om300776a_si_001.cif (89.03 kB)

Synthesis and Application of Strong Brønsted Acids Generated from the Lewis Acid Al(ORF)3 and an Alcohol

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posted on 12.11.2012, 00:00 by Anne Kraft, Jennifer Beck, Gunther Steinfeld, Harald Scherer, Daniel Himmel, Ingo Krossing
The strong neutral Brønsted acids [(R)­OH→Al­(OC­(CF3)3)3] (R = −C­(CF3)3 (1), −C6F5 (2), (−)-menthyl (3)) were synthesized by complexation of perfluoro tert-butyl alcohol, pentafluorophenol, and (−)-menthol with the Lewis superacid Al­(OC­(CF3)3)3. The 1:1 composition of the compounds was proven by NMR (except for compound 2), IR, and partially Raman spectroscopy and X-ray crystallography. Of the structures, 2 crystallized with a coordinated toluene molecule, which might be seen as a frozen intermediate or prestep to form the classical Wheland complex of protonated toluene. This interaction was calculated to be exothermic by 32 (no dispersion) or 88 kJ mol–1 (Grimmes D3 dispersion correction included @ RI-BP86/SV­(P)). 1 proved suitable to protonate mesitylene and Et2O, giving the acidic cationic Brønsted acids [H­(C6H3(CH3)3)]+ (4b) and [H­(OEt2)2]+ (5) with the respective weakly coordinating anion [Al­(OC­(CF3)3)4]. In dichloromethane solution 4b decomposes at room temperature, leaving the room-temperature-stable salt [H­(C6H3(CH3)3)]+[((CF3)3CO)3Al–F–Al­(OC­(CF3)3)3] (4a; XRD). The acidities reached with 4 and 5 are discussed in terms of our recently introduced absolute Brønsted acidity scale. The absolute chemical potentials of a 0.001 M solution of protonated mesitylene and Et2O amount to −944 and −1015 kJ mol–1 orin terms of absolute pHabs valuesto 165 and 178 and thus are at the threshold of superacidity of −975 kJ mol–1 or pHabs of 171.