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.