Ligand Charge Separation To Build Highly Stable Quasi-Isomer of MOF-74-Zn

Built from an unusual high-charge-density ligand 2,5-dioxido-1,4-benzenedicarboxylate (dobdc⁴⁻), MOF-74 M (M₂dobdc) have unsurpassed gas uptake and separation properties. It is thus intriguing to mimic or replicate such ligand properties in other chemical systems. Here, we show a ligand charge separation (LCS) model that could offer one pathway toward this goal. Two new materials (CPM-74 and -75, corresponding to MOF-74 and IRMOF-74-II, respectively) are presented here to illustrate this concept and its feasibility. Specifically, the dobdc⁴⁻ ligand in MOF-74 can be conceptually broken down into OH⁻ and obdc³⁻ (H₃obdc = 2-hydroxyterephthalic acid), which leads to CPM-74, Zn₂(OH)(obdc), that is nearly isomeric with MOF-74-Zn. Different from MOF-74, CPM-74 is made from homohelical rod packing. Moreover, CPM-74 has high hydrothermal and thermal stability uncommon for Zn-MOFs. It contains open Zn sites on 4-coordinated Zn²⁺ and its isosteric heat of adsorption for CO₂ is 22% higher than that of MOF-74-Zn at low pressures.