Tight and Selective Caging of Chloride Ions by a Pseudopeptidic Host |
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| Authors: | Inés Martí Dr. Michael Bolte Prof. Dr. M. Isabel Burguete Dr. Cristian Vicent Dr. Ignacio Alfonso Prof. Dr. Santiago V. Luis |
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| Affiliation: | 1. Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, Avda. Sos Baynat, s/n, 12071 Castellón (Spain), Fax: (+34)?964728214;2. Institut für Anorganische Chemie, J.‐W.‐Goethe‐Universit?t, Max‐von‐Laue‐Strasse 7, 60438 Frankfurt/Main (Germany);3. Serveis Centrals d'Instrumentació Científica, Universitat Jaume I, Avda. Sos Baynat, s/n, 12071 Castellón (Spain);4. Departamento de Química Biológica y Modelización Molecular, Instituto de Química Avanzada de Catalu?a (IQAC‐CSIC), Jordi Girona 18–26, 08034 Barcelona (Spain), Fax: (+34)?932045904 |
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| Abstract: | The selective molecular recognition of chloride versus similar anions is a continuous challenge in supramolecular chemistry. We have designed and prepared a simple pseudopeptidic cage ( 1 a ) that defines a cavity suitable for the tight encapsulation of chloride. The interaction of the protonated form of 1 a with different inorganic anions was studied in solution by 1H NMR spectroscopy and ESI‐MS, and in the solid state by X‐ray diffraction. The solution binding data showed that the association constants of 1 a to chloride are more than two orders of magnitude higher than to any other tested inorganic anion. Remarkably, 1 a displayed a high selectivity for chloride over other closely related halides such as bromide (selectivity=111), iodide (selectivity=719), and fluoride (selectivity >1000). Binding experiments (1H NMR spectroscopy and ESI‐MS) suggested that 1 a has a high‐affinity (inner) binding site and an additional low‐affinity (external) binding site. The supramolecular complexes with F?, Cl?, and Br? have been also characterized by the X‐ray diffraction of the corresponding [ 1 a? nHX] crystalline salts. The structural data show that the chloride anion is tightly encapsulated within the host, in a binding site defined by a very symmetric array of electrostatic H‐bonds. For the fluoride salt, the size of the cage cavity is too large and is occupied by a water molecule, which fits inside the cage efficiently competing with F?. In the case of the bigger bromide, the mismatch of the anion inside the cage caused a geometrical distortion of the host and thus a large energetic penalty for the interaction. This minimalistic pseudopeptidic host represents a unique example of the construction of a simple well‐defined binding pocket that allows the highly selective molecular recognition of a challenging substrate. |
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| Keywords: | anions cage compounds molecular recognition pseudopeptides supramolecular chemistry |
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