Studies of anion binding in pyrrole-containing supramolecular motifs
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Anions are important in a wide range of biological as well as chemical processes. In addition to their importance in the areas of medicine and catalysis, many environmental pollutants are anionic, such as phosphates, nitrates, and pertechnetate. Anions are more difficult to sense via electrostatic interactions than cations because of their lower charge to radius ratio; the majority of neutral anion receptors thus utilize hydrogen bonding as the predominant binding mode. Anions also have a wide range of geometries and require a high degree of complementarity in receptor design in order to achieve selectivity. Additionally, an anion receptor must successfully compete with the solvent environment. Anions are very strongly solvated, and the free energy gained upon binding must exceed the free energy lost as the result of dehydrating the anion. This can make binding in protic or hydroxylic solvents particularly challenging. Because of the inherent difficulties associated with anion recognition, the development of anion binding agents has lagged behind corresponding work on cation receptors. Currently, there are two general classes of synthetic anion receptors, those that are positively charged and those that are neutral. The major advantage of neutral hosts is that the absence of a positive charge generally provides for more selective binding, for the simple reason that positive charges are nondirectional and lead to electrostatic attractions that cannot by definition be selective for a particular anion. Another advantage to neutral hosts is that there is no inherent competition with a receptor associated with a counterion, which can often result in a weaker affinity for the intended guest. This dissertation focuses on the anion binding behavior of neutral, pyrrole containing supramolecular systems. The first chapter will discuss the ability of ferrocene-pyrrole conjugates to sense anions electrochemically. Chapter 2 investigates the anion binding ability of an expanded calixphyrin and a strapped calixpyrrole. Chapter 3 focuses on crown ether appended dipyrrolylquinoxalines and their effectiveness as ditopic receptors. Chapter 4 provides experimental methods and characterization data.