Carboxylic Acid Functionalized Clathrochelate Complexes: Large, Robust, and Easy-to-Access Metalloligands

Polycarboxylate ligands are among
the most important building blocks for the synthesis of metal–organic
frameworks (MOFs). The ability to access these ligands in an efficient
way is of key importance for future applications of MOFs. Here, we
demonstrate that mono- and dinuclear clathrochelate complexes are
versatile scaffolds for the preparation of polytopic carboxylate ligands.
The largely inert clathrochelate complexes have a trigonal-bipyramidal
shape. The synthesis of functionalized clathrochelates with two, three,
four, or five carboxylic acid groups in the ligand periphery can be
achieved in a few steps from simple starting materials. Apart from
being easily accessible, the metalloligands display interesting characteristics
for applications in metallasupramolecular chemistry and materials
science: they are rigid, large (up to 2.2 nm), and robust and they
can show additional functions (e.g., fluorescence or extra charge)
depending on the metal ion that is present in the clathrochelate core.
The utility of these new metalloligands in MOF chemistry is demonstrated
by the synthesis of zinc- and zirconium-based coordination polymers.
The combination of Zn­(NO3)2 with clathrochelates
having two or three carboxylic acid groups gives MOFs in which the
clathrochelate ligands are connected by Zn4O clusters or
zinc paddlewheel links. The structures of the resulting two- and three-dimensional
networks could be established by single-crystal X-ray crystallography.
The reaction of carboxylic acid functionalized clathrochelates with
ZrCl4 gives amorphous powders that display permanent porosity
after solvent removal.