Bis- and tris(amidine)fluoroboron cations and mixed tetrahaloborate anions: NMR studies of mixed boron trihalide adduct redistribution reactions involving amidines as strong nitrogen bases

Abstract

Amidines as strong Lewis bases react with amidine–mixed boron trihalide adduct systems D•BFnCl3−n (n = 0–3) in the presence of excess boron trihalide to give complex mixtures of products including the mixed tetrahaloborate anions BFnCl4−n− and the fluoroboron cations D2BF2+ and D3BF2+, which coexist with the neutral adducts D•BFnCl3−n. Excess amidine rapidly displaces chloride ion from the chlorofluoroborate anions and neutral mixed boron trihalide adducts to give high yields of the fluoroboron cations. The D3BF2+ cations of the amidines 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) and 1,5-diazabicyclo[4,3,0]non-5-ene (DBN) are inert and have been isolated as their hexafluorophosphate salts. (DBN)2BF2+ is inert and has been isolated as its hexafluorophosphate salt, but (DBU)2BF2+ is highly reactive. Key words: amidines, fluoroboron cations, mixed boron trihalide adducts, redistribution reactions, fluorine-19 NMR, boron-11 NMR, 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), 1,5-diazabicyclo[4,3,0]non-5-ene (DBN)