Tailoring the breathing behavior of functionalized MIL-53(Al,M)-NH 2 materials by using the mixed-metal concept

Mixed-metal metal-organic frameworks with two or more metals distributed over crystallographically equivalent sites are gaining growing attention in various research areas. We have extended the range of available mixed-metal frameworks of the MIL-53-NH 2 family by combining aluminum with scandium, vanadium, chromium or iron. In contrast to other already reported mixed-metal MIL-53 materials, the developed MIL-53(Al,M)-NH 2 materials (M = Sc, V, Cr, Fe), which contain 2-aminoterephthalate as linker instead of terephthalate, will enable subsequent post-synthetic modifications. All developed materials have been thoroughly characterized with standard methods as well as X-ray absorption spectroscopy to prove the exclusive incorporation of the employed metals into the MIL-53-NH 2 framework structure and to verify the absence of impurity phases. Nitrogen physisorption measurements revealed that the choice of the metal combinations and ratios had a strong influence on the breathing behavior. Furthermore, post-synthetic modification reactions of the amine groups with maleic anhydride showed that the metal combination had a dramatic impact on the resulting modification degrees. Thus, the flexibility of the pore systems can be tailored by a careful selection of suitable metal combinations for these materials.