Different and Often Opposing Forces Drive the Encapsulation and Multiple Exterior Binding of Charged Guests to a M4 L6 Supramolecular Vessel in Water.

The supramolecular assembly [Ga4 L6 ]12- acts as a nanoscale flask to mediate the reactivity of encapsulated reactive guests and also functions as a catalyst to carry out enzyme-like chemical transformations. The guest binding to the interior cavity and exterior of this host is difficult to untangle because multiple equilibria occur in solution, and only when refining simultaneously data obtained from different techniques, such as NMR, UV/Vis, and calorimetry, can the accurate solution thermodynamics of these host-guest systems be determined. This study reports the driving forces for the inclusion and stepwise exterior guest binding of different aliphatic quaternary ammonium guests to the [Ga4 L6 ]12- assembly. Encapsulation into the host cavity was found to be an entropy-driven process, whereas exterior ion association is driven either by enthalpically favorable attractive forces or by the entropy gain due to desolvation, depending on guest size and character. The analysis of the energetics of reaction may help predicting and understanding the intimate role and contribution of the transition state in those rate-accelerated reactions involving this supramolecular assembly as an enzyme-like molecular flask.