An Exceptionally Stable Metal–Organic Framework Constructed from Chelate-Based Metal–Organic Polyhedra

We report the rational
design and synthesis of a water-stable metal–organic framework
(MOF), Fe-HAF-1, constructed from supramolecular, Fe3+-hydroxamate-based
polyhedra with mononuclear metal nodes. Owing to its chelate-based
construction, Fe-HAF-1 displays exceptional chemical stability in
organic and aqueous solvents over a wide pH range (pH 1–14),
including in the presence of 5 M NaOH. Despite the charge neutrality
of the Fe3+-tris­(hydroxamate) centers, Fe-HAF-1 crystals
are negatively charged above pH 4. This unexpected property is attributed
to the formation of defects during crystallization that results in
uncoordinated hydroxamate ligands or hydroxide-coordinated Fe centers.
The anionic nature of Fe-HAF-1 crystals enables selective adsorption
of positively charged ions in aqueous solution, resulting in efficient
separation of organic dyes and other charged species in a size-selective
fashion. Fe-HAF-1 presents a new addition to a small group of chelate-based
MOFs and provides a rare framework whose 3D connectivity is exclusively
formed by metal–hydroxamate coordination.