Steering Lu3N clusters in C76–78 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@Cs(17 490)-C76, Lu3N@C2(22 010)-C78, and Lu3N@D3h(5)-C78. The three lutetium-based nitride clusterfullerenes (NCFs) with small C76-78 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 Cs(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@D3h(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 Cs(17 490)-C76 to D3h(5)-C78, is dramatically opposite to the cluster expansion with cage elongation found in La2C2- or Y2C2-based metallofullerenes.