Differential Recognition of Anions with Selectivity towards F super(-) by a Calix[6]arene-Thiourea Conjugate Investigated by Spectroscopy, Microscopy, and Computational Modeling by DFT

• Published in: Chemistry : a European journal 2016-06, Vol.22 (26), p.8903-8914
• Main Authors: Nehra, Anita, Bandaru, Sateesh, Yarramala, Deepthi S, Rao, Chebrolu Pulla
• Format: Article
• Language: English
• Subjects: Anions; Binding; Computation; Conjugates; Recognition; Spectroscopy; Strength; Thioureas
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201600844

Anion recognition studies were performed with triazole-appended thiourea conjugates of calix[6]arene (i.e., compound super(6)L) by absorption and super(1)HNMR spectroscopy by using nineteen different anions. The composition of the species of recognition was derived from ESI mass spectrometry. The absorption spectra of compound super(6)L showed a new band at lambda =455nm in the presence of F super(-) 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 H sub(2)PO sub(4) super(-) and HSO sub(4) super(-), exhibit a redshift of the lambda =345nm band and the spectral changes are associated with the formation of an isosbestic point at lambda =343nm. super(1)HNMR studies further confirm the binding of F super(-) 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 super(-). The other anions also showed interactions with compound super(6)L, however, their binding strength follows the order F super(-)>CO sub(3) super(2-)>H sub(2)PO sub(4) super(-) approximately CH sub(3)COO super(-)>HSO sub(4)u- The NMR spectral changes clearly revealed the anion-binding region of the arms in case of all these anions. The anion binding to compound super(6)L indeed stabilizes a flattened-cone conformation as deduced based on the calix-aromatic proton signals and was further confirmed by VT super(1)HNMR experiments. The stabilization of the flattened-cone conformation was further augmented by the interaction of the butyl moiety of the nBu sub(4)N super(+) 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 super(1)HNMR spectroscopic measurements. In addition, SEM images showed a marigold flower-type morphology for compound super(6)L and this has been transformed into a chain-like structure of connected spherical particles in the presence of F super(-). 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 super(6)L were further compared with that of compound super(4)L, a calix[4]arene analogue of compound super(6)L, in order to ad