♦ Unveiling the Unique Structures and Chemical Reactivities of Endohedral Metallofullerenes

Fullerenes, spherical molecules made entirely of carbon, provide highly delocalized and widespread π-electron systems, being regarded as a fascinating element of novel functional materials. Intriguingly, encapsulation of metal atoms inside fullerene cages offers a new class of hybrid molecules, known as endohedral metallofullerenes (EMFs). The electron transfer from encaged metal atoms to the fullerene cages imparts the remarkable characteristics to EMFs. Our efforts have been devoted to unveiling their unique structures and chemical reactivities. Firstly, we have demonstrated that the metal encapsulation is able to stabilize a new type of sp²-carbon cage geometry. 2D-NMR spectroscopic and X-ray crystallographic analyses of dimetallofullerene Sc₂@C₆₆ disclose that the C₆₆ cage contains two sets of unsaturated linear triquinanes (ULTs), in which three pentagons abut one another and two scandium atoms are located within the folds of each of the ULT units. Theoretical calculations suggest that the coordination of the scandium atoms stabilize the anti-aromatic ULT units. The structural elucidation shows that new motifs in carbon cage geometry continue to be discovered and calls attention to the ULT unit, whose aromaticity and coordination chemistry deserve further study.
Secondly, we have shown that the radical reaction of two isomers of La₂@C₈₀, La₂@I_h-C₈₀ and La₂@D_5h-C₈₀, with 3-chloro-5,6-diphenyltriazine proceeded regioselectively to form isolable fullerenyl radicals. The unpaired electron, formed by the radical coupling reaction, is not distributed on the carbon cage but is confined to the internal metal—metal σ-bonding orbital, where an endohedral (La)₂5⁺ dimer is formed. The resulting fullerenyl radicals are stable in air despite their radical character, showing that the internally isolated distribution of the SOMO is an effective means to stabilize the fullerenyl radicals. We also found that the fullerenyl radical of La₂@I_h-C₈₀ reacts further with toluene under thermal conditions, to form a di-substituted derivative that is diamagnetic in solution. Remarkably, this benzylated derivative forms a paramagnetic dimer when it is crystallized. Our results demonstrate the unique ability of EMFs to stabilize an unpaired electron by localizing it on the internal metal atoms. Thirdly, we have found that the thermal reaction of La₂@C₇₂, which contains two pentalene units at opposite ends of the cage, with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine proceeded regioselectively to afford only two separable bisfulleroid isomers. The molecular structure of one isomer was determined using X-ray crystallography. The results indicate that the [4+2] cycloaddition was initiated at the C—C bond connecting two pentagons of C₇₂. Subsequent intramolecular electrocyclization followed by cycloreversion resulted in the formation of an open-cage derivative having three seven-membered ring orifices on the cage. The oxidation potentials of the open-cage derivatives were shifted more negative than those of pristine La₂@C₇₂, pointing out that further oxidation could occur easily in the derivatives with further enlargement of the orifices. In this respect, the reaction can be considered as the first step for the cage opening and eventual transplantation of metal atoms to EMFs for the creation of novel endohedral fullerenes. In conclusion, we believe that our comprehensive studies shed light on fundamental understanding of the unique structures and chemical reactivities of EMFs.