Isolation and Structural Characterization of Er@C2v(9)-C82 and Er

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Isolation and Structural Characterization of Er@C2v(9)‑C82 and Er@Cs(6)‑C82: Regioselective Dimerization of a Pristine Endohedral Metallofullerene Induced by Cage Symmetry Shuaifeng Hu,†,§ Tong Liu,‡,§ Wangqiang Shen,† Zdeněk Slanina,† Takeshi Akasaka,†,* Yunpeng Xie,*,† Filip Uhlik,∥ Wenhuan Huang,*,‡ and Xing Lu*,†

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/18/19. For personal use only.

†

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China ‡ Shaanxi Key Laboratory of Chemical Additives for Industry, College of Chemistry & Chemical Engineering, Shaanxi University of Science & Technology, Xi’an 710021, China ∥ Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, 12843 Praha 2, Czech Republic S Supporting Information *

ABSTRACT: Two Er@C82 isomers have been isolated and unambiguously characterized as Er@C2v(9)-C82 and Er@Cs(6)-C82, respectively, by single-crystal Xray diffraction. Er@Cs(6)-C82 is identified as a dimeric structure in the crystalline state, but dimerization does not occur for Er@C2v(9)-C82 under identical crystallization conditions, indicating a cage-symmetry-induced dimerization process. Density functional theory calculations reveal that the major unpaired spin resides on a special C atom of Er@Cs(6)-C82, which leads to regioselective dimerization. Calculations also found that the dimeric structure of Er@Cs(6)-C82·Ni(OEP) is much more stable than the two monomers, suggesting a thermodynamically favorable dimerization process. Vis−near-IR spectrometric and electrochemical results demonstrate that the electronic structure of Er@C82 isomers is Er3+@C823−, instead of the theoretically proposed Er2+@ C822−.

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INTRODUCTION Encapsulation of metal atoms or metallic clusters into fullerene cages generates novel hybrid molecules named endohedral metallofullerenes (EMFs).1,2 Since the first experimental report on La@C82,3 the C82 cage has become an unique character for EMFs, and a variety of metal atoms, such as rare-earth metals (Sc,4 Y,5 La,6 Ce,7 Pr,8 Nd,9 Sm,10 Eu,11 Gd,12 Er,13 Tm,14 and Yb15) and actinides (Th16 and U17), have been reported to form air-stable mono-EMFs that show 2+, 3+, or 4+ oxidation states. Up to now, molecular structures of M@C2v(9)-C82 (M = ScIII,18 YIII,19 LaIII,20 CeIII,18 GdIII,21 YbII,22 SmII,23 and UIII 17), M@Cs(6)-C82 (M = YbII,24 YIII,19 and SmII 23), M@C2(5)-C82 (M = SmII,23 YbII,24 and UIV 17), SmII@C3v(7)-C82,23 and ThIV@C3v(8)-C8216 with unfunctionalized cages have been unambiguously determined by X-ray crystallography (Roman numerals represent the valence states of the metals). Dimerization of EMFs was first found in the derivative of La@C2v(9)-C82.25 Unexpectedly, a regioselective dimerization was observed in the crystalline states of Y@Cs(6)-C8219 and Ce@C2v(9)-C8218 upon cocrystallization with nickel(II) octaethylporphyrin [Ni(OEP)], in which two fullerene cages are connected via a C−C single bond. However, no dimerization occurs for Y@C2v(9)-C82 under the same crystallization conditions.19 It is proposed that localization of the high spin density on a special cage C atom of Y@Cs(6)-C82 © XXXX American Chemical Society

caused by the steady displacement of the Y atom inside the Cs(6)-C82 cage should be responsible for regioselective dimer formation.19 For the origin of dimerization of Ce@C2v(9)-C82, Suzuki and co-workers pointed out that the high spin density and pyramidalization of a special cage atom are key factors that favor dimerization.18 Consequently, EMFs with other metal atoms trapped should be explored to further understand the dimerization process. Among the EMFs involving rare-earth elements, Er-based EMFs are rarely explored. Only two isomers of Er2@C82 have been characterized as Er2@C3v(8)-C82 and Er2@Cs(6)-C82 by single-crystal X-ray diffraction (XRD).26,27 Very recently, Er2C94 was unambiguously determined as a carbide EMF, Er2C2@D3(85)-C92,28 which utilizes the same cage as Gd2C2@ D3(85)-C92,29 La2C2@D3(85)-C92,30 and Sm2@D3(85)-C92.31 Experimental studies of other Er-based EMFs are limited to spectroscopic characterizations.13,32 For example, two isomers of Er@C82 have been isolated, but only mass spectrometric and UV−vis−near-IR (NIR) electronic spectroscopic results are available,13 with their molecular structures remaining unknown. Herein, we report the unambiguous X-ray crystallographic analyses of their molecular structures, namely, Er@C2v(9)-C82 Received: November 28, 2018

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DOI: 10.1021/acs.inorgchem.8b03313 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry and Er@Cs(6)-C82. Interestingly, a highly regioselective dimerization is detected for Er@Cs(6)-C82 in the crystalline state, in which two fullerene cages are connected via a C−C single bond of 1.604 Å. On the contrary, Er@C2v(9)-C82 remains in the monomeric form under identical crystallization conditions. Further computational analyses reveal that the unpaired spin of Er@Cs(6)-C82 is more localized compared to that of Er@C2v(9)-C82, which makes a significant contribution to the formation of a dimer through a radical coupling reaction.

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RESULTS AND DISCUSSION Soot containing Er-based EMFs was synthesized by a directcurrent (dc) arc discharge method33 and extracted using carbon disulfide (CS2). Subsequent separations using highperformance liquid chromatography (HPLC) gave pure samples of Er@C82(I) and Er@C82(II) (Figure S1) in a ratio of 5:2. The analytical HPLC chromatograms (Figure 1a) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra (Figure 1b) of Er@C82(I) and Er@C82(II) confirmed their high purity.

Figure 2. Vis−NIR absorption spectra of Er@C2v(9)-C82 and Er@ Cs(6)-C82 in CS2 at room temperature. The curves are vertically shifted for ease of comparison.

Y@C2v(9)-C8219 and Y@Cs(6)-C82,19 respectively, indicating the same electronic structure for these fullerene cages. The molecular structures of the two Er@C82 isomers are unambiguously determined by means of single-crystal XRD. Both isomers crystallized in the commonly encountered space group C2/m, in which half of the Ni(OEP) molecule and both halves of the C82 cage are present in the asymmetric unit. However, the crystallographic mirror plane does not coincide with any of the symmetry elements of the C82 cages. Accordingly, two cage orientations are present in the system with an equal occupancy of 0.50. The molecular structure of Er@C82(I) with Ni(OEP) is shown in Figure 3, in which the fullerene cage is clearly

Figure 1. (a) HPLC chromatograms of Er@C82(I) and Er@C82(II) on a Buckyprep column (⌀ = 4.6 × 250 mm). Conditions: 20 μL injection volume; 1 mL/min toluene flow; 330 nm detection wavelength; 40 °C. (b) MALDI-TOF mass spectra of Er@C82(I) and Er@C82(II).

Figure 3. Drawing showing the orientation of the endohedral fullerene with respect to nickel porphyrin in Er@C2v(9)-C82· Ni(OEP)·1.93C6H6·0.57CS2 with 10% thermal ellipsoids. Only the major Er site and one cage orientation are shown. Solvent molecules and H atoms are omitted for clarity.

The Vis−NIR absorption spectra of Er@C2v(9)-C82 and Er@Cs(6)-C82 are shown in Figure 2. Er@C2v(9)-C82 shows three distinct peaks at 639, 995, and 1392 nm, and two characteristic peaks at 708 and 1039 nm are identified for Er@ Cs(6)-C82. The absorption of endohedral fullerenes in the visible and NIR regions are mostly due to π−π* transitions of the fullerene cage, which are determined by the isomeric structure and the charge state of the fullerene cage.34 The spectra of Er@C2v(9)-C82 and Er@Cs(6)-C82 are substantially similar to the spectra of the crystallographically characterized

assigned as C2v(9)-C82. Inside the cage, the Er atom shows eight disordered positions with fractional occupancy values of 0.27 (Er1 and Er1a), 0.06 (Er2, Er2a, Er4, and Er4a), 0.15 (Er3), and 0.07 (Er5), respectively, indicating a motional behavior (Figure S3). The major metal atom (Er1) resides over a pentagon−hexagon−hexagon junction with the shortest metal cage−carbon distance of 2.233 Å (C9−Er1), which validates the theoretical value (2.298 Å).35 B

DOI: 10.1021/acs.inorgchem.8b03313 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Figure 4 depicts the molecular structure of Er@Cs(6)-C82 showing the major components together with the cocrystal-

Figure 5. Structures of (a) [Er@Cs(6)-C82]2·2[Ni(OEP)] and (b) 2[Er@C2v(9)-C82]·2[Ni(OEP)]. Only one cage orientation is shown for each system. Minor metal sites and solvent molecules are omitted for clarity.

interactions, respectively. It should be noticed that the solvent molecules in Er@C2v(9)-C82·Ni(OEP)·1.93(C6H6)·0.57(CS2) are different from the dimer of [Ce@C2v(9)-C82]2·2[Ni(OEP)]·4(C6H6), which may affect the dimerization of EMFs. Although a theoretical study proposed a two-electron transfer from Er to the C82 cages,35 our absorption results revealed that Er@C82 isomers have electronic structures identical with that of Y@C82, confirming a three-electron transfer configuration and an unpaired electron delocalized on the fullerene cage. In order to rationalize the experimental results, the spin-density distribution and p-orbital axis vector (POAV) on both fullerene cages have been calculated (Figure 6 and Table S2). Remarkably, several C atoms on Er@Cs(6)-

Figure 4. Drawing showing the orientation of endohedral fullerene with respect to nickel porphyrin in [Er@Cs(6)-C82]2·2[Ni(OEP)]· 3(C6H6)·2(CS2) with 20% thermal ellipsoids. Only the major Er site and one cage orientation are shown. Solvent molecules and H atoms are omitted for clarity.

lized Ni(OEP) molecule. Six disordered positions of the embedded Er atom with fractional occupancy values of 0.41 (Er1), 0.41 (Er1a), 0.05 (Er2), 0.05 (Er2a), 0.04 (Er3), and 0.04 (Er3a) are identified (Figure S3), which are different from the fixed metal atom found in the crystal structure of Y@Cs(6)C82.19 The major Er (Er1) locates beneath a [5,6] bond (C12−C33) and departs slightly from the mirror plane of the cage. The Er···cage distances are 2.242 and 2.314 Å for Er1··· C12 and Er1···C33, respectively, which are close to that (2.233 Å) in Er@C2v(9)-C82. Interestingly, the crystal results show that two Er@Cs(6)-C82 molecules are connected with a C−C single bond of 1.604 Å to form a dimeric structure, a situation similar to that in Y@ Cs(6)-C82 (1.611 Å).19 The two bonded atoms (C5 and C5′) are far away from the internal metal atom. Additionally, our computational results at the B3LYP/3-21G∼SDD level with the basis set superposition error correction reveal that the Er@ Cs(6)-C82·Ni(OEP) dimer is 42.27 kcal/mol more stable than the two monomers, indicating that the dimerization process is thermodynamically favorable. As shown in Figure 5, the cage−cage distances of Er@Cs(6)C82 and Er@C2v(9)-C82 are 1.604 and 3.155 Å, respectively, confirming that no dimerization occurs for Er@C2v(9)-C82. Compared to Er@Cs(6)-C82, a larger distance between the two Ni(OEP) molecules but smaller Ni···cage distance and cage length can be found in Er@C2v(9)-C82; thus, the interaction between the fullerene cages in Er@Cs(6)-C82 and Er@C2v(9)C82 should be attributed to the covalent bond and π−π

Figure 6. Spin-density distribution on the 24 and 44 nonequivalent cage C atoms for Er@C2v(9)-C82 and Er@Cs(6)-C82, respectively. The numbering schemes for the C atoms are shown in Figure S3. Data are obtained from the optimized structures of monomeric Er@Cs(6)-C82· Ni(OEP) and Er@C2v(9)-C82·Ni(OEP).

C82 exhibit much higher spin-density values, and C5 possesses the highest value of 0.211. The situation is similar to that of Y@Cs(6)-C82, in which the C atom at the site of dimerization possesses the highest spin-density value (0.22) among all of the C atoms.19 Meanwhile, C5 possesses a relatively higher POAV value (10.37°) than other cage C atoms, which more feasibly C

DOI: 10.1021/acs.inorgchem.8b03313 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 1. Redox Potentials (V vs Fc/Fc+)a of Er@Cs(6)-C82 and Er@C2v(9)-C82

bond with each other to release the strain. In the case of Er@ C2v(9)-C82, the highest spin density is 0.077, which belongs to C14, around one-third the value of C5 in the Cs(6) case. Moreover, the C14 atom is actually not accessible because it is covered by the porphyrin unit (Figure 3). Therefore, the different behaviors between Er@C2v(9)-C82 and Er@Cs(6)-C82 in identical crystallization conditions should be attributed to the significant difference in the spin-density distribution. The redox properties of Er@Cs(6)-C82 and Er@C2v(9)-C82 were studied by cyclic voltammetry (CV; Figure 7) and

species Er@Cs(6)-C82 Er@C2v(9)-C82

ox

E1

−0.09 +0.08

red

E1

−0.44 −0.42

red

red

E2

E3

−1.46 −1.40b b

−2.04

red

E4

−2.46 −2.18

ΔEgap 0.35 0.50

a

Half-cell potentials in o-DCB unless otherwise addressed. bTwoelectron process.

crystalline state, but Er@C2v(9)-C82 exists as a monomeric structure under identical conditions. DFT calculations suggest that regioselective dimerization can be attributed to localization of the abnormally high spin density on a special cage C atom of Er@Cs(6)-C82 and the dimerization process is thermodynamically favorable.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Synthesis and Isolation of Er@C2v(9)-C82 and Er@Cs(6)-C82. Soot containing Er-based EMFs was synthesized using a DC arc discharge method. Briefly, a core-drilled graphite rod filled with graphite/Er2O3 (molar ratio: Er/C = 1:15) was burned under a 250 Torr helium atmosphere with a power of 110 A × 20 V. Then, the soot was collected and sonicated in carbon disulfide for 1 h under an argon atmosphere. After solvent removal, the extracted fullerenes were dissolved in toluene, and the solution was subjected to HPLC separations. Experimental details are described in the Supporting Information. General Characterization. HPLC was conducted on an LC-908 machine (Japan Analytical Industry Co., Ltd.) with toluene as the mobile phase. MALDI-TOF mass spectrometry was measured on a BIFLEX III spectrometer (Bruker Daltonics Inc., Germany). Vis− NIR spectra were obtained from a PE Lambda 750S spectrophotometer in carbon disulfide. CV and DPV were measured in o-DCB with 0.05 M TBAPF6 as the supporting electrolyte at a glassy carbon working electrode with a CHI660E workstation. Single-Crystal XRD Measurements of Er@C82 Isomers. Crystalline blocks of two Er@C82 isomers were obtained by layering a benzene solution of Ni(OEP) over a carbon disulfide solution of Er@C82 at 0 °C. Over a 20-day period, the two solutions diffused together, and black crystals formed. Single-crystal XRD measurement of Er@Cs(6)-C82 was performed at 173 K on a Bruker D8 QUEST machine equipped with a CMOS camera (Bruker AXS Inc., Germany). Crystallographic characterization of Er@C2v(9)-C82 was performed at 100 K using synchrotron radiation (0.65250 Å) with a MarCCD detector at the BL17B station of the Shanghai Synchrotron Radiation Facility. The Multiscan method was used for absorption corrections. The structures were solved by direct methods and refined with SHELXL-2018/1.36 CCDC 1878016 (Er@C2v(9)-C82) and CCDC 1878017 (Er@Cs(6)-C82) contain the supplementary crystallographic data for this paper.

Figure 7. CV curves of (a) Er@Cs(6)-C82 and (b) Er@C2v(9)-C82. Conditions: working electrode, glassy carbon electrode; counter electrode, Pt wire; reference electrode, Ag wire; supporting electrolyte, 0.05 M TBAPF6 in o-DCB with ferrocene as the internal standard. Scan rate: 100 mV/s.

differential pulse voltammetry (DPV; Figure S4) in odichlorobenzene (o-DCB) containing 0.05 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. It is clear that Er@Cs(6)-C82 has four reversible reduction steps and one reversible oxidation process, while Er@C2v(9)-C82 exhibits three reversible reduction steps and one reversible oxidation process. The first reduction potential value for Er@Cs(6)-C82 is close to that of Er@C2v(9)-C82, indicating that the reducibility of these two compounds is similar. Interestingly, two peaks were observed in the second reduction process for Er@Cs(6)-C82 by means of DPV, and the shape of the CV curve suggests a two-electron-transfer process. Meanwhile, the DPV curve of Er@C2v(9)-C82 shows that the integral peak area of the second reduction is around twice than that of each of the other redox peaks, and the shape of the CV curve suggests a simultaneous two-electron-transfer process. These results indicate that the lowest unoccupied molecular orbitals of Er@Cs(6)-C82 and Er@C2v(9)-C82 are degenerate (Table 1).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03313. HPLC separation of Er@C2v(9)-C82 and Er@Cs(6)-C82, packing structures of Er@C2v(9)-C82·Ni(OEP) and [Er@Cs (6)-C82]2 ·2Ni(OEP), erbium disorder and atom numbering scheme for (a) Er@C2v(9)-C82 and (b) Er@Cs(6)-C82, DPV curves of Er@C2v(9)-C82 and Er@Cs(6)-C82, crystallographic data of Er@C2v(9)-C82· Ni(OEP)·1.93(C6H6)·0.57(CS2) and [Er@Cs(6)-C82]2· 2Ni(OEP)·3(C6H6)·2(CS2), spin-density and POAV values of C atoms in Er@C2v(9)-C82 and Er@Cs(6)C82, and computational method (PDF)

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CONCLUSIONS In summary, two Er@C82 isomers have been isolated and characterized as Er@C2v(9)-C82 and Er@Cs(6)-C82, respectively. A dimeric structure for Er@Cs(6)-C82 is observed in the D

DOI: 10.1021/acs.inorgchem.8b03313 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Accession Codes

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CCDC 1878016−1878017 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Takeshi Akasaka: 0000-0002-4073-4354 Yunpeng Xie: 0000-0002-4065-9809 Wenhuan Huang: 0000-0003-1474-9373 Xing Lu: 0000-0003-2741-8733 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from NSFC (Nos. 51472095, 51672093, 51772111, and 51602097) is gratefully acknowledged. We thank the staff in BL17B beamline of 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.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03313 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03313 Inorg. Chem. XXXX, XXX, XXX−XXX