Structures of Gd3N@C80 Prato Bis-Adducts: Crystal Structure

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Cite This: J. Am. Chem. Soc. 2019, 141, 10988−10993

Structures of Gd3N@C80 Prato Bis-Adducts: Crystal Structure, Thermal Isomerization, and Computational Study Olesya Semivrazhskaya,† Adrian Romero-Rivera,§ Safwan Aroua,† Sergey I. Troyanov,⊥ Marc Garcia-Borras̀ ,§ Steven Stevenson,‡ Sílvia Osuna,*,§,# and Yoko Yamakoshi*,† †

Laboratorium für Organische Chemie, ETH Zürich, Vladimir-Prelog-Weg 3, CH8093 Zürich, Switzerland CompBioLab, Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus Montilivi, 17071 Girona, Catalonia, Spain ⊥ Department of Chemistry, Moscow State University, Leninskie gory, 119991 Moscow, Russia ‡ Department of Chemistry, Purdue University Fort Wayne, Fort Wayne, Indiana 46805, United States # ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain

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§

S Supporting Information *

Recently, multiple functionalizations of fullerenes have become important for the production of functional and smart materials such as fullerene-based photovoltaic devices.21−23 In a recent study on bis-Prato-addition to M3N@C80 (M = Y, Gd), we observed highly regioselective generation of an asymmetric [6,6][6,6]-bis-adduct as a main product, but without detail of the structure.24 In this study, we solved a crystal structure of Prato-minor-bis-adduct of Gd3N@C80the first structure of Gd3N@C80 derivatives. In combination with visible-near infrared (vis-NIR) spectra, isomerization study, and theoretical calculations, the structure of the asymmetric major-bis-adduct was also elucidated. In the obtained structure of minor-bis-adduct, the Gd3N cluster inside the carbon cage was completely planar and occupied the extra internal space of the Ih-C80 cage formed by the conversion of sp2 carbons to sp3, which determines the regioselectivity of the adduct generation. While tether-based chemistries are generally required for the regioselective synthesis of bis-adducts of empty Ih-C60,25,26 bisaddition reactions of metallofullerenes using Diels−Alder,27 Bingel−Hirsch,28,29 and other reactions30−32 often proceed regioselectively. An initial report on Diels−Alder reaction of Gd3N@Ih-C80 by Stevenson27 provided little structural detail, but further studies on Bingel−Hirsch bis-addition reactions of Sc3N@C78 by Dorn28 provided the structural details revealing regioselective addition sites, controlled by the endohedral trimetallic nitride cluster, which was rationalized by the calculated lowest unoccupied molecular orbital surface electron density of the monoadduct. Furthermore, the X-ray crystal structures of the regioselectively obtained products of biscarbene addition to La2@C72 and La2@Ih-C80 by Akasaka and Nagase30,32 showed longer La−La distances, indicating that regioselectivity was governed by the pyramidalization and charge density of the addition sites on the carbon cage, being controlled by the endohedral metal inside the carbon cage.33 The Prato bis-additions of M3N@Ih-C80 (M = Sc, Er, Lu, Y, Gd) were reported more recently. In our own studies, we reported regioselective generation of one major-[6,6][6,6]-bisadduct from each Y3N@C80 and [email protected] Simulta-

ABSTRACT: The structures of two bis-ethylpyrrolidinoadducts of Gd3N@Ih-C80, obtained by regioselective 1,3-dipolar cycloadditions, were elucidated by single crystal X-ray, visible-near infrared (vis-NIR) spectra, studies on their thermal isomerization, and theoretical calculations. The structure of the minor-bis-adduct reveals a C2-symmetric carbon cage with [6,6][6,6]-addition sites and with an endohedral Gd3N cluster that is completely flattened. This is the first example of a crystal structure of Gd3N@Ih-C80 derivatives. The structure of the major-bisadduct was inferred by the vis-NIR spectrum being corresponded to the structure of a previously reported major-bis-adduct of Y3N@Ih-C80 known to have an asymmetric [6,6][6,6]-structure. Based on experimental results showing that the minor-bis-adduct of Gd3N@IhC80 isomerized to the major-adduct, a possible second addition site was elucidated with support from density functional theory calculations.

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hemical functionalization of trimetallic nitride templateendohedral metallofullerenes (TNT-EMF) is essential for constructing new functional organo-metallofullerene materials for applications such as photovoltaics1,2 and magnetic resonance imaging-contrast agents (MRI-CAs).3−5 The functionalization of TNT-EMF has been achieved through different types of reactions including Diels−Alder reaction,6 1,3-dipolar cycloaddition (Prato reaction),7,8 [2+1]-cyclopropanation,9 [2+2]-cycloaddition,10 and photochemical reactions.11,12 These studies showed that the reactivity of cycloadditions was related to Mayer Bond Order and pyramidalization angle of addition sites.13 In particular, in the widely used Prato reactions, it was proposed that the size of the endohedral metal clusters of M3N@Ih-C80 (or M3N@C80, M (ionic radii (Å)) = Sc (0.75), Lu (0.85), Y (0.90), Gd (0.94)) highly contributed to determining the regio-ratio of monoadducts ([6,6]- versus [5,6]-adduct); thermodynamic products were often obtained after the mutual isomerization of [6,6]- and [5,6]-monoadducts.14−20 © 2019 American Chemical Society

Received: May 31, 2019 Published: July 3, 2019 10988

DOI: 10.1021/jacs.9b05603 J. Am. Chem. Soc. 2019, 141, 10988−10993

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Journal of the American Chemical Society

revealed two rather sharp and blue-shifted (ca. 60 nm) absorption maxima at 572 and 756 nm. This vis-NIR spectrum of minor-2 was not identical to any of previously reported Prato bis-adducts of M3N@C80 (M = Y, Sc, Er, Lu)34,35 and suggested that this minor-bis-2 had a new addition pattern. To obtain structural information on these bis-adducts 1 and 2, we attempted to prepare single crystals. After exploring numerous conditions, we determined a liquid−liquid diffusion technique using a combination of CS2, benzene, and MeOH at 5 °C provided a single crystal of minor-bis-adduct-2 of Gd3N@ C80 suitable for X-ray diffraction study. The structure of minorbis-2 was successfully solved, the first crystal structure of a Gd3N@Ih-C80 derivative, as shown in Figure 2.

neously, the Echegoyen group reported the regioselective generation of bis-adducts from Sc3N@C80 (3 isomers), Lu3N@ C80 (2 isomers),34 additional bis-adducts of Y3N@C80 (totally 3 isomers), Lu3N@C80 (totally 4 isomers), and three isomers for Er3N@C80 (with identical structures to the ones for Y3N@ C80).35 Taking into account the large number of geometrically possible bis-adducts (91 regioisomers) on an Ih-C80 cage34 and that bis-Prato reactions on empty Ih-C60 proceed with low regioselectivity,36 the Prato bis-addition reaction of TNT-EMF can be considered to be highly regioselective due to the endohedral metal cluster. In the present study, Prato reaction of Gd3N@Ih-C80 was carried out in the presence of excess of N-ethylglycine (50 equiv) and paraformaldehyde (400 equiv) at 120 °C for 15 min (Figure 1a).37 As seen in the high performance liquid

Figure 2. (a) X-ray crystal structure of minor-bis-2 Gd3N@ C80[C4H9N]2 (side and top views) and the Schlegel diagram showing the addition sites (red thick bonds). The black dot in the center represents C2 axis. The hexagons coordinated to the endohedral Gd atoms are highlighted with solid or dotted black lines. (b) The geometry of the endohedral Gd3N cluster with out-of-plane displacement (0.52 Å) of the nitrogen atom inside the pristine Gd3N@C80 and the planar configuration inside the minor-bis-adduct2.

Figure 1. (a) Prato reactions of Gd3N@C80, (b) HPLC diagram of reaction mixture (Buckyprep 4.6 mm i.d. × 250 mm, toluene, 1 mL/ min, 390 nm), (c) MALDI MS of minor-bis-adduct-2 with simulated pattern, and (d) vis-NIR spectra of bis-adducts major-1, minor-2 of Gd3N@C80 and major isomer of Y3N@C80 (known as [6,6][6,6]asymmetric adduct).24 aOnly the [6,6]-monoadduct was observed under a short reaction interval (15 min). bIsolated minor-adduct-2 was successfully crystallized and subjected to X-ray crystallography (Figure 2). Vis-NIR spectra were obtained from HPLC PDA detector (eluent: toluene).

The crystal structure of the minor-bis-2 showed a C2symmetric [6,6][6,6]-structure. Two of the three Gd atoms of the endohedral Gd3N were located very close to the sp3 cage carbons on the [6,6]-addition sites being coordinated to two hexagons (the solid black line in the Schlegel diagram) from inside, while the third Gd atom was positioned over the center of hexagon at the bottom (side view in Figure 2a). Interestingly, the Gd3N cluster in the minor-bis-adduct-2 was strictly planar, being much less strained compared to pristine Gd3N@C80, in which the N atom was out-of-plane by 0.52 Å (Figure 2b).39 The Gd−N distances showed some variations with two slightly elongated Gd−N bonds directed toward the sp3 addition sites (2.105 Å) and one relatively shorter (2.07 Å). These Gd−N distances fall into a similar range as the ones in pristine Gd3N@C80 (2.038(8), 2.085(4), and 2.117(5) Å).39 It is known that pyramidalization of the endohedral M3N cluster is related to the thermodynamic stability of the entire M3N@C80 adduct. For example, the [5,6]-monoadduct of

chromatography (HPLC) diagram of the reaction mixture (Buckyprep, toluene, Figure 1b), two peaks with retention times at 12.7 (major-1) and 14.0 (minor-2) min were isolated and identified as bis-adducts by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) (Figures 1c and S1). The peak of the minor-bis-adduct 2 was further purified by HPLC (5PBB column, toluene) (Figure S2). The vis-NIR spectra of bis-adducts major-1 and minor-2 are shown in Figure 1d along with the previously reported asymmetric major-[6,6][6,6]-bis-adduct of [email protected] The spectra of the Gd3N@C80 major-bis-1 and the Y3N@C80 major-bis-adduct were identical with absorption maxima at 636 and 816 nm, indicating that the addition sites of both major Prato-bis-adducts were presumably identical with the same conjugation system. The spectrum of the minor-bis-2 10989

DOI: 10.1021/jacs.9b05603 J. Am. Chem. Soc. 2019, 141, 10988−10993

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Journal of the American Chemical Society

Figure 3. (a) HPLC traces of thermal isomerization of minor-bis-adduct-2 at 120 °C in o-DCB (Buckyprep 4.6 mm i.d. × 250 mm, toluene, 1.0 mL/min, 390 nm).43 (b, c) Relative stabilities of the most stable bis-adducts 2 and 1 and computed isomerization pathways A (shown in pink) and B (in teal) from the linear transit (LT) calculations. All energies are expressed in kcal/mol and are computed with respect to bis-adduct-1. The pyramidalization of the Gd3N unit in the two most stable bis-adducts is also indicated (DFT calculations were done at the BP86-D2/TZP//BP86D2/DZP level of the theory).

described above, based on the vis-NIR spectra, the structure of major-bis-adduct-1 corresponded to the major Y3N@C80 bisadduct with an asymmetric [6,6][6,6]-structure. By the combination of the result from isomerization of minor-2 to major-1 and density functional theory (DFT) calculations on the relative energy of adducts, the structure of major-bisadduct-1 was elucidated as below. The second addition site in minor-2 is shown as bond 53− 54 in Figures 3c and S8. On the basis of DFT calculations, the most favorable Gd3N@C80 Prato-bis-adducts were suggested to be asymmetric [6,6][6,6]-isomer with a second addition site on bond 57−58 (1 in Figure 3 and S8, −0.02 kcal/mol compared to the minor-2) and symmetric [6,6][6,6]-isomer on bond 53−54 (2 in Figure 3 and S8). Both the most favorable major-bis-adduct (bond 57−58) and the minor-bis-adduct (bond 53−54) are almost equally stable and separated by only two bonds (one [6,6] and one [5,6], see Figure 3c). To analyze further how the isomerization process proceeds from bis-adduct 2 to 1, which was observed experimentally, and which pathway is more favorable, linear transit (LT) calculations were performed. Isomerization of minor-bis-2 (second addition on bond 53− 54) to major-bis-1 (second on bond 57−58) can potentially proceed through two different paths (Figure 3c): pathway A that involves the formation of a [5,6][6,6]-bis-adduct on bond 53−59 (2.5 kcal/mol higher in energy than 2) and [6,6][6,6]bis-adduct on 59−57 (+7.4 kcal/mol), and pathway B through the formation of the [6,6][6,6]-bis-adduct on 54−52 (+5.8

Sc3N@C80, which is thermodynamically more stable than the [6,6]-monoadduct, has a perfectly planar Sc3N cluster, while the larger Y3N cluster in the [5,6]-monoadduct of Y3N@C80 showed a slight pyramidalization (0.13 Å) related to its less thermodynamic stability demonstrating the partial reversibility with [6,6]-monoadduct.7,14−17 Several crystallographic studies of pristine metallofullerenes also described that the pyramidalization of endohedral clusters depends on metal size40 and the shape of the carbon cage.41 In the current X-ray crystal structure, the Gd3N cluster adopted a planar geometry inside the Ih-C80 cage to fill out the increased locally available endohedral space in bis-adduct, which was gained by the conversion of sp2 cage carbons to sp3 with releasing the strain. In our previous study, the major-bis-adduct of Gd3N@C80 was stable and did not isomerize at 130 °C at least for 72 h.24 In this study, the stability of minor-bis-2 was tested under a thermal condition (120 °C) to show that a portion of minor-2 isomerized to major-1 within 5 min providing a mixture of 1 and 2 (35:65) (Figures 3a and S6).42 This result indicated that the kinetic major-bis-1 can be obtained also from minor-2 by thermal isomerization presumably through the rearrangement. The isomerization of minor-2 to major-1 proceeded up to 35% and did not go to completion, while 1-to-2 isomerization was not observed as described above. This seems to be due the conversion of isomer 1 to another isomer (isomer 3 observed in Figure S6 in Supporting Information), which further undergoes retrocycloaddition to provide [6,6]- and [5,6]monoadducts during the thermal rearrangement process. As 10990

DOI: 10.1021/jacs.9b05603 J. Am. Chem. Soc. 2019, 141, 10988−10993

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kcal/mol), followed by [5,6][6,6]-bis-adduct on 52−58 (+0.9 kcal/mol). The computed relative stabilities of these intermediate bis-adducts along the 2-to-1 isomerization suggest pathway B to be thermodynamically favored as it proceeds through the more stable bis-adducts (Figure 3b). The LT calculations also indicated that the zwitterionic intermediates formed along the initial isomerization pathways are ca. 19−29 kcal/mol higher in energy for A and ca. 20−28 kcal/ mol for B. The obtained stabilities of the zwitterionic intermediates for the 2-to-1 isomerization in Gd3N@C80 are in line with our previously computed intermediates for the [6,6]-to-[5,6] isomerization process in Sc3N@C80 monoadduct (ca. 22 kcal/mol higher in energy than the [6,6]-monoadduct) and Y3N@C80 monoadduct (ca. 28−29 kcal/mol).38 It is worth mentioning that this isomerization was a [6,6]-to-[6,6] system, which has not been previously reported for TNT-EMF. The similar isomerization of bis-adducts via a “walking on the sphere” rearrangement was observed for six bis-malonate adducts of C60, but being induced electrochemically.44 So far, only [6,6]-to-[5,6] or [5,6]-to-[6,6] isomerizations were reported in endohedral15,16,20,24,38,45 and empty fullerenes.46−49 Most likely in previous work it was impossible to detect the [6,6]-to-[6,6] isomerization in monoadduct of IhC60 or C80 fullerenes, since the starting materials and products are identical. In any case, although the isomerization in this study was observed as a [6,6]-to-[6,6] process, it is expected to proceed in a similar manner to the reported studies on fulleropyrrolidine derivatives of M3N@C80, (M = Sc, Lu, Y, Gd, Er) via the sigmatropic rearrangement.16,20,38 In conclusion, a new bis-adduct of Gd3N@C80 was isolated as minor-adduct-2 and its structure was successfully determined. X-ray crystallographic analysis revealed its C2symmetric [6,6][6,6]-structure and completely flat and less strained geometry of the Gd3N cluster with two Gd atoms coordinated to the C80 cage from inside at the two sp3 addition sites. Further structural elucidation of major-1 was carried out in detail by (1) vis-NIR spectra indicating an asymmetric [6,6][6,6]-bis-structure, (2) the experimental results from thermal isomerization of minor-2, and (3) DFT calculations. Taken together our previous results of regioselective generation of bis-adducts of Y3N@C80 and Gd3N@C80, current results clearly show that the kinetic bis-adducts were obtained regioselectively due to the effect of large metal cluster inside C 80 and isomerization proceeded due to the similar thermodynamic stability of two bis-adducts (1 and 2) in a similar manner to the [6,6]-to-[5,6] sigmatropic rearrangement generally observed in Ih-C80 EMF monoadducts.

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Sergey I. Troyanov: 0000-0003-1663-0341 Marc Garcia-Borràs: 0000-0001-9458-1114 Steven Stevenson: 0000-0003-3576-4062 Sílvia Osuna: 0000-0003-3657-6469 Yoko Yamakoshi: 0000-0001-8466-0118 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the computer resources, technical expertise, and assistance provided by the Barcelona Supercomputing Center - Centro Nacional de Supercomputación. This study was supported in part by the Swiss National Foundation (200021_156097, 205321_17318, IZLJZ2_183660, Y.Y.) and the ETH Research Grant (ETH-21 15-2, ETH-25 11-1, ETH45 19-1, Y.Y.), NSF RUI Grant (1465173, S.S.), the Generalitat de Catalunya for Ph.D. fellowship (2015-FI-B00165, A.R.R.), Spanish MINECO for Juan de la Cierva fellowship (IJCI-2017-33411, M.G.-B), European Research Council Horizon 2020 research and innovation program (ERC-2015-StG-679001, S.O.), Spanish MINECO project (PGC2018-102192-B-I00, S.O.), and the Generalitat de Catalunya for group emergent 2017 SGR-1707.

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(1) Ross, R.; Cardona, C. M.; Sankaranarayanan, S.; Reese, M.; Kopidakis, N.; Peet, J.; Walker, B.; Bazan, G. C.; Van Keuren, E.; Holloway, B. C.; Guldi, D. M.; Drees, M. Endohedral fullerenes for organic photovoltaic devices. Nat. Mater. 2009, 8 (3), 208−212. (2) Liedtke, M.; Sperlich, A.; Kraus, H.; Baumann, A.; Deibel, C.; Wirix, M. J. M.; Loos, J.; Cardona, C. M.; Dyakonov, V. Triplet Exciton Generation in Bulk-Heterojunction Solar Cells Based on Endohedral Fullerenes. J. Am. Chem. Soc. 2011, 133, 9088−9094. (3) Han, Z.; Wu, X.; Roelle, S.; Chen, C.; Schiemann, W. P.; Lu, Z.R. Targeted gadofullerene for sensitive magnetic resonance imaging and risk-stratification of breast cancer. Nat. Commun. 2017, 8 (1), 692. (4) Li, T.; Xiao, L.; Yang, J.; Ding, M.; Zhou, Z.; LaConte, L.; Jin, L.; Dorn, H. C.; Li, X. Trimetallic Nitride Endohedral Fullerenes Carboxyl-Gd3N@C80: A New Theranostic Agent for Combating Oxidative Stress and Resolving Inflammation. ACS Appl. Mater. Interfaces 2017, 9, 17681−17687. (5) Zhang, J.; Fatouros, P. P.; Shu, C.; Reid, J.; Owens, L. S.; Cai, T.; Gibson, H. W.; Long, G. L.; Corwin, F. D.; Chen, Z.-J.; Dorn, H. C. High Relaxivity Trimetallic Nitride (Gd3N) Metallofullerene MRI Contrast Agents with Optimized Functionality. Bioconjugate Chem. 2010, 21, 610−615. (6) Iezzi, E. B.; Duchamp, J. C.; Harich, K.; Glass, T. E.; Lee, H. M.; Olmstead, M. M.; Balch, A. L.; Dorn, H. C. A Symmetric Derivative of the Trimetallic Nitride Endohedral Metallofullerene, Sc3N@C80. J. Am. Chem. Soc. 2002, 124 (4), 524−525. (7) Cardona, C. M.; Kitaygorodskiy, A.; Ortiz, A.; Herranz, M. A.; Echegoyen, L. The First Fulleropyrrolidine Derivative of Sc3N@C80: Pronounced Chemical Shift Differences of the Geminal Protons on the Pyrrolidine Ring. J. Org. Chem. 2005, 70 (13), 5092−5097. (8) Cai, T.; Ge, Z.; Iezzi, E. B.; Glass, T. E.; Harich, K.; Gibson, H. W.; Dorn, H. C. Synthesis and characterization of the first trimetallic nitride templated pyrrolidino endohedral metallofullerenes. Chem. Commun. 2005, No. 28, 3594−3596. (9) Lukoyanova, O.; Cardona, C. M.; Rivera, J.; Lugo-Morales, L. Z.; Chancellor, C. J.; Olmstead, M. M.; Rodríguez-Fortea, A.; Poblet, J. M.; Balch, A. L.; Echegoyen, L. Open Rather than Closed” Malonate Methano-Fullerene Derivatives. The Formation of Methanofulleroid Adducts of Y3N@C80. J. Am. Chem. Soc. 2007, 129, 10423−10430.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b05603. Isolation details, MS, vis-NIR spectra of adducts 1 and 2, and computational details and cartesian coordinates of DFT optimized structures (PDF)

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REFERENCES

Crystal structure of 2 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] 10991

DOI: 10.1021/jacs.9b05603 J. Am. Chem. Soc. 2019, 141, 10988−10993

Communication

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Nitride Fullerene BisAdduct: Exploring the Reactivity of Gd3N@C80. J. Am. Chem. Soc. 2005, 127, 12776−12777. (28) Cai, T.; Xu, L.; Shu, C.; Champion, H. A.; Reid, J. E.; Anklin, C.; Anderson, M. R.; Gibson, H. W.; Dorn, H. C. Selective Formation of a Symmetric Sc3N@C78 Bisadduct: Adduct Docking Controlled by an Internal Trimetallic Nitride Cluster. J. Am. Chem. Soc. 2008, 130 (7), 2136−2137. (29) Feng, L.; Tsuchiya, T.; Wakahara, T.; Piao, Q.; Maeda, Y.; Akasaka, T.; Kato, T.; Yoza, K.; Horn, E.; Mizorogi, N.; Nakahodo, T.; Nagase, S. Synthesis and Characterization of a Bisadduct of La@ C82. J. Am. Chem. Soc. 2006, 128 (18), 5990−5991. (30) Ishitsuka, M. O.; Sano, S.; Enoki, H.; Sato, S.; Nikawa, H.; Tsuchiya, T.; Slanina, Z.; Mizorogi, N.; Liu, M. T. H.; Akasaka, T.; Nagase, S. Regioselective Bis-Functionalization of Endohedral Dimetallofullerene, La2@C80: Extremal La−La Distance. J. Am. Chem. Soc. 2011, 133 (18), 7128−7134. (31) Sawai, K.; Takano, Y.; Izquierdo, M.; Filippone, S.; Martin, N.; Slanina, Z.; Mizorogi, N.; Waelchli, M.; Tsuchiya, T.; Akasaka, T.; Nagase, S. Enantioselective Synthesis of Endohedral Metallofullerenes. J. Am. Chem. Soc. 2011, 133, 17746−17752. (32) Lu, X.; Nikawa, H.; Tsuchiya, T.; Maeda, Y.; Ishitsuka, M. O.; Akasaka, T.; Toki, M.; Sawa, H.; Slanina, Z.; Mizorogi, N.; Nagase, S. Bis-Carbene Adducts of Non-IPR La2@C72: Localization of High Reactivity around Fused Pentagons and Electrochemical Properties. Angew. Chem., Int. Ed. 2008, 47, 8642−8645. (33) Their further work on the Prato addition of monofunctionalized La@C72 also showed regioselectivity.28,30 (34) Cerón, M. R.; Izquierdo, M.; Garcia-Borràs, M.; Lee, S. S.; Stevenson, S.; Osuna, S.; Echegoyen, L. Bis-1,3-dipolar Cycloadditions on Endohedral Fullerenes M3N@Ih-C80 (M = Sc, Lu): Remarkable Endohedral-Cluster Regiochemical Control. J. Am. Chem. Soc. 2015, 137 (36), 11775−11782. (35) Cerón, M. R.; Maffeis, V.; Stevenson, S.; Echegoyen, L. Endohedral fullerenes: Synthesis, isolation, mono- and bis-functionalization. Inorg. Chim. Acta 2017, 468, 16−27. (36) Kordatos, K.; Bosi, S.; Da Ros, T.; Zambon, A.; Lucchini, V.; Prato, M. Isolation and Characterization of All Eight Bisadducts of Fulleropyrrolidine Derivatives. J. Org. Chem. 2001, 66, 2802−2808. (37) Since it is known that the [6,6]-to-[5,6] thermal isomerization of N-ethylpyrrolidine of Gd3N@C80 is generally slow (with t1/2 of ca. 400 min at 120 °C),38 the reaction was stopped in a short interval to obtain mostly the kinetic products. (38) Aroua, S.; Garcia-Borràs, M.; Osuna, S.; Yamakoshi, Y. Essential Factors for Control of the Equilibrium in the Reversible Rearrangement of M3N@Ih-C80 Fulleropyrrolidines: Exohedral Functional Groups versus Endohedral Metal Clusters. Chem. - Eur. J. 2014, 20, 14032−14039. (39) Stevenson, S.; Phillips, J. P.; Reid, J. E.; Olmstead, M. M.; Rath, S. P.; Balch, A. L. Pyramidalization of Gd3N inside a C80 Cage. The Synthesis and Structure of Gd3N@C80. Chem. Commun. 2004, 2814− 2815. (40) Olmstead, M. M.; Zuo, T.; Dorn, H. C.; Li, T.; Balch, A. L. Metal ion size and the pyramidaization of trimetallic nitride units inside a fullerene cage: Comparisons of the crystal structures of M3N@Ih-C80 (M = Gd, Tb, Dy, Ho, Er, Tm, Lu, and Sc) and some mixed metal counterparts. Inorg. Chim. Acta 2017, 468, 321−326. (41) Beavers, C. M.; Chaur, M. N.; Olmstead, M. M.; Echegoyen, L.; Balch, A. L. Large metal Ions in a Relatively Small Fullerene Cage: The Structure of Gd3N@C2(22010)-C78 Departs from the Isolated Pentagon Rule. J. Am. Chem. Soc. 2009, 131, 11519−11524. (42) The same isomerization was observed at room temperature in 2 weeks (Figure S7). (43) The retention time of bis 1 and bis 2 in Figure 3a has a slight shift after 20 min of reaction due to the small amount of the products. But these peaks identifications were clarified by vis-NIR spectra as shown in Figure S6. (44) Kessinger, R.; Gómez-López, M.; Boudon, C.; Gisselbrecht, J.P.; Gross, M.; Echegoyen, L.; Diederich, F. Walk on the Sphere: Electrochemically Induced Isomerization of C60 Bis-Adducts by 10992

DOI: 10.1021/jacs.9b05603 J. Am. Chem. Soc. 2019, 141, 10988−10993

Communication

Journal of the American Chemical Society Migration of Di(alkoxycarbonyl)methano Bridges. J. Am. Chem. Soc. 1998, 120, 8545−8546. (45) Chen, N.; Zhang, E.-Y.; Tan, K.; Wang, C.-R.; Lu, X. Size Effect of Encaged Clusters on the Exohedral Chemistry of Endohedral Fullerenes: A Case Study on the Pyrrolidino Reaction of ScxGd3‑XN@ C80 (x = 0−3). Org. Lett. 2007, 9 (10), 2011−2013. (46) Semivrazhskaya, O. O.; Belov, N. M.; Rybalchenko, A. V.; Markov, V. Yu.; Ioffe, I. N.; Lukonina, N. S.; Troyanov, S. I.; Kemnitz, E.; Goryunkov, A. A. Regioselective Synthesis of [6,6]-Open and [5,6]-Closed C70(CF3)8[CH2] Methanofullerenes with Rapid [6,6]to-[5,6] Phototransformation. Eur. J. Org. Chem. 2018, 2018, 750− 758. (47) Janssen, R. A. J.; Hummelen, J. C.; Wudl, F. Photochemical Fulleroid to Methanofullerene Conversion via the Di-π-methane (Zimmerman) Rearrangement. J. Am. Chem. Soc. 1995, 117 (117), 544−545. (48) Nakahodo, T.; Okada, M.; Morita, H.; Yoshimura, T.; Ishitsuka, M. O.; Tsuchiya, T.; Maeda, Y.; Fujihara, H.; Akasaka, T.; Gao, X.; Nagase, S. [2 + 1] Cycloaddition of Nitrene onto C60 Revisited: Interconversion between an Aziridinofullerene and an Azafulleroid. Angew. Chem., Int. Ed. 2008, 47 (7), 1298−1300. (49) Hall, M. H.; Lu, H.; Shevlin, P. B. Observation of Both Thermal First-Order and Photochemical Zero-Order Kinetics in the Rearrangement of [6,5] Open Fulleroids to [6,6] Closed Fullerenes. J. Am. Chem. Soc. 2001, 123, 1349.

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DOI: 10.1021/jacs.9b05603 J. Am. Chem. Soc. 2019, 141, 10988−10993