Observation of 13C NMR chemical shifts of metal carbides encapsulated in fullerenes: Sc2C2@C82, Sc2C2@C84, and Sc3C2@C80.

Endohedral metallofullerenes have attracted special interest as promising spherical molecules for material and catalytic applications, because of their unique properties that are not expected from empty fullerenes. The successful isolation and purification of endohedral metallofullerenes in macroscopic quantities have made it possible to investigate the structures as well as the electronic properties and reactivities through a close interplay between theory and experiment. Since the first isolation and characterization of a metal carbide encapsulated metallofullerene (Sc2C2@C84) by MEM (maximum entropy method)/Rietveld analysis of synchrotron powder diffraction data, much attention has been paid to encapsulation of metal carbides. Recently, we found by C NMR spectroscopy and X-ray single-crystal structure analyses that Sc3C82 and Sc2C84 have the structures Sc3C2@ C80(Ih) [5] and Sc2C2@C82(C3v), [6,7] encapsulating metal carbides, although it had long been believed from the MEM/Rietveld analyses that they have the conventional structures Sc3@C82 [8] and Sc2@C84 [9] , respectively. The encapsulation of metal carbides has been confirmed by the recent improved MEM/ Rietveld analyses of Y2C2@C82(C3v) [10] as well as Sc2C2@ C82(C3v) [11] and Sc3C2@C80(Ih). [12] However, attempts to detect the C NMR chemical shifts of the C2 units of metal carbides encapsulated in the fullerenes have not been yet successful. This unsuccessful detection has been explained in term of the spin-rotation interaction, because the C2 unit may rotate rapidly inside carbon cages. It is an important task to observe the C NMR chemical shift of the C2 unit in an attempt to provide insight into its electronic and magnetic properties. We present herein the first detection of the C NMR chemical shifts of the C2 unit in Sc2C2@C82(C3v), Sc2C2@ C84(D2d), and [Sc3C2@C80(Ih)] by using C-enriched samples. We also successfully assigned all C NMR chemical shifts of the cage carbon atoms for the three compounds by 2D INADEQUATE (incredible natural abundance double quantum transfer experiment) NMR measurements. The C NMR spectrum of Sc2C2@C82(C3v) (Figure 1a) shows 17 chemical shifts of the C82 cage at d = 134.6– 151.9 ppm, as reported previously. To map the bond connectivity in the carbon cage and assign the C NMR