A Ferrocene Derivative Redox Sensor for Mercuric Ion: Synthesis and Electrochemical Study

Abstract

The sensitive detection of mercuric ion is currently a task of prime importance for environmental or biological concerns. To develop sensitive mercuric ion sensors, various receptors consisting of a mercuric ion recognition unit and a probe exhibiting physical responses upon the coordination of mercuric ion have been reported. Among them, ferrocene derivatives are often applied to redox sensors due to the unique electrochemical property. In the presence of mercuric ions, the ion-recognition event induces the significant electrochemical changes at the redox active ferrocenyl moiety. In addition, the ease of functionalization of ferrocene leads to the synthesis of assorted ferrocene derivatives possessing the metal ion binding unit. Recently, our research has focused on the development of new ferrocene complexes as redox sensors for the mercuric ion detection. In this account, we report the synthesis and electrochemical analysis of 2-(ethylthiomethyl)-N-ferrocenylmethyl aniline in the presence of various metal ions, illustrating this ferrocene complex as a sensitive and selective redox sensor for the detection of mercuric ion. As shown in Scheme 1, 1-(bromomethyl)-2-nitrobenzene 1 was treated with ethanthiol with sodium metal to afford ethyl(2-nitrobenzyl)sulfide 2 in 72% yield. Subsequent reduction of compound 2 provided 2-(ethylthiomethyl)aniline 3 which reacted with ferrocene carboxaldehyde 4, forming imine 5. Due to the instability of compound 5 on the silica gel, compound 5 was not isolated and subject to the reduction conditions using NaB(OAc)3H to give 2-(ethylthiomethyl)-N-ferrocenylmethyl aniline 6 in 80% yield. With compound 6 in hand, electrochemical properties of compound 6 were evaluated in the absence of any metal ion via cyclic voltammetry (CV). In the range of 0.0 to 2.0 V, two oxidation peaks appear at 0.41 V and 1.04 V during the anodic scan, and no reduction peak was observed in the reverse scan, shown in Figure 1. When the anodic scan was reversed at 0.80 V subsequent to the 1 oxidation at 0.41 V, the reversible reduction peak at 0.30 V was detected, with the peak separation of ca. 0.11 V. Accordingly, the first oxidation peak at 0.41 V was speculated to be the oxidation of the ferrocene entity of compound 6. The second oxidation