Fluoride Anion Chelation by a Bidentate Stibonium–Borane Lewis Acid

The stibonium–borane [o-(Ph2MeSb)(Mes2B)C6H4]+ (4+) has been synthesized and isolated as a triflate salt. Competition experiments carried out with [o-(Ph2MeP)(Mes2FB)C6H4] 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.