Yb@C2n (n = 40, 41, 42): New Fullerene Allotropes with Unexplored Electrochemical Properties

摘要

A series of13C-enriched monoytterbium endohedral metallofullerenes (EMFs)—Yb@C2n (n = 40, 41, 42)—was synthesized and isolated. Their cage structures were systematically determined for the first time using computational and experimental 13C NMR studies. The results revealed that all isomers adopt cage structures conforming to the isolated pentagon rule. In detail, Yb@C80 possesses the C2v(3) cage; Yb@C82(I, II, III) bear Cs(6), C2(5), and C2v(9) cages, respectively; and Yb@C84(II, III, IV) have C2(13), C1(12), and C2(11) cage structures, respectively. This is the first report describing C2(13)-C84 and C1(12)-C84 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@C2(13)-C84 resembles that of Yb@C2(5)-C82, 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@C2n (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 (ΔE = oxE1 − redE1). The results show a ΔE value of 0.88−1.41 V for Yb@C2n (2n = 40, 41, 42), which is much larger than the values of trivalent mono-EMFs (ΔE < 0.5 V), but generally smaller than those of metal nitride cluster EMFs (1.4 V < ΔE < 2.1 V). Results further demonstrated that the ΔE values correlate reasonably with their relative abundances.