Cationic boranes for the complexation of fluoride ions in water below the 4 ppm maximum contaminant level.
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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 ((+)), Et ((+)), n-Pr ((+)), and Ph ((+)). 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 (+), 6.92(+/-0.1) for (+), 6.59(+/-0.08) for (+), and 6.08(+/-0.09) for (+), thereby indicating that the Lewis acidity of the cationic boranes increases in following order: (+) < (+) < (+) < (+). 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 (+), 2500(+/-200) M(-1) for (+), 4000(+/-300) M(-1) for (+), and 10 500(+/-1000) M(-1) for (+)) 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 (+) to (+). In turn, (+) is sufficiently fluorophilic to bind fluoride ions below the EPA contaminant level in pure water. These results indicate that phosphonium boranes related to (+) could be used as molecular recognition units in chemosensors for drinking water analysis.
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Kim, Youngmin||Gabbaï, François P