Tuning Lewis Acidity by a Transannular pπ -σ* Interaction between Boron and Silicon/Germanium Atoms Supported by a Cage-Shaped Framework.

Cage-shaped borates tethered by heavier Group 14 elements (Si or Ge) were synthesized. These possess an intramolecularly transannular pπ -σ* interaction between the boron center and the tethered Si/Ge atom, which allows the precise tuning of their Lewis acidity. The Lewis acidity was investigated by the ligand-exchange reaction rate and IR measurements with the help of theoretical calculation. The synthesized borates exhibited catalytic activity. This study demonstrated the effectiveness of the direct orbital perturbation of a metal center by space interaction during fine tuning of the Lewis acidity.