A direct low-temperature carbon-13 and fluorine-19 nuclear magnetic resonance study of boron trifluoride complexes with pyridines

Abstract

Complexes of boron trifluoride with a series of substituted pyridines have been studied using a direct, low-temperature ¹³C and ¹⁹F n.m.r. technique. At temperatures from 0 to –40 °C, ligand exchange is slow enough to permit the observation of separate ¹³C n.m.r. signals for bulk and co-ordinated pyridine molecules. The co-ordinated pyridine shift displacements are interpreted in terms of ligand polarization and a paramagnetic effect at the nitrogen atom. The BF₃¹⁹F n.m.r. chemical shifts were correlated with calorimetric data in several cases, and in general provide a measure of the strength of the interaction but not of ligand basicity. Comparative complexing abilities were evaluated by studying several pyridine mixtures.