Polar covalent apex-base bonding in borapyramidanes probed by solid-state NMR and DFT calculations

Luder D.J.; Terefenko N.; Sun Q.; Eckert H.; Mück-Lichtenfeld C.; Kehr G.; Erker G.; Wiegand T.

Abstract

Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid-state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11B solid-state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around −50 ppm vs. BF3-Et2O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13C−11B spin-spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon-boron bond to the −B(C6F5)2 substituents of the base serves as an example for a classical covalent 2-center-2-electron (2c–2e) sp2-carbon-sp2-boron σ-bond with 1J(13C-11B) coupling constants in the order of 75 Hz, those of the boron(apical)-carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum-chemical calculations.