Fluoride Anion Chelation by a Bidentate Stibonium–Borane Lewis Acid

The stibonium–borane [ o -(Ph 2 MeSb)(Mes 2 B)C 6 H 4 ] + ( 4 + ) has been synthesized and isolated as a triflate salt. Competition experiments carried out with [ o -(Ph 2 MeP)(Mes 2 FB)C 6 H 4 ] indicate that 4 + exhibits a much higher fluoride affinity than its lighter phosphorus congener. Structural studies show that the higher fluoride affinity of 4 + is correlated to the Lewis acidity of the stibonium center, which engages the boron-bound fluoride anion in the formation of a B–F→Sb bridge. These results illustrate that the Lewis acidity of main-group onium ions increases as the group is descended.