Peripheral Charge Gating Enables Redox-Controlled Capture and Complete Release of Phosphate by a Europium(III) Complex in Water

Abstract

Dynamic control of anion binding in competitive media remains a central challenge in supramolecular chemistry. Most responsive hosts alter the protonation, conformation, or donor properties of the binding site itself. Here we establish a peripheral charge gating as a distinct design principle in which a mild reaction remote from the coordination sphere switches guest affinity by changing the net charge of the receptor. The reduced complex EuIII-SH-HOPO binds phosphate with high affinity (log β12 = 8.8) and selectively over competing anions such as bicarbonate, sulfate, or nitrate. Oxidation to a mixed disulfide bearing a terminal sulfonate installs a single peripheral negative charge near the receptor without rearranging the Eu(III) receptor itself. This single negative charge is sufficient to release phosphate completely. Subsequent reduction regenerates the thiol receptor and restores its phosphate response. This reversible on/off cycle translates a peripheral covalent reaction into complete control over guest binding without altering the binding site of the receptor, establishing peripheral electrostatic modulation as a strategy for gating ion recognition.