The design of artificial receptors for sulfate ions is of great interest because of the importance of sulfate ions in environmental and biological systems.[1] One of the applications of sulfate ion receptors is extraction of the sulfate ion from nitrate-rich mixtures in the remediation of nuclear waste. [2] Based on liquid-liquid anion exchange technology, extraction of sulfate ions from an aqueous to an organic phase was realized by using macrocyclic receptors.[ ) of the receptor were needed in this case to ensure applicable extraction. Hence, the extraction efficiency has yet to be improved for sulfate ion extractants. This aim is quite challenging because of the extremely large hydration energy of the sulfate ion (DG h = À1080 kJ mol À1 for SO 4 2À compared to À300 kJ mol À1 for NO 3 À ) [3] according to the Hofmeister series, [4] as well as the high nitrate/sulfate ratios present in the crude waste. To overcome the Hofmeister bias, which disfavors the separation of the extremely hydrophilic sulfate ion from water, the receptor must have both excellent affinity and selectivity for sulfate ions.In recent years, some receptors for sulfate ions have been synthesized by employing different binding groups (mostly NH moieties), such as protonated Schiff base macrocycles, [5] diindolylureas, [6] and an M 4 L 6 cage containing a bipyridinefunctionalized monourea; [7] these receptors bind the anion in the 1:1, 3:1 and 6:1 (host/guest) mode, respectively. The trenbased tripodal trisurea backbone (L 1 ; tren = tris(2-aminoethyl)amine) has also been found to encapsulate the sulfate ion in a 2:1 (host/guest) ratio. [8] Although saturated coordination (12 hydrogen bonds) for sulfate and phosphate ions has been achieved by these receptors, the complementarity for the ions is not optimal in most cases. Calculations have demonstrated that the optimal saturated coordination mode for sulfate ions is binding in a tetrahedral cavity with 12 hydrogen bonds along the edges. [9] In this regard, the ideal sulfate ion receptor would possess a complementary tetrahedral cavity surrounded by 12 optimally arranged binding sites.The chelate effect may also play an important role in the host-guest binding affinity because of the favorable contributions from both entropy and enthalpy. As a typical example of the chelate effect, the Co 2+ complex of the bidentate ligand 1,2-diaminoethane is 10 8 times more stable than that of the unidentate ligand ammonia.[10] Moreover, the hexadentate ligand ethylenediaminetetraacetic acid (EDTA) displays extremely high binding affinities toward most metal ions (for example, 10 14.3 m À1 for Fe 2+ and 10 16.3 m À1 for Co 2+ ).[11]Given the similarities between anion coordination and classical transition-metal coordination chemistry, [12] increasing the number of binding sites to achieve high chelate effects should be an effective way to improve the extraction efficiency of sulfate ion extractants.We have devoted our efforts to the synthesis of selective anion receptors based on the urea functionality. [8a...

A word in urea: A strategy of mimicking the scaffold and metal coordination behavior of oligo‐2,2′‐bipyridine ligands was used to obtain a triple anion helicate from a bis(biurea) receptor and phosphate ions. The successful assembly of the triple helical structure provides guidelines for the design of new anion ligands by taking advantage of the similarities of metal and anion coordination.

A tripodal tris(3-pyridylurea) receptor (L) assembles with metal sulfate salts MSO(4) (M=Mn, Zn) to afford supramolecular cages [SO(4) subset L(2)] that encapsulate the SO(4)(2-) ion via multiple hydrogen bonds in a three-dimensional structure held by second-sphere coordination; (1)H NMR and negative-ion mode ESI-MS spectra reveal significantly strong sulfate binding in solution.

An anion-coordination-based A L ("A" denotes anion and "L" is ligand) tetrahedral cage was constructed by a C -symmetric bis-bis(urea) ligand and phosphate anion, which showed reversible interconversion with the A L triple helicate as a response to the template, concentration, or solvent. Notably, an unusual "peripheral" templation was found to be critical to stabilize the tetrahedral structure. This peripheral effect was utilized to assemble an "empty" A L cage that allows the multi-stimuli-controlled capture/release of biologically important species such as choline and acetylcholine.