Note Cite This: J. Org. Chem. 2017, 82, 11631-11635
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Synthesis and Crystal Structure of Li+@Fluoreno[60]fullerene: Effect of Encapsulated Lithium Ion on Electrochemistry of Spiroannelated Fullerene Hiroshi Ueno,† Hiroshi Okada,‡ Shinobu Aoyagi,§ and Yutaka Matsuo*,†,‡,∥ †
School of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, Jilin 130024, PR China Department of Mechanical Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8565, Japan § Department of Information and Basic Science, Nagoya City University, Nagoya 467-8501, Japan ∥ Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR China ‡
S Supporting Information *
ABSTRACT: The reaction of [Li+@C60]TFSI− (TFSI = bis(trifluoromethanesulfonyl)imide) with 9-diazofluorene directly produced a [6,6]-adduct of lithium-ion-containing fluoreno[60]fullerene, [6,6]-[Li + @C 60 (fluoreno)]TFSI − , which was crystallographically characterized. Cyclic voltammetry of the compound showed a reversible one-electron reduction wave at −0.51 V (vs Fc/Fc+) and an irreversible reduction wave for the second electron. The latter was attributed to opening of the three-membered ring due to strong stabilization of the resulting sp3-carbanion by the encapsulated Li+ and formation of a 14π-electron aromatic fluorenyl anion.
F
luoreno[60]fullerenes1 are an interesting class of fullerene derivatives in structural organic chemistry. In the early days of fullerene chemistry, Wudl and Prato examined periconjugation between the fullerene core and aromatic addends (Figure 1, fluoreno[60]fullerene, HOMO).1,2 The reduction potential at the fullerene core is sensitive to the substituents on the fluoreno group because of this electronic coupling. In addition, it is known that synthesis of fluoreno[60]fullerenes proceeds through an unusual reaction mechanism: [6,6]-fluoreno[60]fullerenes are formed via direct reaction of C60 with diazofluorenes, not via formation of [5,6]-open fulleroids, which are known kinetic intermediates in the synthesis of [6,6]-methano[60]fullerenes. Despite the interesting structural and synthetic chemistry of fluoreno[60]fullerenes, in-depth study on fluoreno[60]fullerenes has still not been sufficient likely because of the complexity in their electrochemistry. The X-ray crystal structure of fluoreno[60]fullerene has not yet been reported. In this study, we investigated the fluoreno[60]fullerene derivative of lithium-ion-containing [60]fullerene (Li+@C60),3 which was first isolated and characterized in 2010 as a stable ionic form of lithium-encapsulating fullerene.4 As a result of basic studies conducted in recent years,5 the fundamental properties of Li+@C60 have been unveiled, and it is now attracting growing attention in electrochemistry,6 supramolecular chemistry,7 physics,8 and organic electronics.9 For © 2017 American Chemical Society
further developing the unique chemistry of ionic fullerenes chemistry, tuning of their physicochemical properties by covalent modification is strongly awaited, but only several Li+@C60 derivatives have reported so far.10 The use of Li+@C60 as an alternative to empty C60 remarkably perturbs the electronic structure of spiro-fullerenes, potentially helping to understand their through-space periconjugation. Here we synthesized lithium-ion-containing fluoreno[60]fullerene [Li+@C60(fluoreno)]TFSI− (TFSI = bis(trifluoromethanesulfonyl)imide), which is the first spiroannelated Li+@C60 derivative to be reported. X-ray structure analysis revealed not only the precise structure of the fluoreno[60]fullerene cage but also unusual localization of the encapsulated Li+. The electrochemistry of this compound was investigated by cyclic voltammetry, and revealed unique electrochemical processes arising from the spiroannelated structure and the encapsulated Li+. We envision that spiroannelated Li+@C60 derivatives can expand basic and applied research on endohedral lithium-containing [60]fullerenes because of the synthetically easy and direct access to the [6,6]-adduct and the electronic interactions arising from its spiro-fullerene structure. Received: July 28, 2017 Published: October 12, 2017 11631
DOI: 10.1021/acs.joc.7b01893 J. Org. Chem. 2017, 82, 11631−11635
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The Journal of Organic Chemistry Scheme 1. Synthesis of Lithium-Ion-Containing Fluoreno[60]fullerene
Figure 1. Molecular orbital distribution of Li+@C60[fluoreno] and empty C60(fluoreno) calculated at the B3LYP/6-31G(d) level of theory.
We began by performing a density functional theory study to elucidate the through-space periconjugation between the C60 cage and the addend of the Li+@C60(fluoreno) cation.11 The structure of Li+@C60(fluoreno) was optimized at the B3LYP/6-31G(d) level. The calculated molecular orbitals (HOMO−1 to LUMO+1) are shown in Figure 1 along with those of empty fluoreno[60]fullerene computed at the same level of theory. In empty fluoreno[60]fullerene, the HOMO was delocalized over not only the C60 cage but also the fluoreno moiety due to the through-space interaction arising from the rigid structure of the fluoreno moiety as predicted previously.1 In Li+@C 60(fluoreno), the through space interaction was also found but is on HOMO−1 because the positive charge of the internal Li+ lowered the energy level of the molecular orbitals on the C60 moiety. It should be mentioned that effect of the positive charge for the addend part should be much smaller due to relatively long distance between the encapsulated Li+ and the addend, thereby the molecular orbital distribution of Li+@C60(fluoreno) did not coincide with that of empty C60(fluoreno). From these theoretical results, Li+@C60(fluoreno) is expected to have electron correlation in the whole molecule including the C60 cage, the fluoreno addend, and the encapsulated lithium ion. [Li+@C60(fluoreno)]TFSI− was synthesized by the reaction of 9-diazofluorene and [Li+@C60]TFSI− (Scheme 1). To avoid a decrease in yield as a result of bis-adduct formation, the diazo compound was slowly added while monitoring the progress of the reaction by analytical HPLC (Figure 2). The target monoadduct was isolated by our recently reported preparative HPLC method.10a By this reaction, only [6,6][Li+@C60(fluoreno)]TFSI− was formed, in the same way that empty C60 reacts directly with 9-diazofluorene to form only the [6,6]-adduct.1a We ascribe this to stabilization of a possible biradical intermediate formed from diazofluorene by 14π aromatization.12 The product was characterized by 1H, 13C, and 7Li NMR, UV−vis absorption spectroscopy, atmospheric pressure chemical ionization time-of-flight (APCI-TOF) mass spectrometry, and X-ray structure analysis. The 1H and 13C NMR
Figure 2. HPLC profile of the reaction of [Li+@C60]TFSI− with 9diazafluorene in CH2Cl2 at room temperature giving [6,6]-[Li+@ C60(fluoreno)]TFSI− directly.
spectra of the product clearly indicated the formation of the C60(fluoreno) structure (Figure S1 and S2). The 7Li NMR spectrum showed a sharp signal at −12.8 ppm (Figure S3), which was shifted slightly upfield compared with the starting material [Li+@C60]TFSI− and almost identical to the signal for [6,6]-closed Li+@C60 derivatives.10a,e This was contrast to the 7Li NMR spectra of [5,6]-open Li+@C60 derivatives, which were shifted downfield compared with Li+@C60 because of the weaker shielding effect of the [5,6]-open fulleroid structure.10a The UV−vis absorption spectra showed broad absorption in the visible region with an absorption maximum at 495 nm (Figure S5). This absorption is a characteristic pattern of [6,6]-methanofullerenes.13 The high-resolution APCI-TOF mass spectrum of the product had a peak at 891.0769 which was assigned to the molecular ion (M+; calcd for C73H8Li, 891.0786). Because Li+@C60 and its derivatives can form an ionic crystal with several kinds of stable counteranions such as PF6− and TFSI−, it is much easier to obtain single crystals suitable for X-ray structure analysis compared with neutral fullerenes. Indeed, we have reported the first crystal structure of a [5,6]open [60]fulleroid containing lithium ion.10a In this study as well, the structure of Li+@C60(fluoreno) was unambiguously confirmed (Figure 3). Interestingly, the C1−C2 bond length was 1.616 Å which was much shorter than that of reported Li+-containing methano[60]fullerene derivatives.10a,e It was also found that the electron density derived from Li+ in the fluoreno[60]fullerene cage was observed at three different positions with the almost equal occupancy of the sites (0.31/ 0.38/0.30) at 93 K, whereas the Li+ in reported Li+@C60 11632
DOI: 10.1021/acs.joc.7b01893 J. Org. Chem. 2017, 82, 11631−11635
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The Journal of Organic Chemistry
almost identical with those observed in the reported Li+@C60 derivatives (Table S1), which suggested that not the electronic effect of the addends but the detraction of π-conjugated system strongly affects the electron accepting property of Li+@C60 derivatives. We also tried the measurement for anodic side to confirm the effect of periconjugation, however, no wave was observed in the range of possible potential window because of much lowered energy level caused by the positive charge of Li+. Another specific electrochemical response was found on the second reduction process. Whereas the other reported Li+@ C60 derivatives have showed strictly reversible second reduction waves, an irreversible reduction wave was observed in [Li+@C60(fluoreno)]TFSI− (Figure 5). Isolation and
Figure 3. Crystal structure of [Li+@C60(fluoreno)]TFSI−. (a) Side view. (b) Expanded view of the fluorene moiety on the fullerene cage.
derivatives is localized near the C1−C2 bond at low temperatures. A possible reason for the unusual disorder of the encapsulated Li+ is decreased localizability caused by the through space periconjugation. As we reported recently, the long C1−C2 bond found in other methano-Li + @C 60 derivatives can be attributed to a partially formed threecenter two-electron bond by the internal coordination of the encapsulated Li+ leading to localization of Li+ at that position.10a On the other hand, the much shorter C1−C2 bond found in this study clearly indicated that the Li+ did not contribute strongly to form the three-center two-electron bond. The coordination ability of Li+ to C1−C2 site is reduced by the periconjugation, which renders the encapsulated Li+ harder to be localized around the C1−C2 bond. Although the reason for the localizability on the other two positions is still not yet clear, the spiroannelated structure could affect the motion/position of inner Li+. The electrochemical activity of the product was investigated by cyclic voltammetry (Figure 4). The redox wave of pristine
Figure 5. A possible explanation of the irreversible electrochemical reduction of [Li+@C60(fluoreno)]TFSI−.
characterization of the reduced product was unsuccessful mainly due to the difficulty of purification. It has been reported that the irreversible electrochemical response observed in spiroannelated fullerenes is due to opening of the three-membered ring in the methanofullerene structure. This reaction could occur if the ring-opened structure is stabilized by conjugation or aromatization of the addends.1 In other words, the energy difference between before and after the ring-opening is an important factor in whether the reaction occurs. Through bond cleavage in the dianion of Li+@C60(fluoreno), the negative charge can be localized on the bridgehead carbon of the fluorenyl moiety and on the sp3 carbon of the C60 moiety. In this state, the two extra electrons can be stabilized by formation of a 14π aromatic system in the fluorenyl group and the electrostatic interaction between the fullerene C(sp3)− and the encapsulated Li+. The latter stabilizing factor should be observed in only the Li+compound. As we recently reported,5b ion pairing between the localized negative charge on the C60 cage and the internal Li+ has a strong stabilizing effect because Li+ cannot be released from the anionic cage and completely forms an ion pair with the localized negative charge. In the case of the empty analogue, on the other hand, delocalization of the negative charge through the fullerene cage should be the dominant effect stabilizing the dianionic state. In summary, we have successfully synthesized the spiroannelated lithium-ion-containing fullerene derivative [Li+@C60(fluoreno)]TFSI−. The reaction directly gave the [6,6]-adduct. X-ray structure analysis revealed that the encapsulated Li+ was located at three different positions inside the fullerene cage with almost equal occupancy, which could be caused by the through space periconjugation seen in spiroannelated C60 derivatives. [Li+@C60(fluoreno)]TFSI− underwent reversible one-electron reduction and irreversible two-electron reduction. Having elucidated these basic properties of the first spiroannelated Li+@C60 derivative with through-space periconjugation, we are now working on the
Figure 4. Cyclic voltammograms of pristine [Li+@C60]TFSI− (red) and [Li+@C60(fluoreno)]TFSI− (blue and purple) in o-DCB containing 50 mm of TBA+TFSI−.
[Li+@C60]TFSI− is also shown for reference. Although the fluoreno-functionalization induced shift of the first reduction potential in empty C60(fluoreno) was only 0.03 V (Figure S6 and Table S1), the first reduction potential (E1red) of [Li+@ C60(fluoreno)]TFSI− was determined to be −0.51 V (vs Fc/ Fc+) which was 0.06 V negative than that of pristine [Li+@ C60]TFSI− (E1red = −0.45 V). The value of cathodic shift is 11633
DOI: 10.1021/acs.joc.7b01893 J. Org. Chem. 2017, 82, 11631−11635
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The Journal of Organic Chemistry design and synthesis of novel charged Li+@C60 derivatives expected to have unique electronic properties.
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Sports, Science and Technology (MEXT), Japan. The computations were performed using Research Center for Computational Science, Okazaki, Japan. We thank Dr. Tsuyoshi Suzuki (The University of Tokyo) for the synthesis of 9-fluorenehydrazone.
EXPERIMENTAL SECTION
General Remarks. [Li+@C60]PF6− was obtained from Idea International Corporation. Anion-exchanged [Li+@C60]TFSI−, diazofluorene, and TBA+TFSI− (TBA = tetrabutylammonium) were prepared according to reported procedures (see refs 7a and 14). All other reagents were commercially available and used as received without further purification. 7Li, 1H, and 13C NMR spectra were respectively recorded at 194.38, 500.16, and 125.77 MHz on a JEOL ECZ-500 system. High-resolution mass spectra were obtained by MALDI using a time-of-flight mass analyzer on a Bruker Ultra exTOF/TOF spectrometer. The UV−visible spectrum was recorded on a JASCO V-570 spectrometer. CV measurements were performed with a HOKUTO DENKO HZ-5000 voltammetric analyzer. Synthesis of [Li+@C60(fluoreno)]TFSI−. 9-Diazofluorene (dichloromethane solution, 0.5 mM, 12 mL, 6.0 μmol) was slowly added to a dichloromethane solution (5.0 mL) of [Li+@C60]PF6− (5.0 mg, 5.0 μmol) and allowed to react at room temperature for 5 min. The reaction mixture was concentrated in vacuo, and the product was purified by HPLC using an ion-exchange column (column: Inertsil CX, φ10 × 250 mm, GL Science) at room temperature with the electrolyte LiTFSI (5 mM) added to chlorobenzene/acetonitrile = 1:3 (v/v) as the mobile phase. After evaporating the solvent in vacuo, diethyl ether was added to the brown residue containing white solids of LiTFSI. After removing the undissolved solid by filtration, recrystallization from the solution by vapor diffusion with diethyl ether at 0 °C gave black crystals of [Li+@C60(fluoreno)]TFSI− (1.9 mg, 32%). 1H NMR (500 MHz, oDCB-d4) δ 8.52 (pseudo d, 2H), 7.93 (pseudo d, 2H), 7.60 (m, 2H), 7.54 (m, 2H). 13C NMR (126 MHz, o-DCB-d4) δ 147.35 (s), 144.18 (s), 144.10 (s), 144.02 (s), 143.83 (s), 143.66 (s), 143.32 (s), 143.25 (s), 142.64 (s), 142.51 (s), 142.21 (s), 142.08 (s), 141.20 (s), 140.09 (s), 138.22 (s), 79.64 (s), the other peaks derived from aromatic carbons overlapped with those of o-DCB-d4 and the concentration was too low to detect the bridgehead carbon of the fluorenyl moiety. 7 Li NMR (194 MHz, o-DCB-d4) δ −12.8 (s). High-resolution MALDI-TOF MS (+) m/z calcd. for C73H8Li, 891.0786, found 891.0769.
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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.joc.7b01893. Mass spectra, 1H, 13C, and 7Li NMR spectra, UV−vis spectrum, X-ray crystal structure data, and theoretical calculations (PDF) Crystal data (CIF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hiroshi Ueno: 0000-0003-3991-0610 Shinobu Aoyagi: 0000-0002-7393-343X Yutaka Matsuo: 0000-0001-9084-9670 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (JSPS KAKENHI Grant Numbers JP15H05760, JP16H04187, JP17K04970, and JP17K19116 (YM), and 17K04970 (HO)) from the Ministry of Education, Culture, 11634
DOI: 10.1021/acs.joc.7b01893 J. Org. Chem. 2017, 82, 11631−11635
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DOI: 10.1021/acs.joc.7b01893 J. Org. Chem. 2017, 82, 11631−11635
