Fluoride ion chelation by a bidentate phosphonium/borane lewis acid

The phosphonium borane [1-Mes₂B-2-MePh₂P-(C₆H₄)]+ ([2]+) has been synthesized as an iodide salt by alkylation of 1-Mes₂B-2-Ph₂P-(C₆H₄) with MeI. This novel cationic borane complexes fluoride to afford the corresponding zwitterionic fluoroborate complex 1-FMes₂B-2-MePh₂P-(C₆H₄) (2-F) with a binding constant in MeOH exceeding that of 1-Mes₂B-₄-MePh₂P-(C₆H₄) ([1]+) by at least 4 orders of magnitude. Structural and computational results indicate that the high fluorophilicity of [2]+ arises from both Coulombic and cooperative effects which lead to formation of a B-F→P interaction with a F→P distance of 2.666(2) Å. These results, which are supported by NBO and AIM analyses, show that the latent phosphorus-centered Lewis acidity of the phosphonium moiety in [2]+ can be exploited to enhance fluoride binding via chelation.