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-(MeS2B)C6H4(PPh2R)](+) 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-(MeS2(HO)B)C6H4(PPh2R). They also react with fluoride ions to form the corresponding zwitterionic fluoride complexes of general formula p-(MeS2(F)B)C6H4(PPh2R). Spectrophotometric acid-base titrations carried out in H2O/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 H2O/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.