Caged supramolecular systems are promising hosts for guest inclusion, separation, and stabilization. Well-studied examples are mainly metal-coordination-based or covalent architectures. An anion-coordination-based cage that is capable of encapsulating halocarbon guests is reported for the first time. This A₄L₄-type (A=anion) tetrahedral cage, [(PO₄)₄L₄]¹²⁻, assembled from a C₃-symmetric tris(bisurea) ligand (L) and phosphate ion (PO₄³⁻), readily accommodates a series of quasi-tetrahedral halocarbons, such as the Freon components CFCl₃, CF₂Cl₂, CHFCl₂, and C(CH₃)F₃, and chlorocarbons CH₂Cl₂, CHCl₃, CCl₄, C(CH₃)Cl₃, C(CH₃)₂Cl₂, and C(CH₃)₃Cl. The guest encapsulation in the solid state is confirmed by crystal structures, while the host–guest interactions in solution were demonstrated by NMR techniques.

By tuning the length and rigidity of the spacer of bis(biurea) ligands L, three structural motifs of the A₂L₃ complexes (A represents anion, here orthophosphate PO₄³⁻), namely helicate, mesocate, and mono-bridged motif, have been assembled by coordination of the ligand to phosphate anion. Crystal structure analysis indicated that in the three complexes, each of the phosphate ions is coordinated by twelve hydrogen bonds from six surrounding urea groups. The anion coordination properties in solution have also been studied. The results further demonstrate the coordination behavior of phosphate ion, which shows strong tendency for coordination saturation and geometrical preference, thus allowing for the assembly of novel anion coordination-based structures as in transition-metal complexes.

Caged supramolecular systems are promising hosts for guest inclusion, separation, and stabilization. Well-studied examples are mainly metal-coordination-based or covalent architectures. An anion-coordination-based cage that is capable of encapsulating halocarbon guests is reported for the first time. This A₄L₄-type (A=anion) tetrahedral cage, [(PO₄)₄L₄]¹²⁻, assembled from a C₃-symmetric tris(bisurea) ligand (L) and phosphate ion (PO₄³⁻), readily accommodates a series of quasi-tetrahedral halocarbons, such as the Freon components CFCl₃, CF₂Cl₂, CHFCl₂, and C(CH₃)F₃, and chlorocarbons CH₂Cl₂, CHCl₃, CCl₄, C(CH₃)Cl₃, C(CH₃)₂Cl₂, and C(CH₃)₃Cl. The guest encapsulation in the solid state is confirmed by crystal structures, while the host–guest interactions in solution were demonstrated by NMR techniques.

A tetrakis(bisurea)-decorated tetraphenylethene (TPE) ligand (L²) was designed, which, upon coordination with phosphate ions, displays fluorescence “turn-on” over a wide concentration range, from dilute to concentrated solutions and to the solid state. The fluorescence enhancement can be attributed to the restriction of the intramolecular rotation of TPE by anion coordination. The crystal structure of the A₄L₂ (A=anion) complex of L² with monohydrogen phosphate provides direct evidence for the coordination mode of the anion. This “anion-coordination-induced emission” (ACIE) is another approach for fluorescence turn-on in addition to aggregation-induced emission (AIE).

A tetrakis(bisurea)-decorated tetraphenylethene (TPE) ligand (L²) was designed, which, upon coordination with phosphate ions, displays fluorescence “turn-on” over a wide concentration range, from dilute to concentrated solutions and to the solid state. The fluorescence enhancement can be attributed to the restriction of the intramolecular rotation of TPE by anion coordination. The crystal structure of the A₄L₂ (A=anion) complex of L² with monohydrogen phosphate provides direct evidence for the coordination mode of the anion. This “anion-coordination-induced emission” (ACIE) is another approach for fluorescence turn-on in addition to aggregation-induced emission (AIE).

A series of dinuclear chloride-binding foldamers – (TEA)₂[L¹Cl₂]·Et₂O (1), (TEA)₂[L²Cl₂] (2), (TBA)₂[L³Cl₂] (3), (TBA)₂[L⁴Cl₂] (4), [K(18-crown-6)]₂[L⁴Cl₂]·H₂O (5), and (TBA)₂[L⁵Cl₂]·H₂O (6) – have been synthesized from naphthyl- and anthracenyl-decorated oligoureas (tetrakisurea to hexakisurea, L¹–L⁵). In the crystal structures of 1–6, the oligoureas adopt helical conformations and two chloride anions are bound inside by the urea groups. The solution binding properties of L¹–L⁵ toward chloride ion were investigated by ¹H NMR and fluorescence spectroscopy. Upon binding of the anions, characteristic downfield shifts of the NH signals and enhanced fluorescent emission relative to the free ligands were observed. In DMSO solution, the ligands each display a 1:1 binding ratio to chloride, with association constants of K = 1.16–7.58 × 10² M^–1, which contrasts with the 1:2 (host to guest) binding mode in the crystal structures. However, the formation of dinuclear foldamers in the less polar solvent CHCl₃ was indicated by the HRMS (ESI) spectra.

The tris(2-aminoethyl)amine (tren)-based tris(4-pyridylurea) receptor L has been synthesized and its anion binding properties were studied. The ligand forms a 2:1 (host/guest) complex with MgSO₄, [SO₄²⁻⊂L₂], in which a sulfate ion is encapsulated by six urea groups from the two ligands through multiple hydrogen bonds. The metal ions do not coordinate to the pyridyl groups but exist as the hydrate [Mg(H₂O)₆]²⁺ and interact with the [SO₄⊂L₂] capsules in the outer coordination sphere to form a three dimensional extended structure. The anion binding behavior of ligand L in solution was studied.

Three ferrocenyl-functionalized tripodal hexaurea anion receptors with ortho- (L²), meta- (L³), and para-phenylene (L⁴) bridges, which showed strong binding affinities toward sulfate ions, have been designed and synthesized. In particular, meta-phenylene-bridged ligand L³, owing to its trigonal bipyramidal structure, can encapsulate two SO₄²⁻ ions in its “inner” and “outer” tripodal clefts, respectively, as supported by their clearly distinct NMR resonances and by molecular modeling. The sulfate complex of ortho-ligand L², (TBA)₂[SO₄⊂L²]⋅2 H₂O (1), displays a caged tetrahedral structure with an encapsulated sulfate ion that is hydrogen bonded by the six urea groups of ligand L². CV studies showed two types of electrochemical response of the ferrocene/ferrocenium redox couple upon anion binding, that is, a shift of the wave and the appearance of a new peak. Quantitative binding data were obtained from the NMR and CV titrations.