Development of the Organic Method for the Synthesis of Endohedral Fullerenes

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(1)The first endohedral fullerene encapsulating molecular hydrogen, H2@C60, was synthesized by so-called "molecular surgical approach". Thus, an 8-membered-ring orifice was formed on the surface of fullerene C60, via cycloaddition of a 1,2,4-triazine derivative to C60, followed by its enlargement into a 12-membered ring by oxidation with singlet oxygen. The opening was further enlarged into a 13-membered ring by insertion of sulfur into the rim of the orifice, and then high-pressure hydrogen was applied to afford an open-cage fullerene encapsulating H2 in 100%. The molecular hydrogen inside the cage was directly observed by means of MEM X-ray analysis using synchrotron radiation. Then the orifice was closed, while retaining the encapsulated hydrogen, by a four-step synthesis to afford H2@C60 in about 9% overall yield from the starting C60.(2)The similar treatment of C70 afforded H2@C70 and also a small amount, of (H2)2@C70. The motion of the two hydrogen molecules in the open-cage [70]fullerene was observed by variable temperature 1H NMR measurement.(3)The electronic interaction between the encapsulated hydrogen and the outer pi-system of fullerene cage was shown to be rather small by electronic measurement and also by 13C NMR spectroscopy.(4)Hydrogen isotopes, i.e., H2, HD, and D2, were also encapsulated into the derivatives of open-cage fullerene. D2@C60 was synthesized and the difference between H2@C60 was clarified.5) Helium was found to be encapsulated into the open-cage fullerene much more readily than hydrogen. [4] The closed-cage fullerene He@C60 was synthesized in the same way as that for H2@C60.