Single-Crystal-to-Single-Crystal Metalation of a Metal–Organic Framework: A Route toward Structurally Well-Defined Catalysts

Metal–organic
frameworks featuring ligands with open chelating
groups are versatile platforms for the preparation of a diverse set
of heterogeneous catalysts through postsynthetic metalation. The crystalline
nature of these materials allows them to be characterized via X-ray
diffraction, which provides valuable insight into the structure of
the metal sites that facilitate catalysis. A highly porous and thermally
robust zirconium-based metal–organic framework, Zr6O4(OH)4­(bpydc)6 (bpydc2– = 2,2′-bipyridne-5,5′-dicarboxylate),
bears open bipyridine sites that readily react with a variety of solution-
and gas-phase metal sources to form the corresponding metalated frameworks.
Remarkably, Zr6O4(OH)4­(bpydc)6 undergoes a single-crystal-to-single-crystal transformation
upon metalation that involves a change in space group from Fm3̅m to Pa3̅.
This structural transformation leads to an ordering of the metalated
linkers within the framework, allowing structural characterization
of the resulting metal complexes. Furthermore, Zr6O4(OH)4­(bpydc)6 yields an active
heterogeneous catalyst for arene C–H borylation when metalated
with [Ir­(COD)2]­BF4 (COD = 1,5-cyclooctadiene).
These results highlight the unique potential of metal–organic
frameworks as a class of heterogeneous catalysts that allow unparalleled
structural characterization and control over their active sites.