Formation of mixed-ligand fluoroboron cations involving C=N donors. A 19F and 11B NMR study

Abstract

Mixed-donor difluoroboron and chlorofluoroboron cations DD'BF2+ and DD'BFCl+ involving at least one C=N donor molecule can be formed by Cl- displacement from the mixed boron trihalide adducts D·BF2Cl and D·BFCl2, provided that the steric hindrance of D' and especially D is not too great. The DD'BF2+ cations can also be formed by selective displacement of one donor molecule in certain D2BF2+ cations, present in acetone as their PF6- salts, allowing isolation of the first salt of a mixed-donor difluoroboron cation, (quinuclidine)(pyridine)BF2+·PF6-. In less favourable cases, however, either no reaction occurs or the reaction continues to give D'2BF2+. Reactivities and 19F and 11B NMR parameters of mixed boron trihalide adducts and fluoroboron cations of a number of C=N donors are reported.Key words: fluoroboron cations, mixed-ligand, fluorine-19 NMR, boron-11 NMR, oxazolines, thiazolines, C=N donors.