Octaalkoxyfullerenes: Widely LUMO-Tunable C2v-Symmetric

‡Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan. #Hefei National Laboratory for Physic...
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Cite This: J. Org. Chem. 2018, 83, 10655−10659

Octaalkoxyfullerenes: Widely LUMO-Tunable C2v-Symmetric Fullerene Derivatives Hiroshi Ueno,*,†,‡ Kouya Uchiyama,‡ Yue Ma,# Keita Watanabe,‡ Kenji Yoza,§ Yutaka Matsuo,*,†,∥ and Hiroshi Moriyama*,‡,⊥ †

School of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, Jilin 130024, China Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan # Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China § Bruker Japan, Moriya-cho, Kanagawa-ku, Yokohama, Kanagawa 221-0022, Japan ∥ Department of Mechanical Engineering, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan

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‡

S Supporting Information *

ABSTRACT: C2v-Symmetric octaalkoxyfullerenes, C60(OR)8 (R = CH3, C2H5, CH2CF3), were synthesized by reacting octabromofullerene with the corresponding alcohols in the presence of AgBF4. The reactions occurred with no change in the addition pattern, and the compounds were unambiguously characterized by NMR spectroscopy and X-ray structure analysis. Electrochemical measurements revealed not only that these derivatives have stable redox properties but also that their LUMO levels can be tuned over a very wide range.

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Poly(halogenated) fullerenes are intrinsically useful precursors for the rational synthesis of multifunctionalized fullerenes with retention of the addition pattern.8 For instance, C60Cl6 reacts with nucleophiles such as MeLi, H2C CHCH2TMS, NaOR, RSH, P(OR)3, and Ar (R = alkyl; Ar = aryl) to produce isostructural C60Me6,9 C60(CH2CH CH2)6,10 C60(OR)5Cl,11 C60(SR)5H,12 C60[P(O)(OR)2]5H,13 and C60Ar5Cl.14 However, isostructural substitution of octa(halogenated) fullerenes C60Cl8 and C60Br8 has yet to be reported. There is one report of a substitution reaction converting C60Br24 to C60F24 and C60Cl24.15 Here, we report C 2v -symmetric octaalkoxyfullerenes C60(OR)8 (R = CH3, C2H5, CH2CF3) synthesized by the substitution reaction of octabromofullerene C60Br88f with the corresponding alcohols in the presence of Ag+ salt. These πconjugated structures were unambiguously characterized and found to have the same conjugation pattern as the starting material, C60Br8. Electrochemical measurements revealed both the stable redox properties of these derivatives and the wide tunability of their LUMO levels depending on the addend.

nergy level control in fullerene derivatives is still an important research topic for their photovoltaic and biomaterial applications.1,2 In particular, the LUMO level of fullerene derivatives governs their electrophilicity and is generally tuned by the addition pattern and electronic properties of addends on the fullerene core.1,3,4 Current research requires the construction of fullerene derivatives with both high- and low-lying LUMO levels5 because of the demand for energy matching with p-type materials in polymer and ternary blend solar cells.6 Although numerous kinds of fullerene derivatives have been reported,4 the approach toward low-LUMO derivatives is relatively limited because the detraction of a 60π-electron system by chemical functionalization generally raises the LUMO level of the derivatives.1,5 Particularly, multifunctionalized fullerenes, which have a lowlying LUMO, are quite rare due to not only this reason but also the difficulty of regioselective syntheses and purification of multiadducts despite their potentials for various applications as highly soluble fullerenes. One possible candidate is poly(trifluoromethyl)fullerene, which has a low-lying LUMO and high solubility.7 However, the rational design of trifluoromethylated fullerenes is usually difficult because the reaction gives thermodynamic products at high temperatures. © 2018 American Chemical Society

Received: June 13, 2018 Published: August 1, 2018 10655

DOI: 10.1021/acs.joc.8b01485 J. Org. Chem. 2018, 83, 10655−10659

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The Journal of Organic Chemistry

structures. Furthermore, 13C-enriched C60 (ca. 5%) enabled us to conduct the full assignment of 13C signals of the carbon skeleton of C60(OMe)8 using 2D INADEQUATE,18 HMBC, and HMQC spectroscopy (Figures 2 and S8). The APCI mass

The C60(OR)8 compounds were synthesized by a substitution reaction of C60Br8 with ROH (R = CH3, C2H5, CH2CF3) in ortho-dichlorobenzene (o-DCB) in the presence of AgBF4 in the dark (Scheme 1). Conversion of the bromo Scheme 1. Ag+-Mediated Substitution of C60Br8 to C60(OR)8

groups into alkoxy groups by treatment with an alcohol in the presence of a silver salt as a halogen scavenger has already been reported for partially brominated fullerene derivatives in solution.16 In contrast to the reported reactions, however, most of the C60Br8 was suspended in the solvent, not dissolved, owing to its extremely low solubility in conventional organic solvents. Nevertheless, the reaction proceeded in this heterogeneous mixture and the small amount of dissolved C60Br8 successively reacted with the alcohol, becoming gradually more soluble as the number of organic addends increased. The final product was obtained entirely in solution. This kind of cation-mediated reaction is a promising approach to obtain new fullerene derivatives, as our laboratory has recently reported.17 Notably, when we attempted the reaction at higher temperatures, the yield of the desired product was much lower. Instead, a mixture of C1-symmetric isomers was obtained, probably due to heat-induced rearrangement (see Figure 1).

Figure 2. Full assignment of 13C NMR signals of C60(OMe)8. The orange colored parts are sp3 carbons. The peaks, j, k, and m, are tentatively assigned because the chemical shifts are close.

spectra (negative mode) of C60(OMe)8, C60(OEt)8, and C60(OCH2CF3)8 showed the [M]− molecular ion peak. These three compounds exhibited characteristic UV−vis absorption spectra, which were similar to that of C2v-symmetric C60Cl8 (Figure S9). The structures of C 60 (OMe)8 and C60 (OEt) 8 were determined by X-ray structure analysis. As shown in Figure 3

Figure 1. A possible mechanism of symmetry lowering by cation rearrangement.

Figure 3. Crystal structure of (a) C60(OMe)8, and (b) C60(OEt)8. (c) Top view of panel a. Hydrogen atoms are omitted for clarity. (d) Schlegel diagram of the compounds. The colored part in panel d indicates the modified six-membered ring (blue) and 46π-conjugated moiety in the C2v-symmetric octasubstituted derivatives (red).

The products were characterized by 1H, 13C, and 19F NMR spectroscopy (Figures S1−S7), 2D NMR (Figure S8), atmospheric pressure chemical ionization (APCI) mass spectrometry, and X-ray structure analysis. The 1H NMR spectrum of C60(OMe)8 exhibited two singlet signals at δ 3.52 and 3.83 in a 1:1 ratio, which were assigned to the two different kinds of methyl protons of the methoxy groups. The 13 C NMR spectrum contained 19 signals derived from the fullerene core and methoxy groups, which is a characteristic spectrum for a C2v-symmetric structure. These results clearly indicate that we successfully obtained the octamethoxyfullerene product that was isostructural with the starting C60Br8. The C60(OEt)8 and C60(OCH2CF3) derivatives also showed characteristic NMR spectra derived from their C2v-symmetric

(and Figure S10), eight alkoxy groups were added at 1,4-sites on each of the four six-membered rings (shown in blue in Figure 3) to form C2v-symmetric structures. The conjugation pattern on the fullerene core was identical to that of the starting material, C60Br8, which had 46 conjugated and 6 isolated π electrons. Because the isolated π electrons were separated from the π-conjugated system, the double bonds were of the ordinary localized ethylene type. In fact, the CC bond lengths of the isolated double bonds ranged from 1.320 to 1.348 Å, almost identical to normal CC bond lengths and shorter than the 6:6 and 6:5 bond lengths of C60. 10656

DOI: 10.1021/acs.joc.8b01485 J. Org. Chem. 2018, 83, 10655−10659

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that of C 60 (CN) 2 and C 60 (CH 2 C 6 F 5 ) 2 , 5a,b which are representative diorgano[60]fullerenes with low-lying LUMO levels. This was due to the strong electron-withdrawing ability of the eight trifluoromethyl groups. In conclusion, we synthesized novel C2v-symmetric octaalkoxyfullerenes C60(OR)8 (R = Me, Et, CH2CF3) by a Ag+mediated substitution reaction of octabrominated fullerene. The products were unambiguously characterized by NMR spectroscopy, APCI mass spectrometry, and X-ray structure analysis, including the first full assignment of 13C signals in a C2v-symmetric octasubstituted fullerene. Electrochemical measurements revealed not only that these derivatives have stable redox properties but also that their LUMO levels can be tuned over a wide range.

Through the substitution of the bromo groups with alkoxy groups, this novel 52π-electron system is now available in highly soluble organofullerene derivatives. In addition, although the starting material, C60Br8, was electrochemically unstable,19 C60(OR)8 showed reversible redox properties (Figure 4). In the reduced state, C60Br8 decomposed via

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EXPERIMENTAL SECTION

General Remarks. The starting octabromofullerene C60Br8 was prepared based on the improved synthetic method of C60Br8.22 All other reagents were commercially available and used as received without further purification. 1H, 13C, and 19F NMR spectra were, respectively, recorded on a AVANCE II 400 system. The UV−visible spectrum was recorded on a UV-3600 spectrometer. High-resolution mass spectra were obtained by APCI using a TOF mass analyzer on a JEOL JMS-T100LC spectrometer. CV measurements were performed with a CHI660E voltammetric analyzer. Synthesis of C60(OMe)8. C60Br8·Br2 (500 mg, 0.329 mmol), CH3OH (2.0 mL), and AgBF4 (872 mg, 4.48 mmol) were added to o-dichlorobenzene (50 mL), and then the solution was stirred at room temperature for 5 h in the dark. The solution was filtered to remove the silver salt. The filtrate was washed with saturated NaHCO3 (aq) and then dried with Na2SO4. The solution was evaporated and then purified using silica gel column chromatography with toluene and chloroform as the eluents to afford C60(OMe)8 (250 mg, 0.258 mmol, 78% yield). 1H NMR (400 MHz, CD2Cl2): δ 3.52 (s, 12H, OCH3), 3.83 (s, 12H, OCH3). 13C NMR (100 MHz, CD2Cl2): all signals represent 4C (sp2) except as noted, δ 55.07 (4C, OCH3), 55.77 (4C, OCH3), 77.91 (4C, sp3), 78.51 (4C, sp3), 134.75, 143.32, 143.57, 144.51, 144.87, 145.05 (2C), 145.74 (2C), 145.89, 146.06, 146.89 (2C), 147.29, 148.83, 149.15, 149.24, 152.56 (2C). APCI-MS (−): calcd for C68H24O8 [M−], 968.1471; found, 968.1466. Synthesis of C60(OEt)8. C60Br8·Br2 (500 mg, 0.329 mmol), C2H5OH (0.50 mL), and AgBF4 (872 mg, 4.48 mmol) were added to o-dichlorobenzene (50 mL), and then the solution was stirred at room temperature for 5 h in the dark. The solution was filtered to remove the silver salt. The filtrate was washed with saturated NaHCO3 (aq) and then dried with Na2SO4. The solution was evaporated and then purified using silica gel column chromatography with toluene and chloroform as the eluents to afford C60(OEt)8 (214 mg, 0.198 mmol, 60% yield). 1H NMR (400 MHz, CD2Cl2): δ 1.23 (t, 12H, OCH2CH3), 1.37 (t, 12H, OCH2CH3), 3.67−3.74 (m, 4H, OCH2CH3), 4.06−4.18 (m, 8H, OCH2CH3). 13C NMR (100 MHz, CD2Cl2): all signals represent 4C (sp2) except as noted, δ 15.89 (4C, OCH2CH3), 15.97 (4C, OCH2CH3), 63.12 (4C, OCH2CH3), 63.99 (4C, OCH2CH3), 77.41 (4C, sp3), 77.98 (4C, sp3), 134.83, 143.33, 144.09, 144.44, 144.82, 145.22 (2C), 145.76 (2C), 145.90, 146.00, 146.82 (2C), 147.16, 148.82, 149.21, 149.54, 152.38 (2C). APCI-MS (−): calcd for C76H40O8 [M−], 1080.2723; found, 1080.2742. Synthesis of C60(OCH2CF3)8. C60Br8·Br2 (500 mg, 0.329 mmol), CF3CH2OH (2.0 mL), and AgBF4 (872 mg, 4.48 mmol) were added to o-dichlorobenzene (50 mL), and then the solution was stirred at room temperature for 5 h in the dark. The solution was filtered to remove the silver salt. The filtrate was washed with saturated NaHCO3 (aq) and then dried with Na2SO4. The solution was evaporated and then purified using silica gel column chromatography with toluene and carbon tetrachloride as the eluents to afford C60(OCH2CF3)8 (281 mg, 0.186 mmol, 56% yield). 1H NMR (400 MHz, CD2Cl2): δ 4.00−4.09 (m, 4H, OCH2CF3), 4.20−4.29 (m, 4H, OCH2CF3), 4.39−4.57 (m, 8H, OCH2CF3). 13C NMR (100 MHz,

Figure 4. Cyclic voltammograms of C60(OR)8 measured in dichloromethane containing 0.1 M TBAPF6 as a supporting electrolyte. The reduction wave of pristine C60 is also shown in the figure as a reference.

elimination of Br−, but the stronger C−OR bonds were not cleaved. Other halogenated fullerenes such as C60Br6 and C60Cln (n = 6, 8, 10, and 12) are also electrochemically unstable.8e,19,20 From the electrochemical measurement results, the LUMO levels of the compounds were estimated according to the following equation:21 E LUMO (eV) = −[4.80 + E1/2 red1(V vs Fc/Fc+)]

Compared with the LUMO level of C60 (−3.77 eV in dichloromethane), the LUMO levels of C60(OMe)8 and C60(OEt)8 were raised to −3.68 and −3.62 eV, respectively, due to the significant contraction of the π-electron system from 60π to 52π (Figure 5). Note that these LUMO levels were deeper than expected, which was attributed to the electronegativity of the oxygen atoms counteracting the effect of the sp3 carbons. On the other hand, despite C60(OCH2CF3)8 having the same 52π-electron system as C60(OMe)8 and C60(OEt)8, it had a much lower LUMO level (−4.02 eV) than

Figure 5. LUMO levels of C 60 , C 60 (OMe) 8 , C 60 (OEt) 8 , C60(OCH2CF3)8, and reference diorgano[60]fullerene derivatives. 10657

DOI: 10.1021/acs.joc.8b01485 J. Org. Chem. 2018, 83, 10655−10659

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The Journal of Organic Chemistry CD2Cl2): all signals represent 4C (sp2) except as noted, δ 64.93 (4C, OCH2CF3), 65.57 (4C, OCH2CF3), 76.92 (4C, sp3), 77.54 (4C, sp3), 123.67 (4C, OCH2CF3), 123.73 (4C, OCH2CF3), 133.54, 141.65, 142.62, 144.44, 144.65 (2C), 145.08 (2C), 145.11, 145.24, 146.39, 147.19 (2C), 147.47, 147.99, 148.83, 148.87, 153.36 (2C). 19F NMR (376 MHz, CD2Cl2): δ −74.67 (s, 12F, OCH2CF3), −74.21 (s, 12F, OCH2CF3). APCI-MS (−): calcd for C76H16O8F24 [M−], 1512.0462; found, 1512.0442.

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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.8b01485. NMR spectra with the full assignment of 13C signals, UV−vis absorption spectra, and detailed X-ray structure analyses (PDF) Crystal data for C60(OMe)8 (CIF) Crystal data for C60(OEt)8 (CIF) Accession Codes

CCDC 1826834 and 1826835 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: [email protected]. *E-mail: [email protected] *E-mail: [email protected]. ORCID

Hiroshi Ueno: 0000-0003-3991-0610 Yutaka Matsuo: 0000-0001-9084-9670 Present Address ⊥

H.M.: Department of Chemistry, School of Science, Yamagata University, Yamagata 990-8560, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Bilateral International Collaborative R&D Program (GT-2009-CL-OT-0058), Ministry of Knowledge Economy, Republic of Korea and the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, Japan (to H.M.), and Japan Society for the Promotion of Science (JSPS) KAKENHI Grants (JP15H05760 and JP16H04187, to Y.M.).

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REFERENCES

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DOI: 10.1021/acs.joc.8b01485 J. Org. Chem. 2018, 83, 10655−10659

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DOI: 10.1021/acs.joc.8b01485 J. Org. Chem. 2018, 83, 10655−10659