engleski

Encapsulation of Fluoride by Protonated Tribenzylammonium Ions

Although molecular recognition and anion recognition has been studied since the 1960-es, the problematics of anion coordination has received a greater attention only in the past decade. This also includes the design of ligands for bonding to specific anions, usually by forming a number of hydrogen bonds between the anion and the ligand. Such ion ligands or receptors may be either unique molecules with a specific hydrogen bonding donor pattern, or supramolecular complexes of several molecules. In the latter case, the desired supramolecular complex forms only in the presence of the specific anion which acts as a template. Such a supramolecular complex acting as a fluoride receptor was detected in the crystal structure of tribenzylammonium fluoride – while one half of fluoride ions are not bonded to cations, the others are placed within capsules composed of two tribenzylammonium cations hydrogen bonded to the same fluoride (d(N—H∙∙∙F) = 2, 467 Å). The capsule is additionally stabilized by six C—H∙∙∙π contacts of 3, 668 Å. When more tribenzylammonium ions is added by co-crystallization with other tribenzylammonium salts (tetraphenylborate, iodide and triodide), so that molar ratios of tribenzylammonium cations to fluoride anions is 2:1, all the fluoride anions are encapsulated as described above, while the counterion remains outside the capsule. If however fluoride is replaced by other halogenide enions, no capsules are formed. Instead, the structures of other tribenzylammonium halogenides comprise simple ion pairs. Therefore the self-assembled bis(tribenzylammonium) receptor appears to be specific for binding of fluoride. This can be attributed primarily to the anion radius, since the increase of central ion size makes the closing of the capsule with C—H∙∙∙π contacts impossible, but also to weaker hydrogen bonding of heavier halogenides to the protonated amine in comparison to the highly basic fluoride.

anion receptors; fluoride; tribenzylamine

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