Silatranes and germatranes as the systems with intramolecular tetrel bonds

We studied intramolecular noncovalent bonds in organogermanium or organosilicon cyclic esters of tris(2-hydroxyalkyl)amines called silatranes and germatranes. We have shown that the N…Si and N…Ge interactions, well known as hypervalent or transannular bonds, can be rightfully categorized as the strong tetrel bonds (TtB). In the wide set of silatranes and germatranes, the TtB strength is under the influence of the Y substituent at Tt atom in the N…Tt–Y fragment and the features of crystalline environment. We have disclosed the quantitative trends in electronic features of N…Tt tetrel bonds and demonstrated the applicability of criteria based on the positions of extremes in electron density and electrostatic potential along the line between N and Tt atoms to compare the strength of the N…Si and N…Ge tetrel bonds. An important observation is that the dependence of gap width on bond lengths is not linear for silatranes. In order to solve this problem we have analyzed the features of total static potential. The gap between positions of extremes in electrostatic and total static potentials is wide for the weak tetrel bonds and narrow for the strong ones.