Abstract:
The preparation of chelated difluoroboron cations (DD)BF2+, where DD is a
saturated polydentate tertiary-amine or polydentate aromatic ligand, has been systematically
studied by using multinuclear solution and solid state nuclear magnetic resonance
spectroscopy and fast atom bombardment mass spectrometry. Three new methods of
synthesis of (DD)BF2+ cations are reported, and compared with the previous method of
reacting a chelating donor with Et20.BF3. The methods most effective for aromatic donors
such as 1,1O-phenanthroline are ineffective for saturated polydentate tertiary-amines like
N,N,N' ,Nil ,Nil-pentamethyldiethylenetriamine.
Polydentate tertiary-amine donors that form 5-membered rings upon bidentate
chelation were found to chelate effectively when the BF2 source contained two leaving
groups (a heavy halide and a Lewis base such as pyridine =pyr or isoxazole =ISOX), i.e.,
pyr.BF2X (X = CI or Br), ISOX.BF2X and (pyr)2BF2+. Those that would form 6membered
rings upon chelation do not chelate by any of the four methods. Polydentate
aromatic ligands chelate effectively when the BF2 source contained a weak Lewis base,
e.g., ISOX.BF3, ISOX.BF2X and Et20.BF3. Bidentate chelation by polydentate tertiaryamine
and aromatic donors leads to nmr parameters that are significantly different then
their (D)2BF2+ relatives (D =monod~ntate t-amines or pyridines).
The chelated haloboron cations (DD)BFCI+, and (DD)BFBr+ were generated from
D.BFX2 adducts for all ligands that form BF2+ cations above. In addition, the (DD)BCI2+
and (DD)BBr2+ cations were formed from D.BX3 adducts by the chelating aromatic
ligands, except for the aromatic ligand 1,8-bis(dimethylamino)naphthalene, which formed
only the (DD)BF2+ cation, apparently due to its extreme steric hindrance.
Chelation by a donor is a two-step reaction. For polydentate tertiary-amine ligands,
the two rates appear to be very dependent on the two possible leaving groups on the central
boron atom. The order of increasing ease of displacement for the donors was: pyr < Cl <
Br < ISOX. The rate of chelation by polydentate aromatic ligands appears to be dependent
on the displacement of the first ligand from the boron. The order of increasing ease of
displacement for the donors was: pyr < CI < ISOX ~ Br < Et20.
