One-pot synthesis of functionalized 4-hydroxy-3-thiomethylcoumarins: detection and discrimination of Co2+ and Ni2+ ions

Abstract

A wide variety of 4-hydroxy-3-thiomethylcoumarin derivatives were synthesized through a one-pot three-component reaction from 4-hydroxycoumarin, aldehydes and thiols, catalysed by L-proline in ethanol at room temperature, in moderate to good yields. The present protocol offers many advantages such as milder reaction conditions, green solvent, easy work up procedure and requirement of a smaller amount of catalyst. Furthermore, one of the derivatives (30a) has been successfully employed as a “turn-off” fluorescence probe that displays remarkable changes in its optical properties only in the presence of cobalt and nickel ions in aqueous based media. The ligand (30a) showed high selectivity towards Co²⁺ & Ni²⁺ without any interference from other commonly coexisting metal ions. Fluorescence quenching of ligand (30a) by Co²⁺ & Ni²⁺ was found to be ∼80% and ∼85% respectively in 9 : 1 DMSO/HEPES buffer (pH = 7.4, 10 mM) at room temperature. Significant changes in the UV-vis spectra with a clear formation of isosbestic points confirms the formation of ligand–metal complexes. The predicted binding mode (2 : 1) for ligand–metal was observed from High Resolution Mass Spectroscopy, Job's plots and single crystal X-ray structures of the complexes. The lower stability of the cobalt(II) complex than the nickel(II) complex provides a reliable platform to distinguish Co²⁺ from Ni²⁺ via a “turn-on” photoluminescence response towards the disodium salt of ethylenediaminetetraacetic acid (EDTA).