Fullerene encapsulation with calix[5]arenes

This paper presents the synthesis of the fullerene hosts based on the calix[5]arenes and their binding properties. Calix[5]arenes la, 2, 3a bind C-60 or C-70 in organic solvents. The solvent effect of the fullerene complexation was clearly observed; the association constant decreases in a solvent with high solubility for C-60. Covalently linked double-calix[5]arenes 4-6 were also investigated on their binding properties for fullerenes in organic solvents. Their binding abilities for both C-60 and C-70 are extremely high in toluene solution. Higher binding selectivity toward C-70 is observed by all the double-calix[5]arenes. The selectivity of 5a toward C-70/C-60 is highest in toluene with a value of 10. The structures of the supramolecular complexes of the calix[5]arene hosts and C-60 or C-70 were investigated by using H-1 and C-13 NMR studies. The molecular mechanics calculation and X-ray structure reveal that the interior of the calix[5]arene is complementary to the exterior Of C-60 molecule. In contrast, the host-guest complexes Of C-70 With the simple calix[5]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 calix[5]arene cavity takes up C-70 molecule, and the C-70 molecule tilts significantly from the C5 axis of the calix[5]arene. In the case of the host-guest complex Of C-70 with the double-calix[5]arene, the molecular dynamics simulation of the host-guest complex represented the realistic movement of the bound C-70 inside the cavity. The combination of the molecular dynamics simulation and the chemical shift simulation of the host-guest complex suggested that the C-70 molecule rapidly moves inside the cavity. (c) 2005 Elsevier Ltd. All rights reserved.