Intermolecular carbon centered tetrel bond complexes – New research tools for investigating fundamental chemical theories

Noncovalent interactions resembling hydrogen bonds is currently a fast-growing area of research. What these chemical bonds have in common is an interaction between a Lewis base Y and an electrophilic center atom X that is connected to a third atom Z (i.e. Z–X···Y). In addition to hydrogen, elements of group IV (tetrels) and group VII (halogens), for example, have been found to engage as electron accepting center atoms in these type of interactions. Notably, in a situation where the Lewis basicity of atoms Z and Y are comparable, the central electrophilic atom X is equally shared between them, which result in bonds of equal strength and length, i.e. Z–X–Y. Such interactions are referred to as three-center, four electron bonds.

In this publication we report the first intermolecular carbon-centered, three-center, four-electron tetrel bond complexes of [N–C–N]+-type (Figure 1). The complexes were prepared by capturing a carbenium ion (C+) with a bidentate Lewis base. Experimental evidence of the existence of the complexes are provided via solution nuclear magnetic resonance (NMR) spectroscopic, titration-calorimetric and reaction-kinetic methods. Furthermore, by switching the bidentate ligand for a monodentate Lewis base, and by varying the R and R' substituents (Figure 1), we show that the bidentate nature of the Lewis bases, as well as the Lewis basicity and the degree of Lewis acidity of the carbenium ion, are critical to the formation of the tetrel bond.

Figure 1. The general structure of the intermolecular carbon-centered tetrel bond complexes under investigation.

Compared to analogous three centered [N–I–N]+ halogen bond complexes studied in the group previously, the [N–C–N]+ complexes were found to exhibit similar energy and geometry. Importantly, the tetrel bond under investigation was found to be significantly stronger than those reported to date. Since the geometry of the tetrel bond complexes mimics that of the SN2 transition state, we foresee that our model system opens doors to new investigations of reaction mechanisms and chemical bonding theories.

Original article: Carbon’s Three-Center, Four-Electron Tetrel Bond, Treated Experimentally. Authors: Alavi Karim, Nils Schultz, Hanna Andersson, Bijan Nekoueishahraki, Anna-Carin C. Carlsson, Daniel Sarabi, Arto Valkonen, Kari Rissanen, Jürgen Gräfenstein, Sandro Keller and Máté Erdélyi.