Perfluorinated alkoxyaluminate salts of cationic Bronsted acids: Synthesis, structure, and characterization of [H(OEt2)2][Al{OC(CF3)3}4] and [H(THF)2][Al{OC(CF3)3}4]

The strong cationic Bronsted acids [H(OEt2)2][Al{OC(CF3)3}4] (1) and [H(THF)2][Al{OC(CF3)3}4] (2) were prepd. in up to 95% yield by reacting stoichiometric amts. of di-Et ether/THF solns. of Li[Al{OC(CF3)3}4] and HX (X = Cl, Br), removing the solvent, and extg. the residue with CH2Cl2. Compds. 1 and 2 were characterized by multinuclear NMR and IR spectroscopy as well as by their single crystal x-ray structures and addnl. quantum chem. calcns. The cation in 1 probably contains an unsym. O-H...O hydrogen bond, while in 2 the protic hydrogen atom was not found in the difference Fourier map. From an anal. of all published [H(OEt2)2]+ structures, for an unsym. H-bridge further rearrangement of the structure occurs leading to the structural type III that may formally be viewed as ethanol coordinating to an Et cation. This point of view has hitherto been neglected in the discussion of the nature of short, strong, low-barrier H-bonds, although it needs to be accounted for. Both 1 and 2 are suitable starting materials to introduce the weakly coordinating and chem. very robust [Al{OC(CF3)3}4]- anion by protonolysis. [on SciFinder (R)]