- Attempts to synthesize diboron tetrafluoride by the reduction of
boron trifluoride with alkali metal solutions in 1, 2-dimethoxyethane
proved to be unsuccessful. No diboron tetrahalide was found when
boron trifluoride, antimony trifluoride, or boron trichloride was
reacted with tetra-(dimethylamino)-diboron; although there appeared
to be some exchange between chloride and amine groups.
Diboron tetrafluoride was prepared by the treatment of diboron
tetrachloride with antimony trifluoride. The diboron tetrachloride
was produced by the acid hydrolysis of tetra-(dimethylamino)-diboron
to hypoboric acid with subsequent dehydration and reaction with
From the results of the formation of Lewis acid-base complexes
and their acid displacement reactions the order of relative acid
strength was found to be: hydrogen chloride, diboron tetrafluoride < boron trifluoride < boron trichloride. Diboron tetrafluoride formed strong complexes with triethylamine,
trimethylamine, 2 , 6-lutidine, N, N, N', N'-tetramethylethylenediamine,
and 1, 2-dimethoxyethane. A weak complex occurred with benzonitrile,
whereas, no complexation was found with p-chlorobenzonitrile.
Boron trifluoride displaced diboron tetrafluoride from its complexes
with 1, 2-dimethoxyethane, trimethylamine, and triethylamine. No
displacement occurred, under the conditions investigated, with the
complexes of N, N, N', N'-tetramethylethylenediamine and benzonitrile.
It was found that p-chlorobenzonitrile serves as an excellent
Lewis base for the separation of boron trifluoride from hydrogen
chloride and boron trichloride impurities by complex formation.
p-Nitroanisole served to separate boron trichloride from hydrogen
chloride and boron trifluoride by the same method.
Inasmuch as diboron tetrafluoride is a weaker Lewis acid than
boron trifluoride and has a short boron-boron bond compared to diboron
tetrachloride, a bond order greater than one is postulated to
exist between the boron atoms. The presence of this partial π bond
would tend to constrain diboron tetrafluoride in planar configuration
in the gaseous state in contrast to the twisted configuration known
to occur in diboron tetrachloride.
The infrared spectral range of 600-4000 cm⁻¹ was scanned with
two principal absorption peaks of 1158 and 1373 cm⁻¹ indicated for
gaseous diboron (11, 11) tetrafluoride. The infrared spectra of solid
diboron tetrafluoride was also obtained over the same range.
An infrared cell with a novel all-metal seal was designed and used
for the spectra determination of gases. The vacuum-tight seal was
obtained by placing an amalgamated lead gasket between a silvered
glass cell and rock salt window. The all-metal seal was found to be
less reactive toward boron halides than other common sealants.