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

As a unique class of molecular adsorbents, porous coordination cages (PCCs) have attracted interest due to their solubility, which allows for various applications. The understanding of cage behavior in solution is crucial for expanding the utility of soluble PCCs. However, the characterization of PCCs in solution has been limited, and there is a lack of definitive evidence. In this study, the combined use of DOSY NMR, TEM, and cryo-TEM is explored to confirm the persistence of soluble PCCs in various solutions.