3He NMR of He@C606- and He@C706-.: New records for the most shielded and the most deshielded 3He inside a fullerene

The reduction of He-3@C-60 and He-3@C-70 by lithium metal to give solutions of the corresponding hexaanions in THF-d(8) has been achieved under gentle conditions, at subambient temperatures, without sonication, by capitalizing on the ability of corannulene (1) to serve as an efficient electron carrier between the lithium metal and the solid fullerenes, which are virtually insoluble in the reaction medium. The He-3 inside the C-60 hexaanion is found to be more strongly shielded (by nearly 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 [delta(He-3@C-70(6-)) = +8.3 ppm], resonating at nearly 15 ppm lower field than any previously reported 3He in a fullerene. These results stand in complete accord with earlier predictions that the magnetic properties of C-60 and C-70 would be altered dramatically, and in opposite directions, by reduction of the fullerenes to their hexaanions. The phenomenal ability of C-60(6-) to Shield an endohedral 3He from a powerful external magnetic field provides the most compelling evidence to date for the ability of electrons to move freely about the surface of a spheroidal pi-system.