NDLI: Differential Recognition of Anions with Selectivity towards $F^{−}$ by a Calix[6]arene–Thiourea Conjugate Investigated by Spectroscopy, Microscopy, and Computational Modeling by DFT

Content Provider	World Health Organization (WHO)-Global Index Medicus
Author	Rao, Chebrolu Pulla Nehra, Anita Yarramala, Deepthi S. Bandaru, Sateesh
Description	Author Affiliation: Nehra A ( Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-, 400076, India.); Bandaru S ( Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-, 400076, India.); Yarramala DS ( Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-, 400076, India.); Rao CP ( Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-, 400076, India. cprao@iitb.ac.in.)
Abstract	Anion recognition studies were performed with triazole-appended thiourea conjugates of calix[6]arene (i.e., compound $^{6}L)$ by absorption and $^{1}H NMR$ spectroscopy by using nineteen different anions. The composition of the species of recognition was derived from ESI mass spectrometry. The absorption spectra of compound $^{6}L$ showed a new band at λ=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 μm. This is associated with a strong visual color change of the solution. Other anions, such as $H_{2}PO_{4}^{−}$ and $HSO_{4}^{−},$ exhibit a redshift of the λ=345 nm band and the spectral changes are associated with the formation of an isosbestic point at λ=343 nm. $^{1}H NMR$ 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 $^{6}L,$ however, their binding strength follows the order $F^{−}>CO_{3}^{2−}>H_{2}PO_{4}^{−}≈CH_{3}COO^{−}>HSO_{4}^{−}.$ The NMR spectral changes clearly revealed the anion-binding region of the arms in case of all these anions. The anion binding to compound $^{6}L$ indeed stabilizes a flattened-cone conformation as deduced based on the calix-aromatic proton signals and was further confirmed by VT $^{1}H NMR$ 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 $^{1}H NMR$ spectroscopic measurements. In addition, SEM images showed a marigold flower-type morphology for compound $^{6}L$ 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 $^{6}L$ were further compared with that of compound $^{4}L,$ a calix[4]arene analogue of compound $^{6}L,$ in order to address the role of the number of arms built on the calixarene platform based on absorption spectroscopy, $^{1}H NMR$ spectroscopy, and DFT computations and it was found that compound $^{6}L$ is a better receptor for $F^{−},$ which extends its interactions from all the three arms.
ISSN	09476539
e-ISSN	15213765
Journal	Chemistry - A European Journal
Issue Number	26
Volume Number	22
Language	English
Publisher	Wiley-VCH;ChemPubSoc Europe
Publisher Date	2016-06-20
Publisher Place	Germany
Access Restriction	Open
Subject Keyword	Chemistry
Content Type	Text
Resource Type	Article
Subject	Organic Chemistry Catalysis

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