Synthesis, Characterization and Anion Binding Properties of Boron-based Lewis Acids
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The recognition and capture of fluoride, cyanide and azide anions is attracting great deal of attention due to the negative effects of these anions on the environment and on human health. One of common methods used for the recognition and capture of these anions is based on triarylboranes, the Lewis acidity of which can be enhanced via variation the steric and electronic properties of the boron substituents. This dissertation is dedicated to the synthesis of novel boron-based anion receptors that, for the most part, feature an onium group bound to one of the aryl substituents. The presence of this group is shown to increase the anion affinity of the boron center via Coulombic effects. Another interesting effect is observed when the onium group is juxtaposed with the boron atom. This is for example the case of naphthalene-based compounds bearing a dimesitylboryl moiety at one of the peri-position and a sulfonium or telluronium unit at the other peri position. Fluoride anion complexation studies with these sulfonium or telluronium boranes, show that the boron-bound fluoride anion is further stabilized by formation of a B-F->Te/S bridge involving a lp(F)->sigma*(Te/S-C) donor acceptor interaction. Some of the sulfonium boranes investigated have been shown to efficiently capture fluoride anions from wet methanolic solutions. The resulting fluoride/sulfonium borane adducts can be triggered to release a "naked" fluoride equivalent in organic solution and thus show promise as new reagents for nucleophilic fluorination chemistry. Interestingly, the telluronium systems show a greater fluoride anion affinity than their sulfonium analogs. This increase is assigned to the greater spatial and energetic accessibility of the sigma* orbital on the tellurium atom which favors the formation of a strong B-F->Te interaction. This dissertation is concluded by an investigation of the Lewis acidic properties of B(C6Cl5)3. This borane, which has been reported to be non-Lewis acidic by other researchers, is found by us to bind fluoride, azide and cyanide anions in dichloromethane with large binding constants. This borane is also reactive toward neutral Lewis bases, such as p-dimethylaminopyridine, in organic solvents.
Zhao, Hai Yan (2012). Synthesis, Characterization and Anion Binding Properties of Boron-based Lewis Acids. Doctoral dissertation, Texas A&M University. Available electronically from