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Title: Differential Recognition of Anions with Selectivity towards F- by a Calix[6]arene-Thiourea Conjugate Investigated by Spectroscopy, Microscopy, and Computational Modeling by DFT
Authors: NEHRA, A
Keywords: Lower Rim
Binding Agents
Issue Date: 2016
Citation: CHEMISTRY-A EUROPEAN JOURNAL,22(26)8903-8914
Abstract: Anion recognition studies were performed with triazole-appended thiourea conjugates of calix[6]arene (i.e., compound L-6) by absorption and H-1 NMR spectroscopy by using nineteen different anions. The composition of the species of recognition was derived from ESI mass spectrometry. The absorption spectra of compound L-6 showed a new band at lambda = 455 nm in the presence of F- due to a charge transfer from the anion to the thiourea moiety and the absorbance increases almost linearly in the concentration range 5 to 200 mu m. This is associated with a strong visual color change of the solution. Other anions, such as H2PO4- and HSO4-, exhibit a redshift of the lambda = 345 nm band and the spectral changes are associated with the formation of an isosbestic point at lambda = 343 nm. H-1 NMR studies further confirm the binding of F- efficiently to the thiourea group among the halides by shifting the thiourea proton signals downfield followed by their disappearance after the addition of more than one equivalent of F-. The other anions also showed interactions with compound L-6, however, their binding strength follows the order F- > CO32- > H2PO4- approximate to CH3COO- > HSO4-. The NMR spectral changes clearly revealed the anion-binding region of the arms in case of all these anions. The anion binding to compound L-6 indeed stabilizes a flattened-cone conformation as deduced based on the calix-aromatic proton signals and was further confirmed by VT H-1 NMR experiments. The stabilization of the flattened-cone conformation was further augmented by the interaction of the butyl moiety of the nBu(4)N(+) counterion. The structural features of the anion-bound species were demonstrated by DFT computations and the resultant structures carried the features that were predicted based on the H-1 NMR spectroscopic measurements. In addition, SEM images showed a marigold flower-type morphology for compound L-6 and this has been transformed into a chain-like structure of connected spherical particles in the presence of F-. The anion-induced microstructural features are reflective of the binding strength, size, and shape of the anions. The binding strengths of the anions by compound L-6 were further compared with that of compound L-4, a calix[4]arene analogue of compound L-6, in order to address the role of the number of arms built on the calixarene platform based on absorption spectroscopy, H-1 NMR spectroscopy, and DFT computations and it was found that compound L-6 is a better receptor for F-, which extends its interactions from all the three arms.
ISSN: 0947-6539
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