Molecular surgical synthesis of H₂@C₆₀: recollections.

The first synthesis of endohedral fullerene containing molecular hydrogen, H₂@C₆₀, is briefly summarized. The synthesis was conducted according to what we call the 'molecular surgical method', that is, opening a hole on a C₆₀ surface, enlargement of the hole, insertion of a guest species and enclosure of the hole without loss of the encapsulated guest. The entire process involves three chemical reactions to open the hole and four reactions to gradually reduce the hole size and finally close the hole. The total yield of the product, H₂@C₆₀, based on consumed C₆₀ was 9%. The encapsulated molecule of hydrogen exhibited a 6 ppm upfield-shifted ¹H NMR signal when compared with free hydrogen, indicating the aromaticity at the inner centre of the C₆₀ cage.