An Efficient Ferrocene Derivative as a Chromogenic, Optical, and Electrochemical Receptor for Selective Recognition of Mercury(II) in an Aqueous Environment

The synthesis, electrochemical, optical, and cation-sensing properties of two triazole-tethered ferrocenyl benzylacetate derivatives, C₃₆H₃₆O₆N₆Fe (<b>2</b>) and C₂₃H₂₃O₃N₃Fe (<b>3</b>), are presented. The binding event of both the receptors can be inferred either from a redox shift (<b>2</b>, Δ<i>E</i>_1/2 = 106 mV for Hg²⁺ and Δ<i>E</i>_1/2 = 187 mV for Ni²⁺; <b>3</b>, Δ<i>E</i>_1/2 = 167 mV for Hg²⁺ and Δ<i>E</i>_1/2 = 136 mV for Ni²⁺) or a highly visual output response (colorimetric) for Hg²⁺, Ni²⁺, and Cu²⁺ cations. Remarkably, the redox and colorimetric responses toward Hg²⁺ are preserved in the presence of water (CH₃CN/H₂O, 2/8), which can be used for the selective colorimetric detection of Hg²⁺ in an aqueous environment over Ni²⁺ and Cu²⁺ cations. The changes in the absorption spectra are accompanied by the appearance of a new low-energy (LE) peak at 625 nm for both compounds <b>2</b> and <b>3</b> (<b>2</b>, ε = 2500 M^–1 cm^–1; <b>3</b>, ε = 1370 M^–1 cm^–1), due to a change in color from yellow to purple for Hg²⁺ cations in CH₃CN/H₂O (2/8)