No abstract

The ability to chemisorb iodine is important for the safe longterm storage of fission products from nuclear reactors. Herein, we successfully used single-crystal X-ray diffraction analysis to crystallographically visualize I 2 binding sites in two isostructural metal−organic frameworks, viz. Co 2 (m-DOBDC) (m-DOBDC 4− = 4,6-dioxo-1,3-benzenedicarboxylate) and Co 2 (p-DOBDC) (p-DOBDC 4− = 2,5-dioxo-1,4-benzenedicarboxylate), with increasing I 2 loading. Interestingly, the C−H bond at the electron-rich carbon (C5) of m-DOBDC 4− is activated toward electrophilic aromatic substitution, forming an aryl C−I bond and I − or I 3 − that coordinates to unsaturated open Co sites. Cooperation between the ligand and the open Co sites leads to rapid chemisorption of I 2 even under mild adsorption conditions, such as room temperature. In contrast, molecular I 2 coordinates to the open Co sites of Co 2 (p-DOBDC).Owing to the chemisorption of I 2 , I 2 @Co 2 (m-DOBDC) decomposes at a much higher temperature than I 2 @Co 2 (p-DOBDC), as revealed by thermogravimetric analysis.

We report the dual postsynthetic modification (PSM) of am etal-organic framework (MOF) involving the microscopic conversion of CÀHbonds into CÀCbonds and the mesoscopic introduction of hierarchical porosity.M OF crystals underwent single-crystal-to-single-crystal transformations during the electrophilic aromatic substitution of Co 2 (m-DOBDC) (m-DOBDC 4À = 4,6-dioxo-1,3-benzenedicarboxylate) with alkylhalides and formaldehyde.The steric hindrance caused by the proximity of the introduced functional groups to the coordination bonds reduced bond stability and facilitated the transformation into hierarchically porous mesostructures by etching with in situ generated protons (hydroniums) and halides.The numerous defect sites in the mesostructural MOFs are potential water-sorption sites.H owever,s ince the introduced functional groups are close to the main adsorption sites, even methyl groups are able to considerably decrease water adsorption, whereas hydroxy groups increase adsorption at low vapor pressures. Scheme 1. Dual PSM of Co 2 (m-DOBDC) with an alkyl halide.

In this work, the effect of functional groups on I 2 sorption kinetics is investigated using two different types of isostructural metal-organic frameworks, UiO-66-X series (X = H, Br, NO 2 , NH 2 , (OH) 2 , and (COOH) 2) and M 2 (m-DOBDC) series, (M = Co2+, Mg2+, and Ni2+; m-DOBDC 4− = 4,6-dioxo-1,3benzenedicarboxylate). Among the UiO-66-X series, UiO-66-(COOH) 2 exhibits the fastest sorption kinetics and the highest sorption capacity due to dipole-induced dipole interactions between carboxylic acid groups and I 2 molecules. In addition, faster I 2 chemisorption is observed in M 2 (m-DOBDC) because of electrophilic aromatic substitution of m-DOBDC 4− with I 2. The I 2 sorption mechanisms are further supported by fitting the I 2 adsorption kinetics data to pseudo-first-order and pseudo-second-order kinetic models.