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 activated carbons, zeolites,
metal–organic frameworks (MOFs), 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.