Crystallographic Characterization of Ti₂C₂@D_3h(5)‑C₇₈, Ti₂C₂@C_3v(8)‑C₈₂, and Ti₂C₂@C_s(6)‑C₈₂: Identification of Unsupported Ti₂C₂ Cluster with Cage-Dependent Configurations

Fullerene cages are ideal hosts to encapsulate otherwise unstable metallic clusters to form endohedral metallofullerenes (EMFs). Herein, a novel Ti₂C₂ cluster with two titanium atoms bridged by a C₂-unit has been stabilized by three different fullerene cages to form Ti₂C₂@D_3h(5)-C₇₈, Ti₂C₂@C_3v(8)-C₈₂, and Ti₂C₂@C_s(6)-C₈₂, representing the first examples of unsupported titanium carbide clusters. Crystallographic results show that the configuration of the Ti₂C₂ cluster changes upon cage variation. In detail, the Ti₂C₂ cluster adopts a butterfly shape in Ti₂C₂@C_3v(8)-C₈₂ and Ti₂C₂@C_s(6)-C₈₂ with Ti–C₂–Ti dihedral angles of 156.35 and 147.52° and Ti–Ti distances of 3.633 and 3.860 Å, respectively. In sharp contrast, a stretched planar geometry of Ti₂C₂ is observed in Ti₂C₂@D_3h(5)-C₇₈, where a Ti–C₂–Ti angle of 176.87° and a long Ti–Ti distance of 5.000 Å are presented. Consistently, theoretical calculations reveal that the cluster configuration is very sensitive to the cage shape which eventually determines the electronic structures of the hybrid EMF-molecules, thus adding new insights into modern coordination chemistry.