Synthesis of Acylhydrazone-Based Macrocycles for Aqueous Sulfate Extraction

Selective extraction of sulfate anions (SO42-) from aqueous media is highly desirable but remains challenging, largely due to the high hydration energy of SO42- (ΔGhyd = -1080 kJ mol-1). Herein, we report the design and synthesis of a series of acylhydrazone-based macrocycles using SO42- as a template, with yields reaching up to 90%. The single-crystal structure of the M12·SO42- complex reveals a sandwich-type 2:1 stoichiometry, in which the SO42- anion is tightly bound by 12 complementary hydrogen bonds. Comprehensive liquid-liquid extraction (LLE) studies demonstrate that M4·Cl- exhibits excellent sulfate-extraction performance (>95% from 1H NMR and 90% determined based on ion chromatography (IC)). Notably, sulfate anions can be efficiently extracted in the presence of competing anions (Br-, I-, NO3-, HCO3-, and H2PO4-), and the extracted sulfate can be released over several consecutive cycles, allowing for recyclable operation. This work offers an alternative strategy for the efficient synthesis of anion receptors and demonstrates their feasibility for the reversible extraction of sulfate anions from an aqueous phase into an organic phase, highlighting their potential for nuclear waste remediation.