Visualization of Iodine Chemisorption Facilitated by Aryl C-H Bond Activation

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-DOBDC^4- = 4,6-dioxo-1,3-benzenedicarboxylate) and Co₂(p-DOBDC) (p-DOBDC^4- = 2,5-dioxo-1,4-benzenedicarboxylate), with increasing I₂ 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 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.