Bonding inside and outside Fullerene Cages - Accounts of Chemical

Feb 27, 2018 - This Account aims to give an advanced summary of the recent achievements in research of EMFs, focusing mainly on the interplay between ...
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Bonding inside and outside Fullerene Cages Lipiao Bao,† Ping Peng,† and Xing Lu* State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, P. R. China CONSPECTUS: Concrete crystallographic results of endohedral metallofullerenes (EMFs) disclose that the bonding within the metallic clusters and the interactions between the metal ions and the cage carbon atoms, which are closely associated with the coordination ability of the metal ions, play essential roles in determining the stability, the molecular structure, and the chemical behavior of the hybrid EMF molecules, in addition to the previously recognized charge transfer from metal to cage. For the carbide cluster metallofullerenes, a “size effect” between the encapsulated metallic cluster and the fullerene cage has been suggested. Thus, through the geometric effect, a series of giant fullerenes (C90−C104) have been stabilized by encapsulating a large La2C2 cluster, which adopts different configurations in accordance with cage size and shape. Interestingly, the crystallographic analysis of La2C2@ D5(450)-C100 has led to the direct observation of the axial compression of short carbon nanotubes caused by the internal stress. Additionally, the defective C2(816)-C104 cage is viewed to be a precursor that can transform into the other three ideal tubular fullerene cages, presenting crystallographic evidence for the top-down formation mechanism of fullerenes. Structural characterization of Y2C2@C108 confirms a linear carbide cluster inside the large cage, indicative of a geometric effect of cage size on the bonding behavior of the internal cluster. Apart from the carbide realm, direct metal−metal bonding is observed between the two seemingly repulsive Lu2+ ions in Lu2@C82−86, adding new insights into current coordination chemistry. Meanwhile, the bonding state between the metal ions inside the cage (e.g., in La2@Ih(7)-C80) and even the configuration of the internal metallic cluster (e.g., in Sc3C2@Ih(7)-C80) can be readily controlled by exohedral radical addition, illuminating their future applications as single molecule magnets and in electronics. In addition, observation of the unexpected dimerization between two paramagnetic Y@Cs(6)-C82 molecules suggests a spin-induced bonding behavior, which depends closely on the cage geometry. In contrast, synergistic effect of both electronic and geometric parameters has led to the formation of the unprecedented [6,6,6]-Lewis acid−base adduct of Sc3N@Ih(7)-C80. However, introduction of an oxygen atom gives rise to the corresponding normal carbene adducts for both Sc3N@Ih(7)-C80 and Lu3N@Ih(7)-C80, presenting an unexpected way of steric hindrance release. Remarkably, the Lewis acid−base complexation is demonstrated to be a facile way toward isomerically pure metallofullerene derivatives with surprisingly high regioselectivity and quantitative conversion yield for Sc2C2@C3v(8)-C82. This Account aims to give an advanced summary of the recent achievements in research of EMFs, focusing mainly on the interplay between the internal metallic species and the surrounding cages through bond formation or cleavage. Perspectives suggesting the future developments of EMFs are also given in the last section.

1. INTRODUCTION The sub-nanometer cavity of fullerenes can host a variety of metal atoms or metallic clusters, forming endohedral metallofullerenes (EMFs),1−4 presenting an ideal platform for elucidating the interactions between metal ions and carbon atoms with, for example, single crystal crystallography, one of the most powerful tools for atomic-resolution structural studies. One of the most brilliant features of EMFs is the charge transfer from metal to cage, which is critical to the stability of the resultant hybrid molecules and even to the chemical reactivity of the cage carbon atoms.5 During the last five years or so, comprehensive results concerning the molecular structures, chemical properties, and formation mechanism of EMFs have considerably enhanced our knowledge about these hybrid materials to a certain extent. Importantly, it is discovered that the coordination behaviors of the encapsulated metal ions © XXXX American Chemical Society

in the sub-nanometer sized space of the cage are usually different from the situations found in bulky complexes.

2. BONDING INSIDE EMFs Not only does the fullerene cage serve as a shelter to protect the intermetallic clusters from being attacked from outside, but a part of the cage also acts as a ligand to coordinate with the metal atoms accompanied by charge transfer. As a result, the bonding inside the cages is somehow complicated and is thus of particular interest. As a matter of fact, upon electron transfer, the metal ions play a central role in determining the bonding inside the cages, which involves two parts: the bonding between the metal ions and the cage carbon atoms and the bonding Received: January 5, 2018

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DOI: 10.1021/acs.accounts.8b00014 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research within the metallic clusters (Figure 1). An interesting finding from the crystallographic results of EMFs is that the metal ions

fullerene is an important factor determining the stability of the resultant EMFs. Indeed, a series of CCMFs with giant cages ranging from C90 to C104 has been successfully obtained by incorporation of a relatively large La2C2 cluster inside.12−14 Both theoretical and experimental results confirm that the carbide cluster form (La2C2@C2n) is more stable than the dimetallic form (La2@ C2n−2) because the C2-unit can coordinate with the La3+ ions from inside so as to compensate the Coulomb repulsion between them. The molecular structures of La2C2@C2n (2n = 90−104) shown in Figure 3 clearly reveal that the configuration of the La2C2 cluster changes gradually from butterfly-like to zigzag along with cage expansion, indicating a size-matching effect between the cluster and the surrounding cage. An exception from the carbide realm for La2C2n is La2@ D5(450)-C100,15 which possesses the same cage as that of La2C2@D5(450)-C100. Actually, this ideally tubular cage can be viewed as a short [10,0]-carbon nanotube (CNT) with two end-caps. A comparison of their X-ray structures reveals that the belt of the cage consisting of continuous hexagons is more inclined to deform upon the internal strain whereas the pentagons are rigid units with negligible deformation. The C2unit acts as a molecular spring to contract the surrounding cage via interactions with the two intercalating La3+ ions. The above result presents an accurate evaluation of the torsion of short CNTs and is useful for the rational design of carbon nanostructures with desired mechanical properties. Very recently, a series of giant metallofullerenes containing yttrium, which has a smaller ionic radius than lanthanum, Y2C2n (2n = 92−130), have been obtained by the treatment of the 1,2-dichlorobenzene fraction with SnCl4. Subsequent chromatographic separation affords a pure sample of Y2C2@C1(1660)C108, representing as the largest metallofullerene identified to date. Remarkably, crystallographic analysis of this EMF reveals a linear configuration of the Y2C2 cluster, confirming the importance of geometric matching between the internal cluster and the cage on controlling the configuration of the metallic cluster.16

Figure 1. Structure of metallofullerene illustrating the bonding between the metal ions and the cage carbon atoms (pink) and the bonding within the metallic cluster (blue). Metal ions are highlighted in green.

are generally moving inside the cages, a phenomenon totally different from that of conventional metallic complexes. 2.1. The Carbide Phenomenon

EMFs with a composition of M2C2n may exist as di-EMFs, M2@ C2n, or as carbide cluster metallofullerenes (CCMFs), M2C2@ C2n−2.6 During the last ten years, it gradually turned out that the latter form is more inclined for rare-earth-metal containing EMFs. In particular, nearly all the EMFs containing two scandium atoms take the carbide form, for example, Sc2C2@ C80−88,6 with only two exceptions, Sc2@C667 and [email protected] For other rare earth metal elements, CCMFs are also common, but a size effect occurs due to the strong interactions between the metallic cluster and the cage (Figure 2). A thorough inspection

2.2. Direct Metal−Metal Bonding

Although the carbide structure is common for M2C2n-type EMFs, it is somehow avoided when Y and Er are encapsulated. In fact, M2C2@C2n−2 and M2@C2n always coexist in the raw soot and in the extract when M is Y or Er. This phenomenon is theoretically explained by considering the ionic energy of the encapsulated metal atoms, which may form a metal−metal bond when taking low valent states. In particular, computational results from Popov and co-workers suggest that lutetium atoms are most inclined to take the +2 state among all rareearth metals, and accordingly, Lu−Lu bonding should be rather favorable in lutetium-containing EMFs.17 Consistently, experimental results of a series of Lu-EMFs confirm that all the isolated isomers with a cage ranging from C82 to C86 take the form of Lu2@C2n instead of Lu2C2@ C2n−2.18 Crystallographic results demonstrate that the Lu···Lu distances between the two lutetium atoms are all in the range of a Lu−Lu single bond length, thus presenting unambiguous experimental evidence for direct Lu−Lu bonding between two repulsive Lu2+ ions (Figure 4). Population analysis shows that a 5d electron and a 6s electron for each Lu atom are transferred onto the cage and the remaining 6s electron is shared with the other Lu atom to form a Lu−Lu single bond. Interestingly, computational results suggest that the carbide form Lu2C2@C2n

Figure 2. Relationship between the ionic radius of encapsulated metal ions and cage size for typical CCMFs that have been crystallographically characterized so far.

of the reported compounds with a composition of M2C2@C2n reveals that the small Sc2C2 cluster prefers to be encapsulated inside the cages smaller than C88, but the relatively large M2C2 (M = Gd, Tb, and Y) clusters tend to choose either the nonisolated pentagon rule (non-IPR) C1(51383)-C84 cage or the even larger D3(85)-C92 cage.9−11 These results indicate that the size matching between the encapsulated metallic cluster and the B

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Figure 3. Molecular structures of La2C2@C2n (2n = 90−104). The internal C2-unit is highlighted in blue and La atoms in green.

Figure 5. Molecular structures of (a) Sc2C2@C82−NHC, (b) Sc3N@ C80−NHC, and (c) Sc3N@C80−O−NHC.

instead of the traditional cycloadducts because of steric hindrance. Interestingly, the reaction of Sc3N@Ih(7)-C80 with NHC under argon produces a sole product with the abnormal carbene center linking to a [6,6,6]-cage carbon (Figure 5b), which has never been observed before. Computational results reveal that steric hindrance between the bulky NHC moiety and the C80 cage prevents the formation of normal carbene adducts, whereas the formation of the unprecedented [6,6,6]adduct is a consequence of an electronic effect induced by the internal cluster. Unexpectedly, the steric hindrance between the NHC moiety and the Ih(7)-C80 cage can be substantially released when an oxygen atom is introduced onto the cage of Sc3N@Ih(7)-C80, which favors the normal carbene addition while maintaining the [6,6,6]-pattern (Figure 5c). It appears that the orientation of the Sc3N cluster is totally different in the abnormal carbene adduct Sc3N@C80−NHC and the oxidized normal carbene adduct Sc3N@C80−O−NHC. These interesting results demonstrate that the bonding behaviors outside EMFs are readily affected by either the inside metallic cluster or the cage geometry or both.

Figure 4. Molecular structures of Lu2@C2n (2n = 82−86) showing direct Lu−Lu bonding. Lu atoms are highlighted in green.

becomes more stable than the dimetallic one when the cage expands, most probably because of the long distance between the two Lu ions, which disables direct Lu−Lu bonding.

3. BONDING OUTSIDE EMFS 3.1. Lewis Acid−Base Complexation

It has been realized that the chemical properties of EMFs are largely affected by the internal metal elements, via charge transfer or geometric effects. A recent typical example is the Lewis acid−base reaction between EMFs and constrained Nheterocyclic carbenes (NHCs), which exhibit both high regioselectivity and high conversion yield. For example, the reaction of Sc2C2@C3v(8)-C82 with NHC proceeds smoothly to produce merely two monoadduct isomers, one of which is a kinetically favorable product that is not isolable.19 X-ray results of the final product show that a polarized C−C single bond is formed between the normal carbene site of the NHC moiety and a specific [5,6,6]-carbon atom out of 17 types of nonequivalent cage carbon atoms of Sc2C2@C3v(8)-C82 (Figure 5a), showing surprisingly high regioselectivity. Theoretical natural bond order analysis confirms that the C−C single bond between the NHC moiety and Sc2C2@C3v(8)-C82 is highly polarized. Total charges of +0.92e and −0.92e reside on the NHC moiety and Sc2C2@C3v(8)-C82, respectively, demonstrating a zwitterionic nature of the adduct. In this reaction, the EMF Sc2C2@C3v(8)-C82 acts as a Lewis acid that accepts an electron from the NHC to form the singly bonded adduct

3.2. Communication between the Internal and the Exohedral

In many cases, the influence of the internal metallic species on the chemical behaviors of the surrounding cage carbon atoms is remarkable.20 A recent work reported the unprecedented dimerization of pristine metallofullerene, namely, the labile Y@Cs(6)-C82 isomer, during its crystallization process.21 In contrast, the stable Y@C2v(9)-C82 isomer does not dimerize under the same experimental conditions, suggesting an obvious cage-symmetry dependence. Theoretical results demonstrate that the steady displacement of the yttrium atom inside the Cs(6)-C82 cage accounts for the regioselective dimerization by C

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endohedrals.24 Further experiments suggest that the charge transfer from the entrapped metal onto the cage25 and the cluster−cage interaction26 are crucial for the bottom-up process. Another argument for the “closed network growth” mechanism is that large U@C2n EMFs are demonstrated to form with the smallest stable fullerene U@C28 as a precursor.27 Subsequent theoretical exploration of this mechanism was conducted on the Ti@C2n (2n = 26−50) family, revealing that all the optimal isomers are unraveled to be formally linked by a simple exergonic/exothermic C2 insertion along with an additional C2 rearrangement in a few cases.28 Indeed, this mechanism is strongly supported by the laser synthesis process of small fullerenes/EMFs, but it seems feeble for understanding the formation process of large fullerenes especially under the arc-discharge conditions, where a top-down process appears reasonable.

localizing high spin density on a special cage carbon atom of Y@Cs(6)-C82. In contrast, recent studies indicate that the behaviors of the internal metallic species can also be controlled by exohedral modification. Radical reaction is found to be an efficient strategy to achieve the mutual communication between the inner and outer moieties. For instance, addition of benzyl radical to La2@C80 leads to the formation of a single-electron La−La bond in the monoadduct. It happens that the unpaired electron is transferred from the exohedral radical to the molecular orbital of the internal La2 cluster (Figure 6a). In

4.2. Top-down Process

Apart from the bottom-up process, the inverse top-down mechanism, which sizes down the cage by elimination of carbon atoms, has also been advanced. Theoretical simulations suggest that the fullerene cage can both expand and shrink under high temperatures depending on the concentration of noncage carbon.29 Moreover, the direct transformation of graphene into fullerene has been observed by transmission electron microscopic visualization,30 and the shrinkage of fullerenes to smaller fullerenes at high temperature has been experimentally demonstrated.31 Based on the structural relationship between Y2C2@C82(III) and Y2@C82(III), a top-down process involving the evaporation of C2 either outward or inward from Y2@C84 was claimed.32 More solid proof for this top-down mechanism comes from the crystallographic results of several new EMFs. Dorn et al. reported that an asymmetric C1(51383)-C84 cage, which is stabilized by the incorporation of a Y2C2/Gd2C2 cluster, is actually a missing link in the formation process of most EMFs, which accounts for the majority of solvent-extractable compounds by well-established C2-loss and Stone−Wales rearrangements.33 Meaningfully, their mass spectroscopic results indicate that the formation process of empty fullerenes also follows the top-down mechanism similar to that of EMFs. Consistently, crystallographic results of La2C2@C100−104 also support the top-down formation mechanism.13,14 Although the possible conversion pathways increase dramatically with the increase of cage size, very close structural connections are still demonstrated for such giant La2C2@C2n (2n = 100−104). Specifically, three ideal tubular fullerenes, namely, D5(450)C100, Cs(574)-C102, and D3d(822)-C104, can all be obtained by rearrangements on the same defective C2(816)-C104 cage (Figure 7). Since these ideal tubular fullerenes can be viewed as the prototypes of finite-length carbon nanotubes (CNTs), the transformation processes may also provide valuable hints for understanding the structural rearrangements from defective CNTs to ideal ones.

Figure 6. Schematic illustrations showing (a) direct metal−metal bond formation in La2@C80−benzyl adduct and (b) configuration control of the inner cluster in Sc3C2@C80 upon exohedral modification.

another effort, La2@C80 was first functionalized with a triazinyl radical, and no doubt, the unpaired electron is trapped inside the cage to form a single-electron La−La bond. The adduct was then allowed to undergo a further reaction with a benzyl radical.22 Interestingly, the result reveals that the trapped electron is extracted from the cage so that the La−La bond is broken. However, during the crystallization process, the bisadduct tends to form a dimer so as to achieve the La−La bonding in each cage, a case similar to that of Y@Cs(6)-C82. In addition to the tuning of the bonding state of the internal metallic species, the configuration of the internal metallic cluster can also be controlled by exohedral modification. A successful result is achieved on Sc3C2@C80 by, again, benzyl radical addition. X-ray crystallographic results reveal that the internal cluster changes from a bat-ray-like shape in pristine Sc3C2@C80 to a trifoliate shape in the adduct as a result of subsequent electron redistribution (Figure 6b).23 Such a precise configuration control of the internal metallic species at atomic level may promise their potential application in the fields of information storage and quantum computer.

4. MECHANISM OF FULLERENE/EMF FORMATION The formation mechanism of fullerenes has not been clearly elucidated due to the challenge in achieving direct experimental evidence. Moreover, the formation process of EMFs seems to be more complicated because of the presence of the metallic species and their hybridization. Nonetheless, recent efforts have led to the advent of two different mechanisms for fullerene formation, namely, the bottom-up process and the top-down one.

5. PERSPECTIVES The metal−carbon hybrid character promises tremendous fundamental insights and practical applications for EMFs. Academically, the sub-nanometer-sized inner cavity of fullerene cages is an ideal platform for elucidating the interactions between a small number of atoms. Practically, the intrinsic properties of the encapsulated metal atoms can be readily preserved, expressed, or even enhanced after being incorpo-

4.1. Bottom-up Process

The so-called “closed network growth” bottom-up formation mechanism by incorporation of atomic carbon or C2 onto small cages was proposed by Dunk and co-workers based on a series of mass spectroscopic results of empty fullerenes or D

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Accounts of Chemical Research Biographies

Lipiao Bao obtained his Ph.D. under the supervision of Professor Xing Lu from Huazhong University of Science and Technology in 2016. From 2014 to 2015, he studied in the group Professors Alan L. Balch and Marilyn M. Olmstead at University of California, Davis, as a visiting Ph.D. student. He is now working at Friedrich-AlexanderUniversität Erlangen-Nürnberg, Germany, as an Alexander von Humboldt (AvH) Fellow. His research interests focus on structures and properties of fullerenes and carbon nanotubes. Ping Peng received his B.Sc. in Chemistry from Northeast Normal University and Ph.D. in Applied Chemistry from Jilin University. After postdoctoral studies, he joined Huazhong University of Science and Technology as an associate professor of School of Materials Science and Engineering in 2017. His current research focuses on carbon materials, including the design and synthesis of empty fullerene and endohedral metallofullerene derivatives, as well as their potential applications in solar cells and energy storage. Xing Lu is currently a Full Professor in Huazhong University of Science and Technology sponsored by The National Thousand Talents Program of China. His research interests lie in the rational design and facile generation of novel hybrid carbon materials with applications in energy storage and conversion and biology. He received a Ph.D. from Peking University in 2004. Then he worked as a COE Postdoctor in Nagoya University. From 2006 to 2011, he was a Senior Scientist in University of Tsukuba. He is the recipient of The Ambassador Award from Chinese Embassy in Japan (2009) and The Osawa Award from Fullerenes and Nanotubes Research Society of Japan (2011). He has published more than 90 peer-reviewed papers in international journals with more than 30 in J. Am. Chem. Soc. and Angew. Chem., Int. Ed..

Figure 7. Structural transformation from the defective C2(816)-C104 cage to the other three ideal tubular cages, namely, D5(450)-C100, Cs(574)-C102, and D3d(822)-C104. Reproduced from ref 14 with permission. Copyright 2016 American Chemical Society.

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rated inside the fullerene cages, thus providing a variety of functions with potentials in photovoltaics, electrooptics, biomedicine, and even superconductors. Bonding is thus the core in these unique metal−carbon hybrid structures. Thanks to the crystallographic structural results, exciting new findings are unceasingly discovered at the atomic level. We can now discuss such interesting topics as the bonding inside and outside fullerene cages, the behaviors of the clusters in a confined space, and the interplay between the endohedral and the exohedral moieties. However, the formation mechanism of EMFs and fullerenes is still controversial. Identification of new structures with relatively small or giant cages shall be an efficient solution. Moreover, the exohedral manipulation of the confined units within the cages is of particular importance because the results may lead to revolutionary applications in, for example, information storage and quantum computation. Finally, intensive exploration into the chemistry of EMFs will present tremendous derivatives with possible applications for target purposes in the near future.

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ACKNOWLEDGMENTS Financial support from NSFC (51472095 and 51672093) is gratefully acknowledged. ABBREVIATIONS EMFs, endohedral metallofullerenes; CCMFs, carbide cluster metallofullerenes; IPR, isolated pentagon rule; THJ, triplehexagon junction; CNTs, carbon nanotubes; HPLC, high performance liquid chromatography; XRD, X-ray diffraction; TEM, transmission electron microscopy; DFT, density functional theory; NHC, N-heterocyclic carbene

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xing Lu: 0000-0003-2741-8733 Author Contributions †

L.B. and P.P. contributed equally.

Notes

The authors declare no competing financial interest. E

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DOI: 10.1021/acs.accounts.8b00014 Acc. Chem. Res. XXXX, XXX, XXX−XXX