Recognition-guided sulfate extraction and transport using tripodal hexaurea receptors

The separation of sulfate anions (SO42-) from water is a great challenge due to its high hydration energy. Using synthetic receptors that are designed with a size-complementary cavity for sulfate binding, sulfate anions could be extracted from water to the organic phase via liquid-liquid extraction (LLE) method. To understand the correlation between sulfate binding (recognition chemistry) and sulfate-separation efficiency across two phases, herein we prepared a family of tripodal hexaurea receptors bearing various terminal substitutions: 4-nitrophenyl substituted L-1, 4-methylphenyl substituted L-2, and hexyl-chain-substituted L-3. The crystal structures of [L-2.SO4](2-) and [L-3.SO4](2-) and H-1 NMR titrations suggested that the sulfate-binding affinity of these receptors were terminal substitution-dependent, where the H-bonding strength and secondary C-H center dot center dot center dot pi interactions were regulated. Comprehensive LLE studies indicated that all three receptors displayed highly efficient sulfate extraction with receptor-loading dependence and concentration independence. Relative sulfate-extraction efficiency was consistent with the sulfate-binding affinity of these receptors. Notably, using the hexyl-chain-substituted receptor L-3, sulfate anions could be extracted and released by acidification for several cycles. Typical U-tube transport experiments demonstrated that over 70% of sulfate anions could be transported from the source phase to the receiving phase in 3 days across a bulk liquid membrane, which comprised the receptor L-3. Our work shows a paradigm of how the sulfate-recognition property is correlated with sulfate separation via LLE, which may help to understand and promote the development of supramolecular recognition-based systems for achieving desired separations.