Quantitative Determination of Anion Binding Strenght with A Tripodal Hexaurea Receptor

Abstract

Anions are ubiquitous, and their related activities are closely related to human life and production. Thus, study of anion recognition is significant by understanding the binding affinity and selectivity. The tripodal hexaurea receptor has three flexible arms that can form an interior cavity for anion binding. Yet its anion binding property remains unclear. Here, by taking hexyl chain substituted hexaurea receptor as an example, we quantitatively determined the anion binding affinities by using ¹H NMR titrations. The chloride binding affinity was determined by direct titration, and the sulfate and phosphate binding constants were determined by competing titrations. In 5% v/v H₂O/DMSO-d₆, the binding affnities of Cl^–, SO₄^2–, and PO₄^3– were determined to be 93 M⁻¹, 7.3×10³ M⁻¹, and 8.4×10⁵ M⁻¹, respectively. The phosphate binding affnity is about 117-fold and 9048-fold stronger than sulfate and chloride binding, respectively. Crystal structure also supports strong binding of phosphate.