Measuring ion-pairing and hydration in variable charge supramolecular cages with microwave microfluidics

Abstract Metal-organic supramolecular cages can act as charged molecular containers that mediate reactions, mimic enzymatic catalysis, and selectively sequester chemicals. The hydration of these cages plays a crucial role in their interactions with other species. Here we use microwave microfluidics to measure the hydration and ion pairing of two metal-organic cage assemblies that are isostructural but have different overall anionic charge. We supplement our measurements with density functional theory calculations to compare binding site energies on model metal-organic cage vertices. We find that the cage with dianionic vertices is more strongly hydrated and forms a distinct ion pair species from the cage with trianionic vertices. We evaluate multi-ion species and distinct ion pair solvations as possible sources for differences in ion dynamics and hydration. Broadly, this work highlights the utility of microwave microfluidics to elucidate the consequences of charge states on metal-organic complexes in solution.