Cationic boranes for the complexation of fluoride ions in water below the 4 ppm maximum contaminant level.

In search of a molecular receptor that could bind fluoride ions in water below the maximum contaminant level of 4 ppm set by the Environmental Protection Agency (EPA), we have investigated the water stability and fluoride binding properties of a series of phosphonium boranes of general formula p-(Mes(2)B)C(6)H(4)(PPh(2)R) with R = Me (1), Et (2), n-Pr (3), and Ph (4). These phosphonium boranes are water stable and react reversibly with water to form the corresponding zwitterionic hydroxide complexes of general formula p-(Mes(2)(HO)B)C(6)H(4)(PPh(2)R). They also react with fluoride ions to form the corresponding zwitterionic fluoride complexes of general formula p-(Mes(2)(F)B)C(6)H(4)(PPh(2)R). Spectrophotometric acid-base titrations carried out in H(2)O/MeOH (9:1 vol.) afford pK(R+) values of 7.3(+/-0.07) for 1, 6.92(+/-0.1) for 2, 6.59(+/-0.08) for 3, and 6.08(+/-0.09) for 4, thereby indicating that the Lewis acidity of the cationic boranes increases in following order: 1 < 2 < 3 < 4. In agreement with this observation, fluoride titration experiments in H(2)O/MeOH (9:1 vol.) show that the fluoride binding constants (K = 840(+/-50) M(-1) for 1, 2500(+/-200) M(-1) for 2, 4000(+/-300) M(-1) for 3, and 10 500(+/-1000) M(-1) for 4) increase in the same order. These results show that the Lewis acidity of the cationic boranes increases with their hydrophobicity. The resulting Lewis acidity increase is substantial and exceeds 1 order of magnitude on going from 1 to 4. In turn, 4 is sufficiently fluorophilic to bind fluoride ions below the EPA contaminant level in pure water. These results indicate that phosphonium boranes related to 4 could be used as molecular recognition units in chemosensors for drinking water analysis.