An improbable monometallic cluster entrapped in a popular fullerene cage: YCN@C(s)(6)-C82

Abstract

Since the first proposal that fullerenes are capable of hosting atoms, ions, or clusters by the late Smalley in 1985, tremendous examples of endohedral metallofullerenes (EMFs) have been reported. Breaking the dogma that monometallofullerenes (mono-EMFs) always exist in the form of M@C2n while clusterfullerenes always require multiple (two to four) metal cations to stabilize a cluster that is unstable as a single moiety, here we show an unprecedented monometallic endohedral clusterfullerene entrapping an yttrium cyanide cluster inside a popular C82 cage--YCN@C(s)(6)-C82. X-ray crystallography and (13)C NMR characterization unambiguously determine the cage symmetry and the endohedal cyanide structure, unexpectedly revealing that the entrapped YCN cluster is triangular. The unprecedented monometallic clusterfullerene structure unveiled by YCN@C(s)(6)-C82 opens up a new avenue for stabilizing a cluster by a single metal cation within a carbon cage, and will surely stimulate further studies on the stability and formation mechanism of EMFs.

Figure 1. Drawing of the crystallographically determined structure of YCN@C_s(6)-C₈₂ with the major C₈₂ cage (0.55 occupancy) and the most abundant yttrium location (0.50 occupancy), and its relation to the Ni^II(OEP) molecule.

Thermal ellipsoids are set at 30% probability level. Solvent molecules, the minor cage and minor metal positions are omitted for clarity. Inset shows the triangular configuration of the internal YCN cluster to ensure a clear visualization. Red: Y; Blue: N; Grey: C.

Figure 2. ¹³C NMR (125 MHz) spectrum of YCN@C₈₂ showing a 38 × 2C (labeled by blue numbers), 6 × 1C (labeled by red numbers) pattern for the sp² carbon atoms of the cage.

The filled triangle marks the ¹³C nuclei signal of the internal YCN cluster, and the asterisk labels an unidentified impurity.

Figure 3. Cyclic voltammogram of YCN@C₈₂ in o-dichlorobenzene (o-DCB) solution with ferrocene (Fc) as the internal standard and tetrabutylamonium hexafluorophosphate (TBAPF₆) as supporting electrolyte.

Scan rate: 100 mV/s. Each redox step is marked with a number and a solid dot to aid comparison. The small peak at around −1.50 V is due to an unknown impurity. The asterisk labels the oxidation peak of ferrocene.