Recognition-guided sulfate extraction and transport using tripodal hexaurea receptors

Abstract

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 L¹, 4-methylphenyl substituted L², and hexyl-chain-substituted L³. The crystal structures of [L²·SO₄]²⁻ and [L³·SO₄]²⁻ and ¹H NMR titrations suggested that the sulfate-binding affinity of these receptors were terminal substitution-dependent, where the H-bonding strength and secondary C–H⋯π 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 L³, 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 L³. 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.