Fluoride ion chelation by a bidentate phosphonium/borane Lewis acid.
Additional Document Info
The phosphonium borane [1-Mes2B-2-MePh2P-(C6H4)]+ (+) 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) (+) by at least 4 orders of magnitude. Structural and computational results indicate that the high fluorophilicity of + 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) A. These results, which are supported by NBO and AIM analyses, show that the latent phosphorus-centered Lewis acidity of the phosphonium moiety in + can be exploited to enhance fluoride binding via chelation.