Naphthol based positional isomers of ferrocene appended benzochromene: Differential selectivity towards Hg(II) ion

Abstract

A set of positional isomers (2 and 4) based on ferrocene appended benzochromene derivative have been designed and synthesized by “click with MCRs” strategy. A facile one pot multi-component reactions (MCRs) protocol followed by click reaction strategy has been utilized for the synthesis of our desired compounds 2 and 4. Interestingly, it was found that compound 4, the beta isomer, is more selective and sensitive towards Hg2+ ion compared to that of its alpha isomer, 2. This fact may be attributed to the participation of naphthol ring (in case of beta isomer) to the attached Hg2+ ion via cation-pi interaction, which is not possible for alpha isomer due to the unfavorable position of the aromatic ring. All the spectroscopic and electrochemical studies along with detection limit (LOD) calculation corroborated the high selectivity of beta isomer for Hg2+ ion over alpha isomer. However, both the compounds exhibited the color change from light yellow to greenish blue upon addition with Fe3+ and Cu2+ ions due to oxidation of ferrocene to ferrocenium ion, which was further supported by the appearance of low-energy (LE) absorption band around ~630 nm. The binding mode was unambiguously determined by 1H NMR titration study and stoichiometric of the present probe (1:1) was confirmed by Job's plot analysis. Both the receptors and their metal complexes are thermally stable up to 80 °C. Additionally, the practical application of receptor 4 was fruitfully investigated towards the detection of Hg2+ ion in real water samples by using the different water sources.