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

The strong neutral Brønsted acids [(R)­OH→Al­(OC­(CF₃)₃)₃] (R = −C­(CF₃)₃ (<b>1</b>), −C₆F₅ (<b>2</b>), (−)-menthyl (<b>3</b>)) were synthesized by complexation of perfluoro <i>tert</i>-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 <b>2</b>), IR, and partially Raman spectroscopy and X-ray crystallography. Of the structures, <b>2</b> 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)). <b>1</b> proved suitable to protonate mesitylene and Et₂O, giving the acidic cationic Brønsted acids [H­(C₆H₃(CH₃)₃)]⁺ (<b>4b</b>) and [H­(OEt₂)₂]⁺ (<b>5</b>) with the respective weakly coordinating anion [Al­(OC­(CF₃)₃)₄]⁻. In dichloromethane solution <b>4b</b> decomposes at room temperature, leaving the room-temperature-stable salt [H­(C₆H₃(CH₃)₃)]⁺[((CF₃)₃CO)₃Al–F–Al­(OC­(CF₃)₃)₃]⁻ (<b>4a</b>; XRD). The acidities reached with <b>4</b> and <b>5</b> 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 pH_abs valuesto 165 and 178 and thus are at the threshold of superacidity of −975 kJ mol^–1 or pH_abs of 171.