Purification of C₆₀ and C₇₀ by selective complexation with calixarenes

MOLECULAR complexation of fullerenes in host-guest complexes has the potential to afford efficient large-scale purification of fullerenes ¹. This method would avoid the losses inherent in the chromatographic techniques currently in use, which arise from irreversible absorption on the stationary phase². Several host-guest interactions have been reported for C₆₀, including the formation of complexes with 1,4-hydroquinone^3,4, azacrown compounds⁵, certain water-soluble macrocycles such as γ-cyclodextrin^6-8, and complexation between C60 and an iridium phosphine in which pendant groups attached to a phosphorus atom form a cradle for an adjacent C₆₀ molecule⁹. Other relevant studies are the inclusion of C₆₀ into a microporous aluminophosphate¹⁰ and the formation of a charge-transfer complex between C₆₀ and a thiafulvalene¹¹. Here we show that both C₆₀ and C₇₀ will form discrete complexes with calixarenes, bowl-shaped macrocycles with hydrophobic cavities. Complexation of p-Bu^'-calix[8]arene with a mixture of the toluene extract of 'crude' fullerene soot, followed by a series of recrystallizations, affords >99.5% pure C₆. Our approach could lead to a substantial reduction in the cost of purifying C₆₀ and C₇₀.