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.