³He NMR of He@C₆₀⁶⁻ and He@C₇₀⁶⁻. New Records for the Most Shielded and the Most Deshielded ³He Inside a Fullerene¹

The reduction of 3 He@C 60 and 3 He@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 3 He inside the C 60 hexaanion is found to be more strongly shielded (by nearly 20 ppm!) than any previously reported 3 He in a fullerene [δ( 3 He@C 60 6 - ) = −48.7 ppm, relative to dissolved 3 He gas in the solution], whereas the 3 He inside the C 70 hexaanion is actually deshielded [δ( 3 He@C 70 6 - ) = +8.3 ppm], resonating at nearly 15 ppm lower field than any previously reported 3 He 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 3 He 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 π-system.