The ability to chemisorb iodine is important for the safe long-term storage of fission products from nuclear reactors. Herein, we successfully used single-crystal X-ray diffraction analysis to crystallographically visualize I2 binding sites in two isostructural metal-organic frameworks, viz. Co2(m-DOBDC) (m-DOBDC4- = 4,6-dioxo-1,3-benzenedicarboxylate) and Co2(p-DOBDC) (p-DOBDC4- = 2,5-dioxo-1,4-benzenedicarboxylate), with increasing I2 loading. Interestingly, the C-H bond at the electron-rich carbon (C5) of m-DOBDC4- is activated toward electrophilic aromatic substitution, forming an aryl C-I bond and I- or I3- that coordinates to unsaturated open Co sites. Cooperation between the ligand and the open Co sites leads to rapid chemisorption of I2 even under mild adsorption conditions, such as room temperature. In contrast, molecular I2 coordinates to the open Co sites of Co2(p-DOBDC). Owing to the chemisorption of I2, I2@Co2(m-DOBDC) decomposes at a much higher temperature than I2@Co2(p-DOBDC), as revealed by thermogravimetric analysis.
Keywords: aryl C−H bond activation; chemisorption; iodine capture; metal−organic frameworks; single crystal−single crystal transformation.