Iron-Based Metal–Organic Frameworks MIL-88B and NH2-MIL-88B: High Quality Microwave Synthesis and Solvent-Induced Lattice “Breathing”

Iron-based MIL-88B and NH2-MIL-88B microcrystals with high dispersibility and uniform size were successfully synthesized by using a rapid microwave-assisted solvothermal method. By carefully controlling the reaction conditions, the microwave method provided superior quality MIL-88B crystals in high yields and excellent phase purity. Framework flexibility was observed for both MIL-88B-Fe and NH2-MIL-88B-Fe frameworks in various solvents, which however significantly differs between the two materials. MIL-88B-Fe shrinks reversibly by about 25% only when it is dispersed in the strongly hydrogen bonding solvents water or methanol. In contrast, NH2-MIL-88B-Fe shrinks up to 33% upon replacement of dimethylformamide (DMF) by any other solvent studied (benzene, chloroform, acetone, acetonitrile, methanol, water). The change in unit cell parameters (shortening of the a axis) can be seen macroscopically, although the overall integrity of the materials is maintained. We suggest that hydrogen bonding between the oxygen atoms of the MIL-88B-Fe framework and solvent molecules plays an important role in the framework shrinkage, while in the NH2-MIL-88B-Fe framework additional hydrogen bonds may form and thus a different breathing behavior is observed.