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

Abstract

Several possible methods of synthesis of chelated fluoroboron cations are explored, using the tert-amines N,N,N',N'-tetramethylethylenediamine (Me4en) and N,N,N',N",N"-pentamethyldiethylenetriamine (Me5dien) as model chelating ligands. Both ligands displace pyridine from (pyr)2BF2+ (as its PF6- salt) to form the bidentate (Me4en)BF2+ and (Me5dien)BF2+ cations. The same cations, as well as the corresponding BFCl+ and BFBr+ cations, can also be prepared by displacement of the donor molecule (D = pyridine or isoxazole) and the heavy halide ion (Cl- or Br-) from the neutral D·BF2X and D·BFX2 adducts. The central nitrogen of Me5dien becomes chiral when it and one terminal nitrogen are coordinated, and the prochiral and magnetically nonequivalent fluorines of (Me5dien)BF2+ give 19F NMR signals separated by 1.2 ppm. In (Me5dien)BFCl+ the boron is a second chiral centre and the two diastereomers, distinguishable by NMR with 19F chemical shifts differing by 3.0 ppm, form in a 3:1 ratio. The bidentate BFBr+ cations of Me4en and Me5dien are insoluble in non-coordinating solvents but have been detected by positive ion FAB mass spectrometry and 11B MAS NMR. The tridentate complex (Me5dien)BF+2 does not form under our conditions.Key words: N,N,N',N'-tetramethylethylenediamine, N,N,N',N",N"- pentamethyldiethylenetriamine, chelated fluoroboron cations, fluorine-19 NMR, boron-11 NMR, pyridine, isoxazole, chiral, magnetically nonequivalent.