In a flame-dried 1L three-necked flask at nitrogen atmosphere equipped with stirrer, reflux condenser, and addition funnel, magnesium (8.6 g, 354 mmol) was grinded by vigorous stirring. After addition of 350 mL of diethylether, 1 mL of pentafluorophenylbromide was added to the magnesium without stirring. Within 15 minutes heat formation was observed. The reaction mixture was cooled to 0°C, and the remainder of the pentafluorophenylbromide (in total: 50.5 g; 295 mmol) was added dropwise in 1h while stirring. When the addition of the arylbromide was completed, the reaction mixture was stirred at room temperature for 30 minutes. The dark-gray Grignard reagent was settled overnight.

The reaction mixture was filtered using a large isolation filter, and cooled to 0°C. To the Grignard solution, 9.9 g (69.5 mmol) of BF3∙Et2O was added while stirring. The reaction mixture was warmed to room temperature and refluxed for 30 minutes. The diethylether solution was cooled to room temperature, and 350 mL of toluene was added. The diethylether solvent was removed by distillation using a 20 cm vigreux. The resulting toluene solution was refluxed for 2 hrs, and the solvent was removed at reduced pressure. The resulting brownish compound was collected on an extraction filter. The compound was extracted overnight using 450 mL of hexane.

The compound was further purified by sublimation at reduced pressure using an IR lamp (10-3 torr, 60 volt), yielding 27.2 g (53 mmol; 76% yield, based on BF3∙Et2O) of tris(pentafluorophenyl)borane. During the sublimation, an unknown colorless oil was condensed prior to the borane. This oil was distilled off first. 19F NMR (toluene-d8, RT, 200 MHz): -130.03 (d, 19.8 Hz, o-F), -143.27 (t, 20.6 Hz, p-F), -161.44 (m, m-F).

NOTE: the entire synthesis was performed at a nitrogen atmosphere. The Grignard reagent C6F5MgBr has to be handled with care, and should be kept in solution throughout the synthesis. The compound is unstable and tends to give fluorine transfer to magnesium.

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