Crystallographic identification of Eu@C2 n (2 n = 88, 86 and 84): completing a transformation map for existing metallofullerenes

Abstract

Revealing the transformation routes among existing fullerene isomers is key to understanding the formation mechanism of fullerenes which is still unclear now because of the absence of typical key links. Herein, we have crystallographically identified four new fullerene cages, namely, C ₂(27)-C₈₈, C ₁(7)-C₈₆, C ₂(13)-C₈₄ and C ₂(11)-C₈₄, in the form of Eu@C_2n , which are important links to complete a transformation map that contains as many as 98% (176 compounds in total) of the reported metallofullerenes with clear cage structures (C_2n , 2n = 86-74). Importantly, the mutual transformations between the metallofullerene isomers included in the map require only one or two well-established steps (Stone-Wales transformation and/or C₂ insertion/extrusion). Moreover, structural analysis demonstrates that the unique C ₂(27)-C₈₈ cage may serve as a key point in the map and is directly transformable from a graphene fragment. Thus, our work provides important insights into the formation mechanism of fullerenes.

Fig. 1. Transformation pathways of fullerenes involving (a) one SWT, (b) one SWT and one subsequent [5,5]-C₂ extrusion or (c) one [5,6]-C₂ elimination and one subsequent SWT. For the ease of explanation, we follow a top-down manner in the figure and context.

Fig. 2. (a) HPLC chromatograms and (b) LDI-TOF mass spectra of purified Eu@C₂(27)-C₈₈, Eu@C₁(7)-C₈₆, Eu@C₂(13)-C₈₄ and Eu@C₂(11)-C₈₄. HPLC conditions: Buckyprep column (φ = 4.6 × 250 mm); eluent, toluene; flow rate, 1.0 mL min^–1; detection wavelength, 330 nm; r.t. Insets show the theoretical and experimental isotopic distributions of the corresponding metallofullerenes.

Fig. 3. Vis-NIR absorption spectra of Eu@C₂(27)-C₈₈, Eu@C₁(7)-C₈₆, Eu@C₂(13)-C₈₄ and Eu@C₂(11)-C₈₄ dissolved in CS₂. The curves are vertically shifted for the ease of comparison.

Fig. 4. Ortep drawings of (a) Eu@C₂(27)-C₈₈, (b) Eu@C₁(7)-C₈₆, (c) Eu@C₂(13)-C₈₄ and (d) Eu@C₂(11)-C₈₄ with thermal ellipsoids set at a 20% probability level. Only one cage orientation and the major metal sites are shown. Solvent molecules, the minor metal sites and H atoms are omitted for clarity.

Fig. 5. Transformations among existing metallofullerenes with C₂(27)-C₈₈ as the starting top point. As many as 98% of metallofullerenes from C₈₆ to C₇₄ with known structures are included in the map. The rearrangement pathways involve one or two well-established steps. IPR cages are marked in green and non-IPR cages are marked in yellow. The cages reported in this work are highlighted with red circles. The blue one-way arrow indicates a [5,6]-C₂ loss with a subsequent SWT while the green one-way arrow refers to a SWT followed by a [5,5]-C₂ loss. The yellow one-way arrow corresponds to merely one C₂ loss while the aqua blue two-way arrow indicates a SWT and the dashed aqua blue two-way arrow refers to two SWTs. The detailed transformation pathways are illustrated in the ESI (Fig. S4–S41).

Fig. 6. Proposed formation of C₂(27)-C₈₈via self-assembly of a graphene fragment.