Encapsulation of an Ionic Bond in Fullerenes: What is the Difference

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Encapsulation of an Ionic Bond in Fullerenes: What is the Difference? Yiyun Wang, Yingying Shi, Xingting Fan, Juan Ren, and Xianglei Kong* State Key Laboratory and Institute of Elemento-Organic Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University Tianjin 300071, China

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S Supporting Information *

how can we restrict a typical ionic bond inside fullerenes, and if so, how can the encaging affect the ionic bond? Considering our recent progress on the generation of new EMFs in the laser ablation experiments with the precursor of graphene,20 here we report some experimental and calculation proofs of engaging an ionic bond in fullerenes, and the cage effects on the ionic bond are also discussed. By laser ablation of a mixture of graphene/LuCl3/LiCl on a metal plate, ions of LuClC2n+ (2n = 90−190) were clearly identified according to their m/z’s and isotropic distributions with the aid of the high mass resolution of an FT ICR mass spectrometer (Figure 1). The mass spectrum obtained in a low mass region is shown in Figure S1, and only fullerene ions were observed there. Also, the mass spectrum obtained using a mixture of graphene/LuCl3 was shown in Figure S2. From it, we can find that the addition of LiCl favors the production of

ABSTRACT: Endohedral metallofullerene ions containing an ionic bond of Lu−Cl were observed in a mass spectrum for the first time. A theoretical calculation has been performed on the example of LuCl@C90. The two most stable isomers are LuCl@C2(99917)-C90 and LuCl@C2(99914)-C90. Interestingly, both encaged Lu− Cl bonds have potential curves quite different from the Morse curve and have some energy-preferential directions relative to their outside carbon cages.

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or many years, encapsulating different atoms or molecules inside fullerenes was a very interesting and challenging task for chemists. Besides the curiosity of scientists, the unique properties and application potentials of these novel materials are also great motives for such studies.1−5 By now, efforts have been greatly devoted to encaging metallic clusters into fullerenes to form different endohedral metallofullerenes (EMFs).1−5 In recent years, with the improved method of molecular surgery, several important molecule endofullerenes, including H2@C60 and H2O@C60, have been synthesized successfully.6−14 These species have provided us a valuable model to understand how nanoscale confinement influences the properties of the enclosed species. The results are important for both EMFs and other materials. For example, increasing attention has been paid recently to the field of applying carbon nanotubes (CNTs) to modulate the properties and behavior of confined molecules and nanoparticles.15−19 Without a doubt, these studies can greatly benefit from the research in molecule endofullerenes and their size effects or shape effects. On the other hand, it is seldom reported or suggested how a typical ionic bond can be encaged by fullerenes or CNTs. People have known ionic complexes for many years, but it is still difficult to isolate and stabilize a single ionic bond. A very interesting and close example is HF@C60, which has been synthesized and studied by Krachmalnicoff et al.14 It has been found that its electric dipole moment is only 25% of that for isolated HF, indicating the strong shielding effect caused by the carbon cage.14 Unlike covalent bonds, ionic bonds have no sharing electrons, thus they have no fixed pairing relationship between ions. The property makes the confinement of a single ionic bond very difficult. However, fullerenes, with well-defined three-dimensional structures, can provide an ideal model for such a kind of study, both for experimental and theoretical chemists. It will be very interesting to think of it in this way: © XXXX American Chemical Society

Figure 1. Mass spectrum of the ions of LuClC2n+, generated by laser ablation of the mixture of graphene/LuCl3/LiCl. The theoretical mass spectra of LuClC122+ and LuClC124+ are shown in the sub-figures, respectively. Received: January 31, 2019

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DOI: 10.1021/acs.inorgchem.9b00296 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry LuClC2n+. Several hints support the endohedral structures of these ions: (1) Oxygen gas was introduced into the FT ICR cell as the reaction gas after the isolation of corresponding peaks; no reaction product ions were observed between the metallofullerene ions and oxygen gas (Figure S3). (2) Only species of LuClC2n+ were identified in these experiments, and no species of LuClC2n+1+ or LuClmC2n+ (m > 1) was observed, indicating that they are not chlorine-doped fullerenes or chlorofullerenes.21−23 (3) The distribution of these ions was consistent with our previous studies on other endohedral species.20 In order to better understand the structures of these ions, LuCl@C90 was selected as an example to be investigated by systematic theoretical calculations here. Density functional theory (DFT) methods of B3LYP,24−26 M06-2X,27,28 and TPSSH29,30 were applied here, and a different basis set of SDD was also applied for Lu atoms with the B3LYP method. The relative energies and HOMO−LUMO gaps of the seven most stable isomers of LuCl@C90 obtained on the level of B3LYP/6311G(d) ∼ CEP-31G are listed in Table 1 (their structures are

LuCl@C2(99917)-C90 and LuCl@C2(99914)-C90 and those for corresponding empty cages are also obtained (Figure S4). The corresponding cations LuCl@C90+ showed similar results (Table S4). In addition, we calculated these isomers with both single and triplet spin multiplicities, and the latter proved to be higher in energy for all of them (Table S5). On the basis of equilibrium statistical thermodynamic analysis, the relative concentrations of these EMF isomers were evaluated to explore their thermodynamic stabilities under different temperatures (Figure S6). It turns out that LuCl@C2(99917)-C90 dominates in a narrow temperature range of 0−500 K. With the temperature increasing, its relative concentration decreases rapidly, then is exceeded by LuCl@ C2(99914)-C90 at 650 K and by LuCl@C2(99913)-C90 at 3000 K. At about 700 K, the relative concentration of LuCl@ C2(99914)-C90 rises to its maximum of 49%. Though its relative concentration decreases gradually, it still occupies the guiding position before 5000 K. Geometry structures of the two lowest energy isomers of LuCl@C90 are presented in Figure 2. It is worth mentioning that for all optimized isomers, the Cl atoms are located in centers of cages and Lu atoms are between Cl atoms and cages. It is well-known that an isolated ionic bond has a typical Morse or Morse long-range potential. The Morse potential curve of ionic species of LuCl2+ is shown in Figure 3; the equilibrium

Table 1. Relative Energies and HOMO-LUMO Gaps of LuCl@C90 Isomers Obtained with the Method of B3LYP/6311G(d) ∼ CEP-31G Sprial ID

IPR ID

sym.

ΔE (kcal/mol)

gap (eV)

C90-99917 C90-99914 C90-99918 C90-99913 C90-99912 C90-99907 C90-99902

45 42 46 41 40 35 30

C2 C2 C2v C2 C2v Cs C1

0 1.1 2.4 5.0 5.6 6.8 10.5

1.61 1.82 1.82 1.35 1.68 1.53 1.45

shown in Figure S4 and more calculation results about other low-energy isomers are listed in Table S1). And the results obtained with the other two DFT methods also give similar results (Tables S2, S3). Among those isomers, LuCl@ C2(99917)-C90, with a HOMO−LUMO gap of 1.61 eV, has the lowest energy. The two isomers with highest HOMO− LUMO gaps (1.82 eV) are LuCl@C2(99914)-C90 and LuCl@ C2v(99918)-C90, whose relative energies are 1.1 and 2.4 kcal/ mol higher than that of LuCl@C2(99917)-C90, respectively. These large HOMO−LUMO gaps indicate their excellent chemical stability. The main frontier molecular orbitals of

Figure 3. Potential curves of LuCl2+, LuCl, LuCl@C2(99917)-C90, and LuCl@C2(99914)-C90.

Figure 2. DFT-optimized molecular structures: (a) LuCl@C2(99917)-C90 and (b) LuCl@C2(99914)-C90. Carbon atoms of the fullerene cages are shown in gray. The Lu atom is red, and Cl atom is green. B

DOI: 10.1021/acs.inorgchem.9b00296 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 4. Potential energy curves of LuCl@C2(99917)-C90 obtained by rotating the Lu−Cl bond in a fixed plane: (a) in the first plane and (b) in the second plane, which is perpendicular to plane one. And those for LuCl@C2(99914)-C90 are shown in c and d, respectively.

bond distance between Lu and Cl is calculated to be 2.35 Å. For the neutral species of LuCl, the Morse potential becomes a little flat, and the equilibrium bond distance is elongated to be 2.55 Å. The encaged ionic bond of Lu−Cl, however, has a different potential curve. Due to the bound of the outside carbon cage, the system has come to be an infinite potential well. The potential cannot be depicted by a Morse or Morse long-range potential, nor by a parabolic potential. Typically, it is thought that the encaged LuCl unit would transfer two electrons to the fullerene cage. Natural population analysis (NPA) shows that the charges of Lu and Cl atoms in the isomer of LuCl@C2(99917)-C90 (Figure 2a) are +1.536 and −0.482, respectively (Table S6). So the equilibrium bond distance of Lu−Cl in LuCl@C2(99917)-C90 should be close to that of LuCl2+. However, calculation shows that the value in LuCl@C2(99917)-C90 is 2.49 Å, very close to that of neutral LuCl. The potential curve of another isomer, LuCl@ C2(99914)-C90, is also shown in Figure 3, and both results are very similar. Interestingly, both potential wells seem deeper than those of LuCl or LuCl2+, indicating the encaged ionic bonds are more stable. Due to the change of their potential curves, it is suggested that the nonharmonic effect in their high vibration modes should be weakened. Because it is only based on electrostatic attraction between oppositely charged ions, ionic bonds have no properties of directionality and saturability. This isotropic property makes the orientation of isolated ionic bonds random. Does that mean the Lu atom, for example, in the isomer of LuCl@ C2(99917)-C90, can rotate freely? The answer depends on the interaction between the metal atoms and the outside carbon cage. In fact, in the process of searching the lowest-energy isomer for the fullerene of LuCl@C2(99917)-C90, several isomers differentiating only by positions of Lu atoms were found with different energies (Figure S7). To get a full picture of it, the potential energy of the system was scanned by rotating the Cl−Lu bond in a fixed plane. A plane was first selected based on two structures of LuCl@C2(99917)-C90

(Figure S8). The potential changes according to the rotation of the Cl−Lu bond in this plane are shown in Figure 4a. Interestingly, up to seven energy minima were found in the process, and their energies are different from each other by 20−220 kcal/mol. If we change the rotation plane to the one perpendicular to the former, the results are quite different. As shown in Figure 4b, only four energy minima exist in this equatorial plane, and the energy barrier is much higher, up to 600 kcal/mol. For the isomer of LuCl@C2(99914)-C90, similar results were observed (Figure 4c,d), except the number of energy minima was much less (based on the plane shown in Figure S9). Surely, the interaction between the ionic bond and fullerenes makes their structures more diverse. Several important points should be mentioned or further discussed: (1) Different from isolated ionic bonds and ionic bonds in crystals, the ionic bonds restricted in fullerenes have quite different potential curves, and several energy-preferential directions exist relative to outside carbon cages. (2) It is also suggested that the electric dipole moment of the ionic bond in endohedral fullerenes would decreased greatly, compared to that of the original ionic bond. (3) The existence of the ionic bond enriches the region selectivity for possible reactions on carbon atoms in fullerenes. (4) The structural diversity of fullerenes makes the potential and pseudodirection of the encaged ionic bond designable. (5) The result also shows a different experimental way to generate these species other than the method of molecular surgery. In summary, a kind of new EMF containing an ionic bond in fullerenes, LuCl@C2n (90 < 2n < 190), has been observed in a mass spectrum for the first time. Systematic theoretical investigation has been performed on the example of endohedral metallofullerene of LuCl@C90 at the B3LYP/6311G(d) ∼ CEP-31G level. The two isomers with the lowest relative energies are LuCl@C2(99917)-C90 and LuCl@ C2(99914)-C90, with large HOMO−LUMO gaps of 1.61 and 1.82 eV, respectively. Interestingly, the potential wells in both C

DOI: 10.1021/acs.inorgchem.9b00296 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(10) Turro, N. J.; Marti, A. A.; Chen, J. Y.-C.; Jockusch, S.; Lawler, R. G.; Ruzzi, M.; Sartori, E.; Chuang, S.-C.; Komatsu, K.; Murata, Y. Demonstration of a chemical transformation inside a fullerene. The reversible conversion of the allotropes of H2@C60. J. Am. Chem. Soc. 2008, 130, 10506−10507. (11) Kurotobi, K.; Murata, Y. A single molecule of water encapsulated in fullerene C60. Science 2011, 333, 613−616. (12) Li, Y. B.; Lou, N.; Xu, D.; Pan, C. W.; Lu, X.; Gan, L. B. Oxygen-delivery materials: synthesis of an open-cage Fullerene derivative suitable for encapsulation of H2O2 and O2. Angew. Chem., Int. Ed. 2018, 57, 14144−14148. (13) Almeida Murphy, T.; Pawlik, Th.; Weidinger, A.; Höhne, M.; Alcala, R.; Spaeth, J.-M. Observation of atomlike nitrogen in nitrogenimplanted Solid C60. Phys. Rev. Lett. 1996, 77, 1075−1078. (14) Krachmalnicoff, A.; Bounds, R.; Mamone, S.; Alom, S.; Concistrè, M.; Meier, B.; Kouřil, K.; Light, M. E.; Johnson, M. R.; Rols, S.; Horsewill, A. J.; Shugai, A.; Nagel, U.; Rõõm, T.; Carravetta, M.; Levitt, M. H.; Whitby, R. J. The dipolar endofullerene HF@C60. Nat. Chem. 2016, 8, 953−957. (15) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Water conduction through the hydrophobic channel of a carbon nanotubes. Nature 2001, 414, 188−190. (16) Serp, P.; Castillejos, E. Catalysis in carbon nanotubes. ChemCatChem 2010, 2, 41−47. (17) Pan, X. L.; Bao, X. H. The effects of confinement inside carbon nanotubes on catalysis. Acc. Chem. Res. 2011, 44, 553−562. (18) Xiao, J. P.; Pan, X. L.; Zhang, F.; Li, H. B.; Bao, X. H. Sizedependence of carbon nanotube confinement in catalysis. Chem. Sci. 2017, 8, 278−283. (19) Agrawal, K. V.; Shimizu, S.; Drahushuk, L. W.; Kilcoyne, D.; Strano, M. S. Observation of extreme phase transition temperatures of water confined inside isolated carbon nanotubes. Nat. Nanotechnol. 2017, 12, 267−273. (20) (a) Kong, X. L.; Li, S. Q.; Zhang, S.; Huang, Y.; Cheng, Y. Large carbon cluster anions generated by laser ablation of graphene. J. Am. Soc. Mass Spectrom. 2011, 22, 2033−2041. (b) Kong, X. L.; Huang, Y.; Chen, Y. S. Difference in formation of carbon cluster cations by laser ablation of graphene and graphene oxide. J. Mass Spectrom. 2012, 47, 523−528. (c) Kong, X. L.; Bao, X. D. Generation of View the MathML source M@C2n+ (M = Ca, Sr, Ba, 2n = 50−230) by laser ablation of graphene. Int. J. Mass Spectrom. 2014, 372, 1−7. (d) Bao, X. D.; Kong, X. L. Generation of M@C2n+ (M = K, Rb, Cs, 2n = 80−220) by laser ablation of graphene. Rapid. Rapid Commun. Mass Spectrom. 2015, 29, 1774−1778. (e) Mu, L.; Yang, S. M.; Feng, R. X.; Kong, X. L. Encapsulation of platinum in fullerenes: is that possible? Inorg. Chem. 2017, 56, 6035−6038. (f) Wang, Y. Y.; Ren, J.; Mu, L.; Kong, X. L. Metallofullerene ions of LunC2m+ (1≤ n≤ 9, 50≤ 2 m≤ 198) generated by laser ablation of graphene/LuCl3. Int. J. Mass Spectrom. 2017, 422, 105−110. (g) Kong, X. L.; Bao, X. D. Formation of endohedral metallofullerene (EMF) ions of (M= La, Y, n ≤ 6, 50 ≤ 2m ≤ 194) in the laser ablation process with graphene as precursor. Rapid Commun. Mass Spectrom. 2017, 31, 865−872. (h) Wang, Y. Y.; Ma, L. F.; Ren, J.; Mu, L.; Kong, X. A systematic study on the generation of multimetallic lanthanide fullerene ions by laser ablation mass spectrometry. Rapid Commun. Mass Spectrom. 2018, 32, 1396− 1402. (21) Xie, S. Y.; Gao, F.; Lu, X.; Huang, R. B.; Wang, C. R.; Zhang, X.; Liu, M. L.; Deng, S. L.; Zheng, L. S. Capturing the labile fullerene [50] as C50Cl10. Science 2004, 304, 699. (22) Troshin, P. A.; Avent, A. G.; Darwish, A. D.; Martsinovich, N.; Abdul-Sada, A. K.; Street, J. M.; Taylor, R. Isolation of two sevenmembered ring C58 fullerene derivatives: C58F17CF3 and C58F18. Science 2005, 309, 278−281. (23) Kuvychko, I. V.; Streletskii, A. V.; Shustova, N. B.; Seppelt, K.; Drewello, T.; Popov, A. A.; Strauss, S. H.; Boltalina, O. V. Soluble chlorofullerenes C60Cl2,4,6,8,10. synthesis, purification, compositional analysis, stability, and experimental/theoretical structure elucidation, including the X-ray structure of C1-C60Cl10. J. Am. Chem. Soc. 2010, 132, 6443−6462.

isomers are deeper than those for naked species of LuCl and LuCl2+, indicating their different properties. What’s more, both Lu−Cl bonds restricted in two fullerenes have some energypreferential directions relative to their outside carbon cages. This study not only opens the possibilities of new access to ionic bond entrapped fullerenes but also helps us to better understand the properties of these “simple” ionic bonds in confined space.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00296. Experimental and calculational details, other mass spectra, DFT-optimized structures and their main frontier molecular orbitals, relative concentrations of LuCl@C90 isomers, the plane determining method, and structure parameters of some species (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +89 22 23509564. E-mail: [email protected]. cn. ORCID

Xianglei Kong: 0000-0002-8736-6018 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Nos. 21475065, 21627801) is gratefully acknowledged.

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REFERENCES

(1) Heath, J. R.; O’Brien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Tittel, F. K.; Smalley, R. E. Lanthanum complexes of spheroidal carbon shells. J. Am. Chem. Soc. 1985, 107, 7779−7780. (2) Chaur, M. N.; Melin, F. A.; Ortiz, L.; Echegoyen, L. Chemical, electrochemical, and structural properties of endohedral metallofullerenes. Angew. Chem., Int. Ed. 2009, 48, 7514−7538. (3) Popov, A. A.; Yang, S. F.; Dunsch, L. Endohedral Fullerenes. Chem. Rev. 2013, 113, 5989−6113. (4) Lu, X.; Feng, L.; Akasaka, T.; Nagase, S. Current status and future developments of endohedral metallofullerenes. Chem. Soc. Rev. 2012, 41, 7723−7760. (5) Zhang, J. Y.; Bowles, F. L.; Bearden, D. W.; Ray, W. K.; Fuhrer, T.; Ye, Y. Q.; Dixon, C.; Harich, K.; Helm, R. F.; Olmstead, M. M.; Balch, A. L.; Dorn, H. C. A missing link in the transformation from asymmetric to symmetric metallofullerene cages implies a top-down fullerene formation mechanism. Nat. Chem. 2013, 5, 880−885. (6) Morinaka, Y.; Tanabe, F.; Murata, M.; Murata, Y.; Komatsu, K. Rational synthesis, enrichment, and 13C NMR spectra of endohedral C60 and C70 encapsulating a helium atom. Chem. Commun. 2010, 46, 4532−4534. (7) Khong, A.; Jiménez-Vázquez, H. A.; Saunders, M.; Cross, R. J.; Laskin, J.; Peres, T.; Lifshitz, C.; Strongin, R.; Smith, A. B. An NMR study of He2 inside C70. J. Am. Chem. Soc. 1998, 120, 6380−6383. (8) Komatsu, K.; Murata, M.; Murata, Y. Encapsulation of molecular hydrogen in fullerene C60 by organic synthesis. Science 2005, 307, 238−240. (9) Zhang, R.; Murata, M.; Aharen, T.; Wakamiya, A.; Shimoaka, T.; Hasegawa, T.; Murata, Y. Synthesis of a distinct water dimer inside fullerene C70. Nat. Chem. 2016, 8, 435−441. D

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Inorganic Chemistry (24) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behaviour. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (25) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (26) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (27) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215−241. (28) Zhao, R. S.; Guo, Y. J.; Zhao, P.; Ehara, M.; Nagase, S.; Zhao, X. Warning to Theoretical Structure Elucidation of Endohedral Metallofullerenes. J. Phys. Chem. C 2016, 120, 1275−1283. (29) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta− Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91, 146401−146404. (30) Dreuw, A.; Headgordon, M. Single-Reference ab Initio Methods for the Calculation of Excited States of Large Molecules. Chem. Rev. 2005, 105, 4009−4037.

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DOI: 10.1021/acs.inorgchem.9b00296 Inorg. Chem. XXXX, XXX, XXX−XXX