Mapping the metal positions inside spherical C80 cages: crystallographic and theoretical studies of Ce2@D(5h)-C80 and Ce2@I(h)-C80.

The dynamic positions of the dimetallic cluster inside the mid-sized spherical cages of C(80)-C(82) have been seldom studied, despite the high abundance of M(2)@C(2n) (2n = 80, 82) species among various endohedral metallofullerenes. Herein, using crystallographic methods, we first unambiguously map the metal positions for both Ce(2)@D(5h)-C(80) and Ce(2)@I(h)-C(80), showing how the symmetry or geometrical change in cage structure can influence the motional behavior of the cluster. Inside the D(5h) cage, the primary cerium sites have been identified along a cage belt of the contiguous hexagons, which suggests the significant influence of such a cage motif on endohedral cluster motion. Further analysis revealed a distorted D(5h) cage owing to the "punch-out" effect of cerium atoms. The consequence is the presence of two localized electrostatic potential minima inside the cage of (D(5h)-C(80))(6-), thus reflecting the primary ionic cerium-cage interaction. In contrast, a different motional behavior of Ce(2) cluster was observed inside the I(h) cage. With the major cerium sites, the molecule of Ce(2)@I(h)-C(80) presented an approximate D(2h) configuration. With the combined theoretical study, we propose that the additional unidentified influence of Ni(II) (OEP) (OEP = octaethylporphyrin) might be also relevant for the location of cerium sites inside the I(h) cage.