Neutral and Cationic Main Group Lewis Acids - Synthesis, Characterization and Anion Complexation
Hudnall, Todd W.
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The molecular recognition of fluoride and cyanide anions has become an increasingly pertinent objective in research due to the toxicity associated with these anions, as well as their widespread use. Fluoride is commonly added to drinking water and toothpastes to promote dental health, and often used in the treatment of osteoporosis, however, high doses can lead to skeletal fluorosis, an incurable condition. Cyanide is also an extremely toxic anion, which binds to and deactivates the cytochrome-c oxidase enzyme, often leading to fatality. The molecular recognition of these anions in water has proven to be challenging. For fluoride, the anion is small, and thus, efficiently hydrated (?H?hyd = -504 KJ/mol), making its complexation in aqueous environments particularly difficult. In addition to being small and efficiently hydrated like the fluoride anion, cyanide has a pKa(HCN) of 9.3 making its competing protonation in neutral water a further complication. Recent efforts to complex fluoride and cyanide have utilized triarylboranes, which covalently bind the anion. Monofunctional triarylboranes display a high affinity for fluoride with binding constants in the range of 105-106 M-1 in organic solvents, and chelating triarylboranes exhibit markedly higher anion affinities. These species, however, remain challenged in the presence of water. This dissertation focuses on the synthesis and properties of novel Lewis acids designed for the molecular recognition of fluoride or cyanide in aqueous environments. To this end, a group 15 element will be incorporated into a main group Lewis acidcontaining molecule for the purpose of: i) increasing the Lewis acidity of the molecule via incorporation of a cationic group, and ii) increasing the water compatibility of the host. Specifically, a pair of isomeric ammonium boranes has been synthesized. These boranes are selective sensors which selectively bind either fluoride or cyanide anions in water. The study of phosphonium boranes has revealed that the latent Lewis acidity of the phosphonium moiety is capable of aiding triarylboranes in the chelation of small anions. Finally, my research shows that Br?nsted acidic H-bond donors such as amides, when paired with triarylboranes, are capable of forming chelate complexes with fluoride.