Fullerene encapsulation with calix[5]arenes

This paper presents the synthesis of the fullerene hosts based on the calix5]arenes and their binding properties. Calix5]arenes 1a, 2, 3a bind C₆₀ or C₇₀ in organic solvents. The solvent effect of the fullerene complexation was clearly observed; the association constant decreases in a solvent with high solubility for C₆₀. Covalently linked double-calix5]arenes 4-6 were also investigated on their binding properties for fullerenes in organic solvents. Their binding abilities for both C₆₀ and C₇₀ are extremely high in toluene solution. Higher binding selectivity toward C₇₀ is observed by all the double-calix5]arenes. The selectivity of 5a toward C₇₀/C₆₀ is highest in toluene with a value of 10. The structures of the supramolecular complexes of the calix5]arene hosts and C₆₀ or C₇₀ were investigated by using ¹H and ¹³C NMR studies. The molecular mechanics calculation and X-ray structure reveal that the interior of the calix5]arene is complementary to the exterior of C₆₀ molecule. In contrast, the host-guest complexes of C₇₀ with the simple calix5]arenes take many conformational options due to its less symmetric shape. The molecular mechanics calculation and our chemical shift simulation nicely worked to estimate the reliable structures; the calix5]arene cavity takes up C₇₀ molecule, and the C₇₀ molecule tilts significantly from the C5 axis of the calix5]arene. In the case of the host-guest complex of C₇₀ with the double-calix5]arene, the molecular dynamics simulation of the host-guest complex represented the realistic movement of the bound C₇₀ inside the cavity. The combination of the molecular dynamics simulation and the chemical shift simulation of the host-guest complex suggested that the C₇₀ molecule rapidly moves inside the cavity.