Property-Tuned, Defective, and Amorphous MOF Adsorbents via Ligand Tethering

The covalent tethering of individual ligands into oligomers prior to metal-organic framework (MOF) synthesis can result in unique structural effects in the resulting oligoMOFs. In this study, oligoMOFs based on the Cu-DMOF-1 framework are synthesized using a series of flexible alkyl-tethered ligand dimers ranging in length from propyl to heptyl. Despite synthesis under identical conditions, these Cu-oligoDMOFs show systematic variations in crystallinity, crystal size, thermal properties, and porosity as a function of tether length. Through microcrystal electron diffraction (uED), powder X-ray diffraction (PXRD), imaging, spectroscopy, and thermal analysis, in combination with periodic DFT calculations, we find that the balance and interplay between the energetics of the metal ion coordination environment and the ligand constraints dictate the synthetic outcomes of these self-assembly reactions. Increasingly severe geometric constraints imposed by shorter tethers result in defective and even amorphous Cu-oligoDMOFs directly from synthesis, thereby outlining a novel, systematic, and reticular approach by which MOF properties, including crystallinity, decomposition temperature, and defectivity, can be incrementally tailored solely through tether sizing. Adsorption studies show that coordinative unsaturation and pore constriction introduced by tethers of various sizes can lead to greatly enhanced CO₂ adsorption performance, with the amorphous propyl(bdc)₂-Cu-DMOF-1 exhibiting an exceptionally high BET surface area (941 m²g^-1) and greatly enhanced affinity toward CO₂ (Q_st = 26.8 kJ mol^-1, IAST CO₂/N₂ selectivity 102) compared to tether-free Cu-DMOF-1.