Cross-Linking of Metal Organic Frameworks

Kazuki Sada

Department of Chemistry, Faculty of Science, Hokkaido University, Japan*E-mail: sadatcm@mail.sci.hokudai.ac.jp, Tel: +81-(0)11-706-3473

Metal organic frameworks (MOFs) have been attracting continuous interest as nano-porous crystalline

materials due to their structural robustness and inclusion of guest molecules for catalysis, storage, and

separation. In this presentation, we demonstrate cross-linking of the organic ligands in MOF crystals to

transform to polymer networks without decomposition of crystalline state by the following two systems;

cyclodextrin MOF (CD-MOF) cross-linked by diglycidyl ethers and (2) azide-tagged MOF (AzM) cross-

linked by multi-alkyne cross-linkers via click reaction.

In the first system, cubic crystals of CD-MOF in the range of several hundred nm to mm were prepared

by reacting -CD with aqueous KOH, followed by vapor diffusion of methanol. The cross-linking reaction

was carried out by treatment with ethylene glycol diglycidyl ether (L) to cross-link between OHs of each -

CD unit in the CD-MOF pores. After degradation of the coordination bonds by repeated soaking in

EtOH/H2O, they became insoluble and swelled in H2O. This indicated that they were converted to polymer

gels by the cross-linking of the organic linkers in the MOF crystals. In the second system, azide-tagged MOF

(AzM) crystals were subjected to click reaction with tetra-acetylene cross-linker (CL4) to cross-link the

organic ligands. After decomposition of the coordination bonds, the MOF crystals gradually swelled and

became insoluble polymer gels. In both cases, the sizes and the shapes of the resulting polymer gels were

very similar to those of the starting MOF crystals, and the chracteristic polyhedral shapes with well-defined

faces originated from the templated MOF crystals were preserved during transformation processes.

In conclusion, the cross-linking of MOF opens a new horizon for the preparation of micro- and nano-sized

polyhedral polymer gels. Our system is the first example for connecting polymer gels as soft and wet

amorphous materials with MOF crystals as robust and crystalline materials with respect to three-dimensional network structures.