Synthesis, Characterization and Anion Complexation of Cationic Main Group Lewis Acids
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Due to favorable Coulombic effects, cationic main group Lewis acids should be more Lewis acidic than their neutral counterparts. To investigate this idea, this dissertation has been dedicated to the synthesis, characterization and anion binding properties of new cationic Lewis acids for selective anion complexation. The cationic borane [p-(Mes2B)C6H4(PPh3)] displays an enhanced anion affinity towards fluoride due to a combination of Coulombic and hydrophobic effects, and can be used to detect fluoride at levels below 4 ppm in water. A related phosphonium borane featuring a chromophoric dansyl amide moiety has been synthesized and used for the fluorescence turn on sensing of CN−. This borane is very sensitive and can be used to measure cyanide concentration in the 20-30 ppb range in water. The bidentate borane [o-(Mes2B)C6H4(PPh2Me)] is selective for N3 − over F− in water/chloroform biphasic mixtures because of the lipophilic character of the azide anion, as well as its ability to interact with both the boron and phosphorus Lewis acidic sites of the receptor via chelation (lp(N)s*(P-C)). Sulfonium borane [o(Mes2B)C6H4(SMe2)] can detect up to 50 ppb of cyanide in water at pH 7 due to favorable Coulombic effects. The sulfonium moiety interacts with the cyanide anion through both bonding and back-bonding interactions, thus enhancing the unusual affinity of [o-(Mes2B)C6H4(SMe2)] towards cyanide. This approach can be extended to Lewis acids containing fluorosilanes such as [1-Ant2FSi-2-Me2S-(C6H4)] whose fluoride affinity exceeds that of neutral fluorosilanes by several orders of magnitude.
Kim, Youngmin (2010). Synthesis, Characterization and Anion Complexation of Cationic Main Group Lewis Acids. Doctoral dissertation, Texas A&M University. Available electronically from