The reduction of 3He@C60 and
3He@C70 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 3He inside the
C60
hexaanion is found to be more strongly shielded (by nearly 20 ppm!)
than any previously reported 3He in a
fullerene
[δ(3He@C60
6-)
= −48.7 ppm, relative to dissolved 3He gas in the
solution], whereas the 3He inside
the C70 hexaanion is actually deshielded
[δ(3He@C70
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 C60 and
C70 would be altered dramatically, and in opposite
directions,
by reduction of the fullerenes to their hexaanions. The phenomenal
ability of C60
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
π-system.
The reduction of corannulene (1) has been followed by parallel detection of optical absorption, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies. The major findings are the existence of a diamagnetic dianion that can be observed in NMR and negligable counterion influences in the monoand dianion. The diamagnetic state of the dianion is also supported by vanishing EPR intensity and semiempirical calculations and exhibits one indistinguishable proton NMR resonance at -5.6 ppm for potassium and lithium as counterions, respectively. The NMR signal is strongly temperature dependent and can only be obtained at low temperature (T < 230 K) where the line width decreases with temperature. Further reduction to the trianion and tetraanion succeeds with Li as the reducing metal, but not with K even at ambient temperatures. The EPR spectra of the trianion show a pronounced temperature dependence with at least two Li cations being strongly coupled.
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