Steering Lu₃N clusters in C_76-78 cages: cluster configuration dominated by cage transformation

While the strong interaction between the internal unit and the fullerene cage inside metallofullerenes is widely acknowledged, how the cage transformation interacts with the cluster configuration remains elusive. For this purpose, we herein synthesized three metallofullerene molecules with an easy-to-compare cluster configuration and cage arrangement, namely Lu₃N@C(17 490)-C₇₆, Lu₃N@C₂(22 010)-C₇₈, and Lu₃N@D₃(5)-C₇₈. The three lutetium-based nitride clusterfullerenes (NCFs) with small C_76-78 carbon cages were synthesized by a modified arc-discharge method and their structures were unambiguously confirmed by X-ray crystallography. Notably, the cage transformation from C(17 490)-C₇₆ to C₂(22 010)-C₇₈via a simple C₂-unit insertion leads to a remarkable configuration change of the encapsulated Lu₃N cluster from an unusual asymmetric plane to a common symmetric one. This close correlation between the cluster configuration and cage transformation is further confirmed by the pyramidal Lu₃N cluster in Lu₃N@D₃(5)-C₇₈ other than the symmetric planar Lu₃N unit in Lu₃N@C₂(22 010)-C₇₈, as a result of an even larger difference in the cage arrangement. Astonishingly, such a cluster shrinkage, accompanied by an increase in the cage size from C(17 490)-C₇₆ to D₃(5)-C₇₈, is dramatically opposite to the cluster expansion with cage elongation found in La₂C₂- or Y₂C₂-based metallofullerenes.