Synthesis and Characterization of the D-5h Isomer of the Endohedral Dimetallofullerene Ce-2@C-80: Two-Dimensional Circulation of Encapsulated Metal Atoms Inside a Fullerene Cage

Herein we show the synthesis and characterization of the second known Ce-2@C-80 isomer. A C-13 NMR spectroscopic study revealed that the structure of the second isomer has D-5h, symmetry. Paramagnetic NMR spectral analysis and theoretical calculation display that the encapsulated Cc atoms circulate two-dimensionally along a band of ten contiguous hexagons inside a D-5h-C-80 cage, which is in sharp contrast to the three-dimensional circulation of two Ce atoms in an I-h-C-80 cage. The electronic properties were revealed by means of electrochemical measurements. The D-5h isomer of Ce-2@C-80 has a much smaller HOMOLUMO gap than cluster fullerenes (M3N@C-80, M=Sc, Tm, and Lu) with the same D-5h-C-80 cages. The chemical reactivity was investigated by using disilirane as a chemical probe. The high thermal reactivity toward 1,1,2,2-tetramesityl-1,2-disilirane is consistent with the trends of the redox potentials and the lower LUMO level of the D-5h isomer of Ce-2@C-80 compared with that of C-60.