Measuring Ion-Pairing and Hydration in Variable Charge Supramolecular Cages with Microwave Microfluidics
Angela C. Stelson, Cynthia M. Hong, Mitchell C. Groenenboom, Charles A. Little, James C. Booth, Nathan D. Orloff, Robert G. Bergman, Kenneth N. Raymond, Kathleen A. Schwarz, F. D. Toste, Christian J. Long
MetaMetalorganic supramolecular cages can act as charged molecular containers that mediate reactions, mimic enzymatic catalysis, and selectively sequester chemicals.1,2 The hydration of these cages in solution plays a crucial role in their interactions with other species in solution (e.g. substrates, counterions, and guest molecules). However, these noncovalent interactions of water molecules and counterions are challenging to detect by standard analytical chemistry techniques.3,4 Here, we use microwave microfluidics to measure the hydration and ion pairing properties of two tetrahedral metalorganic cage assemblies, K12(Ga4L6) and K8(Si4L6), which are isostructural but have different overall anionic charge (12- for K12(Ga4L6), 8- for K8(Si4L6)). Our microwave microfluidics measurements cover 40 kHz to 110 GHz of frequency, far more than typical dielectric spectroscopy techniques, and a reduced measurement volumes of nanoliter. From these broadband measurements, we extract hydration numbers and ion pairing dynamics for the cages. We find that the cage charge can be described as four di- and trianionic vertices rather than an overall large anionic charge. Unexpectedly, the K8(Si4L6) cage is more strongly hydrated and forms a more solvated ion pair compared to the K12(Ga4L6) cage. We show that microwave microfluidics can characterize solvation and ion pairing dynamics for charged supramolecular cages and demonstrate that changes in charge have consequences for the behavior of metal organic cages in solution.