Selective recognition of tetrahedral dianions by a hexaaza cryptand receptor


A hexaamine cage was synthesised in good yield by a [2+3] Schiff-base condensation followed by sodium borohydride reduction to be used as a receptor for the selective binding of anionic species. The protonation constants of the receptor, as well as its association constants with Cl(-), I(-), NO(3)(-), AcO(-), ClO(4)(-), H(2)PO(4)(-), SO(4)(2-), SeO(4)(2-) and S(2)O(3)(2-) were determined by potentiometry at 298.2 +/- 0.1 K in H(2)O-MeOH (50 : 50 v/v) and at ionic strength 0.10 +/- 0.01 mol dm(-3) in KTsO. These studies revealed a remarkable selectivity for dianionic tetrahedral anions by the protonated receptor, with association constants ranging 5.03-5.30 log units for the dianionic species and 1.49-2.97 log units for monoanionic ones. Single crystal X-ray determination of [(H(6)xyl)(SO(4))(H(2)O)(6)](SO(4))(2)center dot 9.5H(2)O showed that one sulfate anion is encapsulated into the receptor cage sited between the two 2,4,6-triethylbenzene caps establishing three N-H center dot center dot center dot O hydrogen bonds with two adjacent N-H binding sites and additional O-H center dot center dot center dot O hydrogen bonding interactions with six water of crystallization molecules. Four water molecules of the (SO(4))(H(2)O)(6) cluster interact with [H(6)xyl](6+) through N-H center dot center dot center dot O hydrogen bonds. Molecular dynamics simulations (MD) carried out with SO(4)(2-) and Cl-anions in H(2)O-MeOH (50 : 50 v/v) allowed the full understanding of anion molecular recognition, the selectivity of the protonated receptor for SO(4)(2-) and the role played by the methanol and water solvent molecules.






Mateus, P; Delgado, R; Brandao, P; Carvalho, S; Felix, V

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The authors acknowledge FCT and POCI, with co-participation of the European Community funds FEDER, for financial support under projects PTDC/QUI/56569/2004 and PTDC/QUI/68582/2006. The NMR spectrometers are part of the National NMR Network and were acquired with funds from FCT and FEDER. We also acknowledge M. C. Almeida for providing elemental analysis and ESI-MS data from the Elemental Analysis and Mass Spectrometry Service at the ITQB. Pedro Mateus and Silvia Carvalho thank FCT for the grants, SFRH/BD/36159/2007 and SFRH/BPD/42357/2007, respectively.

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