Tailoring the breathing behavior of functionalized MIL-53(Al,M)-NH2 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 mixedmetal frameworks of the MIL-53-NH2 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)-NH2 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-NH2 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.