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

Abstract

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.

Fig. 1. (a) MALDI-TOF mass spectrum of purified Ti₃C₃@I_h-C₈₀. The insets show the experimental and theoretical isotopic distributions of Ti₃C₈₃. (b) Vis-NIR absorption spectrum of Ti₃C₃@I_h-C₈₀ recorded in CS₂. A photograph of the corresponding solution is shown in the inset.

Fig. 2. ORTEP drawing of Ti₃C₃@I_h-C₈₀·[Ni^II(OEP)] with 15% thermal ellipsoids. Only the major Ti₃C₃ site is shown. Solvent molecules and H atoms are omitted for clarity.

Fig. 3. Orthogonal views showing the relative position of the major Ti₃C₃ site to the closest cage portions.

Fig. 4. (a) The optimized Ti₃C₃ configuration at the B3LYP/3-21G∼SDD level and (b) geometry configuration of the major site of the Ti₃C₃ cluster from X-ray results.

Fig. 5. CV curve of Ti₃C₃@I_h-C₈₀. Conditions: working electrode, glassy carbon electrode; counter electrode, Pt wire; reference electrode, Ag wire; supporting electrolyte, 0.05 M TBAPF₆ in o-DCB with ferrocene as the internal standard. Scan rate: 100 mV s^–1.