Alkylation of tris(pentafluorophenyl)boron with pentafluorophenyl lithium and metathesis with triphenylmethylchloride; triphenylcarbenium tetrakis(pentafluorophenyl)borate

Dr Simon Lancaster ([email protected]),
A contribution from the Lancaster Group, University of East Anglia

Chemicals Used
BrC6F5 (Fluorochem), n-BuLi (Aldrich), B(C6F5)3 (see synthetic pages preparation), light petroleum (dried over Na / K alloy), dichloromethane (dried over calcium hydride)

Procedure
Li[B(C6F5)4] Dissolve BrC6F5 (2.47 g, 10 mmol) in 100 mL light petroleum and cool to -78 °C on a dry-ice/acetone bath (IT IS VERY IMPORTANT THAT THE BATH TEMPERATURE IS MAINTAINED AT -78 °C, LiC6F5 IS THERMALLY UNSTABLE AND WILL DECOMPOSE, LIBERATING LiF, IN AN EXOTHERMIC AND POTENTIALLY EXPLOSIVE REACTION!) add 6.25 mL of a 1.6 M solution of n-BuLi in hexanes slowly with stirring. The LiC6F5 will precipitate as a white solid. Stir for a further hour at -78 °C to ensure complete reaction (DO NOT WARM). Rinse the dropping funnel with a little anhydrous light petroleum, added only slowly to the flask. Slowly add solid B(C6F5)3 (5.12 g, 10 mmol). Stir the resulting suspension for a further hour at -78 °C before allowing to warm very slowly to room temperature over a period of between four and six hours. At this stage the reaction can safely be left overnight under an inert gas atmosphere. Allow the LiB(C6F5)4 to settle and filter off the light petroleum. [CPh3][B(C6F5)4] Combine the Li[B(C6F5)4] (10 mmol) prepared above with Ph3CCl (2.79 g, 10 mmol) and add dichloromethane (200 mL). The reaction mixture will rapidly turn bright orange. Stir for 2 hours to ensure complete reaction. Separate the insoluble lithium halides from the dichloromethane solution. Concentrate the solution to approximately 30 mL and cool to -20 °C (introduce seed crystals at this point if available) until crystallisation occurs. The resulting crystals should be washed with cold -40 °C dichloromethane and the purity established by 1H and 19F NMR.

Author's Comments
The preparation should be carried out under anhydrous conditions under an inert gas atmosphere using standard vacuum-line/ Schlenk techniques. It is possible to prepare the lithium tetrakis(pentafluorophenyl)borate salt directly through the reaction of four equivalents of pentafluorophenyl lithium with boron trichloride or boron tribromide by following the basic procedure outlined above. However, the reaction of one equivalent pentafluorophenyl lithium and tris(pentafluorophenyl)boron has the advantage of reducing the quantity of the hazardous LiC6F5 and furthermore B(C6F5)3 is much easier to handle and measure accurately than BCl3 and BBr3. The resulting increase in the purity of the LiB(C6F5)4 pays dividends when attempting to crystallise [CPh3][B(C6F5)4]. The preparation of LiB(C6F5)4 should be carried out in light petroleum and subsequent metathesis with Ph3CCl in dichloromethane, in particular the salts should not be exposed at any stage to tetrahydrofuran, which leads to decomposition of the trityl salt. We routinely handle and prepare [CPh3][B(C6F5)4] with the exclusion of light following Tilley�s report of the light sensitivity of [CPh3][B(C6H5)4]. The separation of lithium halides from the viscous dichloromethane solution by conventional filtration may be unsatisfactory. We have successfully employed centrifuge techniques to aid this process. The real difficulty with this preparation is obtaining a very pure sample of [CPh3][B(C6F5)4]. A solid can be obtained by concentrating the solution to a volume of approximately 20 mL and adding it dropwise to 500 mL of rapidly stirred light petroleum, which will cause the trityl salt to precipitate as a fine yellow solid. Chien maintains that this material can be purified by repeated washing with small portions of toluene. It is our experience that material produced in this fashion can be of variable purity, often appearing to be quite clean by 1H NMR spectrum, typically with only traces of hydrocarbon impurities. However, there may be contamination with a second anionic species indicated in the 19F NMR spectrum, which will not be removed using this procedure. Once crystallisation of one sample has occurred use of these crystals to seed subsequent preparations can be beneficial.. Once crystallised, the increased purity leads to a readily recrystallisable solid, with only one set of resonances in the 19F NMR spectrum. The salt crystallises with dichloromethane of crystallisation, and if it is to be used to generate sensitive materials the crystals must be crushed and the solid dried under vacuum to remove the solvent.

Data
1H NMR (400 MHz, CD2Cl2): [CPh3]+ : 8.27 (t, p-H), 7.88 (t, m-H), 7.67 (d, o-H); [B(C6F5)]- : 19F NMR (CDCl3, rel. CFCl3): -125.7 (s, br), -156.1 (t, J 20.6Hz), -160.0 (t, br, J ca. 18 Hz).

Lead Reference
1. J. C. W. Chien, W. M. Tsai, and M. D. Rausch, J. Am. Chem. Soc., 1991, 113, 8570.

Other References
2. M. Bochmann, S.J. Lancaster J. Organomet. Chem. 1992, 434, C1.

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