Monitoring the Solution Persistence of Porous Coordination Cages with Diffusion NMR Spectroscopy and Cryogenic Transmission Electron Microscopy

As a unique class of molecular adsorbents, porous coordination cages (PCCs) have garnered interest because of their potential solubility, which enables applications not available to and covalent organic frameworks (COFs), such as covalent membrane incorporation, solution-based ion binding, and facile incorporation into porous liquids. The identification of key aspects of cage behavior in solution such as the self-assembly process, structural integrity in solution, conformational changes, and cage interactions with guest molecules is necessary to broaden the utility of soluble porous cages. Many characterization strategies for permanently porous coordination cages have been limited to the solid state and have relied heavily on X-ray diffraction, electron microscopy, and infrared (IR), UV-vis, and nuclear magnetic resonance (NMR) spectroscopies. As such, definitive evidence of cage behavior while in solution has been lacking, as the often-paramagnetic nature of permanently porous coordination cages complicates one-dimensional NMR spectroscopy experiments. Here, we explore the combined use of diffusion-ordered spectroscopy NMR (DOSY NMR), transmission electron microscopy (TEM), and cryogenic transmission electron microscopy (cryo-TEM) to confirm the persistence of soluble PCCs in various solutions.

Automated summary

Porous coordination cages (PCCs) are a unique class of molecular adsorbents that have gained attention due to their potential solubility and various applications. However, the characterization of PCC behavior in solution has been challenging. In this study, the combination of DOSY NMR, TEM, and cryo-TEM was used to confirm the persistence of soluble PCCs in different solutions.