Facile Access to Y2C2n (2n = 92â130) and Crystallographic

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Facile Access to Y2C2n (2n = 92-130) and Crystallographic Characterization of Y2C2@C1(1660)-C108: A Giant Nanocapsule with A Linear Carbide Cluster Changwang Pan, Lipiao Bao, Xianyong Yu, Hongyun Fang, Yun-Peng Xie, Takeshi Akasaka, and Xing Lu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00384 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Facile Access to Y2C2n (2n = 92-130) and Crystallographic Characterization of Y2C2@C1(1660)-C108: A Giant Nanocapsule with A Linear Carbide Cluster Changwang Pan†, Lipiao Bao†, Xianyong Yuǂ, Hongyun Fang†, Yunpeng Xie†, Takeshi Akasaka†, and Xing Lu†,* †

State Key Laboratory of Materials Processing and Die & Mold Technology, School of Materials

Science and Engineering, Huazhong University of Science and Technology, 1037, Luoyu Road, Wuhan 430074, China. Email: [email protected] ǂ

Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of Ministry of

Education, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, 411201, China

ABSTRACT: A series of giant metallofullerenes Y2C2n (2n=92-130) have been successfully obtained through the treatment of the fraction enriched by 1,2-dichlorobenzene with SnCl4. Subsequent chromatographic separation gives a pure sample with a composition of Y2C110.

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Crystallographic results reveal that this endohedral takes the carbide form, namely Y2C2@C1(1660)-C108, representing as the largest metallofullerene that have been characterized by crystallography to date. Despite of the disorder of the metal cluster, the major Y2C2 adopts a previously predicted linear configuration, indicating that the compression of the internal cluster by the cage is almost negligible in this giant cage. Electrochemical studies suggest that Y2C2@C1(1660)-C108 is a good electron donor instead of an electron acceptor.

KEYWORDS: fullerene · metallofullerene · carbide cluster · crystallography · nanocapsule

Giant fullerenes, for example, those consisting of 100 or more carbon atoms (C2n≥100) possess a seemingly infinite regime that show close connections with carbon nanotubes (CNTs).1–3 However, the enormous possible isomers of giant fullerenes make the identification of their structures challenging and thus single crystal X-ray diffraction (XRD) crystallography stands as one of the most reliable solutions. Another obstacle impeding final acquisition of giant fullerenes is their poor solubility in common solvents, which is closely associated with their electronic structures.4 Thus electrochemical or chemical modifications have been applied to alter the electronic structures of giant fullerenes to make them more soluble. Though the electrochemical method appeared efficient for the enrichment of giant fullerenes like C2n (100 ≤ 2n ≤ 112), their anionic form hindered further isolation of pure isomers.5 Alternatively, chemical modification has led to the identification of several chlorinated fullerenes, such as C100(18)-Cl28/30,2 C102(603)Cl18/20,6 C104(812)-Cl16/18/20/22,6 C106(1155)-Cl247 and C108(1771)-Cl12,7 but the cages are distorted because of the severe exohedral modification and sometimes cage shrinkage also took place.


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Meanwhile, endohedral metal-doping represents as another effective way to alter the electronic structures of giant fullerenes by forming endohedral metallofullerenes (EMFs).8–10 Many giant EMFs have been isolated and crystallographically characterized during recent years. Although the first two examples are both di-EMFs, namely, Sm2@D3d(822)-C10411 and La2@D5(450)-C100,3 it has been revealed that carbide cluster metallofullerenes (CCMFs) are also preferred because of the

strong

metal-cage

interactions.

Successful

examples

are

La2C2@D5(450)-C100,12

La2C2@Cs(574)-C10213 and La2C2@C2(816)-C104.13 In these compounds, the C2-unit plays an essential role in stabilizing the giant cages by coordinating with the metal ions so as to compensate the repulsion between them. As a direct proof of the strong metal-carbon interactions, La2C2@D5(450)-C100 shows an anomalous axial compression as compared with La2@D5(450)-C100, indicating that giant fullerenes can be viewed as capped short CNTs to investigate the intrinsic properties of CNTs.12 However, in spite that great efforts have been devoted to the acquisition of giant EMFs, the largest cage to be clearly characterized to date is limited to C104.11,13 Herein, we report a Lewis-acid-precipitation strategy for the facile enrichment of a series of diyttrium EMFs with a cage ranging from C92 to C130. Subsequent chromatographic separations give a pure compound, Y2C2@C1(1660)-C108, representing as the largest metallofullerene with definite molecular structures being elucidated to date. RESULTS AND DISCUSSION Soot containing yttrium-EMFs produced from the arc-discharge method was extracted with carbon disulfide. After solvent removal, the residue was redissolved in toluene and the solution was subjected to high performance liquid chromatographic (HPLC) separation. Herein we


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innovatively injected a volume of 10 mL 1,2-dichlorobenzene (o-DCB) to wash out the giant fullerenes/EMFs from the elution. The collected o-DCB fraction was labeled as F-o (Figure S1). The mass spectrum of F-o shows a series of empty fullerenes C2n (2n=94-150) and EMFs Y2C2n (2n=92-130) with giant cages (Figure 1a). Subsequently, the o-DCB fraction (F-o) was treated with SnCl4 which lead to the rapid enrichment of diyttrium EMFs as a precipitate (F-op).14 The mass spectrum of F-op reveals that empty fullerenes have been completely removed and only Y2C2n (2n=92-130) are present in the sample (Figure 1b). Finally, we have been able to get a pure sample of Y2C110 through multistage HPLC separations. The experimental details are put in the Supporting Information (Figures S1-S4).

Figure 1. Mass spectra of (a) the fraction enriched with o-DCB (F-o) and (b) the precipitate after treatment with SnCl4 (F-op). HPLC charts of Y2C110 on different columns verify its high purity (Figure 2a). Consistently, the laser desorption/ionization time-of-flight mass spectroscopic (LDI-TOF MS) result shows a single peak at m/z 1498 corresponding to Y2C110. To the best of our knowledge, this is the largest


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metallofullerene isolated to date which is almost double the size of the most abundant C60. Figure 2b shows the Vis-NIR absorption spectrum of Y2C110, which displays absorption bands at 533 nm, 654 nm, 852 nm and 1037 nm with an onset at around 1400 nm, indicative of a small optical gap (0.89 eV).

Figure 2. (a) HPLC profiles and (b) Vis-NIR absorption spectrum of Y2C110 (the LDI-TOF mass spectrum of Y2C110 is shown in the inset). HPLC conditions: 1.0 mL/min chlorobenzene flow, 330 nm detection wavelength, 40 °C. The molecular structure of Y2C110 is unambiguously determined by single-crystal XRD crystallographic study performed on a piece of co-crystals of Y2C110·2Ni(OEP) (OEP = octaethylporphyrin). The details of crystallographic data are listed in Table S1. The system displays some degree of disorder with regard to the metal atoms which show twelve positions in total with occupancy values ranging from 0.080 to 0.204 (Figure S5 and Table S2). Figure 3 shows a drawing of the molecular structure of Y2C110·2Ni(OEP) containing only the major metal site. This endohedral lies slantwise between the two surrounding Ni(OEP) molecules with respective Ni-cage distances of 2.895 Å and 3.054 Å, featuring π-π interactions.


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This sandwich-like arrangement is similar to the situations reported for La2@D5(450)-C1003 and Sm2@D3d(822)-C10411 which also require two Ni(OEP) molecules to confine the giant cages. Evidently, Y2C110 is a carbide cluster EMF with an asymmetric chiral cage, namely Y2C2@C1(1660)-C108, which is one of the 1799 isomers of C108 that obey the isolated pentagon rule. Unexpectedly, this cage shows a relatively round appearance because of the absence of a band of contiguous hexagons and thus the more evenly distributed pentagons. In contrast, the other reported giant EMFs, such as La2@D5(450)-C100,3 La2C2@D5(450)-C100,9 La2C2@Cs(574)C10210 and Sm2@D3d(822)-C1048 all possess tubular cages with pentagons concentrated on the two ends. Accordingly, a relatively short length of Y2C2@C1(1660)-C108 (10.04 Å) is observed although it is the largest cage to be crystallographically characterized for EMFs to date, whereas the other smaller cages are longer (10.28 Å for La2@D5(450)-C100,3 10.26 Å for La2C2@Cs(574)C102,10 10.27 Å for La2C2@C2(816)-C104,10 and 10.80 Å for Sm2@D3d(822)-C1048).


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Figure 3. ORTEP drawing of Y2C2@C1(1660)-C108•2Ni(OEP) showing ellipsoids at the 5% probability level. Only one cage orientation and the predominant carbide cluster site are shown whereas the minor components are omitted for charity. Interestingly, the internal Y2C2 cluster adopts a linear configuration along the long cage axis as a consequence of sufficient inner space provided by the giant cage. Dorn and coworkers have proposed a nanoscale compression of a Y2C2 cluster by the fullerene cage based on the NMR and DFT results of a series of Y2C2@C2n isomers. They demonstrate that the Y2C2 cluster tends to adopt a nearly linear configuration inside C100 or larger cages but it changes to a butterfly-like structure in small cages like [email protected] Prior to our work, butterfly-like or zigzag (planar) configurations have been found in CCMFs with relatively small cages (e.g. Sc2C2@C80-88).13–17 Our crystallographic results present the first experimental evidence of a linear M2C2 cluster which is fully consistent with the theoretical prediction by Dorn et al. Figure 4 shows an in-depth inspection of the configuration of the major Y2C2 cluster. The YC-C-Y dihedral angel is 173.1°, suggesting a linear structure. The C-C distance of 1.08 Å is even shorter than a typical C≡C triple bond. In addition, the Y-Y distance found here is 5.85 Å which is similar to the theoretical value of free Y2C2 cluster (5.83 Å), but is clearly longer than the theoretical values for Y2C2 in Y2C2@C3v(8)-C82 (3.74 Å), Y2C2@D3(85)-C92 (4.92 Å) and Y2C2@D5(450)-C100 (5.51 Å) reported by Dorn and coworkers,15 confirming experimentally that the compression of the internal cluster induced by the cage is negligible in Y2C2@C1(1660)-C108.


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Figure 4. (a) A view of Y2C2@C1(1660)-C108 showing a linear Y2C2 cluster inside the cage. (b) Structural parameters of Y2C2. The electrochemical properties of Y2C2@C1(1660)-C108 is investigated with cyclic voltammetry (Figure 5). This compound displays a reversible oxidation step and four reversible reduction processes. The oxidation potential appears at +0.06V, indicating that this EMF is a good electron donor. The four reduction potentials are present at -0.90 V, -1.39 V, -1.83 V, -2.11 V, and thus an electrochemical bandgap of 0.96 V is deduced. Since there is no report on the electrochemical properties of other Y2C2@C2n isomers, we compare the above values with those of typical scandium-containing CCMFs (Table 1). In general, the first reduction potential of Y2C2@C1(1660)-C108 is similar to the values of the scandium-containing CCMFs but its first oxidation potential is cathodically shifted, resulting in a smaller electrochemical gap for this giant fullerene. We conclude herein that the difference in the oxidation potentials between Y2C2@C1(1660)-C108 and Sc2C2@C72-88 is caused by the cage size and cage symmetry because the type of the internal metallic units normally exerts negligible influence on the electronic


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properties of the EMFs. For example, the redox potentials of M@C82 (M = Sc, Y, La, etc.) isomers are nearly identical though different metal atoms have been encapsulated.21–23

Figure 5. Cyclic voltammogram of Y2C2@C1(1660)-C108 measured in o-DCB at 100mV/s. Table 1. Redox potentials (V vs Fc/Fc+)a of Y2C2@C1(1660)-C108 and Sc-containing CCMFs.

a

E4

Gap (oxE1red E 1)

ref

-1.83

-2.11

0.96

this work

-1.54

-1.75

-2.23

1.60

17

-0.80

-1.19

-1.58b

-1.80b

1.30

24

0.41

-0.74

-1.33

-1.71

-2.04

1.15

25

Sc2C2@C3v(8)-C82

0.47

-0.94

-1.15

-1.60

－

1.41

26

Sc2C2@C2v(9)-C82

0.25

-0.74

-0.96

－

－

0.99

18

Sc2C2@Cs(6)-C82

0.42

-0.93

-1.30

－

－

1.35

20

Sc2C2@C2v(9)-C86

0.47

-0.84

-1.11b

-1.63b

-2.54b

1.31

27

Sc2C2@Cs(hept)-C88

0.38

-0.78

-1.76b

-2.00

-2.25

1.16

16

EMFs

ox

red

Y2C2@C1(1660)-C108

0.06

-0.90

-1.39

Sc2C2@Cs(10528)-C72

0.41

-1.19

Sc2C2@D3h(3)-C74

0.50

Sc2C2@C2v(5)-C80

E1

E1

red

E2

red

E3

red

Half-cell potentials in o-DCB unless otherwise noted. b Irreversible peak values.


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CONCLUSION We have developed a facile method for the rapid enrichment of giant EMFs Y2C2n (2n=92130) by the combination of o-DCB washing and Lewis acid precipitation. By virtue of this strategy, a metallofullerene with a giant cage has been successfully isolated and crystallographically

identified

as

Y2C2@C1(1660)-C108,

representing

as

the

largest

metallofullerene with a clear structure reported to date. Moreover, the theoretically predicted linear Y2C2 cluster is observed inside the giant C1(1660)-C108 cage, indicating that the nanoscale compression of the internal cluster by the cage is negligible for this endohedral. Electrochemical studies demonstrate one reversible oxidation process and four reduction ones for Y2C2@C1(1660)-C108. Our separation method is so facile and efficient that it is expected that more giant EMFs will be discovered in the near future some of which may show clear connections with CNTs. METHODS High performance liquid chromatography (HPLC) was conducted on an LC-908 machine (Japan Analytical Industry Co., Ltd.). Laser desorption/ionization time-of-flight (LDI-TOF) mass spectrometry was measured on a MICROFLEX spectrometer (Bruker Daltonics Inc., Germany). Vis-NIR spectrum was obtained with a PE Lambda 750S spectrophotometer in carbon disulfide (CS2). Co-crystallization of Y2C2@C1(1660)-C108·2Ni(OEP). Black blocks of Y2C2@C1(1660)C108·2Ni(OEP) were obtained by allowing a CS2 solution of Y2C2@C1(1660)-C108 and a benzene solution of Ni(OEP) to diffuse together. The crystallographical characterization was performed at 100K in beamline station BL17B at Shanghai Synchrotron Radiation Facility. The multiscan


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method was used for absorption corrections. The structure was solved by direct method and refined with SHELXL-2016/7.28 In addition, the program SQUEEZE, a part of the PLATON package of crystallographic software, was used to calculated the solvent disorder area and to remove its contribution from the intensity data.29,30 CCDC-1573049 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. ASSOCIATED CONTENT Supporting Information. CIF data, supplementary Figure S1-S6 and Table S1-S2. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENT Financial support from NSFC (Nos. 51472095, 51672093, 51602112, 51772111 and 21771071) and Ministry of Science and Technology of the People's Republic of China (2015AA034601) is gratefully acknowledged. We thank the staffs in BL17B beamline of National Center for Protein Sciences Shanghai (NCPSS) at Shanghai Synchrotron Radiation Facility for the assistance with data collection. REFERENCES 1.

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