Tetrazoles are potent anion recognition elements in a variety of structural contexts




Pinter, Thomas

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In efforts to expand the limited amount of functional groups available for anion recognition, a series of highly acidic, strongly hydrogen bond-donating groups were envisaged as suitable candidates. These included the thoroughly studied N-aryl sulfonamides along with the less utilized N-acyl sulfonamides and tetrazoles. These groups were affixed to a well-understood supramolecular platform in calix[4]arene and their binding affinities for various halides and oxyanions probed. It was found that although in its least energetically favourable conformation that is orthogonal to the aryl group to which it was bound, the tetrazole proved a superior anion-binding element. Noting that tetrazoles prefer co-planarity with aryl neighbours, a series of pyrrolyl-tetrazole anion binding compounds were prepared, first a simple bidentate pyrrolyl-tetrazole which when tested for anion binding affinity demonstrated some of the strongest binding with anions for a bidentate compound ever observed, especially chloride. It was then conceived to hybridize this new binding motif with the well-known amidopyrrole moiety and two new tetrazolyl-amidopyrroles were constructed. When compared to an ester-functionalized pyrrolyl-tetrazole, binding strength with halides was not much different, leading to the postulation that the amide N-H may just be a spectator in the binding event, and the electron-withdrawing nature of the adjacent carbonyl was what led to the binding potency. Nonetheless, a new class of diversifiable anion binders with superior strength to analogous amidopyrroles has been constructed and could perhaps be used in a variety of functional applications.



tetrazoles, anion-binding