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J. Phys. Chem. C 2008, 112, 12188–12194
Regioselective Chemical Modification of Fullerene by Destructive Electrophilic Reaction in Polyphosphoric Acid/Phosphorus Pentoxide Dae-Hyun Lim,† Christopher B. Lyons,‡ Loon-Seng Tan,*,§ and Jong-Beom Baek*,† School of Chemical Engineering, Chungbuk National UniVersity, Cheongju, Chungbuk, 361-763 South Korea, Southwestern Ohio Council for Higher Education (SOCHE), Dayton, Ohio, and Nanostructured and Biological Materials Branch, Materials and Manufacturing Directorate, AFRL/RXBP, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433-7750 ReceiVed: February 29, 2008; ReVised Manuscript ReceiVed: May 3, 2008
An adduct of electrophilic reaction between C60 and 4-(2,4,6-trimethylphenoxy)benzamide was afforded in a polyphosphoric acid (PPA)/phosphorus pentoxide (P2O5) medium at 130 °C. The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrum of the adduct showed that multiple destructive acylation reactions of 4-(2,4,6-trimethylphenoxy)benzamide on C60 had occurred to give hexakis(4-(2,4,6-trimethylphenoxy)benzoyl)-substituted C54. On the basis of the combined results of optical studies, it could be presumably concluded that the regioselective destruction and addition pathway on the C60 framework might have predominantly occurred to six-membered rings. Introduction Buckminsterfullerene (C60), which is of the most abundant carbon sphere, is generally considered as a stable electrondeficient material. Nevertheless, the strained bonds of the convex surface of C60 offer new possibilities of chemical reactions.1 Due to the electron affinity, C60 is thus considered to be more susceptible to nucleophilic attacks than to electrophilic ones.2 In fact, it is found to be effective for nucleophilic addition reactions on C60.3 Consistent with its electronic nature and chemical reactivity, C60 has formed a multitude of organic/ organometallic derivatives via such reactions as cycloaddition,4 nucleophilic addition,5 free-radical addition,6 and metal complexation/coordination.7 However, a well-known exception is the nitration of C60 in nitric/sulfuric acid medium to generate hexanitrofullerene [C60(NO2)6] by electrophilic substitution reaction.8 This exception provides the possibility that in a strongly acidic environment with the presence of electrondeficient cationic species (e.g., nitronium, NO2+, as present in HNO3/H2SO4), certain electrophilic reactions may take place and lead to interesting functionalized fullerene products. Here, we are first to report the destructive electrophilic functionalization of C60 via Friedel-Crafts acylation using an optimized reaction medium in polyphosphoric acid/phosphorus pentoxide, PPA/P2O5,9 which is a much milder acidic condition in comparison to previously reported reaction methods and conditions.8 Experimental Section Materials. All reagents and solvents were purchased from Aldrich Chemical Inc. and used as received, unless otherwise specified. Fullerene (C60, >99.9% purity) was purchased from Strem Chemicals and used as received.10 * Corresponding authors. Tel.: +82-43-261-2489(J.-B.B.), +1-937-2559141(L.-S.T.). Fax: + 82-43-262-2380 (J.-B.B.), +1-937-255-9157 (L.S.T.). E-mail:
[email protected] (J.-B.B.),
[email protected] (L.-S.T.). † Chungbuk National University. ‡ Southwestern Ohio Council for Higher Education. § Wright-Patterson Air Force Base.
Instrumentation. Fourier transform infrared (FT-IR) spectra were recorded on a Jasco FT-IR 480 Plus spectrophotometer. Elemental analysis and mass spectral analysis were performed by the System Supports Branch, Air Force Research Laboratory, Dayton, Ohio. The melting points (mp) of all the compounds were determined on a Mel-Temp melting point apparatus and are uncorrected. A Shimadzu Axima-CFR plus matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometer (MS) was used to determine masses using a reflection mode. Dithranol was used as matrix. Thermogravimetric analysis (TGA) was conducted both in air and nitrogen atmospheres with a heating rate of 10 °C/min using a PerkinElmer TGA7. UV-vis spectra were obtained from a PerkinElmer Lambda 35 UV-vis spectrometer. Fluorescence experiments were conducted with a Perkin-Elmer LS 55 fluorometer. The applied excitation wavelength was the UV-absorption maximum. The field emission scanning electron microscope (FE-SEM) used in this work was a LEO 1530FE. The field emission transmission electron microscope (FE-TEM) employed in this work was an FEI Tecnai G2 F30 S-Twin. 4-(2,4,6-Trimethylphenoxy)benzonitrile (1). 2,4,6-Trimethylphenol (28.1 g, 0.21 mol), 4-fluorobenzonitrile (24.9 g, 0.21 mol), potassium carbonate (34.0 g, 0.25 mol), and a mixture of NMP (350 mL) and toluene (80 mL) were placed in a 500 mL three-necked round-bottomed flask equipped with a magnetic stir-bar, nitrogen inlet, and a condenser. The reaction mixture was then heated and maintained around 140 °C for 8 h with vigorous nitrogen flow. The dark solution was filtered while it was warm, and the filtrate was poured into distilled water containing 5% hydrochloric acid. The solution was separated into an organic layer and an aqueous layer. The organic layer was diluted with dichloromethane and separated. The solvent was removed to dryness. The light orange oily residue was dissolved in warm heptane and allowed to cool room temperature to give 47.7 g (98% yield) of white needles: mp 89.5-91 °C. Anal. Calcd for C16H15NO: C, 80.98%; H, 6.37%; N, 5.90%; O, 6.74%. Found: C, 80.31%; H, 6.37%; N, 5.75%; O, 6.46%. FT-IR (KBr, cm-1): 3092, 3040, 2923, 2227 (CtN stretch), 1594, 1485. Mass spectrum (m/e): 237 (M+, 100% relative
10.1021/jp801772r CCC: $40.75 2008 American Chemical Society Published on Web 07/19/2008
Regioselective Chemical Modification of Fullerene
J. Phys. Chem. C, Vol. 112, No. 32, 2008 12189
SCHEME 1: Synthesis of 4-(2,4,6-Trimethylphenoxy)benzamide 2a
a
(a) K2CO3, NMP/toluene, 130 °C; (b) phosphoric acid, 130 °C.
SCHEME 2: Proposed Mechanism for the Generation of the Carbonium Ion of 4-(2,4,6-Trimethylphenoxy)benzamide in a PPA/P2O5 Medium
SCHEME 3: Synthesis of 4-(2,4,6-Trimethylphenoxy)benzoyl Functionalized Fullerenea
a
(a) PPA/P2O5, 130 °C.
abundance), 222, 204, 194. 1H NMR (CDCl3, ppm) δ 2.05 (s, 6H, CH3), 2.30 (s, 3H, CH3), 6.81-6.84 (d, 2H, Ar), 6.91 (s, 2H, Ar), 7.53-7.56 (d, 2H, Ar). 13C NMR (CDCl3, ppm) δ 16.10, 20.79, 115.48, 129.07, 129.15, 129.88, 130.48, 134.25, 147.84, 150.03, 161.44. 4-(2,4,6-Trimethylphenoxy)benzamide (2). 4-(2,4,6-Trimethylphenoxy)benzonitrile (49.8 g, 0.21 mol) and phosphoric acid (190 g) were placed in a 250 mL three-necked roundbottomed flask equipped with a magnetic stir-bar, nitrogen inlet, and a condenser. The reaction mixture was then heated at 120 °C for 22 h. During this time, the benzamide product crystallized out of the reaction mixture. The crystals were collected by suction filtration, air-dried, dissolved in warm ethanol, and filtered. The filtrate was allowed to cool to room temperature to afford 37.0 g (69% yield) of white crystal: mp 244–246 °C. Anal. Calcd for C16H17NO2: C, 75.27%; H, 6.71%; N, 5.49%. Found: C, 74.92%; H, 6.62%; N, 5.29%, FT-IR (KBr, cm-1): 1642 (CdO stretch), 3215, 3386 (N–H stretch). Mass spectrum (m/e): 255 (M+, 100% relative abundance). 1H NMR (DMSO-
d6, ppm) δ 1.20 (s, 6H, CH3), 2.26 (s, 3H, CH3), 3.40 (s, NH2), 6.73–6.75 (d, 2H, Ar), 6.97 (s, 2H, Ar), 7.81–7.83 (d, 2H, Ar). 13C NMR (DMSO-d , ppm) δ 15.84, 20.43, 113.83, 127.68, 6 129.75, 130.16, 131.71, 134.54, 147.99, 159.77, 167.41. PPA Treatment of Fullerene (C60) in a Reaction Condition (1). Fullerene (C60, 0.15 g, 0.21 mmol) and PPA (83% assay, 20 g) were placed in a 100 mL resin flask equipped with a hightorque mechanical stirrer and a nitrogen inlet and outlet. The mixture was stirred under dried nitrogen purging at 130 °C for 3 h, and then P2O5 (5.0 g) was added in one portion. The initial heterogeneous dark mixture became a homogeneous brown solution after 8 h. The temperature was maintained 130 °C for 48 h. After cooling down to room temperature, water was added. The resulting black needle-like precipitates were collected, washed with diluted ammonium hydroxide, Soxhlet-extracted with water for 3 days to remove residual PPA and methanol for 3 days to remove other low molar mass impurities, and finally dried over phosphorus pentoxide under reduced pressure (0.05 mmHg) at 50 °C for 72 h to give 0.13 g (87% crude yield)
12190 J. Phys. Chem. C, Vol. 112, No. 32, 2008
Figure 1. MALDI-TOF mass spectra: (a) as-received C60; (b) C60(TMPBA)x adduct 3.
TABLE 1: Elemental Analysis sample as-received C60 PPA-treated C60 C54(TMPBA)6 a
elemental analysis C (%) H (%) O (%) P (%) calcd found calcd found calcd found
100.00 99.40 100.00 98.29 86.68 85.19
0.00 1.05 0.00 1.03 4.05 4.40
Lim et al.
Figure 2. Mass spectra of C60 treated in PPA/P2O5 at 130 °C for 48 h: (a) the first attempt; (b) the second attempt.
SCHEME 4: Possible Structure of PPA-Treated C60a
0.00 0.00
