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-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.