A Highly Sulfate-Selective Ionic Framework with Interconnected Packed Cavities Based on Tetraphenylmethane-Derived Octa-urea Receptors

A tetraphenylmethane-derived octaurea receptor L was synthesized, characterized, and assembled into a framework of ionic compound 1 by leveraging its high selectivity for sulfate binding. The compound 1 was constructed through sulfate- and 18-crown-6-directed assembly of the receptor L. Single-crystal X-ray diffraction analysis confirmed the structure of 1, revealing that two L molecules, each containing four bis-urea arms, adopt a crossed arrangement in the solid state. At spatially observed intersection regions, two bis-urea arms from each molecule converge, enabling a total of eight urea groups to engage a central sulfate anion via multiple N–H···O hydrogen bonds. Concurrently, K+(18-crown-6) units coordinate to a urea group at the terminal of one bis-urea arm from receptor L, as well as to acetone molecules, occupying localized molecular cavities. Propagation of these interactions further drives the assembly, ultimately forming a stable and densely packed framework. The composition and phase purity of ionic compound 1 were verified by elemental analysis, Fourier transform infrared spectroscopy, and powder X-ray diffraction. Its thermal behavior was assessed by thermogravimetric analysis, demonstrating notable framework stability. In addition, solution-phase UV–vis absorption studies revealed that receptor L exhibits high binding affinity and selectivity toward sulfate over other competing anions.