Chelated fluoroboron cations. I. Synthesis and NMR studies involving the tertiary-amine ligands N,N,N ',N '-tetramethylethylenediamine and N,N,N ',N '',N ''-pentamethyldiethylenetriamine

Several possible methods of synthesis of chelated fluoroboron cations are e xplored, using the tert-amines N,N',N,N'-tetramethylethylenediamine (Me(4)e n) and N,N,N',N",N"-pentamethyldiethylenetriamine (Me(5)dien) as model chel ating ligands. Both ligands displace pyridine from (pyr)(2)BF2+ (as its PF6 - salt) to form the bidentate (Me(4)en)BF2+ and (Me(5)dien)BF2+ cations. Th e same cations, as well as the corresponding BFCl+ and BFBr+ cations, can a lso be prepared by displacement of the donor molecule (D = pyridine or isox azole) and the heavy halide ion (Cl- or Br) from the neutral D.BF2X and D.B FX2 adducts. The central nitrogen of Me(5)dien becomes chiral when it and o ne terminal nitrogen are coordinated, and the prochiral and magnetically no nequivalent fluorines of (Me(5)dien)BF2+ give F-19 NMR signals separated by 1.2 ppm. In (Me(5)dien)BFCl+ the boron is a second chiral centre and the t wo diastereomers, distinguishable by NMR with F-19 chemical shifts differin g by 3.0 ppm, form in a 3:1 ratio. The bidentate BFBr+ cations of Me(4)en a nd Me(5)dien are insoluble in non-coordinating solvents but have been detec ted by positive ion FAB mass spectrometry and B-11 MAS NMR. The tridentate complex (Me(5)dien)BF+2 does not form under our conditions.