Ligand Rigidification for Enhancing the Stability of Metal–Organic Frameworks

Metal–organic frameworks (MOFs)
have been developing at
an unexpected rate over the last two decades. However, the unsatisfactory
chemical stability of most MOFs hinders some of the fundamental studies
in this field and the implementation of these materials for practical
applications. The stability in a MOF framework is mostly believed
to rely upon the robustness of the M–L (M = metal ion, L =
ligand) coordination bonds. However, the role of organic linkers as
agents of stability to the framework, particularly the linker rigidity/flexibility,
has been mostly overlooked. In this work, we demonstrate that a ligand-rigidification
strategy can enhance the stability of MOFs. Three series of ligand
rotamers with the same connectivity but different flexibility were
prepared. Thirteen Zr-based MOFs were constructed with the Zr6O4(OH4)­(−CO2)n units (n = 8 or 12) and
corresponding ligands. These MOFs allow us to evaluate the influence
of ligand rigidity, connectivities, and structure on the stability
of the resulting materials. It was found that the rigidity of the
ligands in the framework strongly contributes to the stability of
corresponding MOFs. Furthermore, water adsorption was performed on
some chemically stable MOFs, showing excellent performance. It is
expected that more MOFs with excellent stability could be designed
and constructed by utilizing this strategy, ultimately promoting the
development of MOFs with higher stability for synthetic chemistry
and practical applications.