Direct Observation of Enhanced Iodine Binding within a Series of Functionalized Metal-Organic Frameworks with...

Optimization of active sites and stability under irradiation are important targets for sorbent materials that might be used for iodine (I₂) storage. Herein, we report the direct observation of I₂ binding in a series of Cu(II)-based isostructural metal-organic frameworks, MFM-170, MFM-172, MFM-174, NJU-Bai20, and NJU-Bai21, incorporating various functional groups (-H, -CH₃, - NH₂, -C≡C-, and -CONH-, respectively). MFM-170 shows a reversible uptake of 3.37 g g^-1 and a high packing density of 4.41 g cm^-3 for physiosorbed I₂. The incorporation of -NH₂ and -C≡C- moieties in MFM-174 and NJU-Bai20, respectively, enhances the binding of I₂, affording uptakes of up to 3.91 g g^-1. In addition, an exceptional I₂ packing density of 4.83 g cm^-3 is achieved in MFM-174, comparable to that of solid iodine (4.93 g cm^-3). In situ crystallographic studies show the formation of a range of supramolecular and chemical interactions [I···N, I···H₂N] and [I···C≡C, I-C═C-I] between -NH₂, -C≡C- sites, respectively, and adsorbed I₂ molecules. These observations have been confirmed via a combination of solid-state nuclear magnetic resonance, X-ray photoelectron, and Raman spectroscopies. Importantly, γ-irradiation confirmed the ultraresistance of MFM-170, MFM-174, and NJU-Bai20 suggesting their potential as efficient sorbents for cleanup of radioactive waste.