Fluorescent Charge-Assisted Halogen-Bonding Macrocyclic Halo-Imidazolium Receptors for Anion Recognition and Sensing in Aqueous Media


The synthesis and anion binding properties of a new family of fluorescent halogen bonding (XB) macrocyclic halo-imidazolium receptors are described. The receptors contain chloro-, bromo-, and iodo-imidazolium motifs incorporated into a cyclic structure using naphthalene spacer groups. The large size of the iodine atom substituents resulted in the isolation of anti and syn conformers of the iodo-imidazoliophane, whereas the chloro- and bromo-imidazoliophane analogues exhibit solution dynamic conformational behavior. The syn iodo-imidazoliophane isomer forms novel dimeric isostructural XB complexes of 2:2 stoichiometry with bromide and iodide anions in the solid state. Solution phase DOSY NMR experiments indicate iodide recognition takes place via cooperative convergent XB-iodide 1:1 stoichiometric binding in aqueous solvent mixtures. H-1 NMR and fluorescence spectroscopic titration experiments with a variety of anions in the competitive CD3OD/D2O (9:1) aqueous solvent mixture demonstrated the bromo- and syn iodo-imidazoliophane XB receptors to bind selectively iodide and bromide respectively, and sense these halide anions exclusively via a fluorescence response. The protic-, chloro-, and anti iodo-imidazoliophane receptors proved to be ineffectual anion complexants in this aqueous methanolic solvent mixture. Computational DFT and molecular dynamics simulations corroborate the experimental observations that bromo- and syn iodo-imidazoliophane XB receptors form stable cooperative convergent XB associations with bromide and iodide.



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Zapata, F; Caballero, A; White, NG; Claridge, TDW; Costa, PJ; Felix, V; Beer, PD

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F.Z. acknowledges Ministry of Education of Spain for a Postdoctoral contract (Programa Nacional de Movilidad de Recursos Humanos del Plan Nacional I+D+I 2008-2011). A.C. acknowledges the European Union for a Marie Curie Postdoctoral Fellowship. N.G.W. acknowledges Clarendon Fund and Trinity College for a studentship. P.J.C. thanks FCT for the postdoctoral grant SFRH/BPD/27082/2006. We acknowledge Oxford University Crystallography Service for instrument use.

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