A series of(13)C-enriched monoytterbium endohedral metallofullerenes (EMFs)-Yb@C(2n)
(n = 40, 41, 42)-was synthesized and isolated. Their cage structures were systematically
determined for the first time using computational and experimental (13)C NMR studies.
The results revealed that all isomers adopt cage structures conforming to the isolated
pentagon rule. In detail, Yb@C(80) possesses the C(2v)(3) cage; Yb@C(82)(I, II, III)
bear C(s)(6), C(2)(5), and C(2v)(9) cages, respectively; and Yb@C(84)(II, III, IV)
have C(2)(13), C(1)(12), and C(2)(11) cage structures, respectively. This is the first
report describing C(2)(13)-C(84) and C(1)(12)-C(84) cage structures. It is noteworthy
that the cage structures found for mono-EMFs generally differ from either the corresponding
empty fullerenes or the related EMFs encapsulating more than one metal atom, indicating
that the metal atom inside the fullerene cage plays an important role in determining
the EMF structure. On the basis of the fact that the structure of Yb@C(2)(13)-C(84)
resembles that of Yb@C(2)(5)-C(82), a metal-templated growth process was proposed
as a kinetic factor controlling EMF formation. Furthermore, previous electrochemical
studies of divalent EMFs have failed to observe their oxidation potentials, which
have raised the assumption that such species are large-bandgap molecules. This study
revealed that all isomers of Yb@C(2n) (n = 40, 41, 42) display one or two reversible
oxidation steps together with four reversible reduction processes in 1,2-dichlorobenzene,
even at a low scan rate (20 mV/s), which enables estimation of their electrochemical
bandgaps (DeltaE = (ox)E(1)-(red)E(1)). The results show a DeltaE value of 0.88-1.41
V for Yb@C(2n) (2n = 40, 41, 42), which is much larger than the values of trivalent
mono-EMFs (DeltaE < 0.5 V), but generally smaller than those of metal nitride cluster
EMFs (1.4 V < DeltaE < 2.1 V). Results further demonstrated that the DeltaE values
correlate reasonably with their relative abundances.