Anion-Induced Intramolecular Cyclization of Urea-Based Receptors: Entrapment of Cyclic [SO₄(H₂O)₄^2–]_n Cluster and Encapsulation of Anions with Varied Dimensionality

Two unsymmetrical dipodal monourea anion receptors BPU-1 and BPU-2 with varying π-acidic terminal electron-withdrawing substituents (–NO₂ and –CF₃) can interact with anions of varied geometries (spherical, trigonal planar, and tetrahedral). Receptors are designed with an arm having a flexible urea unit, and the other arm contains a semirigid fluorophoric benzimidazole unit. BPU-1 could entrap cyclic sulfate water cluster, [SO₄(H₂O)₄^2–]_n, resulting in R₄⁴(8) and R₄⁴(12) arrangements in the solid state. Anion-sensing behavior of both receptors has been investigated in the aqueous medium, where BPU-1 could selectively sense SO₄^2–/HSO₄^– among other biologically relevant anions. Anion-induced intramolecular cyclization of both receptors in the presence of F^–/HO^– to yield benzimidazo[1,2-c]quinazolin-6-ones has also been investigated in detail.