Selective Sorption of Organic Molecules by the Flexible Porous Hybrid Metal−Organic Framework MIL-53(Fe) Controlled by Various Host−Guest Interactions

The structures of four iron(III) carboxylate materials with the MIL-53 structure and the guest molecules water, pyridine, or 2,6-dimethylpyridine (2,6-lutidine) have been solved and refined from high-resolution powder X-ray diffraction data. These materials have porous structures constructed from infinite chains of trans -corner-shared {FeO 4 (OH,F) 2 } octahedra cross-linked by benzene-dicarboxylate ligands. With pyridine, the unit cell is partially expanded with respect to the hydrated phase, with hydrogen bonds between N donors and OH framework atoms, whereas with lutidine, water is co-sorbed to give a fully expanded version of the MIL-53(Fe) structure in which water molecules bridge the N donors of the organic molecule and the framework OH groups by hydrogen bonding. Careful dehydration of MIL-53(Fe)[2,6-lutidine,H 2 O] provides a new material, MIL-53(Fe)[2,6-lutidine 0.5 ], with an intermediate unit cell volume: in this case no host−guest hydrogen bonds are formed. Time-resolved in situ energy-dispersive X-ray diffraction shows that when exposed to an aqueous mixture of pyridine and 2,6-lutidine the hydrated MIL-53(Fe) takes up both guest molecules initially to give two distinct phases, but the ultimate product is only MIL-53(Fe)[2,6-lutidine,H 2 O].