Cage-Based Metal-Organic Framework Featuring a Double-Yolk Core-Shell U₆L₃@U₁₈L₁₄ Structure for Iodine Capture

Cage-based MOFs, with their customizable chemical environments and precisely controllable nanospaces, show great potential for the selective adsorption of guest molecules with specific structures. In this work, we have constructed a novel cage-based MOF [(CH₃)₂NH₂]₂[(UO₂)₂(TMTTA)]·11.5DMF·2H₂O (IHEP-51), utilizing a triazine derivative poly(carboxylic acid), 4,4',4″-(((1,3,5-triazine-2,4,6-triyl)tris(((4-carboxycyclohexyl)methyl)azanediyl))tris(methylene))tribenzoic acid (H₆TMTTA), as an organic ligand and uranyl as a metal node. The 2-fold interpenetrated (3,6,6)-connected framework of IHEP-51 features two types of supramolecular cage structures: the Pyrgos[2]cage U₆L₃ and the huge cage U₁₈L₁₄. They are further assembled into a double-yolk core-shell U₆L₃@U₁₈L₁₄ structure, making it suitable for I₂ capture. The maximum adsorption capacities of IHEP-51 for iodine in solution and gaseous iodine are 420.4 and 1561.2 mg·g^-1, respectively. XPS, Raman spectra, single-crystal X-ray diffraction, and DFT calculations reveal that the adsorbed iodine is located inside the U₆L₃ Pyrgos[2]cage in the form of I₃^-, thus resulting in the formation of a (I₃)₂@U₆L₃@U₁₈L₁₄ ternary core-shell structure.