Solvent-free and time efficient postsynthetic modification of amino-tagged metal-organic frameworks with carboxylic acid derivatives

A fast, simple, and effective approach for postsynthetic modification (PSM) of amino-tagged metal-organic frameworks (MOFs) with carboxylic acids, acid anhydrides, and acid chlorides without additional solvent at elevated temperature was developed. In a comparative study optimal synthesis conditions for PSM were determined by systematic variation of the reactants in terms of reactivity and size for MIL-53(Al)-NH₂, UiO-66(Zr)-NH₂, and MIL-101(Al)-NH₂. For this purpose, an acid-free digestion was prepared that allows accurate yield comparison of synthesized acid sensitive amides employing solution nuclear magnetic resonance (NMR). For MIL-53(Al)-NH₂ and UiO-66(Zr)-NH₂ best results were obtained using acid chlorides or anhydrides at 100 °C. For MIL-101(Al)-NH₂ the application of acid anhydrides up to 100°C was a suitable pathway for an efficient PSM. Optimization of all reaction parameters led to a postfunctionalization yield with acetic anhydride at 100°C of 94.9 ± 0.5% for 2 h reaction time with MIL-53(Al)-NH₂, 98 ± 3% for 10 min reaction time with UiO-66(Zr)-NH₂, and 90 ± 1% yield for 10 min reaction time with MIL-101(Al)-NH₂. Reactions with small carboxylic acid reactants or byproducts like acetic acid led to damage or decomposition of UiO-66(Zr)-NH₂ and MIL-101(Al)-NH₂ probably due to postsynthetic ligand exchange. However, this limitation can be overcome by using carboxylic acids with high steric demand that might decelerate or disable ligand exchange and hence prevent the MOF scaffold from damages.