Facile and systematic access to the least-coordinating WCA [(RFO)₃Al–F–Al(ORF)₃]− and its more Lewis-basic brother [F–Al(ORF)₃]− (RF = C(CF₃)₃)

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Abstract

By reaction of the Lewis acid Me₃Si-F-Al(ORF)₃ with a series of [PF₆]- salts, gaseous PF₅ and Me₃Si-F are liberated and salts of the anion [F-Al(ORF)₃]- ([f-al]-; RF = C(CF₃)₃) can be obtained. By addition of another equivalent of Me₃Si-F-Al(ORF)₃ to [f-al]-, gaseous Me₃Si-F is released and salts of the least coordinating anion [(RFO)₃Al-F-Al(ORF)₃]- ([al-f-al]-) are formed. Both procedures work for a series of synthetically useful cations including Ag+, [NO]+, [Ph₃C]+ and in very clean reactions with 5 g batch sizes giving excellent yields typically exceeding 90%. In addition, the synthesis of Me₃Si-F-Al(ORF)₃ has been optimized and scaled up to 85 g batches in an one-pot procedure. These anions could previously only be obtained by difficult to control decomposition reactions of [Al(ORF)₄]- or by halide abstraction reactions with Me₃Si-F-Al(ORF)₃, generating relatively large countercations that are unsuited for further use as universal starting materials. Especially [al-f-al]- is of interest for the stabilization of reactive cations, since it is even weaker coordinating than [Al(ORF)₄]- and more stable against strong electrophiles. This bridged anion can be seen as an adduct of [f-al]- and Al(ORF)₃. Thus, it is similarly Lewis acidic as BF₃ and eventually reacts with nucleophiles (Nu) from the reaction environment to yield Nu-Al(ORF)₃ and [f-al]-. This prevents working with [al-f-al]- salts in ethereal or other donor solvents. By contrast, the [f-al]- anion is no longer Lewis acidic and may therefore be used for reactions involving stronger nucleophiles than the [al-f-al]- anion can withstand. Subsequently it may be transformed into the [al-f-al]- salt by simple addition of one equivalent of Me₃Si-F-Al(ORF)₃.

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