Crystallographic Visualization of Distinct Iodic Aggregations in Isostructural Metal–Organic Frameworks

Precisely determining the location of adsorbed molecules is essential for illuminating the mechanisms underlying molecular confinement within porous metal-organic frameworks (MOFs). Here, we present the pore-filling and reactive adsorption of iodine in ALP-MOF-1 and its isostructural redox-active ALP-MOF-2. The adsorbed iodine molecules (I₂) are unaffected by Zn(II) in ALP-MOF-1 and are exclusively confined into an unusual three-dimensional (3D) iodine aggregation due to the 3D cross-linked pore topology and multiple I₂-framework interactions. Conversely, in ALP-MOF-2, the adsorbed I₂ enables the oxidation of Co(II) to Co(III), which is accompanied by the reduction of I₂ to I₃^- and the formation of I₅^- and I₂ during continuous I₂ loading. Identification of distinct iodine adsorption processes in ALP-MOF-1 and -2 motivated tuning of the metal ion composition to adjust the adsorption mechanism. The iodic aggregations in both MOFs are unambiguously confirmed by the combination of single crystal X-ray diffraction and spectroscopic characterization. The presence of multiple adsorption sites facilitate rapid iodine uptake of ∼179 wt % in ALP-MOF-1 and ∼150 wt % in ALP-MOF-2 within ∼5 h, which could be advantageous for applications requiring rapid and energy-efficient iodine capture.