Sulfate Recognition in Water via Charge‐Assisted Hydrogen Bonding
Alexander Mariscal, Luzelena Sagal, Carson Doan, Canjia Zhai, Dexin Liu, Łukasz Wojtas, Wenqi Liu
Abstract
Abstract Achieving selective molecular recognition of hydrophilic anions in water remains a formidable challenge due to the competitive nature of water and the high hydration energies of target anions such as sulfate. Here, we report the design, synthesis, and characterization of a simple dicationic tetralactam macrocycle (BPTL 2 ⁺·2Cl⁻) capable of binding highly hydrated anions in water via charge‐assisted hydrogen bonding. Structural, spectroscopic, thermodynamic, and computational studies reveal that BPTL 2 ⁺ exhibits a strong binding affinity for sulfate ( K a = 2892 M⁻¹), driven primarily by entropic gain from water release and reinforced by electrostatic and hydrogen bonding interactions. Single‐crystal X‐ray diffraction and DFT‐optimized structures confirm the formation of directional [N─H•••O] and [C─H•••O] hydrogen bonds. Comparative studies with a control macrocycle (6Na + •HCTL 6− ) that has a charge‐neutral binding cavity underscore the essential role of cationic charge in overcoming desolvation enthalpic penalties. The receptor displays anti‐Hofmeister selectivity, preferentially binding more hydrophilic anions. This work provides fundamental insights into the mechanisms of anion recognition in water. It establishes charge‐assisted hydrogen bonding as a powerful strategy for developing next‐generation receptors for sensing, separation, sequestration, transport, and catalysis in aqueous environments.