Theoretical Insight for the Metal Insertion Pathway of Endohedral Alkali Metal Fullerenes

We have investigated the mechanism of alkali metal incorporation into C60 fullerene by density functional theory (DFT) at the UB3LYP/6-31G* level of theory. Calculations were performed to study the insertion pathways of Li(+), Na(+), and K(+) through six- or five-membered rings of fullerene, and the computed energy barriers of metal ion insertion are compared with the available experimental data. Between the two possible insertion pathways, metal ion insertion through [2 + 2 + 2] ring opening of the six-membered ring is found to be more favored than the insertion through the ring opening of the five-membered ring. The size of the ring openings generated by the three metal ions is likely to be correlated with their ionic size, which shows the smallest opening for Li(+) and the largest for K(+) cation. The insertion energy barriers of the ions are found to be increased in the order of Li(+) < Na(+) < K(+) in line with the experimental results. The ring opening made by breaking of C-C bonds during the metal ion insertion in six- or five-membered rings can cause the ring to be rearranged and convert back into a closed fullerene cage to form a stable endohedral metal-fullerene complex.