5-(1H-Indol-3-yl)-pyrazolyl derivatives as colorimetric sensor for anions

Abstract

The synthesis, characterisation and binding and deprotonation studies with anions for four 5-(1H-indol-3-yl)-pyrazolyl derivatives (2–5) have been described. It is worthy to mention that sensor 2 shows a drastic change in absorption spectrum (ca. 335 nm) and colour (colourless to blue) upon addition of F− in DMSO solution due to the deprotonation of indole –NH proton, as confirmed by 1H NMR titration. Sensor 4 recognizes F− and CN− ions by deprotonation mechanism with visible colour change of the solution in a similar manner to that of 2. However, in contrary to 2 and 4, sensor 3 binds with F−, CN−, H2PO4 −, AcO− and PhCOO− ions exploiting hydrogen-bonding interaction with the shifting of absorption band to longer wavelength and subsequent colour change of the solution. Compound 5 recognizes F− without any visual colour change and its binding is studied by 1H NMR titration to acquire the important information about the nature of binding between F− and 5.