Chloride-based salts are widely used for a variety of purposes including de-icing. Despite their utility, many of these salts pollute lakes and rivers due to runoff. The resulting elevated chloride concentrations are acutely toxic to freshwater aquatic life. Better-control of chloride concentration is crucial for the use of water in agriculture, consumption, and industrial applications. With this in mind, we seek to develop anion receptors that can capture chloride ions upon external stimuli for better controlled chloride separation, as well as offering modes of chloride sensing. As a prototype receptor, we have synthesized a molecule that has multiple hydrogen bonding donor sites for anion recognition with a ferrocene moiety to reversibly control the receptor’s affinity for anions via redox processes. In this study, we investigated the electrochemical and anion binding properties of the receptor using cyclic voltammetry (CV). We collected CV data of the receptor at various tetrabutylammonium salt (TBACl, TBASO₄) concentrations at a glassy carbon electrode (GCE) in Ar-bubbled acetonitrile (MeCN) and dichloromethane (DCM) solution containing tetrabutylammonium hexafluorophosphate (TBAPF₆) as a supporting electrolyte. Pt counter and pseudo-reference electrodes were used with decamethylferrocene as an internal standard. Preliminary experimentation suggested negligible adsorption of the receptor onto the GCE occurs. This finding allowed us to unambiguously assess how the following affected receptor binding: 1) [supporting electrolyte], 2) anion charge, and 3) solvent polarity. The effect of [supporting electrolyte] on receptor binding was probed by varying the [TBAPF₆]. The receptor peak potential appeared to shift less at higher [TBAPF₆]. This indicates that the supporting electrolyte interferes with anion binding. The PF6^-is suspected to completely bind to the receptor binding site. TBACl and TBASO₄were titrated into the receptor solution to assess anion charge’s effect on receptor binding. SO₄^2- displayed larger peak potential shifts than Cl^-, suggesting greater anion affinity. Both the greater charge and binding stoichiometry of SO₄^2- and the receptor are thought to cause this behavior. In DCM, no potential shift was observed for the receptor, this indicates that no anion binding occurs. Cl^- and TBA⁺ are thought to form an ion pair in solution, preventing receptor coordination. Solvation in MeCN prevents this ion pair from forming in polar solvents.

Takashi Ito

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