title: Cationic boranes for the complexation of fluoride ions in water below the 4 ppm maximum contaminant level.
abstract: 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.
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA.
authors.forename: Youngmin
authors.initials: Y
authors.lastname: Kim
authors.forename: François P
authors.initials: FP
authors.lastname: Gabbaï
mesh_terms: D001880:Boranes / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D018360:Crystallography, X-Ray; D005459:Fluorides / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D007477:Ions / Q000032:analysis; D008958:Models, Molecular; D015394:Molecular Structure; D013237:Stereoisomerism; D014867:Water / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D001880:Boranes; D002412:Cations; D007477:Ions; D014874:Water Pollutants, Chemical; D014867:Water; D005459:Fluorides