Ammonium Boranes for the Selective Complexation of Cyanide or Fluoride Ions in Water

With the recognition of aqueous fluoride and cyanide ions as an objective, we have investigated the anion binding properties of two isomeric ammonium boranes, namely [p-(Mes₂B)C₆H₄(NMe₃)]⁺ ([1]⁺) and [o-(Mes₂B)C₆H₄(NMe₃)]⁺ ([2]⁺). These cationic boranes, which could be obtained by reaction of the known 4- and 2-dimesitylboryl-N,N-dimethylaniline with MeOTf, have been investigated both experimentally and computationally. They both react with fluoride and cyanide ions in organic solvents to afford the corresponding fluoroborate/ or cyanoborate/ammonium zwitterions 1F, 1CN, 2F, and 2CN. In aqueous solution, however, these cationic boranes behave as remarkably selective receptors. Indeed, [1]⁺ only complexes cyanide ions while [2]⁺ only complexes fluoride ions. In H₂O/DMSO 60:40 vol (HEPES 6 mM, pH 7), the cyanide binding constant of [1]⁺ and the fluoride binding constant of [2]⁺ are respectively equal to 3.9 (±0.1) × 10⁸ and 910 (±50) M^-1. Structural and computational studies indicate that both steric and electronic effects contribute to the unusual selectivity displayed by these cationic boranes. Owing to favorable Coulombic effects, the para-derivative [1]⁺ has a very high affinity for cyanide; yet these effects are not sufficiently intense to allow complexation of the more efficiently hydrated and less basic fluoride anion. In the case of the ortho-derivative [2]⁺, the proximity of the ammonium moiety leads to an increase in the Lewis acidity of the boron center thus making fluoride binding possible. However, steric effects prevent cyanide coordination to the boron center of [2]⁺. Finally, cation [1]⁺ and [2]⁺ bind their dedicated anions reversibly and show a negligible response in the presence of other common anions including Cl^-, Br^-, I^-, NO₃^-, OAc^-, H₂PO₄^-, and HSO₄^-.