Nuclear Quadrupole Double Resonance of Compounds with Transannular Boron—Nitrogen Bonds

Abstract

The boron and nitrogen nuclear quadrupole double resonance spectra of several ring compounds with transannular boron-nitrogen bonds are reported. The electron donation from nitrogen to boron as seen by their quadrupole coupling parallels the boron-nitrogen bond lengths. One of the compounds exhibits a transannular valence topomerisation between two identical boron-nitrogen pairs in solution which is frozen in the solid state but may possibly exist in a preformed state of this equilibrium from its quadrupole coupling. The oxygen-boron π-bond in boroxines, whose extent is deduced from the quadrupole coupling in one of the compounds with a boroxine-like structure at boron and in (PhBO)3, is approximately half as strong as the nitrogen-boron π-bond in borazine.