Modulating MOFs

By modifying amine groups in a metal-organic framework (MOF), Zhenqiang Wang and Seth M. Cohen of the University of California, San Diego, have shown that it’s possible to activate the “breathing” behavior of pores in the materials to selectively adsorb gases or other molecules. The finding marks the first time researchers have used a postsynthetic modification to trigger and control the flexibility of MOFs (J. Am. Chem. Soc., DOI: 10.1021/ja907742z). MOFs are porous materials with high surface areas that are ideal for catalysis, separation, sensing, and gas-storage applications. Most MOFs are rigid crystalline solids, but a small subset of the materials has unique flexible frameworks that permit the pores to open and narrow. Cohen’s group built a flexible MOF from zinc carboxylate units joined by 1,4-diazabicyclo[2.2.2]octyl groups in one direction and by 2-amino-1,4-benzenedicarboxylates in the other direction (Inorg. Chem. 2009, 48, 296). Wang and Cohen then treated the MOF with alkylanhydrides of various lengths (O[CO(CH₂)_nCH₃]₂) to form amide groups. The short-chain amide (n = 0) narrows the pores and the longer chain amides (n = 3–5) hold the pores open, but the medium-length chains (n = 1 or 2) are just right, creating a bistable framework that leads to controlled breathing.