Preparation and Lewis acid properties of diboron tetrafluoride and related compounds Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/h989r617d

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  • 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 boron trichloride. 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.
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