Crystallographic identification of Eu@C_2n (2n = 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, C₂(27)-C₈₈, C₁(7)-C₈₆, C₂(13)-C₈₄ and C₂(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 C₂(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.

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Introduction

Fullerenes are a collection of spherical pure-carbon molecules consisting of exactly 12 pentagons and a variable number of hexagons.¹ Although C₆₀ and C₇₀ are abundantly produced and have received extensive attention,² pure isomers of higher fullerenes (C_2n, 2n > 70) are rarely reported because of their low solubility in organic solvents and the tremendous number of possible isomers that may exist in reality.³ Alternatively, endohedral metal-doping has had great success in stabilizing higher fullerenes to produce a new family of hybrid molecules named endohedral metallofullerenes (EMFs).^4,5 Up to now, hundreds of EMF-isomers have been isolated and structurally characterized.^4–7 Their cage sizes range from C₆₀ to even C₁₀₈ and the metallic species are also diverse and can be one or two pure metal atoms or a cluster of metal carbides, nitrides, oxides, sulfides and even cyanides.^4–7 It is noteworthy that although many possible cage structures are available for a given C_2n,³ the experimentally obtained species are limited to a small number of compounds.^4–7 For instance, as many as seven isomers that obey the ‘isolated pentagon rule’ (IPR)⁸ are available for C₈₀,³ but only the I_h-C₈₀ and D_5h-C₈₀ cages have been found for M₃N@C₈₀,^4,9–11 and C_2v(5)-C₈₀ has been found for Sc₂C₂@C₈₀.^12,13

It is certainly of special interest to understand the inter-cage transformation between isolated fullerene/EMF isomers which is key to clarifying the formation mechanism of fullerenes. In the very beginning of fullerene research, the structural transition from C₆₀ to C₇₀ was described like this: C₆₀ is cut in half, and one half is rotated by 36 degrees relative to the other, and then a 10-carbon ring is added in between, and thus C₇₀ forms.¹⁴ Obviously, this route is too complicated to happen in reality. In fact, both theoretical and experimental results suggest that the transformations between fullerene isomers may involve merely two processes: Stone–Wales transformation (SWT)¹⁵ and C₂-extrusion or insertion (Fig. 1). Theoretical calculations suggest that the energy barrier of such facile transformations might be easily overcome in the very hot environments of fullerene formation¹⁶ and the reported transformation reactions are favored in terms of entropy.¹⁷ The proposed SWT and C₂ elimination/addition could generate labile pentalene/heptagon containing intermediates, which has been demonstrated by the experimental identification of heptagon^18,19 and fused-pentagon^20–26 containing metallofullerenes with the endohedral metallic unit or exohedral functionalized moieties^27–29 as stabilizers.

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.

During recent years, more and more higher-fullerene cages have been captured and structurally identified, mainly in the form of EMFs which enables in-depth investigation of the inter-cage transformations among existing fullerene cages.^4,5 For example, Balch and co-workers identified Sc₂S@C_s(6)-C₈₂ and Sc₂S@C_3v(8)-C₈₂ and proposed the interconversion process between C_s(6)-C₈₂ and C_3v(8)-C₈₂ through two SWTs³⁰ with the commonly encountered C_2v(9)-C₈₂ as the intermediate. In another work, they found that four Sm@C₉₀ isomers can be related pairwise to one another through sequential SWTs.³¹ Besides, Feng et al. revealed that Sc₂O@C_2v(3)-C₇₈ and Sc₂O@D_3h(5)-C₇₈ are closely related via a single SWT transformation.³² A more brilliant study was from Dorn and co-workers who captured and identified M₂C₂@C₁(51383)-C₈₄ (M = Y and Gd) which can transform into the other seven smaller cages, namely C₁(51383)-C₈₄, C_3v(8)-C₈₂, C_2v(9)-C₈₂, C_s(6)-C₈₂, C_s(39663)-C₈₂, I_h(7)-C₈₀ and D_5h(6)-C₈₀, implying a ‘top-down formation mechanism’ for fullerenes.²⁶ In that report, the authors also provided concrete mass spectrometric results to confirm that the formation route of empty fullerenes is the same as that of EMFs.²⁶ Recently, our group reported that the defective C₂(816)-C₁₀₄ cage can change to the other three ideal cages, namely D₅(450)-C₁₀₀, C_s(574)-C₁₀₂ and D_3d(822)-C₁₀₄, via multiple SWTs and C₂-extrusion.³³ It is clear that the transformation scheme of existing fullerenes/EMFs is rather fragmentary with the entire map remaining far from complete.

Herein, we have made a solid step towards completing the transformation map of existing metallofullerenes by capturing four key cages, namely, C₂(27)-C₈₈, C₁(7)-C₈₆, C₂(13)-C₈₄ and C₂(11)-C₈₄ in the form of europium-containing metallofullerenes. These four cages are key links in the map that covers up to 98% of known metallofullerenes with a C_2n (2n = 74–86) cage through very facile transformation routes. In addition, we propose that C₂(27)-C₈₈ is a starting point of the map, and is not obtainable from any of the existing C₉₀ isomers through facile steps, but is a possible product of self-assembly of a graphene fragment.

Results and discussion

Soot containing europium metallofullerenes was synthesized by vaporizing graphite rods containing a mixture of Eu₂O₃ and graphite powder (molar ratio of Eu/C = 1 : 50) under 300 mbar helium in an arc-discharge chamber. The soot was collected and extracted with carbon disulfide. Upon solvent removal, the crude mixture was redissolved in toluene and subjected to multi-stage high performance liquid chromatographic (HPLC) separation (Fig. S1†). The purity of the isolated Eu@C_2n (2n = 88, 86 and 84) isomers is confirmed by both HPLC (Fig. 2a) and laser desorption/ionization time-of-flight (LDI-TOF) mass spectroscopic analyses (Fig. 2b). The vis-NIR absorption spectra of the four compounds are shown in Fig. 3. All show obvious absorptions in the range from 400 to 2000 nm with relatively large onsets, suggesting small optical bandgaps (<1.0 eV) and correspondingly low thermodynamic stability. More details are listed in Fig. S2 and Table S1.†

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.

The molecular structures of the four europium-containing metallofullerenes are unambiguously determined by X-ray crystallography as Eu@C₂(27)-C₈₈, Eu@C₁(7)-C₈₆, Eu@C₂(13)-C₈₄ and Eu@C₂(11)-C₈₄. Notably, this is the first crystallographic identification of europium-containing EMFs since they were first reported over two decades ago.³⁴ More significantly, the C₂(27)-C₈₈ and C₁(7)-C₈₆ cages are unprecedented even without theoretical prediction.

Fig. 4 shows the molecular structures of these four endohedrals co-crystallized with Ni^II(OEP) (OEP is the dianion of octaethylporphyrin). The porphyrin moiety faces a relatively flat region of each cage with the shortest Ni-to-cage-carbon distances ranging from 2.825 to 2.926 Å, indicative of π–π interactions. In the case of Eu@C₂(27)-C₈₈ and Eu@C₁(7)-C₈₆, each fullerene cage is surrounded by two nonparallel Ni^II(OEP) molecules in a sandwich-like arrangement and the ethyl groups of one Ni^II(OEP) molecule face towards opposite sides to enhance the π–π interactions. In contrast, only one Ni(OEP) molecule is required to assist in the crystallization of the smaller Eu@C₂(13)-C₈₄ or Eu@C₂(11)-C₈₄ molecules. These results demonstrate that the stacking mode in the co-crystals depends on the shape and size of the endohedral. The Eu atom in each compound shows some degree of disorder (Fig. S3†), suggesting a motional behavior.

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.

Based on these four new cages, we are now able to complete a transformation map of existing metallofullerenes which includes up to 98% of the reported metallofullerenes (metal@C_2n, 2n = 74–86) with clear cage structures (176 compounds in total; see Table S2† for details). More importantly, the inter-cage transformations require only one or two well-established steps (Stone–Wales transformation or C₂ extrusion/insertion). Note that the transformation map matches with both the bottom-up and top-down formation mechanisms. For the ease of explanation, we follow a top-down manner in the map and context.

Fig. 5 depicts the transformation map and the detailed pathways are illustrated in the ESI (Fig. S4–S41).† The transformation from C₂(27)-C₈₈ to C₁(7)-C₈₆ is straightforward by a direct [5,6]-C₂ loss and a subsequent SWT. Then a [5,6]-C₂ loss and a SWT on C₁(7)-C₈₆ generate C₂(13)-C₈₄. A SWT on C₂(13)-C₈₄ affords C₁(12)-C₈₄ and a further SWT on the latter produces C₂(11)-C₈₄. Two SWTs convert C₂(11)-C₈₄ and C₁(12)-C₈₄ into D_3d(19)-C₈₄ and D_2d(23)-C₈₄, respectively. Elimination of a C₂-unit from a pentalene unit generated by a SWT on C₁(7)-C₈₆ produces the non-IPR missing link C₁(51383)-C₈₄.²⁶ A subsequent SWT on C₁(51383)-C₈₄ gives another non-IPR C_s(51365)-C₈₄.²⁶

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).†

The transformations from C₈₄ to C₈₂ and the isomerization between C₈₂ cages are rather clear. A [5,6]-C₂ loss with a subsequent SWT on C₂(13)-C₈₄ produces C₂(5)-C₈₂ while a SWT with a [5,5]-C₂ loss converts C₂(13)-C₈₄, C₁(12)-C₈₄ and C₂(11)-C₈₄ into C_3v(8)-C₈₂, C_2v(9)-C₈₂ and C_s(6)-C₈₂, respectively. Alternatively, elimination of the remaining pentalene unit on C₁(51383)-C₈₄ and C₁(51365)-C₈₄ affords C_s(6)-C₈₂ and C_2v(9)-C₈₂, respectively.²⁶ A [5,6]-C₂ extrusion plus an additional SWT on C₁(51383)-C₈₄ generates the non-IPR C_s(39663)-C₈₂ as well as C_3v(8)-C₈₂ which are related with a single SWT.²⁶

Furthermore, C_2v(5)-C₈₀ is obtainable via one SWT and one subsequent [5,5]-C₂ extrusion from either C₂(5)-C₈₂ or C_3v(7)-C₈₂, while D_5h(6)-C₈₀ is generated from C_s(6)-C₈₂ through a [5,6]-C₂ loss and a subsequent SWT or from C_s(39663)-C₈₂via merely a [5,5]-C₂ elimination.²⁶ Besides, the C_3v(8)-C₈₂ cage can shrink into the popular I_h(7)-C₈₀ cage via a [5,6]-C₂ loss plus a SWT²⁶ or into the heptagon-containing C_s(hept)-C₈₀ cage¹⁸via merely a [5,6]-C₂ extrusion. Structural transformation can also be demonstrated among three isomeric C₈₀ cages (D_5h(6)-C₈₀, C_2v(5)-C₈₀ and C_2v(3)-C₈₀) via merely one SWT.

In further steps, C₈₀ can shrink to smaller fullerene cages such as C₇₈, C₇₆ and C₇₄. A SWT plus a [5,5]-C₂ loss converts C_2v(5)-C₈₀, C_2v(3)-C₈₀ and I_h(7)-C₈₀ into D_3h(5)-C₇₈, C_2v(3)-C₇₈ and C₂(22010)-C₇₈, respectively. Alternatively, the non-IPR C₂(22010)-C₇₈ is also obtainable from C_s(hept)-C₈₀via a [5,5]-C₂ loss. The transformation between D_3h(5)-C₇₈ and C_2v(3)-C₇₈ can be realized by only one SWT.³² Furthermore, T_d(2)-C₇₆ and the non-IPR C_2v(19138)-C₇₆ are formed with D_3h(5)-C₇₈ (ref. 32) and C_2v(3)-C₇₈ as their respective precursors through a SWT plus a [5,5]-C₂ elimination. Both T_d(2)-C₇₆ and C_2v(19138)-C₇₆ can transform into D_3h(1)-C₇₄via a SWT and a [5,5]-C₂ extrusion. Thus, the transformations among up to 98% of the identified C_2n (2n = 86–74) cages employed by EMFs have been uncovered with the inter-cage transformations involving merely one or two well-established steps (Fig. 1). Depending on the encapsulated species, different transformation routes can be expected.²⁶ However, it should also be mentioned that the encaged metallic unit may change during the transformation process. For example, Y₂@C₈₄ is shown to encapsulate a C₂ unit inside the cage to form Y₂C₂@C_3v(8)-C₈₂.³⁵

One may think that the top C₂(27)-C₈₈ cage is possibly converted from a C₉₀ cage. However, although as many as six isomeric C₉₀ cages (C₁(21)-C₉₀, C₂(40)-C₉₀, C₂(42)-C₉₀, C₂(45)-C₉₀, C_2v(46)-C₉₀ and C₂(41)-C₉₀) have been identified in the form of EMFs,^31,36–38 none of them could transform into the C₂(27)-C₈₈ cage via the defined facile routes (Fig. 1). However, our topological analysis reveals that the C₂(27)-C₈₈ cage may be directly obtained by the self-assembly of a graphene fragment (Fig. 6), indicating the top-down formation process of fullerenes. Another argument for the unique role of the C₈₈ cage is the templating effect of Nd₃N leading to the formation of M₃N@C₈₈, while the smaller Sc₃N and the larger La₃N clusters adopt the C₈₀ and C₉₆ cages, respectively.^9,39,40 This interesting C₈ interval (C₉₆–C₈₈–C₈₀) phenomenon is further corroborated by the fact that the defective C₂(816)-C₁₀₄ cage (obtained in the form of La₂C₂@C₂(816)-C₁₀₄) can change to the other three ideal cages, namely D₅(450)-C₁₀₀, C_s(574)-C₁₀₂ and D_3d(822)-C₁₀₄via facile routes.³³ Accordingly, we propose that the C₈ interval may play an important role in determining the starting points of metallofullerene transformation. In this regard, the missing transformation between C₇₄ and C₇₂ in the map can be understood by considering a C₈ interval between C₈₀ and C₇₂, which implies that C₇₂ may be included in a different map. We acknowledge that the experimentally observed C_2v(9)-C₈₆, D₃(19)-C₈₆, D₂(35)-C₈₈ and C_s(hept)-C₈₈ are not included in our map, which might be a sign of the presence of other transformation routes¹⁹ and hence uncovering new transformation links is still of vital importance.

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

Conclusions

In summary, we have captured and crystallographically identified four fullerene cages, namely, C₂(27)-C₈₈, C₁(7)-C₈₆, C₂(13)-C₈₄ and C₂(11)-C₈₄, in the form of europium-containing metallofullerenes. With their accession, a transformation map covering as many as 98% of the experimentally identified EMFs with clear C_2n (2n = 86–74) cage structures is presented, in which the inter-cage transformations are limited to merely one or two very favorable steps. Structural analysis demonstrates that the unique C₂(27)-C₈₈ cage may serve as the key point in the map and is directly transformable from a graphene fragment. Completing the transformation map is key to solving the long-standing puzzle of fullerene formation and could shed light on the construction and applications of novel nano-carbon molecules.

Experimental

Raw soot containing europium-metallofullerenes was synthesized by a direct-arc discharge method and extracted with carbon disulfide. After solvent removal, the mixture was dissolved in toluene and subjected to multi-stage high performance liquid chromatographic (HPLC) separation which gave the desired compounds (Fig. S1†).

Black crystals were obtained by slow diffusion of a benzene solution of Ni^II(OEP) into a carbon disulfide solution of each metallofullerene over four weeks. Single-crystal X-ray data were collected at 100 K using synchrotron radiation (0.65250 Å) with a MarCCD detector at beamline BL17B of the Shanghai Synchrotron Radiation Facility. A multi-scan method (SADABS) was used for absorption corrections. The structures were solved with direct methods and were refined with SHELXL-2016.⁴¹

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the NSFC (No. 51472095 and 51672093) is gratefully acknowledged. We acknowledge the staff at the BL17B beamline of the National Center for Protein Sciences Shanghai (NCPSS) at the Shanghai Synchrotron Radiation Facility for assistance during data collection, and the Analytical and Testing Center in the Huazhong University of Science and Technology for all related measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Spectroscopic results, crystal structures, metallofullerenes included in the transformation map, and transformation routes. CCDC 1578319–1578322. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc04906h

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