Bis(tertiary amine)dihaloboron cations and related species: nuclear magnetic resonance and fast atom bombardment mass spectrometry studies

Abstract

The formation of four-coordinate haloboron cations from aliphatic tertiary amine adducts of the mixed boron trihalides by heavy halogen displacement has been systematically studied by 19F and 11B nuclear magnetic resonance and positive ion fast atom bombardment mass spectrometry (FAB). Low-steric-hindrance donor molecules readily displace bromide ion from tertiary amine – bromodifluoroborane adducts, D•BF2Br, to form difluoroboron cations D2BF2+ and DD′BF2+, but the corresponding dibromofiuoroborane and tribromoborane adducts are highly resistant to bromide ion displacement. Bis(tertiary amine)dichloroboron and -chloroiodoboron cations can be obtained by selective iodide displacement from D•BCl2I and D•BClI2. Fast atom bombardment mass spectrometry selectively detects the haloboron cations in preference to the neutral adducts in mixtures, and is a valuable complement to nmr in monitoring formation of the haloboron cations as well as any ionic by-products. Keywords: difluoroboron cations, dihaloboron cations, NMR, 11B, 19F, fast atom bombardment (FAB), ligand substitution, redistribution reactions.