Fluoride ion chelation by a bidentate phosphonium/borane Lewis acid

The phosphonium borane [1-Mes(2)B-2-MePh2P-(C6H4)]+ ([2]+) has been synthesized as an iodide salt by alkylation of 1-Mes(2)B-2-Ph2P(C6H4) with Mel. This novel cationic borane complexes fluoride to afford the corresponding zwitterionic fluoroborate complex 1FMes(2)B-2-MePh2P-(C6H4) (2-F) with a binding constant in MeOH exceeding that of 1-Mes(2)B-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) angstrom. 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.