Transformation of Zirconium Metal-Organic Cages to Metal-Organic Gels

Zirconium metal-organic cages (Zr-MOCs) that assemble from zirconocence-based clusters and carboxylate-based ligands have been widely used as building blocks for constructing various MOC-based materials with diverse applications because of their high stability and solubilities. However, in this work, we found that the structures of a series of Zr-MOCs are quite flexible when heated at 60 °C in an N,N'-dimethylformamide solution, in which they are completely disassembled into fragments and reassembled into amorphous metal-organic gels (MOGs). The transformation mechanism was investigated via proton nuclear magnetic resonance, electrospray ionization mass spectrometry, infrared spectroscopy, X-ray photoelectron spectroscopy, and other techniques, which revealed that cyclopentadiene (CP) breaks out of the [Zr₃CP₃(μ₂-OH)₃(μ₃-O)] cluster and remains in solution, whereas the trinuclear [Zr₃(μ₂-OH)₃(μ₃-O)] cluster dimerizes into a hexanuclear [Zr₆(μ₂-OH)₆(μ₃-O)₂] cluster and forms amorphous MOGs with terephthalic acid-based ligands. The reported Zr-MOCs based on terephthalic acid-based ligands with different substituent groups, longer biphenyl dicarboxylic acids, and trimesic acids all undergo such MOC-to-MOG transformations, indicating good versatility for this transformation. In addition, the posttreatment of amorphous MOGs with an appropriate amount of acetic acid could regulate the crystallinity of the MOGs, thus giving rise to MOGs containing nano-UiO-66-type MOF crystals and xerogels with improved porosity.