IMPROVED PURIFICATION OF C-60 AND FORMATION OF SIGMA-HOMOAROMATIC AND PI-HOMOAROMATIC METHANO-BRIDGED FULLERENES BY REACTION WITH ALKYL DIAZOACETATES

A rapid and inexpensive method for the large-scale purification of C60 is the simple filtration of the toluene-soluble extract of commercial fullerene soot through a short plug of charcoal/silica gel with toluene as the eluent. Reactions of C60 with ethyl and tert-butyl diazoacetates in refluxing toluene lead to the formation of the (alkoxycarbonyl)methylene-bridged isomers 1a-3a and 1b-3b, respectively, which can be equilibrated, upon further heating, into the single compounds 1a and 1b, respectively. Isomers 1a/b possess the methano bridge at the 6-6 ring junction, whereas structures 2a/b and 3a/b are bridged at the 6-5 junction. A dramatic influence of local and x-ring current anisotropic effects of the fullerene sphere on the NMR chemical shifts of the methine protons in the bridge is observed: the chemical shifts of the protons located over a pentagon ring in 2a/b and over a hexagon ring in 3a/b differ by DELTAdelta = 3.47 and 3.45 ppm, respectively. The analysis of the C-13-NMR chemical shifts of the bridgehead C-atoms and the 1J(C,H) coupling constants for the methano-bridge atoms reveals conclusively that the 6-5-ring-bridged structures 2a/2b and 3a/3b are pi-homoaromatic ('open' transannular bond) and the 6-6-ring-bridged structures 1a/b are sigma-homoaromatic ('closed' transannular bond). The electronic absorption spectra show that pi-homoconjugation in 2a/b and 3a/b represents a much smaller electronic perturbation of the original C60 chromophore than sigma-homoconjugation in 1a/b. The results of this study demonstrate an impressive linkage between the chemistry of methano-bridged annulenes and methano-bridged fullerenes.