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

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 similar to 80% and similar to 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).