Trapping an unprecedented Ti₃C₃ unit inside the icosahedral C₈₀ fullerene: a crystallographic survey

The sub-nanometer cavity of fullerene cages is an ideal platform to accommodate otherwise unstable species for accurate structural characterization with, for example, rather accurate single crystal X-ray diffraction (XRD) crystallography. Herein, we report the successful entrapment of an isolated Ti₃C₃ moiety inside the icosahedral-C₈₀ cage to form Ti₃C₃@I_h-C₈₀ via an arc-evaporation process in the gas phase. The single crystal XRD crystallographic results unambiguously reveal that the C₃-unit adopts an unprecedented cyclopropane-like structure which coordinates with the three titanium atoms in an unexpected fashion where the triangular C₃-unit is nearly perpendicular to the Ti₃-plane. The intercalation of a cyclopropanated C₃-unit into the titanium layer is thus unambiguously confirmed. The theoretical results reveal that the Ti₃C₃ cluster transfers six electrons to the I_h-C₈₀ cage so that each titanium atom has a positive charge slightly above +2 and the C₃-unit is negatively charged with about -1. It is noteworthy that this is the first observation of the cyclopropane-coordination fashion in any reported organometallic complex, providing new insights into coordination chemistry.