An experimental and theoretical study of LuNC@C76,82 revealing a cage-cluster selection rule

Abstract

Monometallic cyanide clusterfullerenes (CYCFs) with a chemical formula of MNC@C_2n represent a new type of clusterfullerene discovered in recent years; the encapsulated metal types are still limited to yttrium and terbium thus far. This makes the stabilization mechanism of the host fullerene cages upon the variation of internal MNC clusters unclear. Herein, for the first time, four novel Lu-based CYCFs, namely LuNC@C_2v(19138)-C₇₆, LuNC@C₂(5)-C₈₂, LuNC@C_s(6)-C₈₂, and LuNC@C_2v(9)-C₈₂, have been successfully obtained and characterized using a combination of mass spectrometry, vis-NIR, single-crystal X-ray diffractometry, and cyclic voltammetry. In particular, the X-ray results of LuNC@C_2v(19138)-C₇₆ and LuNC@C₂(5)-C₈₂ unambiguously demonstrated that the internal LuNC unit assumes a nearly linear configuration in the small C₇₆ cage but a confined triangle in the large C₈₂ one, demonstrating the critical role of the cage structure in regulating the cluster configuration. Further theoretical calculations reveal that the linear and triangular configurations of LuNC clusters are governed by the strong and weak Lu-cage coordination interactions in the C_2v(19138)-C₇₆ and C₂(5)-C₈₂ cages, respectively. In addition, the exact positions of the internal N and C atoms inside the two cages were well defined by combining the crystallographic data with theoretical calculations, showing that the N atom always resides closer to the metal atom to pursue stronger electrostatic interactions.