Lewis acids. The ongoing debate centered on determining the most suitable bonding description and the potential use of both models.

Meanwhile, one of the earliest instances leading to the discovery of tetrel bonds was when Devendra Mani and Arunan observed that adding an electron-withdrawing group to methane created a positively charged region on its opposite side. This positive area could interact favorably with nucleophiles like water, initially termed a carbon bond by these authors.

Later on, Bauzá identified similar interactions between Si, Ge, and Sn with oxygen or halogen-containing molecules, expanding the concept of tetrel bonds for the carbon family. All together, their research proposed a comprehensive definition of tetrel bonds based on existing literature evidence, outlining donors, acceptors, and key features observed in various phases and computational studies.

Tetrel Bond donors and acceptors
A considerable amount of research has been carried out into the nature of the Tetrel bond donors (TtBD) and acceptors (TtBA) that interact attractively to form Tetrel bonds. To understand tetrel bonds and the complexity of their interactions, it's essential to understand the role of donors and acceptors.

Let's take a look at carbon. Carbon behaves like an acceptor, as it generally tends to attract electrons towards itself. However, there are some contradictions with this statement. Take, for example, methyl radicals and carbenes, which possess extra electrons and can therefore act as unconventional electron donors in tetrel bonding scenarios. The same applies to acceptors, which can react with electron-rich donors.

The donors and acceptors in non-covalent interactions in the family of σ-hole interactions are essentially the same. Where (H=Hydrogen, Hlg=Halogen, Ch=Chalcogen, Pn=Pnictogen, Tr=Tetrel)

The concept of electron acceptance in tetrel bonding aligns with the broader understanding of non-covalent interactions, where electrostatic attraction serves as the fundamental driving force. It provides insights into how polarization and dispersion also influence the stability of tetrel bonds.

The roles of donor and acceptor can then provide insights into the mechanisms underlying tetrel bonding and its various applications in different chemical systems. This concept is most widely used in fields ranging from crystal engineering to catalysis and photovoltaics.

Donor characteristics
The tetrel bond donor tends to approach the tetrel bond acceptor along the outer extension of a σ covalent or coordinate bond,the angular deviation from the bond extension is often more pronounced in tetrel bonds than in halogen bonds, chalcogen bonds and comparable to pnictogen bonds.

Some examples of tetrel bond donors : fullerene derivatives (C60, C70 and C80), pyrene-F10.

Acceptor characteristics
When the nucleophilic region on the tetrel bond acceptor, is a lone pair orbital, or a (negative) π region, the tetrel bond donor tends to approach the tetrel bond acceptor the axis of the lone pair, or orthogonal to the π bond plane.

Some examples of tetrel bond acceptors : halide anions X-, N in pyridines or amines.

Hypervalent Halogen Compounds
Hypervalent halogen compounds form tetral bonds (TB) with distinct characteristics depending on the variation in electronegativity of the atoms involved. In the molecule PhXF2Y(TF3), where X and Y are halogens and T can be C or Si, increasing the electronegativity of the halogen atom X strengthens the TB, while increasing the electronegativity of the halogen atom Y diminishes this strength.

In the case where -TF3 is adjacent to a hypervalent halogen atom, the strength of the TB formed varies with the electronegativity of the hypervalent halogen atom X, increasing with it, but decreasing with the electronegativity of the halogen atom Y. This trend is observed for both C and Si systems. Electrostatic interactions play a major role in the formation of these bonds.

TB strength varies according to position in the periodic table, with a tendency to increase in the order Si < Ge < Sn, and a noticeable change when hydrogen atoms are replaced by halogens. Molecular deformation is also an important factor, and can lead to charge transfer between molecules.

In chemical reactions such as SN2, tetral bond strength plays a crucial role in the transfer of -TX3 groups. Because of their oxidizing properties and special structures, hypervalent halogens are important in organic synthesis.

Characteristics of hypervalent halogen compounds include variation in electrostatic potential and specific geometric structures, revealing significant differences between tetral bonds formed with carbon and silicon atoms.

Comparison of tetrel with other non-covalent bonds
There are various distinctions and similarities among non-covalent bonds. We can wonder which bonds are the strongest or the weakest. Thanks to few studies, we can compare these different non covalent bonds.

Here on this scheme, we can observe the molecular electrostatic potentials (MEPs) of SiH3Br in Figure 3. This map is a specific representation that can be used to illustrate tetrel bonds with different atom. A σ-hole occurs along the Br-Si link which is on the opposite side of the Br atom. The σ-hole is due to the Br electronegativity. Indeed, the electronegativity of the Br leads to a σ-hole which can act as the Lewis acid site for the formation of a halogen bond with a Lewis base such as a fluor atom. Furthermore, the lone pair on the other Br atom acts as the Lewis base thanks to its negative MEP to form a hydrogen bond with an hydrogen.

Now, we will focus on the σ-hole of TtB. The table 2 table shows electrostatics potentials of different tetrel bonds, YH3X (X=halogen, Y=C and Si). We remark that, when the halogen atomic mass increases, the surface of the positive MEP, Vmax(X), on the halogen atom rises. To illustrate, Vmax(X) is the small green part on the halogen atom of the figure 3. Moreover, when Y=C is changed to Y=Si, we observe the inverse effect, the Vmax(X) is reduced due to the lower electronegativity of Si. Whereas, the negative MEP, Vmin(X) (the blue part of the halogen atom) becomes less negative with the increasing halogen atomic number. In the periodic table, electronegativity is inversely proportional to the atomic number in a column.

If we look at the Y atom (C or Si), the positive MEP, Vmax(Y), is bigger when it is linked with a lighter X and a heavier Y atom.

We can acknowledge important information thanks to table 2. The Vmax(Y) is larger than Vmax(X) except if X=I and Y=C. Consequently, the tetrel atom is a much better Lewis acid than the halogen atom.

Application
Tetrel Bond applications cover diverse fields such as crystal engineering, biology, materials science, catalysis, photovoltaics, medicinal and supramolecular chemistry, drug discovery and biomolecular design.