An unprecedented C₈₀ cage that violates the isolated pentagon rule

Abstract

Two Lu₂O@C₈₀ isomers have been successfully isolated and unambiguously assigned as Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀, respectively, by X-ray crystallography. Interestingly, C₁(31876)-C₈₀ is an unprecedented cage with a pair of adjacent pentagons, which can be closely connected with C_2v(5)-C₈₀via a two-step Stone–Wales transformation. More importantly, the C₁(31876)-C₈₀ cage is a key point in the transformation map of oxide cluster fullerenes, filling the vacancy in the formation process.

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Introduction

Endohedral metallofullerenes (EMFs) are a collection of novel molecules formed by the encapsulation of metal ions or clusters in fullerene cages.^1,2 One of the most prominent features of these compounds is the charge transfer from the inner metallic species to the outer cages, thus stabilizing many cages that are unstable as empty fullerenes.³ For example, D_5d(1)-C₈₀ and D₂(2)-C₈₀ were separated as empty cages, but the encapsulation of M₃N clusters accompanied by 6-electron transfer (M = Lu, Dy) promoted the formation of D_5h(6)-C₈₀ and I_h(7)-C₈₀.^4–8 In particular, the pristine cages of metallofullerenes can violate the isolated pentagon rule (IPR) that all empty fullerenes must obey because local steric strain on the adjacent pentagons (pentalene unit) can be released through interaction with the encapsulated metal.^9–12 Since the first isolation of Sc₂@C_2v(4348)-C₆₆ and Sc₃N@D₃(6140)-C₆₈ in 2000, endohedrals that do not obey the IPR have received wide attention due to their unusual structures.^13,14

The formation mechanism of fullerenes remains unclear and controversial because their formation process cannot yet be directly observed. Fortunately, comprehensive inter-cage transformation pathways with cages containing pentalene units as key points are conducive to understanding the rearrangement process of fullerenes.¹⁵ Dorn et al. have shown that the unique asymmetric cage of M₂C₂@C₁(51383)-C₈₄ (M = Y, Gd) is a “missing link” in well-established conversion pathways to form many high-symmetry fullerene cages, which provides structural evidence for the top-down mechanism.¹⁶ Meanwhile, the bottom-up mechanism has also been well developed; Echegoyen et al. suggested that the nonclassical Sc₂C₂@C_s(hept)-C₈₈ with a heptagon could be obtained from Sc₂C₂@C_2v(9)-C₈₆ through a simple C₂ insertion.¹⁷ Recently, the interconversions between all isolated uranium-based mono-metallofullerenes reported by Chen et al. demonstrated that some cages with a pentalene unit can serve as precursors to form larger or smaller cages, indicating the simultaneous top-down and bottom-up processes.^18,19 Accordingly, disclosing new cages that violate the isolated pentagon rule is crucial for completing the transformation map and revealing the formation mechanism of fullerenes.

Herein, we report the synthesis and isolation of two C₈₀ isomers containing a Lu₂O cluster. X-ray crystallographic results unambiguously reveal their molecular structures as Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀, respectively. It is rather surprising that a Lu₂O@C₁(31876)-C₈₀ isomer with a pair of adjacent pentagons has a relatively high yield, and its cage can be interconverted to the C_2v(5)-C₈₀ cage through a two-step Stone–Wales transformation. Importantly, with the discovery of the unique C₁(31876)-C₈₀, a transformation map including the major cages of the reported dimetallic oxide cluster fullerenes is completed.

Results and discussion

Lutetium-based metallofullerenes were synthesized in an arc-discharge reactor under a He/CO₂ atmosphere (270/15 Torr).²⁰ After ultrasonic extraction in carbon disulfide (CS₂), multistage high-performance liquid chromatography (HPLC) separation gave pure Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀ samples (Fig. S1†). The analytical HPLC chromatograms and the laser-desorption ionization time-of-flight (LDI-TOF) mass spectra confirmed their high purity (Fig. S2†).

Absorption measurements were carried out for the two Lu₂O@C₈₀ isomers (Fig. 1a), which present obviously different characteristic absorptions in CS₂. In detail, the Lu₂O@C₁(31876)-C₈₀ isomer shows seven absorption peaks at 531, 592, 640, 834, 882, 940, and 1376 nm, whereas for Lu₂O@C_2v(5)-C₈₀, six absorption peaks at 470, 618, 651, 688, 834, and 1399 nm are observed. Meanwhile, the absorption onsets of Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀ are around 1459 and 1600 nm, corresponding to small optical band gaps of 0.85 and 0.78 eV, respectively. The absorption features of Lu₂O@C_2v(5)-C₈₀ resemble those of Sc₂O@C_2v(5)-C₈₀ ²¹ and Sc₂C₂@C_2v(5)-C₈₀,²² indicating a four-electron transfer from the Lu₂O cluster to the cage.

Fig. 1 (a) Vis-NIR absorption spectra of (I) Lu₂O@C₁(31876)-C₈₀ and (II) Lu₂O@C_2v(5)-C₈₀ in CS₂ at room temperature (concentration: 0.2 mg mL⁻¹). Inset: Photograph of the corresponding sample solutions. (b) Cyclic voltammograms of Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀ in 0.05 M TBAPF₆/o-DCB solution (scan direction: negative; scan rate: 100 mV s⁻¹).

The electrochemical properties of the Lu₂O@C₈₀ isomers were studied by cyclic voltammetry (Fig. 1b) and their redox potentials are summarized in Table 1 along with those of Sc₂O@C_2v(5)-C₈₀. In the anodic region, both Lu₂O@C₈₀ isomers exhibit two fully reversible oxidation processes. The first and second oxidation potentials of Lu₂O@C₁(31876)-C₈₀ are very close to the corresponding potentials of Lu₂O@C_2v(5)-C₈₀. On the other hand, in the cathodic region, four and five reversible reduction processes are observed for Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀, respectively. The first reduction potential of Lu₂O@C₁(31876)-C₈₀ shifts by −240 mV relative to that of Lu₂O@C_2v(5)-C₈₀. Consequently, the electrochemical gap of Lu₂O@C₁(31876)-C₈₀ (1.14 V) is larger than that of Lu₂O@C_2v(5)-C₈₀ (0.95 V), which is in line with the absorption results. These results indicate that the difference in the cage structure has a considerable effect on the electrochemical behavior of the two molecules, especially on their lowest unoccupied molecular orbital energy, which is also reflected in the frontier molecular orbital analysis (Fig. S5†). Moreover, the redox behavior of Lu₂O@C_2v(5)-C₈₀ resembles that of Sc₂O@C_2v(5)-C₈₀,²¹ indicating their similar electronic structure. Thus, it can be concluded that the outer cage structure instead of the metallic unit has important influences on the electrochemical behavior of dimetallic oxide cluster fullerenes.

Table 1 Redox potentials (V vs. Fc/Fc⁺) and electrochemical bandgaps of Lu₂O@C₁(31876)-C₈₀, Lu₂O@C_2v(5)-C₈₀ and Sc₂O@C_2v(5)-C₈₀

Species	^ox E 2	^ox E 1	^red E 1	^red E 2	^red E 3	^red E 4	^red E 5	ΔEgap ^c	Ref.
a Half-wave potential in volts (reversible redox process). b Peak potential in volts (irreversible redox process). c ΔEgap = ^oxE1 − ^redE1.
Lu2O@C1(31876)-C80	0.53^a	0.15^a	−0.99^a	−1.38^a	−2.05^a	−2.36^a		1.14	This work
Lu2O@C2v(5)-C80	0.56^a	0.20^a	−0.75^a	−1.12^a	−1.44^a	−1.78^a	−2.19^a	0.95	This work
Sc2O@C2v(5)-C80	0.56^a	0.24^a	−0.89^b	−1.48^b	−1.75^b	−1.96^b	−2.13^b	1.13	20

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High-quality co-crystals of the two Lu₂O@C₈₀ isomers and Ni^II(OEP) (OEP = 2,3,7,8,12,13,17,18-octaethylporphin dianion) were obtained by layering a benzene solution of Ni^II(OEP) over a CS₂ solution of each endohedral. Their molecular structures were unambiguously determined by single-crystal X-ray diffraction (XRD) crystallography as Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀, respectively. In particular, this is the first identification of the C₁(31876)-C₈₀ cage, which contains a pair of adjacent pentagons. The details of the crystallographic data are listed in Table S1.† The refined structures of Lu₂O@C₁(31876)-C₈₀·Ni^II(OEP) and Lu₂O@C_2v(5)-C₈₀·Ni^II(OEP) with the major metal sites are shown in Fig. 2. The porphyrin moiety leans towards the relatively flat region of each fullerene cage with the shortest Ni-to-cage-carbon distance of 2.959 Å and 2.899 Å for Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀, respectively, indicating substantial π–π interactions between the cluster fullerenes and Ni^II(OEP) molecules.²³ Inside the two fullerene cages, the central oxygen atoms are both fully ordered, but the Lu atoms exhibit a certain degree of disorder (Fig. S3 and Table S2†), implying the motional behavior of the Lu atoms. Interestingly, it was found that one Lu atom prefers to vibrate near the adjacent pentagons in Lu₂O@C₁(31876)-C₈₀, which is similar to the motion of metal atoms in the IPR-violating Dy₂O@C₂(13333)-C₇₄.²⁴ This phenomenon implies that the stabilization of cages that violate the isolated pentagon rule requires sufficient metal–pentalene coordination interactions.²⁵ For the major metal sites (Fig. S4 and Table S3†), the average distance between Lu2 and the pentalene motif in Lu₂O@C₁(31876)-C₈₀ is 2.389 Å, shorter than the other metal–cage distances inside the two isomers (2.570 Å for Lu1 in Lu₂O@C₁(31876)-C₈₀; 2.526 Å for Lu1 and 2.581 Å for Lu2 in Lu₂O@C_2v(5)-C₈₀). The Lu1–O–Lu2 angle in Lu₂O@C₁(31876)-C₈₀ is 157.05°, larger than the value observed in Lu₂O@C_2v(5)-C₈₀ (141.80°). These phenomena suggest that the pentalene unit in Lu₂O@C₁(31876)-C₈₀ could provide stronger metal–pentalene interactions and extra space through protruding the cage.²⁶

Fig. 2 ORTEP drawings of (a) Lu₂O@C₁(31876)-C₈₀·Ni^II(OEP) and (b) Lu₂O@C_2v(5)-C₈₀·Ni^II(OEP). Thermal contours are drawn at the 10% probability level. Only one fullerene cage and the predominant metal sites are shown, whereas minor metal sites, solvent molecules and H atoms are omitted for clarity.

A complete transformation map of fullerenes can provide reliable evidence for understanding the formation mechanism. Importantly, the unique C₁(31876)-C₈₀ cage under study is essential to complete a transformation map for the major cages of the reported dimetallic oxide cluster fullerenes. The transformation processes involving C₁(31876)-C₈₀ simply require at most two well-established steps, i.e., the Stone–Wales transformation (SWT) or C₂ loss.^27,28Fig. 3 depicts the transformation map related to the C₁(31876)-C₈₀ cage and the detailed path of each step is illustrated in the ESI (Fig. S6–S10†). Structural rearrangements are demonstrated among three isomeric C₈₂ cages, namely C_s(6)-C₈₂, C_2v(9)-C₈₂ and C_3v(8)-C₈₂, via merely one SWT. C₁(31876)-C₈₀ is obtainable through a C₂-unit loss in the pentalene motif generated by a SWT on C_s(6)-C₈₂. Then two successive SWTs on C₁(31876)-C₈₀ afford C_2v(5)-C₈₀. In addition, the transformation from C₁(31876)-C₈₀ to C_2v(3)-C₇₈ is straightforward via a C₂-unit loss on the pentalene motif, and then a further SWT on C_2v(3)-C₇₈ produces D_3h(5)-C₇₈. Interestingly, it is evident that the formation process of dimetallic oxide cluster fullerenes is different from that of well-studied monometallic actinide metallofullerenes, in which IPR-violating C₁(17418)-C₇₆ and C₁(28324)-C₈₀ cages are considered as the key intermediates.^19,29 Thus, the present results also corroborate that endohedral metallic species affect the choice of the transformation pathways.¹⁶

Fig. 3 Fullerene transformation map related to the C₁(31876)-C₈₀ cage. Colors are used to visualize the motifs involved in the steps indicated by matching colored arrows.

Theoretical calculations were performed at the PBE/6-31G(d)∼SDD level to investigate the origin of stability of the two Lu₂O@C₈₀ isomers. Fig. 4a and Table S4† show the relative energy and the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Lu₂O@C₈₀ isomers. Although Lu₂O@I_h(7)-C₈₀ and Lu₂O@D_5h(6)-C₈₀ possess the lowest energy of 0.0 and 3.4 kcal mol⁻¹, respectively, their HOMO–LUMO gaps are too small (0.13 eV and 0.20 eV for Lu₂O@I_h(7)-C₈₀ and Lu₂O@D_5h(6)-C₈₀, respectively), which means that the electrons on the HOMOs of Lu₂O@I_h(7)-C₈₀ and Lu₂O@D_5h(6)-C₈₀ are easily excited, and thus these two isomers show high reactivity. On the basis of relative energy, Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀ are thermodynamic candidates with large HOMO–LUMO gaps. To verify this prediction, statistic thermodynamic analysis (Fig. 4b) considering the entropy–enthalpy effect, which has been verified as an effective method to determine the stable isomers of metallofullerenes in theory, such as dimetallic oxide cluster fullerene Sc₂O@C₇₈,^30,31 was carried out. It is noteworthy that fullerenes are formed at very high temperatures (1500–3000 K) in an arc-discharge chamber. As shown in Fig. 4b, except for the highly reactive Lu₂O@I_h(7)-C₈₀ and Lu₂O@D_5h(6)-C₈₀ isomers, the two Lu₂O@C₈₀ isomers under study, namely Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀, are the dominant species at 1500–3000 K. Consequently, the more competitive Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀ are obtained in the experiment instead of other isomers.

Fig. 4 (a) Relative energy (ΔE/kcal mol⁻¹) and the energy gaps (Gap/eV) between the HOMO and the LUMO, and (b) relative concentration (ω/%) at different temperatures (T/K) of Lu₂O@C₈₀ isomers on the PBE/6-31G(d)-SDD. Isomers are numbered according to the spiral algorithm of Fowler and Manolopoulos.⁸ The simplified number is used for the IPR isomers.

Conclusions

In summary, two isomers of Lu₂O@C₈₀, namely Lu₂O@C₁(31876)-C₈₀ and Lu₂O@C_2v(5)-C₈₀, have been successfully isolated and fully characterized by mass spectrometry, Vis-NIR absorption spectroscopy, cyclic voltammetry, single-crystal X-ray diffraction, and density functional theory calculations. Notably, this is the first observation of the C₁(31876)-C₈₀ fullerene cage, which contains a pair of adjacent pentagons. Further structural study reveals that the C₁(31876)-C₈₀ cage can serve as a key point in the transformation map for dimetallic oxide cluster fullerenes, filling the gap in the existing interconversion process. The current work demonstrates that the unique cage with adjacent pentagons may play a key role in the understanding of the long-standing puzzle about the formation mechanism and stimulates further interest in the exploration of more intermediates in the formation process.

Experimental

Materials

Lutetium(III) oxide (Lu₂O₃, 99.999%) was purchased from Suzhou Lanxi New Material Co., Ltd. Graphite rods (∅ 8 × 115 mm, 99.999%) and graphite powder (320 mesh, 99.999%) were purchased from Shanghai Fengyi Carbon Co., Ltd. Carbon disulfide (CS₂, 99.9%) was purchased from Aladdin company. Toluene (99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Chemicals were used as-received if not stated otherwise.

Preparation and isolation

A core-drilled graphite rod (6.88 g) was filled with a homogeneous mixture of Lu₂O₃ (4.26 g) and graphite powder (2.19 g). The rods were annealed in a tube furnace at 1000 °C for 12 hours under an argon atmosphere and then vaporized in a Krätschmer–Huffman-type fullerene generator with an arc current of 100 A under a mixture atmosphere of 270 Torr helium and 15 Torr CO₂. The as-produced fullerene soot was collected and sonicated in carbon disulfide for 1 h. After filtration, CS₂ was removed using a rotary evaporator. The solid residue was dissolved in toluene and filtered. The isolation of Lu-based metallofullerenes was achieved by a multiple-stage HPLC process conducted on an LC-918 machine (Japan Analytical Industry Co. Ltd). In these processes, a Buckyprep column (∅ 20 × 250 mm), a Buckyprep-M column (∅ 20 × 250 mm), and a 5PBB column (∅ 20 × 250 mm) (all Cosmosil, Nacalai Tesque, Japan) were used. More details about the HPLC processes are described in the ESI.† In this study, 300 rods were used to obtain the desired amount of the two metallofullerene samples.

Spectroscopic and electrochemical studies

LDI-TOF mass spectrometry was performed on a BIFLEX III spectrometer (Bruker Daltonics Inc., Germany). Vis-NIR spectra were obtained from a Lambda 35 spectrophotometer (PerkinElmer, USA) in CS₂. CV curves were obtained in o-dichlorobenzene with 0.05 M TBAPF₆ as the electrolyte using a CHI-660E instrument.

Crystallographic characterization

Black co-crystals of the metallofullerenes and Ni^II(OEP) were obtained by layering a benzene solution of Ni^II(OEP) over a CS₂ solution of the respective endohedrals in a glass tube at 0 °C for 30 days. Single-crystal X-ray data were collected at 100 K using a radiation wavelength of 0.73360 Å with a MarCCD detector at beamline BL17B in the Shanghai Synchrotron Radiation Facility. A multi-scan method was used for absorption corrections. The structures were solved by direct methods and refined with SHELXL-2014.³² Hydrogen atoms were inserted at the calculated positions and constrained with isotropic thermal parameters. The details of crystal data are listed in the ESI.†

Computational studies

The optimizations of Lu₂O@C₈₀ isomers were carried out at the PBE/6-31G(d)∼SDD level without any imaginary frequency,^33–36 where 6-31G(d) was for carbon and oxygen atoms and SDD with pseudopotentials was for lutetium atoms. PBE has been previously proved as a suitable functional for lutetium-based metallofullerenes.³⁷ Statistic thermodynamic analysis including the entropy–enthalpy effect was carried out to determine the thermodynamically stable Lu₂O@C₈₀ isomers based on PBE/6-31G(d)∼SDD. Furthermore, single point calculations were conducted for thermodynamically stable Lu₂O@C₈₀ isomers on PBE/6-311G(d,p)∼def2TZVP, where 6-311G(d,p) was for carbon and oxygen atoms and the def2TZVP basis set with a small-core relativistic pseudopotential (14s13p10d8f6g)/[6s6p5d4f3g] was for lutetium atoms.^38–41 All of the above calculations were performed with Gaussian 16 software except for the specific illustration.⁴²

Author contributions

X. Lu, L. Bao and P. Yu designed the research. P. Yu carried out the experiments. M. Li carried out the calculations. P. Yu, M. Li, S. Hu, P.-Y. Yu, X. Tian and W. Shen carried out the analysis. X. Lu and X. Zhao supervised the project. P. Yu and M. Li co-wrote the paper. X. Lu, L. Bao and X. Zhao guided and revised the paper. All authors read and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from NSFC (No. 21925104, 21573172, and 21773181) and the Hubei Provincial Natural Science Foundation of China (No. 2021CFA020) is gratefully acknowledged. We thank the staff at the BL17B beamline of the National Center for Protein Sciences Shanghai (NCPSS) at Shanghai Synchrotron Radiation Facility for the assistance with data collection. We thank the Analytical and Testing Center in Huazhong University of Science and Technology for all related measurements. X. Z. acknowledges financial support from the Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2131027 and 2131029. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi00410k

‡ These authors contributed equally.

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