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Keywords:

  • Acidity;
  • Aluminum;
  • Brønsted acids;
  • IR spectroscopy;
  • Hydrogen bonds

Abstract

The strong cationic Brønsted acids [H(OEt2)2][Al{OC(CF3)3}4] (1) and [H(THF)2][Al{OC(CF3)3}4] (2) were prepared in up to 95 % yield by reacting stoichiometric amounts of diethyl ether/THF solutions of Li[Al{OC(CF3)3}4] and HX (X = Cl, Br), removing the solvent, and extracting the residue with CH2Cl2. Compounds 1 and 2 were characterized by multinuclear NMR and IR spectroscopy as well as by their single crystal X-ray structures and additional quantum chemical calculations. The cation in 1 probably contains an unsymmetrical O–H···O hydrogen bond, while in 2 the protic hydrogen atom was not found in the difference Fourier map. From an analysis of all published [H(OEt2)2]+ structures, we show that for an unsymmetrical H-bridge further rearrangement of the structure occurs leading to the structural type III that may formally be viewed as ethanol coordinating to an ethyl 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 chemically very robust [Al{OC(CF3)3}4] anion by protonolysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)