Open-cage fullerenes: Synthesis, structure, and molecular encapsulation

The carbon-carbon bond scission of the fullerene cage offers an open-cage fullerene derivative having an opening on the fullerene surface. This account summarizes our recent studies on the syntheses of open-cage C-60 derivatives as well as their properties of molecular encapsulation. The photochemical rearrangement of the cyclohexadiene derivative of C-60 gives bis(fulleroid), the precursor for the cage scission. The photooxygenative carbon-carbon bond cleavage of bis(fulleroid) affords an open-cage diketone derivative having a 12-membered ring. The reaction of the diketone derivative with aromatic hydrazine or hydrazone allows the ring expansion of the orifice by regioselective cage scission to yield a product bearing a 16-membered ring. The orifice in the product is large enough to pass a hydrogen molecule, producing an endohedral hydrogen complex. In the case of the reaction with ortho-phenylenediamine, two carbon-carbon bonds are sequentially cleaved to afford a product having a 20-membered ring orifice. This product spontaneously encapsulates one water molecule to form an endohedral water complex.