Boron-11 chemical shifts in the determination of donor-acceptor strengths—VII: Pyridine-borane and phenylborane complexes

Abstract

The 1H and 11B spectra of a series of pyridine- and substituted pyridine-borane and phenylborane (Py, BH 3 and Py, PhBH 2) complexes have been measured. The 14N chemical shifts of pyridine, 2-, 3- and 4-methyl and 4-ethyl-pyridines and the borane complexes and the hydrochlorides have also been determined. The 11B chemical shifts have been related to the donor strengths of the pyridines and, as for the boron trihalide complexes, the order of donor strengths was found to be 2-alkyl- > 4-alkyl- > 3-alkyl-pyridines and pyridine, an order contrary to thermochemical correlations. The merits of using the 11B chemical shifts to determine donor-acceptor interaction are considered.