Molecular Encapsulation in Pyrogallolarene Hexamers Under Nonequilibrium Conditions

Pyrogallol[4]arene is a macrocycle with a concave surface and 12 peripheral hydroxyl groups that mediate its self-assembly to form hexamers of octahedral symmetry in the solid state, in solution, and in the gas phase. These hexamers enclose approximately 1300 Å3 of space, which is filled with small molecules. In this study, we show that solvent-free conditions for guest entrapment in these hexamers, using molten guest molecules as solvent and allowing the capsules to assemble during cooling, results in exceptionally kinetically stable encapsulation complexes that are not formed in the presence of solvent and are not thermodynamically stable. The capsules’ kinetic stabilities are strongly dependent on the size and shape of both guest and solvent molecules, with larger or nonplanar molecules with rigid geometries providing enhanced stability. The greatest observed barrier to guest exchange, ΔG ⧧ = 32 ± 0.7 kcal mol–1 for encapsulated CCl4 → encapsulated pyrene, is, to the best of our knowledge, indicative of the most powerful kinetic trap ever observed for a synthetic, hydrogen-bonded encapsulation complex. Detailed NMR studies of the structures of the assemblies and the kinetics and mechanisms for guest exchange reveal that subtle differences in guest and solvent structure can impart profound effects on the behavior of the systems. Kinetic and thermodynamic stability, capsule symmetry and structure, guest tumbling rates, susceptibility to disruption by polar solvents, and even the mechanism for equilibrationthe presence or absence of supramolecular intermediatesare all greatly influenced. The strongest observed kinetic traps provide encapsulation complexes that are not at equilibrium but are nonetheless indefinitely persistent at ambient temperatures, a property that invites future applications of supramolecular chemistry in open systems where equilibrium is not possible.