HE-3 NMR OF HE-AT-C-60(6-) AND HE-AT-C-70(6-) - NEW RECORDS FOR THE MOST SHIELDED AND THE MOST DESHIELDED HE-3 INSIDE A FULLERENE

The reduction of He-3@C-60 and He-3@C-70 by lithium metal to give solu tions of the corresponding hexaanions in THF-d(8) has been achieved un der gentle conditions, at subambient temperatures, without sonication, by capitalizing on the ability of corannulene (1) to serve as an effi cient electron carrier between the lithium metal and the solid fullere nes, which are virtually insoluble in the reaction medium. The He-3 in side the C-60 hexaanion is found to be more strongly shielded (by near ly 20 ppm!) than any previously reported 3He in a fullerene [delta(He- 3@C-60(6-)) = -48.7 ppm, relative to dissolved 3He gas in the solution ], whereas the 3He inside the C-70 hexaanion is actually deshielded [d elta(He-3@C-70(6-)) = +8.3 ppm], resonating at nearly 15 ppm lower fie ld than any previously reported 3He in a fullerene. These results stan d in complete accord with earlier predictions that the magnetic proper ties of C-60 and C-70 would be altered dramatically, and in opposite d irections, by reduction of the fullerenes to their hexaanions. The phe nomenal ability of C-60(6-) to Shield an endohedral 3He from a powerfu l external magnetic field provides the most compelling evidence to dat e for the ability of electrons to move freely about the surface of a s pheroidal pi-system.