Carbon’s Three-Center, Four-Electron Tetrel Bond, Treated Experimentally

Tetrel
bonding is the noncovalent interaction of group IV elements
with electron donors. It is a weak, directional interaction that resembles
hydrogen and halogen bonding yet remains barely explored. Herein,
we present an experimental investigation of the carbon-centered, three-center,
four-electron tetrel bond, [N–C–N]+, formed
by capturing a carbenium ion with a bidentate Lewis base. NMR-spectroscopic,
titration-calorimetric, and reaction-kinetic evidence for the existence
and structure of this species is reported. The studied interaction
is by far the strongest tetrel bond reported so far and is discussed
in comparison with the analogous halogen bond. The necessity of the
involvement of a bidentate Lewis base in its formation is demonstrated
by providing spectroscopic and crystallographic evidence that a monodentate
Lewis base induces a reaction rather than stabilizing the tetrel bond
complex. A vastly decreased Lewis basicity of the bidentate ligand
or reduced Lewis acidity of the carbenium ion weakensor even
prohibitsthe formation of the tetrel bond complex, whereas
synthetic modifications facilitating attractive orbital overlaps promote
it. As the geometry of the complex resembles the SN2 transition
state, it provides a model system for the investigation of fundamental
reaction mechanisms and chemical bonding theories.