Crystallographic Characterization of Ti₂C₂@<i>D</i>_3<i>h</i>(5)-C₇₈, Ti₂C₂@<i>C</i>_3<i>v</i>(8)-C₈₂, and Ti₂C₂@<i>C</i>_<i>s</i>(6)-C₈₂: Identification of Unsupported Ti₂C₂ Cluster with Cage-Dependent Configurations

Abstract

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@<i>D</i>_3<i>h</i>(5)-C₇₈, Ti₂C₂@<i>C</i>_3<i>v</i>(8)-C₈₂, and Ti₂C₂@<i>C</i>_<i>s</i>(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@<i>C</i>_3<i>v</i>(8)-C₈₂ and Ti₂C₂@<i>C</i>_<i>s</i>(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@<i>D</i>_3<i>h</i>(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.